# How to test 25 kvar capacitor

A three-phase motor works because three-phase alternating current (meaningfully called three-phase current) generates a rotating field in the motor. The three phases are shifted by 1/3 to each other. Single-phase voltage does not meet this requirement. A capacitor is required to generate the necessary phase shift.

Dimensioning of motor capacitors:
The right choice of a running capacitor is quite difficult when the reference capacitance is unknown. The exact capacity of the capacitor depends on the motor power, the torque, the structure of the motor and the design of the winding. However, much of this data usually remains unknown to the user. Therefore, only general statements can be made here, which do not claim to be complete or correct in all cases.
For the operation of a three-phase motor on a single-phase network (so-called Steinmetz circuit), around 70 µF per kW should normally be provided.
The capacity of a capacitor for a single-phase motor is around 30-50µF per kW output, provided the main and auxiliary windings have the same number of turns. This value changes inversely proportional to the turns ratio of the auxiliary to the main winding. In nominal operation, the voltage on the capacitor is about a factor of 1.41 higher than the applied mains voltage. When idling, the capacitor voltage is about 15% higher. If the auxiliary winding has more than twice as many turns as the main winding, a factor of 1 plus the turns ratio squared should be used instead of the factor 1.41. Usually the number of turns of the auxiliary winding is significantly higher than that of the main winding.
Did not understand ? Never mind. We have never wanted or had to calculate that.

And now the redemption:
By looking at many commercially available single-phase motors, it can be said that usually capacitors with around 25-30µF per kW output are used. So if you can't read anything on your capacitor, this statement can be a guide here. However, we cannot guarantee this. If anything is unclear and if the capacitance cannot be read either on the capacitor or on the motor (this can sometimes be found on the rating plate), information from the manufacturer or the original motor dealer may help.

Testing of motor capacitors:
If a motor capacitor is not visibly damaged, it must be measured. Since it is not to be expected that expensive high-tech will be available from the end user, it can be reassuringly stated that quite meaningful measurement results can be achieved with simple analog measuring devices. Digital measuring devices are unsuitable. The capacitor must not be connected to the motor winding for a measurement. It must also be completely discharged! Set your meter to a continuity test and measure the capacitor. The pointer initially deflects to the right and then returns (possibly slowly) to the “0” position. Depending on the capacity, the deflection is different. The time it takes for the pointer to reach the “0” position also varies. Repeat the test by swapping the connections. Please note that the pointer deflection may be quite small with small capacities. If the pointer does not deflect at all or remains in the right position, then the capacitor is defective. Please ensure that you do not touch the measuring points during the measurement. The volume resistance of your body would falsify the measurement result and possibly lead you to wrong conclusions.
You can of course make comparisons with functioning capacitors of comparable capacity. If the pointer deflection is roughly the same and the pointer needs roughly the same time to reach the “0” position, then it can be assumed that the dubious capacitor is OK. For an accurate measurement of the capacitance, however, you will need a capacitance measuring device.

Danger ! During all measurements, make sure that there are no voltages and that the capacitors are completely discharged! There is a risk of serious electric shock!

Use of motor capacitors.
When to start capacitor? When to run capacitor?

Many users have problems deciding whether they need a starting capacitor or a running capacitor. For this reason, starting capacitors are often used by mistake, although an operating capacitor would have been correct. The result is that the starting capacitors immediately fail and the supplier is wrongly accused of having delivered a "crutch".
Basically, starting capacitors are only connected to voltage when an electric motor starts up and are disconnected from the voltage immediately after start-up. If these remain energized, the capacitor will fail quickly. Run capacitors remain connected to the motor winding and voltage 100% of the time.
We now want to briefly explain why this is so.
As a rule, operating capacitors are used on single-phase asynchronous motors. These generate a phase shift and thus a torque, which is the basic prerequisite for this single-phase motor to turn at all. Notes such as DB (for continuous operation) or ED 100% (duty cycle 100%) can sometimes be read on these capacitors. Since motor capacitors come from all countries, this information is by no means always available.
Single-phase motors that only get by with a starting capacitor are much rarer. This is then only used in the starting phase to give the electric motor a "kick" in the right direction. Immediately after the start, these capacitors are separated from the voltage in one or two poles by a mechanism of whatever type. These mechanisms can be e.g. centrifugal switches or so-called Be a start-up relay (usually a black box with three connections). The latter recognize the drop in the motor current after run-up (mostly by means of a stupid heating coil) and switch off the starting capacitor accordingly. If this shutdown does not take place (e.g. because these mechanisms are defective), the failure of the starting capacitor that has not been switched off occurs shortly afterwards, which then usually goes to the capacitor paradise with a roar and smelly. Therefore, before you replace a defective starting capacitor, please check whether the automatic switch-off is functional.
You can usually recognize a starting capacitor by the specification of the duty cycle. On the starting capacitors we sell there is, for example, the indication: 1.7% ED. This means that the capacitor can only be left on for 1.7% of the time. In principle there are approx. 20 regular switch-ons of 3 seconds each. Duration, spread over an hour. In addition to the indication of the switch-on time, the starting capacitor often also has the word "motor start ..." or something similar. In addition, starting capacitors are significantly smaller compared to operating capacitors of the same capacity.
As the last field of application for starting capacitors, we want to mention motors for difficult starting conditions. These have both an operating capacitor, which is permanently connected to the motor winding, and an additional starting capacitor of high capacity. This gives the motor a particularly strong starting torque in the starting phase, especially since single-phase capacitor motors that only have one operating capacitor have a very low starting torque. After run-up, the starting capacitor in these motors is removed from the voltage by the shutdown mechanisms mentioned above.

Conclusion: in most cases, operating capacitors are required!

The picture example shows the typical motor circuit of a single-phase electric motor for heavy starting with operating capacitor (C1) and starting capacitor (C2) with shutdown.