Capacitor Size for Old Single-Phase Motor

Can anybody tell me the factory specification for the motor's capacitor? If not, can anybody tell me how to calculate the proper size capacitor? Or, is it largely empirical? Is there something else I have missed?

15uF sounds about right for a 3/4 hp single phase capacitor run motor (capacitor size isn't to critical and capacitor selection is the same for currently sold single phase motors), your fan startup speed does not however.

Based on the excessively high startup time and high starting and normal operating currents it sounds like the fan motor is overloaded. Possible causes are - Faulty or mis-wired start winding, motor bearing issue or other mechanical issue preventing motor turning freely, motor is too small for application.

Since this is only a small fan motor (inexpensive and not application critical) it may be time to buy a new motor (especially if the rewound start winding can not be replicated), least you waste more money on a motor that is just too old to repair economically.

j-o-a-t,

Thanks for the reply. I suspect that the combination of 115V winding with 840 RPM (8-pole) is a real killer for finding a new motor. That is probably why the old one was rewound. Considering that the rewind shop said they had a lot of trouble in reading the winding data from the old motor, it is quite possible that there is a winding problem as you have hinted. Friends in another rewind shop where I worked years ago suggested that the capacitor size may have to be substantially increased.

Thanks-JMM

Friends in another rewind shop where I worked years ago suggested that the capacitor size may have to be substantially increased.

While capacitor size isn't too critical, increasing the capacitor size excessively can have a detrimental effect on the life of the motor (and the capacitor).

Don't forget you may have the option of changing to a similar (but more commonly available) motor such as a single phase 6 pole (faster) motor, 110/220V winding, or even switching to a three phase 8 pole motor. It depends on your application of course (but in the end it is still just a fan).

Is it a capacitor start & capacitor-run motor or capacitor start and run motor? Check it got any centrifugal switch. If you find switch then capacitor value will be around 80MFD and if not 4MFD.

11.8 mfd capacitor will solve your problem

Why is everyone assuming that the magnet wire and the capacitor value are for 120vac? Maybe the long startup is due to undervoltage. Was it designed to operate on 230 vac?

Luther M

http://www.amazon.co.uk/Electric-Motors-Home-Workshop-Practice/dp/1854861336 this book is all you are gonna need.

Friends,

I appreciate the range of comments and suggestions.

To Kvsubramanyam: It is a permanent split capacitor type of motor, not capacitor start and capacitor run. There was and is no centrifugal switch. The motor terminal box is on the side of the motor, not in an end-bell; it has 4 leads only. No terminals or ability to change direction.

To Jack Of All Trades: The incoming power there was and is 115V/1-phase. Any change to that would require running a new circuit from the distribution panel, etc. Using a faster motor (6-pole) would require the use of a matching blade, etc. The location is near the ceiling of a fairly tall manufacturing facility just inside a tall roll-up door for truck entry/exit. The heater has a live steam coil for supplying the heat, so the immediate environment can be quite hot when the fan is off (most of the time). Therefore, I doubt that a belt drive from a 4-pole or 6-pole motor would survive. I appreciate the warning about life-span if a too-large capacitor is used.

To Luther M: I quoted the original nameplate data on the thread's original post. The motor was rewound for this 115/230 voltage.

To Guest (rated off-topic): This particular unit heater is used as an air curtain, and sees intermittent use. I would agree with your comments about ignition systems for gas fed appliances. However, all the unit heaters in this facility (over 100) use steam through heat exchange coils. Your comment has merit, but does not apply here.

To all: The problem appears to be a too low starting torque. The motor rewind shop said it started OK by itself. They did not have the fan blade with the motor, so did not test the motor with its actual load. The facilities maintenance manager has the option of asking the mechanical contractor to return the motor and fan blade to the motor rewind shop so they could test the combination and perhaps improve it. I don't know if this will happen. This manager is fully aware of my concerns about the startup time, etc. (and is generally a pretty level-headed and reasonable person; far from "just-a-penny-pincher").

Thanks--John M.