View Full Version : swapping start capacitor

11-15-2004, 06:45 PM
I've got an 8 inch craftsman bench grinder that does the job for me. It was inexpensive, but pretty heavily built. Can I swap the 25mf 250v start capacitor for something that will get it up to speed faster?

11-15-2004, 09:51 PM
Most likely not,as I understand the use of capacitors in a split-phase motor they are used to shift one or more fields out of phase to start the motor.Putting a larger capacitor in place of the one you have may make it start faster,but it may also shift the phase too far and not start at all.

J Tiers
11-15-2004, 11:03 PM
That is just a factor of the wheel weight (mass) and diameter....the inertia.

The motor is only a certain power and torque, so the speed of start is limited.

That said....if it is really slow, meaning that it isn't up to speed in a FEW seconds, then probably either:

1) the start cap is actually bad, and needs replaced,
2)the wheel is a lot larger than it is supposed to be for that model.

11-15-2004, 11:22 PM
while waiting for a reply I got bold/stupid and tried the 540mf 120v cap from another motor I had under the bench. The grinder whipped up to speed lik'ty'split... and then stopped about five seconds later. No unwelcomed smells or heat.. so i plugged it in again and the same thing happened. Must have tripped some protection circuitry. I Put the old cap back in. I'll just be satisfied with waiting eight seconds for it to get up to speed.... unless anyone wants to weigh in with anything further?

11-15-2004, 11:48 PM
540mf http://bbs.homeshopmachinist.net//eek.gifyour lucky you didn't see flames.I have a Wal-mart B&D special,1/2hp and it snap starts(2seconds tops)apparently the Chiwan models vary considerably.

11-16-2004, 02:29 AM
Yikes. Try maybe going to around 50 mfd. That is usually within the tolerance range of the caps anyway, more or less. Most caps have a very generous over tolerance value and the motor will be designed to accomodate that.

J Tiers
11-16-2004, 01:59 PM
You should hear a click as the start switch cuts out when it is speeding up. if not, then you should hear it as the grinder slows down. (May be no click if it is a timer type)

I would expect it to cut out a couple seconds at most into starting. Much more, and it strains the cap, which isn't made for long duty.

If the motor doesn't cut out the switch for 5 or 6 seconds, the cap may be dried out and effectively too small.

Some of those do apparently have timed start cap switches, that cut out at a fixed tome. My radial arm saw does. That's a different deal.

11-16-2004, 02:07 PM
Yep, same as the pump motor in my well. So as to avoid starting contacts etc the cap is switched in for two seconds for starting.

Jim Hubbell
11-16-2004, 05:09 PM
If you experiment with start caps. use only caps. with same or higher voltage rating.
The capacity (mfd) determines how much a/c current flows in start winding and cent. switch. Too high current may weld cent. switch contacts. Try a new cap. of same rating before experimenting. Then try maybe 25% more MFDs.

11-16-2004, 08:01 PM
Oh,forgot to mention,some of the import grinders are showing up with pc board caps,not good for motor service IMHO,if it happens to be one of those,you might get an oil filled cap from an AC supply or Graingers.