Often, when an electric motor fails, it is difficult to understand why just by looking at it. An abandoned engine in a warehouse may or may not work, regardless of its outward appearance. With a simple tester you can do a quick check of the engine, but before you can actually use it, you need to get and evaluate more information.
Steps
Method 1 of 4: Check the Engine Exterior
Step 1. Check the exterior of the motor
If any of the following problems exist, the motor may have a limited life due to overloads or misuse in the past. Check if there are:
- Broken feet or mounting holes.
- Blackened paint in the center of the engine (indicates overheating).
- Dust residues or other foreign materials got into the motor through the ventilation openings.
Step 2. Read the motor nameplate
It is located on the stator, that is, on the external container or frame of the motor, and is made of metal or another resistant material; it contains all its nameplate data, without which it would be very difficult to determine if the motor is suitable for a certain application. Usually the data contained are (but there may also be others):
- Manufacturer's Name - the name of the company that manufactured the engine.
- Model and Serial Number - information identifying the engine model.
- Revolutions per minute - the number of revolutions the rotor makes in one minute.
- Power - the amount of mechanical power it is capable of delivering.
- Connection diagram - how to connect the motor to obtain different rotation speeds, different voltages and choose the direction of rotation.
- Voltage - operating voltage and number of phases.
- Current - current value required for maximum power.
- Frame - overall dimensions and type of fixing.
- Type - indicates whether it is open structure, splash proof, fully enclosed with cooling fan, etc.
Method 2 of 4: Check the Bearings
Step 1. Start checking the motor bearings
Many of the failures of an electric motor are caused by broken bearings, which serve to smoothly and accurately rotate the motor shaft in the center of the stator. The bearings are located at both ends of the motor, which are sometimes called "lanterns".
There are several types of bearings. Two very common types are the bushing and the steel ball bearing. Bearings that require lubrication have special couplings, while those that do not have it are called "maintenance free" and are permanently lubricated during construction
Step 2. Perform a bearing check
To perform a quick bearing check, place the motor on a hard surface and place one hand on top of the motor while rotating the shaft with the other. Pay close attention to any symptoms of rubbing, creeping, or rotation irregularity. The rotor should turn silently, smoothly and freely.
Step 3. Next, push and pull the tree
A small inward and outward movement (for most appliance motors this should be half a millimeter or less) is acceptable, but the smaller it is, the better. A motor with bearing problems will be noisy during operation, cause the bearings to overheat and could fail catastrophically.
Method 3 of 4: Check the Windings
Step 1. Check that the windings are not grounded
Most household appliance motors, when they have a winding shorted to ground, i.e. towards the casing or frame, do not start and trip the circuit breaker (some industrial-type motors may be ungrounded, so they could also work with a short-circuited winding without tripping any protection).
Step 2. Check the resistance with a tester
Set the tester to measure the electrical resistance (check in the tester manual that the probe plugs are in the correct sockets, they are usually indicated as COM and V) to the highest available range (can be indicated as R x 1000 or M). If possible, reset the reading by touching the probes together and adjusting the pointer to zero. Find the screw for the ground connection of the motor (usually indicated by the colors green and yellow), or any non-insulated metal part of the housing (you can scratch the paint in one place if necessary) and touch it with one of the probes, while with the other you touch the winding clamps one at a time. Be careful not to touch the metal part of the probes with your fingers, as this would make the measurement wrong. Theoretically, the pointer should not deviate from the maximum resistance value that the tester can measure.
- The hand may actually move a little, but the reading should always remain in the million ohms (called megohms) range. Exceptionally, even values of a few hundred thousand ohms (for example 500,000) could be acceptable, but higher values would be better.
- Most digital testers do not allow you to zero the reading, so skip the zeroing step if your device is this type.
Step 3. Check that the windings are not "open" or "blown"
Many of the simpler motors, be they single-phase or three-phase (used at home or industrial level respectively), with windings directly connected to the power supply, can be easily controlled. Just change the tester range to the lowest resistance value, reset the reading again and measure the resistance between the winding terminals. Check on the connection diagram which pairs of terminals are connected to the individual windings.
Expect very low resistance values. You will read very low values, with a single digit. Always be careful not to touch the test leads to avoid distorting the measurement. Higher than expected values indicate a problem, and very high values mean that a winding is broken. A motor with a high resistance winding will not run, or it will not run at a smooth speed (as happens to a three-phase motor when the windings break during operation)
Method 4 of 4: Identify Other Possible Problems
Step 1. Check the starting or power factor correction capacitor, if present
Most capacitors are protected by a metal screen on the outside of the motor. Remove the screen to check and test the capacitor. You may notice oil leaks, bulges, holes, a burning smell, or combustion residues, all of which can indicate problems.
You can check the operation of the capacitor with the tester. By connecting the test leads to the capacitor terminals and measuring the resistance, this should start from a low value and then gradually increase, as the small current generated by the tester charges the capacitor. If the reading remains at zero or in any case does not increase, the capacitor is broken and must be replaced. You must wait at least 10 minutes before repeating this test to give the capacitor time to discharge
Step 2. Check the rear seat of the motor
Some motors have a centrifugal switch to connect or disconnect the capacitor at a precise RPM. Check that the switch contacts are not welded or contaminated with dust and grease, which would prevent effective electrical contact. With a screwdriver, check that the switch mechanism and any other springs present are free to move.
Step 3. Check the fan
A TEFC-type motor is completely enclosed and has a cooling fan, the blades of which are at the rear of the motor enclosed by a metal cage. Check that the fan is securely attached to the rotor and that it is not blocked by dust or other debris. Air must be able to pass through the fan cage freely, otherwise the motor may overheat and be damaged.
Step 4. Choose the right engine for your application
If the engine will be subject to splashing water or moisture, choose a suitable type; if you use an open motor, make sure it never comes in contact with water or moisture.
- The splash-proof motor can be installed in humid or wet environments, as long as it is not subjected to direct jets of water (or other liquids) and no liquid must fall into it.
- Open engines, as the name indicates, are completely open. The end parts of the motor have rather wide openings and the stator windings are clearly visible; the openings of this type of motor must never be closed or obstructed and the motor must never be installed in humid, dirty or dusty environments.
- TEFC type motors, on the other hand, can be used in all the environments indicated above, but they must not be submerged, unless they are specially designed for this purpose.
Advice
- It is not so strange that a winding is interrupted and shorted to ground at the same time. It might seem a contradiction, but it is not: for example a foreign object can fall or be magnetically attracted inside the motor and cut the wire of a winding, or an excessive supply voltage can burn a winding; at this point, if one of the free ends that are created comes into contact with the motor casing, there is a short circuit to ground. Situations like this are infrequent, but they can happen.
- Consult a list drawn up by NEMA for information on motor sizing standards.
