About Motor Capacitors
Motor capacitors are wearing parts on so-called capacitor motors, a subset of electric motors. They are differentiated into operating capacitors and start-up capacitors.
By the cost-effective replacement of a motor capacitor, electric motors on small devices with 230 volt voltage, such as pumps, lawn mowers or ground grinding machines, often run smoothly for a long time. However, the replacement should only be carried out by a specialist, as there are a number of things to consider.
- How do motor capacitors work?
- What motor capacitors are there?
- Caution Risk of fatal injury: Therefore, motor capacitors should only be replaced by professionals
- When must a motor capacitor be replaced?
- Find a suitable replacement for a motor capacitor
How do motor capacitors work?
Motor capacitors are the name-giving components for capacitor motors, which in turn are among the electric motors. They are operated as AC motors with 230 V household current. Usually, motor capacitors are located on the outside of the electric motor under a separate metal cover.
Our practical tip: Advantages of a motor capacitor
Capacitor motors achieve only about 65 percent of the output of three-phase asynchronous motors. However, as low-maintenance motors for the standard household voltage of 230 volts, they are popular with devices such as lawn mowers and pumps.
Capacitor motors have two windings, which results in a nominal voltage of 450 volts during operation. The first of the two windings is supplied directly from the AC grid, while the second winding is connected in series behind it. The operating capacitor on the second winding generates the additional phase required for the rotating field itself by a 90 degree phase angle.
What motor capacitors are there?
The two variants are offered as types:
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| Plug-in motor capacitors for easy replacement | Radial leaded motor capacitors with two connection cables |
Capacitor motors require at least one operating capacitor, sometimes an additional starting capacitor.
- Operating capacitor: The operating capacitor is permanently connected to the motor terminal board. He is in constant operation. If two capacitors are present, the operating capacitor is smaller than the starting capacitor.
- Starting capacitor: The starting capacitor is not always present. Machines where it is installed are usually heavy or have to work against high pressure, such as grinding machines or compressors. The starting capacitor is switched on in the starting phase of the motor parallel to the operating capacitor in order to achieve a high torque. Caution: If the rated motor speed is reached, the starting capacitor must be switched off, otherwise the electric motor will burn out.
Caution Risk of fatal injury: Therefore, motor capacitors should only be replaced by professionals
Capacitors are considered to be passive components, but they can store electrical energy in dangerous quantities! An electric shock from a motor capacitor can be much higher than that from a normal socket. Work on motor capacitors should never be carried out by non-specialists.
All safety measures must be taken when removing the capacitor cover: Disconnect the device from the mains, remove any fuse, do not work in damp environments or with damp hands, use special tools better.
When must a motor capacitor be replaced?
Motor capacitors are among the wear parts with a limited life. With a spare capacitor, electric motors often continue to run smoothly for a long time. They are available at low cost.
Signs of a defective motor capacitor may be:
- The electric motor hinders or shruks after switching on, but does not run properly round or at all.
- The electric motor is tough or powerless during start-up; under load, the speed drops quickly. The electric motor runs in the wrong direction. (However, these two signs may also indicate a defective auxiliary winding.)
When looking for a replacement, the defective motor capacitor must first be inspected – in compliance with diverser safety rules:
- Apparently defective capacitors such as pierced, deformed, discharged or broken capacitors must be replaced.
- Sometimes damage can be detected by the capacitor cover, which was a mechanical impairment.
- In case of insulation damage, pressure and high temperatures often lead to discoloration at the capacitor or to inflating with belly-like deformations.
- If there is a large amount of oil or other liquids in the recess for the condenser, this could have led to a short circuit.
- If liquid escapes from the condenser itself, it is certainly defective and you should exercise caution, as some of the components may have an corrosive effect.
If no clear damage can be detected, the motor capacitor may still be defective. For further testing, it must be removed in order to be able to measure it electrically with measuring devices. To do this, the capacitor is disconnected from the auxiliary winding and then removed from the motor.
Caution: Ensure that the motor capacitor is electrically discharged, even if it is removed. Otherwise, there is a considerable risk of injury and could destroy the test equipment.
Find a suitable replacement for a motor capacitor
Our practical knowledge:
The rule of thumb for dimensioning motor capacitors is: µ20 F per kW motor power. Always observe all specifications carefully when using capacitors, otherwise there is a risk of fatal injury.
On most capacitors, the technical specifications for capacity in micro-ferrad (µF), tolerance in percent, rated voltage or rated voltage and service life in operating hours can be read directly. Alternatively, the corresponding information can also be found on the cover flap of the motor capacitor.
WB 40300 MK SH = TYPE DESIGNATION
30µF = Capacity
5% = tolerance
P0 = Safety class
AC 400V B
AC 450 V C = lifetime classes at different voltages
40/085/21 = climate class
50 60 Hz = operating frequency
The term “operating capacitor”, “motor capacitor” or starting capacitor, starting capacitor, motor start capacitor is often printed directly on the original part. A power capacitor must not be replaced by a starting capacitor, as the second is not designed for continuous operation. In this respect, there is a DB for the continuous operation of an operating capacitor and OFF for the intermittent operation of a start-up capacitor.
Safety Note: Only install spare motor capacitors that have the same electrical parameters as the original capacitor to avoid hazardous incidents with the electric motor. In case of any doubt, please consult the operating instructions of the device concerned or contact the manufacturer of the electric motor.
(This text is machine translated.)