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Motor without start capacitor not generating enough starting torque

cheeky

New member
I have a old Century Centurion switchless motor that has a run capacitor, but no start capacitor. It doesn't generate enough starting torque for my application. Sounds like I need a new motor, but I'm curious if there is an advantage to the design and why Century went with that approach? Cheaper?
 

Bill D

Active member
Low starting torque. good for fans and some bench grinders. No centrifugal switch, no start winding, no extra cap saves money and time on assembly.
Bill D
 

Madis Reivik

New member
Also, you can wire the motor with the 3 pos switch, extra cap at pos 3, so after start you release the switch to pos. 2
Almost like ignition on car.
 

JST

Moderator
Probably a motor with a high resistance rotor. My RPC has a dual cage motor, the outer start cage is high resistance, and the motor has only one non-switched capacitor, functioning as a start and "balance" capacitor.

You may want to check resistance of the winding the capacitor connects to. If open, OK, it's effectively toast. But even if the windings are OK, and connecting wires are OK, the capacitor may not be. Try another one.
 

cheeky

New member
Also, you can wire the motor with the 3 pos switch, extra cap at pos 3, so after start you release the switch to pos. 2
Almost like ignition on car.

Interesting idea, perhaps you could use a momentary switch that engages cap while held down and then switch to pos 2 upon release.
 

cheeky

New member
Low starting torque. good for fans and some bench grinders. No centrifugal switch, no start winding, no extra cap saves money and time on assembly.
Bill D

Make sense, it is cost savings, but also purpose built. A little more research shows these motors were popular for pools and spas, so there you go.
 

cheeky

New member
What HP? What voltage are you trying to run it on? Is it tripping breaker? Caps can go bad.

It runs fine when up to full speed and I tested the run cap with an esr tester and it's exactly at the rated capacitance. No tripping breakers or anything like that.
 

cheeky

New member
Probably a motor with a high resistance rotor. My RPC has a dual cage motor, the outer start cage is high resistance, and the motor has only one non-switched capacitor, functioning as a start and "balance" capacitor.

You may want to check resistance of the winding the capacitor connects to. If open, OK, it's effectively toast. But even if the windings are OK, and connecting wires are OK, the capacitor may not be. Try another one.

Interesting, I'll check to see if it's dual cage and the windings. The cap is 25uf 370v and tested ok with my esr tester so I think it has to be something else, or it was designed this way for pools and spas.
 
Permanent Split Capacitor motors are very popular for use in fans. Other advantages include very smooth, low-inrush starting, high efficiency, and quiet operation.
 

JST

Moderator
Yes, those typically are not wonderful on start torque, although they can be made to have plenty.....

We tested one as part of a MIL product. It operated an oil burner (pump and fan). It had to start at -45C using JP-8 as the fuel. It drew plenty of current, but it started up acceptably.
 








 
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