How to check the quality of an electric motor repair

While the motor is still on the repair shop floor, any shortcoming — from an incorrect bolt torque to an unstable no-load current — can be corrected on the spot, by the same people who just did the repair.

Once the motor is installed on site, coupled to its driven mechanism and running, the picture changes: sending it back for a re-check means stopping the process, removing the coupling or pulley, disconnecting cables, and sometimes dismantling part of the adjoining equipment. If an unnoticed fault causes an unplanned shutdown, the resulting losses can dwarf the cost of the repair itself.

Checking at handover is not a sign of distrust in the repair shop — it is standard practice: it confirms that the work performed matches the agreed scope, and that the machine is ready for extended service with no hidden defects.

A reputable repair shop hands over a full document package with a repaired motor — one that supports both the reason for the repair and its outcome.

• a fault-detection (defect) report — the fault found at incoming inspection and the justification for the scope of repair;
• a full set of test protocols — insulation resistance, a high-voltage (HiPot) test, a no-load run, a load test, vibration and balancing readings — depending on the machine type and the work performed;
• a handover certificate listing the parts and materials replaced;
• the warranty terms — what period and scope it covers, what it excludes, and how to file a claim.

• More on repair warranty terms

If any of these documents is missing, ask for it directly before signing the handover certificate — once the motor has been paid for and taken away, getting the protocols afterward is much harder.

Part of the check needs no instruments at all — just a careful look at the machine.

• overall cleanliness and finish — no dirt, metal shavings or oil residue, and covers and end shields fastened securely;
• the condition of the terminal box — an intact housing and seals, terminals properly tightened, no signs of overheating or scorching;
• the condition of the commutator or slip rings, brushes and brush holders — where the motor’s design lets you see them without disassembly;
• no fresh paint over spots that mask old damage — dents, cracked housing, or signs of previous overheating should not simply be painted over;
• the nameplate data matching the actual specification stated in the protocols and the order.

This does not replace electrical and mechanical testing, but it lets you rule out obvious assembly shortcomings on the spot.

A protocol is not just a formality — it is a set of actual measurements that can be checked against the reference values for that machine’s insulation class, voltage and power.

• insulation resistance between the windings and the frame — meeting the minimum acceptable level for the insulation class and voltage, with the measurement temperature noted;
• a high-voltage (HiPot) test — the winding withstands the applied voltage with no breakdown and no sharp drop in insulation resistance during the test;
• no-load current — stable, balanced across the phases, with no sudden spikes;
• load-test results — current, heating and speed matching what is expected for that duty;
• vibration and balancing readings — no signs of imbalance, run-out or resonance at running speed;
• no excessive sparking at the commutator or slip rings, for machines of that design.

Even with every protocol in order, the first hours and days of operation in the motor’s actual location are a continuation of the quality check, not the end of it.

After installation and startup, keep an eye on:

• the temperature of the frame and bearing housings — it should level off gradually rather than keep climbing;
• vibration and noise levels — any increase compared with the first minutes of running deserves attention;
• load current — it should match what is expected for the process it drives;
• any smell, smoke color or other sign of overheated insulation or lubricant.

This running-in period matters because some defects — brushes that are not fully bedded in, a small residual imbalance, a bearing fit that is not quite right — do not show up immediately, but only after a few hours or days under the real load that a test bench never fully reproduces.

Treat any of the following, appearing soon after the motor goes back into service, as a warning sign:

• excessive noise, vibration or heating showing up soon after restart;
• sparking at the brushes or commutator that was not there before the repair;
• the same fault the motor was sent in for coming back within a short time;
• test documentation that is missing or incomplete — no actual measurements, just a general conclusion;
• the repair shop being reluctant to hand over protocols, evasive about warranty terms, or unwilling to put the terms in writing.

None of these on its own always means a botched repair, but together they are a reason to contact the repair shop right away rather than wait and see.

1. 01Shut the motor down if the signs are dangerous — heavy vibration, a burning smell, sparking, a sharp temperature rise.
2. 02Document the problem: the date and time it was noticed, the operating mode, instrument readings, photos or video.
3. 03Compare the current readings against the test protocols received at handover — this helps establish whether the problem is actually related to the repair.
4. 04Contact the repair shop as soon as possible, referencing the warranty terms and the protocols on file.
5. 05Do not attempt to fix the fault yourself before agreeing on it with the repair shop — doing so can be grounds for denying a warranty claim.

The sooner a problem is documented and the more complete the evidence, the easier and faster it is to resolve under warranty.

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EPR (Elektropromremont) hands over a full set of test protocols with every completed repair, regardless of the machine type or the scope of work performed. This lets the customer judge the outcome of the repair objectively before the motor is even installed on site, and keeps a documented history of the machine for future service.

The document package the customer receives includes:

• a fault-detection report describing the fault found;
• insulation-resistance and high-voltage test protocols;
• a no-load test protocol and, where applicable, a load-test protocol;
• vibration monitoring and rotor balancing results;
• the warranty terms for the work performed.

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