Sizing motor start and/or run caps

Status
Not open for further replies.
PetrosA

PetrosA

Senior Member
Location
Lancaster, Pennsylvania
I recently had a customer who wanted me to hook up an old drill press from the Westinghouse factory in Philly at his shop outside of the city. It turned out that the drill press has a two-phase motor on it while the customer has single phase service. Based on advice from a local motor repair shop that's been around forever, I got a capacitor and a timer, rewired the leads coming out of the motor and made it work. Customer's happy.

What I'm curious about is how the motor guy knew what size cap to sell me? It's a 3 HP 2 phase motor rated for 220V. Is there some kind of table for this or was he just going off of years of experience? I've been digging on google and here, but I can't find anything helpful. Anyone have any ideas? Thanks.
 
LarryFine said:
Um, ask the guy who selected the cap?

What did he need to know about the motor?
Click to expand...

I provided a photo of the nameplate, so he had everything I knew about it. He didn't want to tell me how he figured it out. He sat for a minute thinking, then went back in the shop to "double check" something, so it could have been a cheat list, handy table or some other motor. The only rule of thumb I've found online (unsubstantiated) is 80 mF/1000W for a start cap. Run caps seem to be sized by manufacturers based on performance so there may not be a rule of thumb for them - but I didn't need one of them anyway.
 
2 phase power is so rare and specific (I think Philly is the only place left where they have it?) that most likely the guy knows something from experience that is no longer taught or available to the general engineering community. Something to be said for the value of working experience.
 
101108-1942 EST

A two phase motor will have two rather identical stator windings. What you would like is one of these to be shifted 90 deg relative to the other. But it does not have to be precisely 90 deg. Any single phase motor is essentially a two phase motor, at least for starting. A single phase capacitor run motor is really a continuous two phase motor. However, the winding with the capacitor will be optimized in its design for use with a specific capacitor.

For a discussion see Chapter 17 of "Alternating-Current Machinery", by Bailey and Gault, McGraw-Hill, 1951. Only cost $6 originally.

.
 
Status
Not open for further replies.
Top