Capacitor - Why Does Rpm Change?

Definitely strange, I've switched out PF capacitors many times and it's never affected the running speed of drives.

I assumed the capacitor is connected close to the motor and the change of motor rpm is not so significant.

Here is the explanation:

Due to the capacitor, the power factor is increased and the motor cable current is decreased and hence the motor cable voltage drop is decreased, which eventually increases the motor terminal voltage. The increase of motor terminal voltage increases the motor rpm (see the thread http://cr4.globalspec.com/thread/56346/Induction-Motor-Voltage-RPM, especially the comment # 2 to see how voltage affects motor rpm).

When capacitor is removed, the opposite actions happen and the motor terminal voltage is decreased and the rpm decreases to some extent.

This effect can be seen only when the capacitor is connected near the motor. If the capacitor is away from the motor (connected to the MCC or power supply), the rpm change will happen.

- MS

I think our "Guest" is having the opposite problem. Motor speed is falling with the capacitor connected.

I to could have understood it rising, but falling?

It would be interesting to know by how much.

I didn't think of the capacitor values before. The other way around is also may be true. The 4 x 16.4 KVAR (total 65.6 KVAR) capacitor is too much for 75 HP motor. Generally, for improving the power factor, the size of capacitor (in KVAR) requires in the range of 25% to 40% of the motor KW value. In this case, the capacitors exceeds the required value and it draws excessive reactive (capacitive) current through the motor cable and voltage drop is much more than the motor current without capacitor.

- MS

The KVAr will be lower than 16.4 as the capacitors are working at 415V and not 525V, but as you say they are far to big.

A strange one this, all down to volt drop, but not from where you expected it.

The capacitors as per OP is rated at 16.4KVAr @525 V but at the actual voltage of 415V it will be much smaller (around 13KVAr) and that also will depend on how he is connecting them.

In fact I wonder about the capacitors- are they four 3 phase banks or are the individual cartridges - then how four are connected across three phases.

Even assuming these are banks - it will be a bit too high even at this value (51KVAr)

Another factor that must be occurring here is the self excitation of the motor by the bank. One of the thumb rules are when the fixed bank is connected across motor, its current should not exceed about 90% of the motor no load current.

You are definitely exceeding this value unless we are missing some data.

Here capacitance value is very high than required. So Ferranti effect will take place, ie receiving end voltage is higher than sending end voltage. Because of Ferranti effect receiving end voltage got high and speed was more. After removing capacitor there will be no more Feranti effect, so speed got less.

Try reading the thread from the beginning and try again

there is a 75HP 960RPM 440VOLT 3PHASE rated Star/delta SQ.INDUCTION MOTOR which run through only direct MCCB WITH OUT STARTER.we apply capacitor on terminal of motor in parellel ratting of capacitor is this 16.4/17 kvar on 525/550 volt 50 Hz marlin grain make all capacitor apply in parellel position with motor it means motor & 4 capacitors are connected parellel on 3 phase supply

1 Are you OP ?

2 You are connecting 4 individual capacitors or 4 three phase capacitor banks?

3 You are yet to reply - how much is the RPM change - is it high?

I still believe it is the self excitation of the motor and that is creating the RPM drop (it is acting as motor cum generator)

please clear that what happen when we applly 16.4/17 kvar 525/550volt 50Hz 3phase rated capacitor on 415-440 volt 50Hz 3phase supply