Starting large motors/loads with capacitor assist

[raisinbran]

In the mining industry, motors that can satisfactorily move the load have difficulty starting the load because of voltage drops in the system. These particular motors are mechanically connected to the load using fluid couplers to take advantage of the motor breakdown torque. The motors are started across the line. The reduced voltage (80%) lowers the torque enough that sometimes the load (chain conveyor) will not start and must be partially emptied by hand.
I was thinking about capacitor assisted starting to boost the motor voltage until the load is accelerated. The capacitors would be switched on with the motor and switched off sometime after the load is up to speed. With a fluid coupling, the motors come up to spedd very quickly, and the load is accelerated over a 10 second (approx.) time frame. I was not planning on doing anything too sophisticated, possibly just using a timer to turn the capacitors off. I also would assure that the capacitors were off before the motor was switched off.
From what I read in previous threads, there are concerns about over-voltages and over-excitation. Is this just a concern when the motor is switched off, or could I get into problems when the motor is running?
Thanks in advance,
Raisinbran

 

[raisinbran]

Davva,
No, we have not implemented the capacitors yet, I have tried to order them, but budgeting has them held up.

My thoughts on this are:
As it is, the power supply supplies both the real (kW) and the reactive (kVAR) power needed to start the motor. The power system will always have to supply the real power regardless of the size of the capacitor installed. The voltage drop due to start-up will be about twice what it is to run the motor. That is about as good as it gets.

If the motor is a 600 kVA (kW?) motor, at six times current, the motor draws 3600 kVA. At 0.25 pf, this is 900 kW and 3485 kVAR. The largest capacitor that makes sense is 3000 kVAR, but smaller is probably much more practical.

I would have a contactor to connect the capacitors to the load side of the motor contactor and close it before the motor is started. Then start the motor, while watching the motor current and line voltage. Disconnect the capacitor when the motor gets up to speed. Disconnect the capacitors (in whole or part) if the voltage rise is excessive.

I don't think that this idea would apply to star-delta starting, as the capacitors may be left on-line during the transition, and that may cause problems.

I do plan to install capacitors as I described in my original post, I just don't know when it will happen. Curious about your thoughts.
Raisinbran

 

[davva]

Raisinbran,

Thanks for the reply. I will be interested to know the outcome. Out of interest what sort of voltage drop improvement (currently 80%? at the motor terminals?) are you hoping to achieve by the capacitor mod?


With regards to our installation....

Motor = 480kW

The motor develops enough starting torque in star to accelerate the load to near full speed. I think I would remove the capacitors before the transition to delta.[The classification societies allow a transient event (less than 2 secs duration) during which the supply can drop to 15% as long as it recovers to the steady state 10% after than duration. The motor reaches full speed within this duration following the delta changeover.]