Thread: Motor capacitor 3HP DC

I now have two 30uf 450V caps in parallel (ones stated to have a built in safety device), they run cold. I'm more worried about the motor, cant really get air into the back of it VIA an intake port into the fan intake as the cabinet is slightly pressurised and any intake, even if close fitting around the motor intake, is still fighting the influence of the DC exhaust air pressure.

Bob just thinking about airflow past the rear of the motor where the motor intake fan is mounted, I'm going to try a small 6" x 6" vent behind and slightly below the rear of the motor, the air flow exiting there will be pretty slow as the main DC housing exhaust port is very large, that should allow the motor intake fan to grab that slow moving air and it should remove any chance of air recirculating around motor, I'll also put a thermocouple probe on the finned motor housing and monitor the temp.

BTW the old cap was mounted inside a closed plastic housing bolted to the side of the motor, so it was designed with no air flow over it from day one.

Originally Posted by MandJ

Bob just thinking about airflow past the rear of the motor where the motor intake fan is mounted, I'm going to try a small 6" x 6" vent behind and slightly below the rear of the motor, the air flow exiting there will be pretty slow as the main DC housing exhaust port is very large, that should allow the motor intake fan to grab that slow moving air and it should remove any chance of air recirculating around motor, I'll also put a thermocouple probe on the finned motor housing and monitor the temp.

Good idea. I have been also dabbling with temp monitoring of my DC motor. Its more because of when I run it at higher speeds.

BTW the old cap was mounted inside a closed plastic housing bolted to the side of the motor, so it was designed with no air flow over it from day one.

I wasn't suggesting remote location of the cap to keep it cool but more in case it exploded again.

FYI: Just did a test to duplicate the conditions when the cap blew the other day: My wife was using the scroll saw and the DC flow is throttled back on that because of dual 100mm pick-up ports, the DC current was only 7.4A instead of nearly 9A using the Lathe 150mm bell-mouth, the current shot from 7.4A to 11.7A when the caps were disconnected (I have a temporary switch for testing) so the motor ran at 11.7A for almost an hour after the cap blew, like I said, I could still hold my hand on it (not for to long though) so cooling air flow can't be to bad.

EDIT: 11.7A is around 3.7HP by my calculations.

Originally Posted by BobL

I wasn't suggesting remote location of the cap to keep it cool but more in case it exploded again.

Ok understand, I've got them in a plastic container at the moment, will get something stronger and add a vent to release pressure if they go again.

Check that the running capacitor is not a normal AC electrolytic type but is a metallized polypropylene unit. These are rated for continuous operation whereas the electrolytic ones are usually for starter circuits which are an intermittant use.

Yes, specified as Polypropylene motor Run capacitors.