Comparison between Capacitor and Synchronous motor (overexcited mode)

Hi,

Can any one please tell me what is the core difference of why we use synch. motor to improve power factor and not capacitor banks other than the size issue?

Obviously a sync. motor is much more complicated than a capacitor bank. But it can generate the equivalent capacitance of a very large capacitor. Otherwise the effect of sync. capacitor on the power line power factor is the same as a capacitor bank of the same equivalent capacitance.

So which one you use is mostly which of the two approaches costs less for a given amount of power factor correction.

Can the capacitance of capacitor vary according to temperature?
Also if one cap blows off in the cap bank, will the complete circuit fail or just the PF will be affected?

Is there a limit to which the sync. motor can provide capacitance or will it adjust PF automatically by varying its speed?

The advantage of a synch motor is that one can continuously adjust the actual KVAR depending on load conditions, whereas on a capacitor bank, you can only vary in steps by actually switching capacitors in and out. If the actual KVAR rating is large, this is not a trivial task.

Now..... on a synchronous motor to change the KVAR one adjusts the field current...the speed remains the same, it is synchronous with the mains frequency. A field regulator varies the field to maintain the required conditions

haxan said:

Can the capacitance of capacitor vary according to temperature?
Also if one cap blows off in the cap bank, will the complete circuit fail or just the PF will be affected?

Is there a limit to which the sync. motor can provide capacitance or will it adjust PF automatically by varying its speed?

Click to expand...

That capacitance can vary slightly with temperature, but generally not enough to be a problem.

If a cap blows, it will just reduce the PF correction by the value of the capacitor.

The limit on a syn. motor capacitance is the amount of field current it can safely carry without overheating. The greater the field current, the more equivalent capacitance the motor has.