When an electric motor fails, it is often difficult to determine the cause of the problem by performing a visual examination. Upon commissioning, an electric motor, which has been in storage for a long period of time, may or may not operate, regardless of its physical appearance. A quick check can be done with a simple ohmmeter, but in fact, more information needs to be gathered and evaluated, before the engine is put into service.
Part 1 of 4: examine the exterior of the engine
Step 1. Examine the engine externally
The following clues may have consequences that may shorten the life of the motor, these clues indicate previous overloads or a malfunction or both. Look for:
- damaged brackets or fixing holes,
- blackened paint on the engine frame, which indicates excessive heating,
- the presence of dirt and other foreign matter which has been thrown into the motor windings, through the crankcase openings.
Step 2. Check the motor nameplate
This plate is either made of metal or made of another durable material; it is riveted or fixed to the fixed part or "stator" of the motor. This plate contains important information, which makes it easier to determine the engine's aptitude for a given service. Typical information found on most nameplates is given in the list below, which is not exhaustive. This is the following data:
- the name of the manufacturer: this is the name of the company that built the engine,
- the model and serial number: this information identifies a particular engine,
- the revolution per minute or rpm: this is the number of revolutions made by the rotor in one minute,
- power: this is the work done by the engine per unit of time,
- the wiring diagram: this diagram shows the different possible connections, depending on the different voltages and speeds and the direction of rotation desired,
- The voltage: it indicates the requirements concerning the supply voltage and the phase requirements,
- intensity: it indicates the requirements concerning the supply current,
- size and frame: these are the physical dimensions and the engine mounting model,
- the type of construction: it describes whether the motor is open or closed, whether it is leak-proof and whether it is completely closed and ventilated, etc.
Part 2 of 4: check the bearings
Step 1. Start by checking the motor bearings
Many electric motor failures are caused by bearing failures. Bearings allow the shaft or rotor to rotate freely and smoothly inside the stator. The bearings are located at both ends of the motor, in a housing, which is sometimes referred to as a “rolling sleeve”.
There are several types of bearings. The two most common types of bearings are brass needle bearings and steel ball bearings. Many of them are fitted with greasing accessories, while others are permanently lubricated, so they are said to be "maintenance free"
Step 2. Check the bearings
To perform a quick bearing check, place the motor on a solid surface and place one hand on top of the motor, then turn the shaft or rotor with the other hand. Try to watch, smell, and listen carefully for any signs of friction, scraping, or spinning rotor imbalance. The rotor should turn quietly, freely and evenly.
Step 3. Next, push the shaft in and pull it out of the stator
A small play is allowed, of the order of 3 mm, for the motors of household appliances, but it would be best to “cancel” this play. A motor which has bearing problems will make noise while rotating, in addition, its bearings will heat up and it will eventually fail seriously.
Part 3 of 4: check the windings
Step 1. Check the insulation of the windings from the frame
Most home appliance motors will not run if they have a shorted winding, they will instantly blow the fuse or trip the circuit breaker, however, 600 volt machines are not "grounded" and therefore a 600 volt motor with a shorted winding can operate without tripping the protection fuse or circuit breaker.
Step 2. Use an ohmmeter to check the insulation resistance
Set your device to operate as an ohmmeter, then connect the measurement probes to the appropriate sockets, most often the sockets marked "common" and "ohms". Consult the device's operation manual, if necessary. Choose the highest scale (R × 1000 or similar scale) and check the zeroing of the device, pressing the two probes against each other. Adjust the needle position to zero, if necessary. Look for a ground screw, often it's a green hex head screw or choose a metal part of the frame. Scrape off the paint if necessary, to have good contact with the metal, and place one of the measuring probes there and the other on each motor terminal, in turn. Ideally, the ohmmeter needle should barely move from its initial position, which is maximum resistance. Make sure that your hands do not touch the metal tips of the probes, as this could cause a measurement error.
- The needle may move slightly, but the meter should always indicate a value in the range of a few million ohms or “megohms”. Sometimes values as low as several hundred thousand ohms, about 500,000, may be acceptable, but it is desirable to have a higher resistance.
- Many digital measuring devices do not offer the possibility of zeroing, so do not take this into account if you are using a digital device.
Step 3. Check that the windings are not open circuit or cut
Sometimes, single-phase or three-phase motors of common design, used in household appliances or in industry, can be controlled simply by changing the ohmmeter calibration to the lowest (R × 1), then the reset is carried out. zero of the device and the resistance between the motor terminals is measured. In this case, it is recommended to consult the motor wiring diagram to be sure to check all windings.
Expect to measure a very low resistance in ohms. Most likely, you will have low single digit values. Make sure your hands are not touching the tips of the probes, as this can cause reading errors. Values greater than this order of magnitude indicate a potential problem, and values significantly higher indicate that the winding is open circuit. A motor, whose resistance is high, will not or will work, but without speed regulation, as is the case with a three-phase motor, when one of its windings breaks, during operation
Part 4 of 4: Solve Other Potential Problems
Step 1. Check the starting capacitor or the running capacitor on some motors, if applicable
Most capacitors are protected from damage by a metal cover placed on the outside of the motor. Remove the cover to access the capacitor for inspection and testing. A visual inspection may reveal an oil leak, deformation or holes in the casing, as it is also possible to smell a burning smell or to find residue of fumes or other signs indicating potential problems..
Electrical verification of a capacitor can be done with an ohmmeter. When placing the measuring probes on the capacitor terminals, the resistance should be low at first, then it increases as the low voltage supplied by the meter battery gradually charges the capacitor. If the measured resistance remains low or does not increase, the capacitor probably has a problem and should be replaced. Before attempting a new check, allow the capacitor to stand still for at least 10 minutes, to allow it to discharge
Step 2. Check the box located on the back of the motor
Some motors have centrifugal switches, which control the power supply to the capacitors and windings, depending on the operating speed of the motor. Check that the switching contacts are not welded in the closed position or clogged with dust and grease which can prevent good contact. Check with a screwdriver that the switching mechanism and possibly its spring can operate freely.
Step 3. Check the fan
A “TEFC” type motor is a “totally enclosed and ventilated motor”. The fan blades are protected by a metal cover at the rear of the motor. Make sure the fan is securely attached to the frame and that it is not clogged with dirt or other debris. Air must be able to flow freely and completely through the openings in the rear metal cover, otherwise the engine will overheat and eventually fail.
Step 4. Choose the right engine based on the expected operating conditions
Check that dripping-tight motors are exposed to a directed spray of water or moisture and that open motors are absolutely not exposed to splashing water or moisture.
- Drip-proof motors can be installed in damp or wet locations, as long as they are positioned to prevent the entry of water or other liquids by gravity, these motors should not be subjected to direct spraying. water or other liquids.
- Open motors are, as the name suggests, completely open. The ends of the motor are provided with large openings, through which the stator and rotor windings can be clearly seen. On these engines, the openings must remain completely free, moreover these engines must not be installed in humid, dirty or dusty areas.
- Totally Enclosed Ventilated Motors (TEFC) can be used in all areas mentioned in the previous paragraph, but these motors should not be submerged, unless they are specially designed for this purpose.
- It is not uncommon for a motor's windings to be both "open" and "shorted" at the same time. At first glance, this may seem paradoxical, but it really is not. For example, imagine an electrical failure caused by a foreign object, which falls into the motor or which is magnetically attracted inside the casing or else by an excessive surge which causes the breaking or melting of a winding, such Failure will have the effect of interrupting the current flow path and thus creating "an open circuit". On the other hand, a "short circuit" can occur when one end of the broken conductor comes in contact with the motor frame or another part of the motor, which is grounded. This does not happen frequently, but it is not to be ruled out.
- A list from the American Association of Electrical Equipment Manufacturers "NEMA" can be a quick reference, this list is useful for obtaining data on electric motors of all sizes.