Synchronous motors: design, operating principle and repair

A synchronous motor is an AC electrical machine whose rotor, in steady state, rotates in synchronism with the stator’s magnetic field. Synchronous speed is defined by the supply frequency and the number of poles:

nₛ = 120f / P

where:

• nₛ — synchronous speed, rpm;
• f — supply frequency, Hz;
• P — number of poles.

For example, at 50 Hz:

In normal synchronous operation, actual rotor speed equals these values and is practically independent of mechanical load, as long as the motor does not fall out of synchronism. The rotor field can be produced by a DC winding, permanent magnets, a reluctance rotor design, or a combined magnetic system.

Main types of synchronous motors:

• electromagnetically excited;
• with a brush excitation system;
• with brushless excitation;
• permanent-magnet;
• synchronous reluctance;
• hysteresis;
• salient-pole;
• non-salient-pole (cylindrical rotor);
• low-speed and high-speed;
• high-voltage and explosion-proof;
• variable-frequency-drive motors;
• synchronous condensers.

General rated characteristics, duty types and requirements for most rotating electrical machines are set out in IEC 60034-1:2026. Methods for determining the parameters of electrically excited synchronous machines from test results are covered by IEC 60034-4 and IEC 60034-4-1.

The word "synchronous" means that the rotor and the stator’s magnetic field move at the same angular speed. The stator creates a rotating magnetic field, and the rotor has its own magnetic field. In operation, the rotor field "locks" onto the stator field, and the rotor rotates together with it.

Unlike an induction motor:

• a synchronous motor does not need constant slip to produce torque;
• rotor current or magnetic flux is produced by a separate system;
• rotor speed exactly matches the stator field frequency;
• a change in load mainly shifts the angle between the fields, not the speed.

The operation of an electromagnetically excited three-phase synchronous motor can be described as follows:

1. 01Three-phase voltage is applied to the stator winding.
2. 02Currents shifted in time flow in the stator phases.
3. 03The stator winding produces a rotating magnetic field.
4. 04DC field current is supplied to the rotor.
5. 05The rotor winding produces a magnetic field fixed relative to the rotor.
6. 06Once the rotor accelerates to a speed close to synchronous, its field locks into synchronism with the stator field.
7. 07The interaction of the two magnetic fields produces electromagnetic torque.
8. 08The rotor continues to rotate exactly at synchronous speed.

When load increases, the rotor does not begin to continuously rotate slower, as an induction motor would. Instead, the angular lag of the rotor’s magnetic axis behind the stator field increases. This angle is called the load angle, torque angle, power angle, or electrical angle δ. As long as the angle stays within allowable limits, the motor remains in synchronism.

A synchronous motor can include:

• a frame, stator core, stator winding;
• a rotor, rotor poles, field winding;
• a damper cage;
• slip rings and a brush gear;
• a brushless exciter and a rotating rectifier;
• an automatic voltage/excitation regulator;
• a shaft, bearings, bearing brackets or pedestal bearings;
• fans and heat exchangers;
• a terminal box, temperature, vibration and rotor-position sensors;
• a starting system and a protection system.

Stator

In general design, the stator of a synchronous motor is similar to that of an induction machine: a frame, an electrical-steel core, slots, teeth, a stator winding, insulation, wedges, bandages, ventilation ducts and temperature sensors.

The stator winding is usually three-phase and can be random-wound or form-wound, single- or double-layer, lap or wave, with round or rectangular wire, low- or high-voltage. Large high-voltage motors use form-wound coils or bars with multi-layer mica ground insulation.

Stator core

The core is made of thin electrical-steel laminations, insulated from one another to limit eddy currents. In large machines, the core can be built from segments with radial ventilation ducts between packs.

Core condition affects iron losses, no-load current, heating, magnetic symmetry, vibration level, local hot spots and synchronous-operation stability.

Damage to the interlaminar insulation is especially dangerous after rotor-to-stator rubbing, a short circuit, uncontrolled winding burnout, mechanical removal of conductors, or welding work on the core.

The rotor produces the magnetic field that interacts with the stator field. By the principle of flux generation, rotors are divided into electromagnetically excited, permanent-magnet, reluctance, hysteresis and combined types. By magnetic-system geometry, they are salient-pole or non-salient-pole (cylindrical).

Salient-pole rotor

A salient-pole rotor has separately distinguishable magnetic poles. Each pole usually consists of a pole body, a pole shoe, a field coil, insulation, fastenings, damper bars and interpole connections.

Salient-pole design is typical of low-speed, multi-pole motors — drives for mills, pumps, compressors, low-speed industrial machinery, and synchronous condensers.

Advantages:

• ability to accommodate a large number of poles;
• relatively easy access to coils and convenient repair of individual poles;
• ability to build a rotor of large diameter;
• suitability for low speeds without a gearbox.

Disadvantages:

• more complex mechanical construction;
• non-uniform air gap;
• significant centrifugal forces on pole fastenings;
• limits on very high speeds.

Pole shoe and field coil

The pole shoe distributes magnetic flux, shapes the air gap, affects the field shape, supports the damper bars, and reduces local saturation and synchronous reactances. Its damage can cause field asymmetry, vibration, uneven heating, torque pulsation and a risk of stator rub.

The field coil of a salient-pole rotor can be made from rectangular copper wire, copper busbar, strip conductor, or a rigid multi-turn construction. Coils are connected so neighboring poles alternate: N, S, N... During repair, the number of turns, winding direction, polarity, conductor cross-section, resistance, insulation, geometry, interpole connections and mechanical fastening must all be precisely restored.

Non-salient-pole (cylindrical) rotor

A non-salient-pole, smooth, or cylindrical rotor has no separate projecting poles — the field winding is embedded in slots of a solid rotor body. This design is used mainly in high-speed synchronous machines, two- and four-pole motors, compressor drives, and turbomachinery.

Advantages:

• high mechanical strength;
• good suitability for high speed;
• relatively uniform air gap;
• lower windage losses and lower field ripple.

Disadvantages:

• complex rotor and winding-repair design;
• high balancing requirements;
• significant mechanical stresses;
• complex fastening of winding end-turns.

The choice between salient-pole and cylindrical construction depends primarily on speed, power, mechanical loads and the required number of poles.

The field winding produces the rotor’s constant magnetic field. Field current determines magnetic flux, internal EMF, reactive power, power factor, static stability margin, and rotor heating. Too little field current causes under-excitation, too much causes over-excitation.

Main excitation system options:

1. 01Brush excitation from an external DC source.
2. 02Excitation from a static thyristor or transistor converter.
3. 03Brushless excitation.
4. 04Excitation from an auxiliary generator.
5. 05Permanent magnets.
6. 06Combined systems.

Brush excitation system

DC is delivered to the rotor winding through slip rings, brushes, brush holders and current-carrying bars.

Advantages:

• a relatively simple electrical scheme;
• direct control of rotor current;
• quick measurement of excitation parameters;
• convenient forcing and field suppression.

Disadvantages:

• brush wear, dust, sparking;
• a need for regular maintenance;
• slip-ring wear, sensitivity to contamination;
• restrictions in explosive atmospheres.

Slip rings can be made of copper, bronze, steel or special alloys and must have correct geometry, minimal radial runout, a smooth working surface, reliable insulation and a stable electrical connection to the winding. Typical defects: grooving, burning, ovality, waviness, cracks, loose fastening, insulation breakdown, contamination with copper-graphite dust.

The brush gear includes brushes, brush holders, springs, a rocker ring, flexible leads and insulators. Maintenance checks brush grade, pressure, contact, height, free movement, temperature, sparking and current-sharing uniformity. Excessive pressure causes mechanical wear and heating; insufficient pressure causes sparking, unstable contact and ring burning.

Brushless excitation system

In a brushless system, an exciter is mounted on the same shaft as the main machine: an exciter field winding on the stator, a rotating armature winding, a rotating diode rectifier, the main rotor’s field winding, and sometimes a permanent-magnet pilot exciter. AC produced by the rotating exciter armature is rectified by diodes directly on the rotor and fed to the main field winding.

Advantages:

• no operating brushes or brush dust;
• less regular maintenance;
• suitability for harsh conditions;
• higher operational reliability with proper maintenance.

Disadvantages:

• more complex diagnostics;
• rotating diodes, difficulty measuring rotor current directly;
• a need to inspect the rotating rectifier;
• greater repair complexity.

Static excitation system and AVR

In a static system, DC is produced by a stationary controlled rectifier and fed to the rotor through slip rings. This provides fast regulation, forcing capability, precise control and rapid field suppression, but retains the brush gear and depends on power electronics and the control system.

An automatic voltage regulator (AVR) can control field current, power factor, reactive power, voltage, stability margin, field forcing and rotor-current limiting — maintaining a target cos φ, reactive power, field current, or bus voltage.

In permanent-magnet synchronous motors, the rotor field is produced by magnets — neodymium-iron-boron, samarium-cobalt, ferrite, or other magnetic materials. Magnets can be placed on the rotor surface, inside the rotor, in special magnetic barriers, or in a combined reluctance-magnetic design.

Advantages:

• no losses in a copper field winding;
• high efficiency and high power density;
• compactness, high power factor;
• precise speed control and good dynamic performance.

Disadvantages:

• high magnet cost, risk of demagnetization;
• difficult rotor repair;
• dangerous attraction forces during assembly;
• temperature-dependent properties;
• the need for a frequency converter in most industrial drives.

Surface-mounted and interior magnets

Surface magnets sit on the outer rotor surface: a relatively simple magnetic circuit and small inductive asymmetry, but mechanical retention is difficult and there is a risk of detachment at high speed, requiring a retaining sleeve or bandage.

Interior magnets are embedded inside the rotor stack: better mechanical protection and the ability to use the reluctance torque component, but more complex geometry, more complex repair and high assembly requirements.

Demagnetization

Can occur due to overheating, excessive stator current, an incorrect current angle, short circuits, a strong opposing field, mechanical damage, or magnet aging or corrosion.

Symptoms: reduced torque, increased current, uneven back-EMF, vibration, torque ripple, phase-parameter asymmetry.

In a synchronous reluctance motor, the rotor does not necessarily have permanent magnets or a field winding — torque arises from the rotor’s tendency to align with the position of minimum magnetic reluctance. The rotor has different magnetic permeance along the direct axis d and the quadrature axis q; this difference is created by flux barriers, special rotor shaping, anisotropic structure, cutouts and bridges.

Advantages of reluctance synchronous motors:

• no rotor winding and no permanent magnets;
• low rotor losses, high efficiency;
• a relatively simple active part, no brushes;
• suitability for variable-frequency operation.

Disadvantages:

• requires a frequency converter;
• torque ripple, acoustic noise;
• complex electromagnetic design and demanding rotor geometry;
• lower power density than some magnet motors.

In a hysteresis synchronous motor, torque is created through the rotor’s magnetic hysteresis. Advantages: smooth starting, low noise, uniform torque, precise synchronous speed. Used mainly in low-power drives, clock mechanisms, instruments, audio equipment, and special precision installations.

The damper winding, or amortisseur/starting cage, sits in the pole shoes of a salient-pole rotor and consists of bars connected by short-circuiting rings or segments. Its functions: asynchronous starting, damping rotor oscillations, reducing speed ripple, improving dynamic stability, damping hunting, and stabilizing during load changes. During starting it behaves much like the squirrel cage of an induction motor.

Asynchronous (induction) starting

If a synchronous motor has a damper cage, it can be started as an induction motor:

1. 01The stator is connected to the supply.
2. 02The rotating field induces current in the damper cage.
3. 03An asynchronous torque appears.
4. 04The rotor accelerates to a speed close to synchronous.
5. 05Field current is applied at the right moment.
6. 06The rotor pulls into synchronism.

Successful synchronization depends on the load torque, mechanism inertia, voltage, damper-cage condition, field current, the timing of excitation application, rotor angle, and acceleration time.

Why a synchronous motor does not always self-start

If a stationary rotor has only a fixed magnetic field, the stator’s rotating field rapidly reverses the direction of electromagnetic torque relative to the rotor, and the average starting torque can be insufficient or close to zero. This is why starting relies on a damper cage, an auxiliary motor, a frequency converter, a special ramp-up system, a starting induction winding, or a ramp from zero frequency.

Starting from a frequency converter

The converter starts at low frequency — the stator field rotates slowly, and the rotor can stay synchronized with it during acceleration. This gives smooth starting, current limitation, torque control, and the ability to start under load, and is especially important for permanent-magnet motors, reluctance synchronous motors, large compressors, mills and servo systems.

Alternatively, the rotor can be accelerated to near-synchronous speed by an auxiliary motor, after which excitation is applied, the synchronous motor is connected to the supply, and the auxiliary drive is disconnected. To ease starting, guide vanes may also be closed, the compressor unloaded, the process load disconnected, or a pump started against closed or throttled valves.

Pulling into synchronism

Successful synchronization requires a sufficiently small speed difference, correct field current, an allowable load, sufficient synchronizing torque, a sound damper system, and correct control. If the motor fails to synchronize, it can continue running asynchronously, draw high current, heat the damper cage, produce torque pulsations, and trip protection.

In steady synchronous operation, slip s = 0, since n = nₛ. But during asynchronous starting, transients, loss of synchronism, or rotor hunting, the instantaneous speed can differ from the field speed — this is not the constant working slip characteristic of an induction motor.

Unlike an induction motor, the rated speed of a synchronous motor will not be, for example, 1480 rpm for four poles at 50 Hz — it will be close to exactly 1500 rpm, allowing for measurement tolerance and grid frequency.

Load angle δ

The angle δ is the angle between the rotor field’s magnetic axis and the resultant rotating stator field. At no load the angle is small; as mechanical load increases, the angle grows — the more torque the shaft requires, the more the rotor field lags the stator field. Up to a certain point, a larger angle increases electromagnetic torque, but once the stability limit is exceeded the motor can fall out of synchronism.

In the simplified case of a cylindrical-rotor synchronous machine, electromagnetic power is approximately:

P ∼ (UE / Xₛ) sin δ

where:

• U — stator voltage;
• E — internal EMF related to excitation;
• Xₛ — synchronous reactance;
• δ — load angle.

This shows that transferred power depends on grid voltage, field current, machine parameters, and the angle between the fields. A salient-pole machine additionally has a reluctance torque component arising from the difference between the d- and q-axis reactances.

Maximum torque and loss of synchronism

The maximum torque beyond which the motor loses synchronism is called the pull-out torque or maximum synchronous torque. If load exceeds this limit, angle δ keeps increasing, the rotor is no longer held by the field, torque impacts occur, speed becomes unstable, stator current rises, and protection must trip the machine.

Causes of loss of synchronism: a sudden overload, a voltage dip, loss of excitation, a rotor short circuit, incorrect regulator operation, faulty rotating diodes, too large a load angle, a supply disturbance, a mechanical shock, or mechanism jamming.

Symptoms: current pulsations, loud noise, torque impacts, active- and reactive-power swings, vibration, unstable speed, damper-cage heating, tripping of the out-of-step protection.

Rotor hunting

After a load change, the rotor can oscillate angularly around the stator field — hunting, or electromechanical oscillation. Causes: abrupt load changes, a pulsating mechanism torque, weak damping, a faulty damper cage, unstable excitation control, or grid fluctuations. The damper cage is meant to suppress such oscillations.

Mechanical shaft load mainly determines the active power the motor draws. Field current largely determines reactive power. By adjusting excitation, a synchronous motor can operate with a lagging current, near-unity power factor, or a leading current — one of the key advantages of an electromagnetically excited synchronous motor.

Under-excitation, normal excitation, over-excitation

With insufficient rotor current, the motor draws reactive power from the grid, and stator current typically lags voltage. Excessive under-excitation raises stator current, increases stator heating, reduces stability margin, and can trip loss-of-excitation protection.

At a certain field current, the reactive component of current can be minimal, power factor approaches unity, and for a given active power, stator current is at its minimum value.

With increased field current, a synchronous motor can deliver reactive power to the grid — stator current leads voltage. This allows it to compensate the reactive power of induction motors, reduce the grid’s reactive burden, and improve a plant’s power factor. Excessive over-excitation can overload the field winding, heat the stator, exceed the allowable field current, and shorten insulation life.

V-curves

A V-curve shows stator current versus field current at constant mechanical load: at low excitation, stator current is elevated and the motor absorbs reactive power; at optimal excitation, stator current is minimal and power factor approaches unity; at excessive excitation, current rises again and the motor delivers reactive power. The plot resembles the letter V.

Synchronous condenser

A synchronous condenser is a synchronous machine operating with essentially no useful mechanical load that controls reactive power: it supports voltage, compensates reactive power, raises short-circuit power, provides electromechanical inertia, and improves grid stability, particularly with a high share of converter-based generation. Once synchronized to the grid, such a machine behaves electrically like a synchronous motor with no useful load, while controlled excitation lets it regulate reactive power.

Efficiency

High-power synchronous motors can achieve very high efficiency. Losses include stator copper losses, field-winding losses, iron losses, mechanical and windage losses, brush-contact losses, exciter and rotating-rectifier losses, and additional stray load losses. Permanent-magnet motors have no continuous copper losses in a field winding, but can have eddy-current losses in the magnets, bandage losses, and additional harmonic losses from the converter.

Advantages of synchronous motors:

• precise constant speed, no rated slip;
• high efficiency, ability to run near-unity power factor;
• ability to generate reactive power;
• high efficiency at large power ratings;
• ability to drive low-speed loads directly, without a gearbox;
• high torque in variable-speed drives, precise position control;
• usability as a reactive-power compensator.

Disadvantages:

• more complex line starting;
• the need for an excitation system in electromagnetic machines;
• a more complex rotor design, risk of losing synchronism;
• more complex protection and a need to regulate excitation;
• brush maintenance or rotating diodes, depending on design;
• demanding frequency-converter tuning;
• complex permanent-magnet repair;
• stricter balancing requirements.

High-voltage synchronous motors

Industrial synchronous motors can operate at 3, 6, 10, 11, 13.8 kV and other special voltages, driving compressors, pumps, mills, blowers, exhausters, rolling-mill equipment and large fans. Features: form-wound stator coils, mica ground insulation, vacuum-pressure impregnation, corona-protection coatings, partial-discharge monitoring, an advanced cooling system, a dedicated excitation system, and complex relay protection.

Low-speed and high-speed synchronous motors

A low-speed motor has a large number of poles and can couple directly to a mill, compressor, pump, or crusher — eliminating a gearbox and reducing mechanical losses and maintenance, but requiring a large diameter, significant mass, and a reinforced foundation.

High-speed synchronous motors are used for centrifugal and turbo compressors and other high-speed units. Features: a non-salient or special rotor, high-precision balancing, critical-speed control, reinforced bandages, sliding or special rolling bearings, complex cooling, and precise control systems.

Vertical synchronous motors

Used mainly in large vertical pumps, pumping stations, water-supply systems, irrigation, and hydraulic structures. May have upper and lower guide bearings, a thrust bearing, a dedicated oil system, water cooling, and a hollow shaft. The thrust bearing carries the rotor mass, the pump’s axial thrust, and hydraulic loading.

Explosion-proof synchronous motors

During repair of an explosion-proof machine, flameproof gaps, frame materials, fastenings, seal type, cable entries, the fan, maximum surface temperature, winding data, the impregnation system, bearing design, and the excitation method must not be arbitrarily changed — repair must follow the documentation and the requirements of the applicable explosion-protection certification.

Variable-frequency-drive synchronous motors

A converter can control frequency, voltage, stator current, torque, flux, load angle, rotor position, speed, and field current using scalar or vector control, direct torque control, sensorless control, or control with an encoder or resolver.

Precise control uses encoders, resolvers, Hall sensors, inductive or magnetic sensors, or sensorless algorithms. Incorrect rotor-position sensor phasing can cause failure to start, reverse torque, overcurrent, vibration, jerking, magnet demagnetization, or converter failure.

Operation above base speed requires reducing effective magnetic flux (field weakening) — in an electromagnetically excited machine, this is done by reducing rotor current, while in a magnet machine the converter injects a current component that partially opposes the magnet field. Limits: converter voltage, stator current, rotor mechanical strength, heating, demagnetization risk, critical speeds, and bearings.

Possible cooling arrangements:

• natural air cooling, self-ventilation, independent (forced) ventilation;
• a totally enclosed air-to-air heat exchanger arrangement;
• an air-to-water heat exchanger;
• water cooling of the stator or rotor;
• special gas cooling or combined cooling.

Air flow, water temperature, heat-exchanger cleanliness, fan rotation direction, pump operation, filter condition, leaks, and condensation must all be monitored.

Synchronous motors use the IEC S-series duty types: S1 (continuous), S2 (short-time), S3 (intermittent periodic), S4 (intermittent periodic with starting), S5 (intermittent periodic with starting and electric braking), S6 (continuous periodic), S7 (continuous periodic with starting and braking), S8 (continuous periodic with speed/load changes), S9 (non-periodic with overloads), S10 (discrete constant loads). General requirements for rated duty types and machine characteristics are set out in IEC 60034-1:2026.

Faults can be grouped into stator, rotor, excitation, damper, mechanical, bearing, ventilation, control, operational, and insulation faults.

Stator faults

A stator interturn fault results from insulation aging, overheating, voltage surges, partial discharges, vibration, poor impregnation, contamination, or coil movement, leading to a local circulating current, overheating, conductor destruction, a phase-to-phase fault, or a ground fault.

A phase-to-phase fault can occur in the slot, in the end-winding, in inter-coil connections, in the terminal box, or where a coil exits the slot — especially dangerous for high-voltage form-wound windings. A stator ground fault is caused by damaged slot insulation, moisture, contamination, a sharp core edge, coil vibration, partial discharges, terminal damage, or overheating. An open stator phase shows up as current asymmetry, reduced torque, pulsations, overheating and inability to synchronize.

Field-winding and damper-cage faults

A field-winding ground fault can occur relative to the pole core, the rotor body, the shaft, an interpole connection, or a slip ring. A single ground fault does not always immediately stop operation but creates a dangerous precondition for a second ground fault, which can short-circuit part of the winding, cause field asymmetry, trigger vibration, and damage the rotor.

A field-winding interturn fault shows up as reduced resistance, increased current at the same voltage, magnetic-field asymmetry, vibration, and local pole overheating. An open field winding (from interpole-connection fatigue, a poor solder joint, a conductor crack, centrifugal loading, or overheating) causes loss of field, a sharp rise in reactive current, loss of synchronism, and rotor heating.

Loss of excitation is one of the most dangerous faults: causes — exciter failure, a rotor open circuit, brush failure, diode damage, loss of regulator power, thyristor-rectifier failure, or an automation error. Consequences: a shift to asynchronous operation, significant reactive-power consumption, heating of the damper cage and poles, torque ripple, loss of synchronism, and rotor damage.

A rotating-diode fault (open circuit, breakdown, elevated forward resistance, poor heatsink contact, lead crack, loose bolted connection) shows up as unstable excitation, increased vibration, exciter heating, current ripple, and insufficient torque.

A broken damper bar (from frequent or prolonged starts, asynchronous operation, thermal cycling, metal fatigue, or a solder defect) worsens starting, increases synchronization time, and causes hunting and local pole-shoe overheating. A cracked short-circuiting segment causes uneven current distribution, heating of adjacent bars, reduced damping, and joint failure.

Mechanical faults and permanent-magnet faults

Pole looseness is a dangerous mechanical defect (loose bolts, broken wedges, fastening fatigue, corrosion, incorrect assembly, overspeed) that alters the air gap, causes vibration, stator rub, and can lead to catastrophic rotor failure. Pole-coil deformation (centrifugal forces, short circuit, loose fastening, thermal expansion) can lead to rubbing, breakdown to the core, and interturn faults.

Permanent-magnet faults: demagnetization, cracking, corrosion, delamination, displacement, detachment, bandage damage, non-uniform flux. Slip-ring faults: wear, grooving, burning, ovality, runout, cracks, insulation breakdown, loose fastening, contamination.

Excessive brush sparking is caused by the wrong brush grade, insufficient or excessive pressure, ring runout, contamination, an uneven surface, vibration, uneven current sharing, poor seating, or damage to a flexible lead.

A non-uniform air gap (bearing wear, pole looseness, a bent shaft, frame misalignment, incorrect assembly) causes magnetic-field asymmetry, unbalanced magnetic pull, vibration, local heating, and rubbing.

Synchronous-motor overheating can be caused by overload, incorrect excitation, voltage asymmetry, cooling contamination, fan failure, a stator defect, a rotor interturn fault, diode failure, asynchronous operation, frequent starts, elevated harmonics, or mechanical friction.

Increased vibration can be caused by imbalance, misalignment, soft foot, a bent shaft, a non-uniform gap, rotor-field asymmetry, shorted turns, a damper-cage defect, hunting, a loose pole, a bearing defect, resonance, or a pulsating load.

If the motor fails to synchronize, check supply voltage, load torque, acceleration time, the damper cage, field current, the timing of excitation application, rotating diodes, the regulator, phasing, mechanical jamming, and unit inertia. If the motor falls out of synchronism under load, possible causes include excessive load, insufficient excitation, reduced voltage, a rotor-winding defect, faulty diodes, a weak damper cage, mechanical-torque fluctuations, incorrect regulator tuning, or a grid short circuit.

Diagnostics should cover the stator, rotor, excitation system, damper winding, mechanical parts, cooling system, control system, and the unit as a whole.

Visual inspection

Check the stator end-windings, bandages, wedges, terminals, contamination, signs of partial discharge, pole coils, interpole connections, damper bars, pole fastenings, slip rings, brushes, diodes, fans, bearings, and heat exchangers.

Electrical measurements

Stator insulation resistance is measured phase-to-phase and phase-to-ground, plus for auxiliary windings and sensors, taking temperature, humidity, rated voltage, and prior measurements into account. Rotor insulation resistance is checked for the field winding to ground, slip rings to shaft, the exciter, the rotating rectifier, and interpole connections.

DC resistance is measured for the stator phases, field winding, pole coils, interpole connections, and exciter winding, with results referred to a common temperature — asymmetry can indicate an open circuit, a poor joint, shorted turns, or a rewinding error.

A surge (impulse interturn) test is applied to the stator winding, individual coils, pole coils, and the exciter winding, and can reveal interturn shorts, weakened insulation, a turn-count mismatch, or an incorrect connection. A hipot (high-voltage withstand) test checks the dielectric strength of the stator, rotor, auxiliary circuits, and exciter insulation, following a controlled procedure that accounts for insulation condition and history.

Partial-discharge monitoring is especially important for high-voltage windings and can be done offline or online, during operation or after repair — it helps detect internal voids, delamination, slot-section defects, degraded corona protection, and contamination. For machines with converter-fed stator or rotor windings, IEC TS 60034-27-6 on online partial-discharge detection and monitoring was issued in 2026.

After repair, pole alternation polarity must be verified — a single reversed pole causes gross field asymmetry, absence of normal torque, vibration, elevated current, and inability to run synchronously.

Shorted rotor turns are identified by comparing resistance, measuring the voltage drop across coils, monitoring magnetic flux, thermal imaging, vibration analysis, flux-density measurement, special impulse methods, or field-current monitoring.

Rotating diodes are checked for forward conduction, reverse blocking, leakage, breakdown, thermal contact, and mechanical fastening — checking with an ordinary multimeter alone is not always enough to reveal a defect that only shows up under working current or temperature.

Mechanical and thermal diagnostics

The damper cage is checked by visual inspection, dye-penetrant and eddy-current testing, ultrasonic testing, electrical-continuity measurement, thermal imaging during testing, and analysis of the starting characteristic and oscillations.

The air gap is measured at several points around the circumference and along the length, accounting for each pole’s geometry, rotor position, axial offset, and runout on salient-pole machines. Large asymmetry can indicate rotor displacement, a bent shaft, pole looseness, stator deformation, or bearing wear.

Vibration diagnostics can assess imbalance, misalignment, soft foot, resonance, bearings, electromagnetic asymmetry, hunting, pulsating torque, foundation looseness, and coupling defects. Thermal imaging reveals hot poles, overheated brushes, uneven slip-ring heating, defective terminals, overheated bearings, heat-exchanger failure, stator asymmetry, and a faulty diode.

Current and power analysis tracks phase currents, active and reactive power, cos φ, field current and voltage, harmonic content, and fluctuations under changing load. Unstable reactive power can indicate a faulty regulator, an excitation defect, hunting, grid instability, or mechanical oscillation.

1. 01Equipment receiving and completeness check.
2. 02Failure-history review and nameplate reading.
3. 03Excitation-scheme analysis and initial electrical measurements.
4. 04Inspection of the brush or brushless assembly.
5. 05Vibration check if a run is possible.
6. 06Marking part positions.
7. 07Disconnecting and discharging the excitation circuits.
8. 08Removing the brush gear and opening the frame.
9. 09Removing the rotor.
10. 10Inspecting the stator and rotor for defects, checking the poles.
11. 11Checking the damper cage and slip rings.
12. 12Checking the exciter and rotating diodes.
13. 13Checking the shaft, bearing assemblies, and air gap.
14. 14Checking the stator core.
15. 15Repairing or rewinding the windings.
16. 16Restoring mechanical parts.
17. 17Impregnation and curing.
18. 18Verifying polarity and restoring interpole connections.
19. 19Balancing the rotor.
20. 20Fitting bearings and assembling the machine.
21. 21Adjusting axial position and checking the gap.
22. 22Commissioning the excitation system.
23. 23Start-up testing and pull-in to synchronism.
24. 24Checking currents and cos φ.
25. 25Vibration and temperature testing.
26. 26Preparing the report.

Stator repair

Can include cleaning, drying, local insulation repair, wedge replacement, bandage restoration, terminal repair, sensor replacement, rewinding, manufacturing form-wound coils, vacuum or vacuum-pressure impregnation, and core repair.

When recording stator winding data, note the number of slots, poles, and phases, winding pitch, turn count, wire cross-section, the number of parallel conductors and parallel paths, connection scheme, group direction, phase interconnections, sensor placement, coil dimensions, insulation construction, corona protection, and copper mass.

Rotor and excitation-system repair

Pole-coil repair can include cleaning, replacing turn and ground insulation, manufacturing a new coil, restoring leads, repairing interpole jumpers, impregnation, testing, and geometry checks. When manufacturing a new coil, the turn count, copper cross-section, winding direction, resistance, dimensions, lead position, cooling scheme, and insulation class must all be restored.

Interpole connections are subject to centrifugal forces, vibration, thermal expansion, and electrodynamic loads — adequate cross-section, flexibility, mechanical fixing, electrical contact, insulation, and cyclic durability must be ensured.

Damper-cage repair can include bar replacement, solder restoration, short-circuiting-segment repair, flexible-jumper replacement, and pole-shoe restoration — replacing a single bar without checking the whole cage may fail to address the root cause.

Slip-ring repair includes turning, grinding, polishing, insulation restoration, ring replacement, lead restoration, balancing, and runout checks; after machining, allowable diameter, surface cleanliness, absence of waviness, and minimal runout must be ensured.

Brushless-exciter repair includes checking the stator and rotating windings, diode and fuse checks, connection repair, insulation checks, balancing, and checking the regulation system.

Permanent-magnet rotor repair requires special safety measures against strong attraction forces, the risk of tool damage, and hazards to electronic devices; possible work includes flux monitoring, magnet inspection, replacement of damaged segments, adhesive restoration, bandage repair, retaining-sleeve restoration, balancing, and back-EMF checks.

Balancing and bearing assemblies

Balancing is mandatory after pole repair, coil replacement, damper-cage repair, diode or heatsink replacement, slip-ring repair, fan replacement, bandage repair, machining, magnet replacement, or shaft repair. Large rotors may require low-speed and high-speed balancing, multi-plane balancing, and a mechanical-condition check at operating speed.

Bearing-assembly repair checks shaft journals, fits, bearing brackets, pedestals, shells, oil rings, seals, lubrication channels, axial position, and electrical insulation. Large synchronous motors often use sliding (sleeve) bearings — advantages: high load capacity, suitability for large shafts, good damping, ability to run at high speed, repairability of the shell; faults: babbitt wear, overheating, scoring, oil-film breakdown, oil contamination, incorrect clearance, misalignment, and electrical erosion.

Shaft currents can arise from magnetic asymmetry, shaft voltage, converter-fed operation, incorrect grounding, or damaged bearing insulation. Insulated bearings and shells, shaft grounding, grounding brushes, proper cable shielding, and converter filters are used to address this.

Can include stator and rotor insulation resistance, absorption ratio, polarization index, phase and field DC resistance, surge testing, hipot testing, polarity checks, diode and damper-cage checks, air-gap control, balancing, rotation-direction checks, a starting test, a pull-in-to-synchronism check, field-current and cos φ control, reactive-power measurement, vibration and temperature monitoring, and a load test.

1. 01Removing poles without marking them.
2. 02Losing polarity data.
3. 03Changing the number of field-coil turns.
4. 04Reducing conductor cross-section.
5. 05Connecting poles incorrectly.
6. 06Loose coil fastening.
7. 07Ignoring the damper cage.
8. 08Repairing one bar without checking the others.
9. 09Fitting diodes with the wrong ratings.
10. 10Incorrect diode polarity.
11. 11Not checking diodes under load.
12. 12Over-machining the slip rings.
13. 13Using the wrong brush grade.
14. 14Uneven brush pressure.
15. 15Skipping balancing.
16. 16Ignoring the air gap.
17. 17Uncontrolled winding burnout.
18. 18Skipping core inspection.
19. 19Fitting the rotor with a displaced pole.
20. 20Not checking the interpole jumpers.
21. 21Incorrect axial position.
22. 22Ignoring bearing insulation.
23. 23Not commissioning the excitation regulator.
24. 24Starting under excessive load.
25. 25No loss-of-excitation protection.
26. 26Not verifying pull-in to synchronism.
27. 27Incorrect position-sensor phasing.
28. 28Exceeding the allowable magnet temperature.
29. 29Assembling a magnet rotor without special tooling.
30. 30Testing with a megohmmeter alone.

What not to do:

• change pole polarity without an electromagnetic calculation;
• apply excitation through a random scheme;
• bypass loss-of-excitation protection;
• start the motor with a faulty damper cage;
• use arbitrary diodes;
• replace brushes based only on physical size;
• operate the motor with excessive ring runout;
• ignore a loose pole;
• grind magnets without the proper technology;
• heat a magnet rotor to an unknown temperature;
• increase speed without checking mechanical strength;
• change the cooling system without a thermal calculation;
• burn out the stator with an open flame;
• limit testing to an insulation-resistance measurement.

Motor fails to synchronize after rotor repair

Check the polarity of all poles, turn count, coil resistance, interpole connections, the damper cage, diodes, field current, the timing of excitation application, and mechanical load.

Motor draws elevated current after a stator rewind

Check turn count, pitch, connection scheme, parallel paths, phasing, core condition, air gap, voltage, field current, and mechanical load.

One pole runs hotter than the others

Possible causes: an interturn fault, a poor interpole connection, incorrect conductor cross-section, a contact defect, a partially shorted coil, or impaired cooling.

Motor runs, but power factor has sharply worsened

Check field current, the automatic regulator, brushes, rings, diodes, the exciter winding, shorted turns, and grid voltage.

Sparking appeared after replacing the brushes

Possible causes: the wrong grade, insufficient seating, incorrect pressure, sticking in the brush holder, an uneven ring surface, uneven brush length, or incorrect material polarity for the operating conditions.

Motor periodically falls out of synchronism

Check load peaks, voltage dips, regulator operation, loss of brush contact, rotating diodes, mechanism fluctuations, compressor surges, protection, and the damper cage.

1. 01Determine the required mechanical power.
2. 02Determine the required speed.
3. 03Determine the number of poles.
4. 04Assess the load torque.
5. 05Determine the unit’s inertia.
6. 06Analyze the starting conditions.
7. 07Choose the rotor type.
8. 08Choose the excitation system.
9. 09Determine the need for reactive-power compensation.
10. 10Choose the voltage.
11. 11Determine the control method.
12. 12Check whether a frequency converter is needed.
13. 13Choose the cooling system.
14. 14Account for the mounting position.
15. 15Determine the required ingress-protection rating.
16. 16Account for explosion-hazard requirements.
17. 17Choose the bearings.
18. 18Provide for sensors.
19. 19Calculate stability margin and overload capacity.
20. 20Plan for service and maintenance.

Synchronous-motor protection can include:

• overcurrent and differential protection;
• ground-fault protection;
• stator and rotor thermal protection;
• loss-of-excitation and out-of-step protection;
• protection against a prolonged start and rotor lock;
• protection against phase asymmetry, undervoltage, and overvoltage;
• monitoring of bearing temperature, vibration, cooling, the brush gear, diodes, and rotor insulation.

Monitoring systems can use Pt100/Pt1000 sensors, thermocouples, PTC sensors, vibration sensors, axial-displacement sensors, air-gap sensors, field-current sensors, shaft-voltage sensors, partial-discharge sensors, water-flow sensors, oil-pressure sensors, and rotor-position sensors.

Routine maintenance regularly checks stator currents, field current and voltage, active and reactive power, cos φ, stator and rotor temperature, bearings, vibration, brushes, rings, diodes, the cooling system, insulation resistance, air gap, foundation condition, alignment, and regulator operation.

• manufacturer, machine type, serial number;
• power, voltage, current, frequency, speed, pole count, power factor;
• field current and voltage, exciter type, rotor type;
• stator winding data, pole-coil data;
• damper-cage, diode, slip-ring, and brush inspection results;
• core-inspection and balancing results, bearing data;
• air gap, insulation resistance, DC resistances;
• surge test results, hipot test results;
• no-load current, reactive power, vibration, temperature;
• the pull-in-to-synchronism result and the conclusion.

• How does a synchronous motor differ from an induction motor?Coming soon
• What is synchronous speed?Coming soon
• What is the load angle?Coming soon
• How does brushless excitation work?Coming soon
• How do you check the rotating diodes?Coming soon
• What is a damper winding?Coming soon
• What is loss of excitation?Coming soon
• What are V-curves?Coming soon
• What is a synchronous condenser?Coming soon
• How are pole coils repaired?
• How is a damper cage repaired?
• How are synchronous rotors rewound?
• How is rotor balancing performed?
• How is the air gap diagnosed?
• How is a turbogenerator built?
• How is a hydrogenerator built?

Electropromremont LLC performs comprehensive diagnostics, repair, rewinding, and testing of industrial synchronous electrical machines.

The scope of work can include:

• incoming diagnostics, disassembly, defect inspection, failure-cause analysis;
• stator, rotor, and excitation-system checks;
• slip-ring, brush-gear, and brushless-exciter inspection;
• rotating-diode and damper-cage checks, pole-fastening inspection;
• air-gap control;
• stator rewinding, manufacture of stator coils, high-voltage insulation repair;
• pole-coil rewinding, manufacture of field coils;
• restoration of interpole connections, repair of damper bars and short-circuiting segments;
• slip-ring repair, turning and grinding, brush-holder repair;
• exciter-winding repair, rotating-diode replacement, rotor-winding repair;
• vacuum and vacuum-pressure impregnation, active-steel repair, core inspection;
• shaft repair, restoration of fits, repair of bearing brackets and pedestal bearings;
• shell restoration, bearing replacement, restoration of bearing-assembly insulation;
• lubrication-system, fan, and heat-exchanger repair, dynamic balancing;
• installation of temperature sensors, insulation-resistance and DC-resistance measurement;
• surge testing, hipot testing;
• pole-polarity verification, diode checks, and pull-in-to-synchronism checks;
• field-current monitoring, power-factor and reactive-power measurement;
• vibration and temperature testing, preparation of the repair report.

A synchronous motor is an AC electrical machine whose rotor, in steady state, rotates exactly at the speed of the stator’s magnetic field. Its defining feature is the absence of working slip. Electromagnetic torque arises from the interaction of the stator’s rotating field and the rotor’s magnetic field; when mechanical load changes, it is not the speed that changes, but the rotor’s angular position relative to the stator field.

Synchronous motors offer important advantages:

1. 01Precise, constant speed.
2. 02High efficiency.
3. 03Ability to run at a high power factor.
4. 04Ability to compensate reactive power.
5. 05Efficiency in large low-speed drives.
6. 06Precise frequency and position control.

At the same time, such a machine requires monitoring of the stator winding, pole coils, damper cage, excitation system, slip rings, brushes, rotating diodes, air gap, balancing, bearings, cooling system, excitation regulator, and out-of-step protection.

Replacing bearings without checking the air gap, rewinding the stator without diagnosing the rotor, or repairing the excitation system without testing pull-in to synchronism does not deliver predictable reliability.

A proper repair should include:

• identifying the root cause of the fault;
• a comprehensive defect inspection;
• checking the stator, poles, and damper cage;
• diagnosing the exciter and checking the diodes;
• restoring mechanical geometry and balancing;
• electrical testing and excitation commissioning;
• verifying synchronous operation;
• documenting the results.

Disclaimer

This article is for general informational purposes. The specific choice of synchronous motor, excitation system, starting method, protection, winding data, rotor current, cooling method, and repair technology must be based on the manufacturer’s documentation, factory drawings, the rated nameplate, electromagnetic calculations, the excitation-system diagram, mechanism characteristics, grid conditions, actual operating duty, ambient conditions, explosion-protection requirements, applicable standards, and the results of professional diagnostics.

Without engineering justification, it is not permitted to change the number of stator or pole turns, conductor cross-section, polarity, the excitation scheme, fit diodes with different ratings, increase rotor current or speed, change the air gap, the damper-cage design, the cooling system, permanent-magnet materials, or protection settings, or operate the machine without loss-of-excitation and out-of-step protection.

Work on high-voltage, explosion-proof, high-speed, magnet, and large-frame synchronous motors must be carried out by qualified personnel using special tooling, controlled processes, calibrated measuring equipment, and appropriate test rigs.