Sizing the start capacitor for an induction motor

  • Thread starter Guineafowl
  • Start date
  • #1
630
310
Summary:
How to spec a start capacitor for an asynchronous induction motor, and the consequences of getting the value too high and too low.
A friend has a single phase, cap start induction motor with the following characteristics:
V 240/250
A 2.6
Hp 1/3
Rpm 1435 @ 50Hz

As for as I know it’s not fitted to a machine, but given the low hp rating I’d guess it’s for a drill press. So it would need only moderate to low starting torque. At the moment it won’t start spinning unless given a turn in either direction, and then only spins very slowly.

It’s so old the markings have worn off the original capacitor. Looking at various guides I have suggested a 50uF replacement, rated appropriately for voltage and designed for motor starting. It’s on the higher end of the suggested values.

While we wait for that to arrive, what would happen if the value is wrong? My guess:
Too high - excessive inrush current on starting
Too low - inadequate starting torque.
 

Answers and Replies

  • #2
Averagesupernova
Science Advisor
Gold Member
3,848
774
Your guess about sizing is correct, to a point. Taken to the extreme, imagine an infinite capacitance resulting in no phase shift yet will allow excessive current in the start winding. I can recall there is a general rule of X uF per horsepower. I can't remember the actual numbers. You may also want to consider the starting torque required. If it is very little and it does not matter if it spins up slowly, you could err on the low side of capacitance. Make sure you don't have an open starting switch before you go swapping out capacitors.
 
  • #3
630
310
Your guess about sizing is correct, to a point. Taken to the extreme, imagine an infinite capacitance resulting in no phase shift yet will allow excessive current in the start winding. I can recall there is a general rule of X uF per horsepower. I can't remember the actual numbers. You may also want to consider the starting torque required. If it is very little and it does not matter if it spins up slowly, you could err on the low side of capacitance. Make sure you don't have an open starting switch before you go swapping out capacitors.
Thanks. I vaguely remember a Jim Hardy post about X uF per horsepower.

The ‘friend’ is actually a user on another forum, and given the description of the existing capacitor (“ancient”) and the difficulties of remote diagnosis, I thought the easiest step was a replacement cap. If no good, I’ll get him to strip the motor down.
 

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