What is buzzing in an electric motor

Günter Pauli GmbH

From catering extractor hoods to high-pressure cleaners, from circular saws to forklifts - a large number of devices in professional use are driven by electric motors. As a rule, we operate a switch, work with the device and do not worry about the motor. Only when one day when we press the on-switch that the electric motor does not start do we notice how dependent we are on the electric motors working.

What to do if the engine strikes First take a look!

The electric motor won't start? Instead of directly replacing the entire motor, it is often worthwhile first to investigate the cause of why the electric motor does not start. Often the problem is not with the engine itself but with the upstream system.

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Let's take the lawn mower as an example. If the lawnmower suddenly stops during operation, we first check the supply line and the plugs to ensure that they are securely in place. Almost every motor has overload protection that protects the motor from overheating in the event of an overload (for example, if the lawn is too high). Let's give the engine a rest period of 10 to 15 minutes and then check it again for functionality.

If this is unsuccessful, the next step is to check the fuse or try a different socket with a different circuit. Running? No? Then the next thing we do is swap the supply cable. Even if the engine is running after that, please note the following:

Thorough cleaning often works wonders

It goes without saying that work tools get dirty is part of normal operation. However, if particularly coarse, sticky or oily substances get inside the machine, this can lead to serious damage. The same applies to particularly fine dusts that can penetrate into the interior of the electric motor through openings. If such soiling is already clearly visible from the outside, especially in the area of ​​the motor openings, the drive must be cleaned at least thoroughly. There is also the risk that more serious damage has already occurred, such as a short circuit.

Visual inspection from the outside: Broken mountings, dark areas, heavy soiling, clogged fans

In tools and machines, electric motors are usually firmly connected to the housing and the other components at predetermined points, for example by screw connections. Mechanical stress can lead to such brackets knocking out or even breaking. The result can be that parts of the machine jam and, for example, the drive shaft can no longer move. So the first look is at the assembly.

Are there any bent or broken screws or brackets? Is the whole motor sitting crookedly in the housing? If this is the case, a new screw connection or a bracket welded by a specialist may help to repair the damage.

You should also check whether there are any dark discolorations on the housing in the area of ​​the middle of the motor. The discoloration can occur in the paintwork as well as in the metal and is an indication of a great deal of heat development. This may indicate a major malfunction in the engine.

Dirt can quickly become a problem, especially with fully encapsulated and fan-cooled motors. From the outside, the fan covers are clearly visible as a metal grille. If these are clogged or clogged with dirt, oil or dust, this can lead to the motor overheating. Cleaning the fan grille (preferably regularly) can help if the electric motor is stuck to the rotating fan wheel due to dirt and therefore does not start.

All data at a glance - the nameplate

It makes perfect sense to look at the motor nameplate. All important data can usually be found there at a glance - important if spare parts or even a replacement motor are required. The name of the manufacturer and the model and serial number make it possible to search the Internet for the relevant documentation for the engine. Concrete construction or circuit diagrams from the manufacturer, which are often made available for download, can be helpful.

The nameplate of the electric motor often also provides information on correct installation - if it looks significantly different in the device, the fault may already have been found. Circuit diagrams showing wiring at different speeds, voltages, or applications can also be helpful. Furthermore, the standard information on voltage and current can usually be found on the nameplate, which should be noted down for the corresponding test with measuring devices.

In addition, it is important to see which type of housing is present before opening it. An open housing is more sensitive to dirt, but easier to open. Drip-proof or even fully encapsulated housings are much more resistant to the ingress of moisture or dirt - when opening, care must be taken not to damage the existing seals. Sometimes fully encapsulated elements can only be completely replaced.

The following work may under no circumstances be carried out by laypeople.

Defective capacitor - if the fault is outside the motor

In the case of a single-phase AC motor with a squirrel-cage rotor, it may help to replace a more inexpensive part such as the starting or operating capacitor so that the motor can run again. However, a defect can have many different causes.

Capacitors can store energy and release it in a directional manner - and they are considered wear parts in the motor. If they are broken, this may mean that the electric motor no longer starts. Capacitors are one of the cheaper replacement parts and are relatively common causes of problems with electric motors.

They are usually attached to the outside of the engine, for example in a separate box with a metal cover. If there are clear bumps on the container, puddles of oil or dirt on the inside, or if the condenser is darkly colored, this indicates possible damage at this point.

Be careful when working on the capacitor!

Extreme caution is required when handling capacitors. Since these are physical passive energy stores, they can also be charged when the entire device is disconnected from the mains. The electric shock from a charged capacitor, no matter how handy the component is, can far exceed that from a socket in intensity and be fatal.

Running or starting capacitor - what's the difference?

Many electric motors are equipped with operating capacitors, which ensure that the motor starts, turns in the right direction and develops its power evenly. Some particularly powerful, difficult-to-start motors also have a so-called starting capacitor. This is used, for example, in compressors or heavy machinery such as floor grinders. In contrast to the running capacitor, the starting capacitor is only active when the motor is started and is then automatically disconnected from the mains.

Signs of a defective operating capacitor

There are a number of signs that a run capacitor is defective. Evidence could be, for example, that the motor only hums and twitches, that it is only starting very slowly or in the wrong direction. If the electric motor does not start up as usual because the capacitor is defective, the motor can often be started by hand in any direction. This means that after the shaft is started, the motor runs by itself.

In general, it is better not to start machines by hand. This is especially true for sharp components such as saw blades or the blades of lawnmowers. Because if the engine is started by hand after it has been started, it is very likely that the fingers are still in the working area of ​​the cutting edges. It goes without saying that the worst injuries can result here.

With the right measuring devices, it is in principle possible to measure capacitors. Since a capacitor is an energy storage device that is potentially charged, there is a real risk that the electronics of the measuring devices will be damaged. If you want to be on the safe side, leave the test to a trained specialist.

Should the capacitor be replaced? All important information that is necessary for an adequate replacement is then on the capacitor itself. These are the capacitance, usually in µF (microfarads), the operating mode (DB for continuous operation with operating, AB for intermittent operation with starting capacitors) and the nominal voltage in volts (V).

Check the centrifugal switch on motors with a starting capacitor

Larger devices such as compressors, which in addition to the operating capacitor also have a starting capacitor, usually have a centrifugal switch in the rear end shield. This separates the starting capacitor from the mains as soon as the motor has reached a certain speed. Grease, dirt or excessive heat can damage or even melt the switch. At this point you can check whether the switching mechanism can be moved freely. If he does not, this can be the reason why the electric motor is not running.

A look inside - the bearings of the electric motor

The bearings of the central drive axle are located at both ends of the electric motor. Often things get stuck at this point in the truest sense of the word. If the axis (rotor) cannot rotate freely in the frame of the motor housing, then nothing runs smoothly. Different types of bearings made of different materials such as steel or brass are installed. The bearings that do not have permanent lubrication may have to be lubricated manually as part of maintenance.

Even a layperson can test on the motor whether the bearings can move freely. To do this, you turn the drive shaft or the rotor by hand and feel whether the whole thing is difficult to move. You can also listen to see whether there is grinding or scraping in the area of ​​the bearings. If a rotor wobbles or shows imbalances, this can also be an indication of a problem with the bearings. Ideally, it should be quiet, smooth and easy to move.

You can also check how much play the shaft has in the motor. In the axial direction it can be up to 1mm. In the vertical direction, however, it should not be more than 0.02mm. In the event of deviations, this can be an indication of a broken part, defective bearings or worn bearing seats.

Last but not least, a check can be made in the rear area of ​​the engine to see whether any fan is running smoothly. If it doesn't, the engine can overheat or even burn out.

At this point I would like to point out once again that everything may only be checked in a voltage-free state. It should also be noted that, for example, lawnmowers have a built-in brake that does not allow the bearings to be checked by rotating them freely.

A look at the windings

The most important elements in an electric motor are the copper windings through which the driving current flows. During the visual inspection you can already see whether there is contact with the housing of the motor due to dirt. A short circuit would then be programmed. The result here too: the electric motor does not start and the fuse trips. Another possible problem is that parts of the winding are burned or are no longer closed in the circuit due to mechanical or thermal damage. A measurement with a special measuring device provides information about this. But visual signs such as significant blackening or mechanical damage can also speak in favor of it.

Heat and stench are not good signs

In some cases, an electric motor that no longer starts can simply no longer be repaired with reasonable effort. If the engine becomes excessively hot, smokes or smells of scorched metal and plastic, there is probably not much that can be done. Then the device is almost certainly irreparably broken and has to be replaced altogether.

The electric motor does not start - if in doubt, a specialist will help

The electric motor does not start and you have not found a fault despite our diagnostic tips? Then only one thing can help - the way to a specialist or the replacement of the entire engine. Whether a repair is worthwhile often depends on the size of the electric motor. Specialized industrial service providers are the first point of contact for electric motors in industrial plants. Günter Pauli GmbH from Dortmund solves problems with defective industrial systems competently and quickly. We offer our service in large parts of North Rhine-Westphalia. Please feel free to contact us.

If there is a fault in smaller electric motors that is difficult to diagnose, replacing the motor can be the more cost-effective solution. Almost all types of electric motors can also be found online in the electric motor shop of your choice.

Save costs in the long term with professional maintenance

Well-maintained engines make a decisive contribution to avoiding malfunctions in industrial plants. Günter Pauli GmbH from Dortmund specializes in industrial services, electric motors and control technology. We offer our services in large parts of North Rhine-Westphalia. On our industrial service page, we present our entire range of products to you. If you like, please contact us directly. We are happy to help!


Tags: starting capacitor, operating capacitor, defect, diagnosis, check electric motor, electric motors, check capacitor, cleaning, nameplate