Hi All, when considering static compensation for a screw compressor what is the best method for calculating capacitor size? As the load varies considerably as the machine loads and unloads I would imagine that care must be taken to avoid any resonance problems. Also the use of a seperate contactor seems to be essential as often when the machine cycles off the compressor will reverse direction for a short period due to the pressure equalising in the system.

All help appreciated.
Kens

Hi Kens

For static correction, you should correct 80% of the magnetising current of the motor.
The magnetising current is constant, load independent so there wil not be any resonance issues at line frequency.
The only variation in reactive current drawn by the motor is due to the load current and the leakage reactance. This is generaly very small relative to the magnetising current.
If you require to correct to 0.95 or better using static correction, this can be a problem as you may well correct to better than 80% of the magnetising current and this can cause resonance issues at ramp down.

I always recommend that a separate contactor is employed for the capacitor control when using static correction. This will diconnect the capacitors from the motor and eliminate resonsance issues during motor shut down.

Best regards,

Hi Marke, thanks, my next question is how can we find the magnitising current. We cannot disconnect motor from load to find open shaft current.

Thanks
Kens

Hi Kens

If you have accurate data on the motor, you can use my electrical calcs software to give you a good recommendation using method 2. - you need to know full load and half load efficiencies and power factors.
Other alternative is to use a suitable meter and measure the VARs at the lowest load you can achieve.

Best regards,