Recommendations on the choice of induction motor capacitor parameters

The current, exceeding the rated one by 20-40%, passes through motor winding in the process of its operation, therefore when operating electric motor in underloaded state or in idle mode, the capacity of running capacitor should be reduced.

For the safety purposes all the starting capacitors shall be used with discharge resistors. Discharge resistance is selected in such a way as to ensure complete release of residual stresses from capacitor within 50 seconds.

In case if a capacitor is used in series connection with electric motor auxiliary winding, the voltage on capacitor terminals under operating speed may cause considerable increase of network voltage.

In the process of operation, capacitors may be installed directly in physical contact with electric motor. In this case, when choosing capacitor type, it’s necessary to consider that it will be subject to the influence of elevated temperature and vibrations – both from the motor itself and from other passive components of different types of devices, the part of which a capacitor will constitute.

Various complicated commutation processes take place during motor capacitor operation, as a result of which step-like voltage changes on capacitor terminals occur. In this connection the capacitor rated voltage shall be chosen with the following condition: in the process of product operation operating voltage must not exceed capacitor rated voltage by more than 10%.

When choosing necessary capacity and operating voltage, resonance factor shall be considered, i.e. when the voltage of electric motor auxiliary winding and capacitor voltage are in the near-resonant point. In this case increasing of voltage on product terminals takes place.

Limit voltage on starting capacitor terminals shall not exceed 450V, and its capacity is chosen, as a rule, to be two and more times more than that of a running capacitor.

As practice shows, 6-7 mF is required for each 100W of electric motor power.

In case if it turns out to be impossible to select capacity in one case, combining using series connection of capacitors is allowed - Сtotal=С1+С2….+Сn

If case of correct capacitor choice, the power of three-phase motor connected to single-phase network shall not decrease by more than 30%.