Start Capacitor Keeps Blowing, Powermatic
Hi there, I have a 20″ Powermatic disc sander (DS20) in my shop that has an electrical issue I’m struggling to troubleshoot. It is taking a very long time (about 45 sec to a minute) to get up to speed at start up and is blowing through start capacitors. I know that start capacitors aren’t meant to be in service for that long and that’s probably why they keep blowing but I can’t figure out why the motor is struggling to get up to speed in the first place. Some pertinent info;
1.) Mod. DS20, HP 2, Phase 1, Volts 230, Amps 14, Hz 60, Rpm 1720, Class A, Pole 4, Date 2016/7, Serial K00587
2.) It ran perfectly for years on the same circuit without any issues. Nothing on the power supply side has changed ex. wire size, length, breaker size (30A) and so on. I plopped a 40A breaker in as an experiment but there was no improvement.
3.) Motor turns freely and is uninhibited aside from it’s own inertia and that of the disc. Bearings feel great.
4.) Centrifugal switch seems to be in good condition but has been replaced anyway. I’m not certain it could even be the issue as it is supposed to trigger at speed and we just aren’t getting there quickly enough.
5.) I don’t know of any event that damaged the machine. I’ve looked at the wraps and internals and can’t see anything frayed, disconnected broken, etc. To my knowledge it just began to have trouble starting and began blowing through capacitors.
6.) I’ve been replacing the caps with these 500uf, 250V, non-polar, from grainger as specified in the operator and parts manual.
(https://www.grainger.com/product/DAYTON-Motor-Start-Capacitor-220-6FLW6?cm_sp=Home-_-Products_You_Have_Recently_Viewed-_-AZHPRV_HPAD-080620&cm_vc=AZHPRV_HPAD-080620)
I see that there are also capacitors with a “bleed resistor” but I don’t know what this means. Perhaps this is my issue? Anyone with knowledge on this that can explain it to me like I’m five?
I also can’t seem to find a wiring diagram for the motor itself. Just the typical power switch wiring schemes that I believe aren’t pertinent to my issue.
Any help would be appreciated. Thankyou!
Replies
If you do a search on start vs run capacitors, google will have wiring diagrams that are quite simple, they show the run capacitor always in the cycle and as you know, the start capacitor only in the circuit at start until the centrifugal switch opens. Could it be that the switch, or it’s wiring to the capacitor is always open and the motor starts on the run capacitor alone ? Easy way to find out is to disconnect the start capacitor and see if you are getting the same slow start, then it’s a matter of troubleshooting the start capacitor side, it should show voltage during startup only. The resistor is there to discharge the capacitor so you don’t get electrocuted when the power is turned off, be careful when handling capacitors, they act as a battery if you short their terminals.
With the start capacitor disconnected, the motor will not start turning at all. It will just hum. And you risk burning out some run wirings as well.
If the starting circuit wirings in the motor are damaged, there may be enough left to start the motor slowly, but not enough to get it up to speed in a normal amount of time. Being 240 volts, there are almost certainly two legs to the starting windings, one for each leg of the 240. If one of them is bad, it might still start but poorly. Check the starting windings for either a short or open circuit.
Take the motor to have it checked.
It might still be the centrifugal switch.
I understand you've replaced it but most require adjustment to work properly. Usually adjustment involves loosening it and sliding it along the shaft.
The goal is:
1. Have the contacts pressed together tightly when the switch is at rest. This assures the start capacitor is "in the loop" when you hit the start button. If it is not, the motor will struggle to come up to speed or just make a humming sound.
2. Have the contacts fully opened when the centrifugal switch is rapidly rotating. This assures the start capacitor is "out of the loop" once the motor is up to speed. If it is not, it will eventually damage the capacitor.
Note: You can simulate the activated position by pulling on the weights.
Also, I agree with folks that have said the motor windings may have shorted out. Let's hope that isn't the problem as that involves rebuilding or replacing.
Mike
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