What causes hum in an AC motor

Günter Pauli GmbH

More than 50 percent of German electricity consumption goes into the operation of electric motors. There are a number of different types of construction. Most of the electric motors that are operated with normal 230 volt alternating current from the socket are so-called capacitor motors. When such a capacitor motor goes on strike, the component that gives it its name is often to blame. It is therefore a good idea to test the electric motor capacitor. In this blog post we explain how experts proceed when you check the motor capacitor. We go into other possible causes in our article on the subject of "Electric motor won't start".

The good news is that capacitors are easy to replace and usually inexpensive. They are considered to be wearing parts with a limited service life. With a new capacitor, the electric motor often runs without any problems for a long time. In order to find out whether this relatively inexpensive component is actually to blame for a defect, we now want to check the motor capacitor.

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

Where can I find the motor capacitor?

The motor capacitor - or several capacitors in heavy machinery - is usually located on the outside of the electric motor under its own metal cover. As always when handling electricity, the necessary safety measures should be taken before opening the cover. This means: Disconnect the device from the mains by pulling the plug, pressing the off switch or removing the corresponding fuse. It is better not to remove the cover with wet hands or in a damp environment. In addition, it is of course always advisable to work with suitable tools.

safety first

Despite these measures, great caution should still be exercised when checking the motor capacitor. After all, capacitors are passive electrical devices that can store considerable amounts of energy. They can be life-threatening even if the entire motor is disconnected from the mains. The electric shock from a charged capacitor can be much stronger than that from an electrical outlet. The nameplate usually provides information about the voltage applied. But we advise: play it safe. When in doubt, it is always better to have the electric motor capacitor checked by a specialist.

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How do I recognize a motor capacitor?

Capacitors in electric motors are usually a few centimeters in size, cylindrically shaped components. Often, but not always, the words “operating capacitor” (see below), “starting capacitor”, “starting capacitor”, “motor capacitor”, “motor capacitor” or “motor start capacitor” are printed on it. In the vast majority of cases, technical information is printed on the capacity, rated voltage, service life and operating mode.

How do I know when the capacitor is broken?

There are some quite characteristic signs that the motor capacitor is defective. On the one hand, one can conclude from the behavior of the motor that the capacitor is defective. On the other hand, the capacitor can often be seen from the outside when it is no longer in order.

Motor behavior when the capacitor is defective

If the electric motor hums after switching on and possibly still twitches, but does not run properly or does not run at all, a motor capacitor is probably broken. If the motor can then still be started in any direction - i.e. set in constant rotation by a short jerk on the rotating element, this is another very reliable indication of a defective motor capacitor. By the way, you should be very careful with this type of test, as there is a risk of injury. Above all, never throw saw blades or blades, for example on lawnmowers, in this way. Many people overestimate their reflexes and cannot take their fingers out of the danger area quickly enough when the engine suddenly starts. Unfortunately, many accidents with severed fingers speak for themselves here.

If the motor runs in the wrong direction, this can also indicate a defective capacitor. The same applies to a very tough or powerless start-up of the machine. When the machine is loaded, the speed drops very quickly in such a case. If your electric motor runs the wrong way round or is powerless, a defective auxiliary winding of the motor can be to blame in addition to a defect in the capacitor.

External signs on the capacitor

If the engine behavior is normal, the next step is the visual inspection. Before we check the motor capacitor, let's first inspect the cover. If the container in which the capacitor or capacitors are located is already dented or perforated, there could be a purely mechanical impairment of the electrical components.

Broken, dented, deformed, or perforated capacitors should be replaced. Even if there is oil or large amounts of other liquid in the recess for the capacitors, this can have led to a short circuit.

However, the motor capacitor can also have broken during normal operation without any external influence - after all, the components are considered wear parts with a limited service life. In this case, too, we often notice external signs when we check the electric motor capacitor. If, for example, there is insulation damage inside the component, this leads to the development of heat and pressure. Discoloration due to high temperatures is therefore an indication of damage to the capacitor. The same applies if the pressure inside the cylindrical component has been inflated and deformed into a bulbous shape.

Another indication of a defective capacitor is when liquid oozes out of the capacitor itself. This is an insulating liquid that is often slightly corrosive. It can attack hands or paintwork. If the capacitor is leaking, you can assume that the component is defective and needs to be replaced. If nothing can be seen from the outside, we continue troubleshooting by checking the motor capacitor with suitable measuring instruments.

Checking a motor capacitor with the measuring device - how does it work?

If there is no visible damage to a motor capacitor, the capacitor can be measured to rule out a defect. However, the following also applies here: Caution, danger to life! You are on the safe side if you have this activity carried out by a trained electrical specialist.

Before a capacitor can be measured, it must be disconnected from the auxiliary winding, i.e. removed from the motor. Once this is done, make sure that the motor capacitor is discharged. Otherwise there is not only the possibility that the component can destroy the electronics of a measuring device, but there is also the risk of a very serious electric shock, which can sometimes be fatal.

Not all measuring devices are suitable

Not all measuring devices are suitable for checking motor capacitors. Digital multimeters, for example, are completely unsuitable. Simple analog devices, on the other hand, basically do their job. However, with most devices the test voltage is not sufficient to provide reliable absolute values. The test only works approximately and can at most provide an indication of whether the component is defective.

To measure the discharged and removed capacitor, the measuring device is set to a continuity test. The test probes are placed on the two contacts. If there are four contacts, two of them are always connected to one another - from an electrical point of view, capacitors only have two connections. It is important not to touch the contacts with your fingers when measuring, as the resistance of the body would falsify the result.

During the continuity test, the pointer of the analog measuring device should first deflect all the way to the right and then slowly return to the zero position. The capacitor builds up an electrical potential in the direct current circuit, fed by the battery of the measuring device. In proportion to this, the current finally drops to zero until the maximum capacity of the functioning capacitor is reached.

With small capacities, the initial pointer deflection can be very small. If the pointer stops on the right, the capacitor is not storing any energy and is probably defective. If there is no rash at all, there is probably also a defect.

Check the motor capacitor with an ohmmeter

You can also check the capacitor in an electric motor by measuring the resistance with an ohmmeter. In this measurement, the resistance should start low and gradually increase as the capacitor charges.

The most meaningful for both measurement methods is the comparison with a definitely functioning motor capacitor with the same technical values. If the pointer deflections behave the same in terms of intensity and time, the capacitor is probably OK.

However, in order to be able to measure the capacitance of a capacitor, which may have decreased in the course of operation, a special capacitance measuring device is required, which gives the value in Farad, the unit of measurement for capacitance.

What does the capacitor in the electric motor actually do?

Two different types of capacitors can be summarized under the umbrella term motor capacitor: the operating capacitor and the starting capacitor. Almost all capacitor motors have an operating capacitor. Only the electric motors of some special machines also have a starting capacitor. This second capacitor is necessary, for example, in machines that are very heavy or have to work against high pressure. Furthermore, an interference suppression capacitor is often found in electric motors, but this has nothing to do with the actual function of the motor and is therefore not dealt with here.

The running capacitor

In most electric motors, a rotating element, the rotor, moves in an electromagnetic field that is generated in the stationary part of the motor, the stator. In order to generate a directed rotary movement, a dynamic in the electromagnetic field is required that generates a torque. For motors that are operated with normal 230 volt household alternating current, current must flow in two different phases in two different copper windings in the stator, the main and auxiliary winding.

However, the alternating current offered only offers a defined phase. It is passed through the main winding. The intermediate operating capacitor is responsible for the phase shift of 90 degrees of the current flowing through the auxiliary winding. The electrical behavior of the capacitor in the AC circuit is responsible for the phase shift. The capacity of a capacitor for this use in a "single-phase motor" is usually between 25 and 30 µF (micro farads) per kW of motor power, but it depends on many different factors.

In addition, there are so-called three-phase motors with Steinmetz circuit, which can be operated both on the normal AC network and with three-phase high-voltage current. The capacity of the operating capacitor there is around 70 µF per kW motor output. Operating capacitors are constantly in the circuit in so-called continuous operation.

The starting capacitor

The motors of machines that have to start against a high resistance, such as floor grinding machines or compressors, also have a starting capacitor. This has a high capacity and is only active when the engine is starting. Then shutdown mechanisms such as centrifugal switches ensure that the capacitor is removed from the circuit. If this does not happen, these capacitors, which are not suitable for continuous operation, can burn out. They are used to give the motor a particularly strong starting torque - a kind of nudge. Under no circumstances may starting capacitors be installed as operating capacitors, as they do not work continuously.

There are also motors that only have a starting capacitor, i.e. only need a push to start and can generate the torque during operation without a capacitor. However, these are rarely used.

Where can I find all the information about the capacitor and what does it mean?

In most cases, all information about the capacitor is on the component itself. The capacity in micro farads (µF) and the tolerance in +/- percent are indicated there. Furthermore, the rated or nominal voltage in volts (V) and a service life class in hours (h). However, the service life of a capacitor always depends on external influences, such as the ambient temperature.

The abbreviations AB or DB indicate the operating mode. AB stands for intermittent operation and identifies starting capacitors that are not suitable for continuous operation (DB). Operating capacitors therefore have a DB.

Furthermore, the corresponding test marks and information about the manufacturer should be found on the capacitor. If this information is not available on the component itself, it is likely to be found on the nameplate of the motor or on a separate sticker in the area of ​​the capacitor cover. If necessary, the engine documentation will help. The manufacturer's website or hotline is also a possible source if you are missing information.

If I can't find any information, can I calculate it?

Theoretically yes. As an approximate rule of thumb, but it is very imprecise, for a single-phase motor 5µF capacity per 100 watts of motor power, for a three-phase motor with Steinmetz circuit 7µF per 100 watts. In addition, there are calculation options that are approximately dependent on the torque, structure of the motor and the number of windings in the main and auxiliary windings and must be found empirically for each type of motor through measurement.

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