Hello, I'm new to this forum but its my last ditch attempt to get a formula that I can't seem to find but I know exists. A friend of mine has a motor that has a start and run capacitor. He wants to replace to the caps because they are old but he can't read the number on them. I know there is a formula out there that calculates the required uF rating to put a single phase motor current to about 90degrees out of phase to increase starting torque. If anyone knows that formula could you post it.. I've used the formula before but its been so long that I can't remember it. Any help would be great!

K

I know I originally found the forumal in a Thompson education text book for basic electricity.

Thanks people!

I have a formula but you would have to know about the reactive power demand of coil where you want to connect the capacitor

C = (QC/V² x W) x 1000000

Where

C = Value of capacitor in uf
QC = Reactive power demand of load
V = terminal voltage
W = 2ðf (ð = 3.14, f = Line frequency)

Dear KenB,

I can give you a practical way of finding the required start and run capacitor assuming that the motor under question has only one capacitor for starting and running (Permanent split capacitor type)
The formula is taken from famous design book by Veinott

Xc = Ec squared /V/Io (1)
Now I will explain each of the terms and how to find out them

Xc = Capacitive reactanc =1/( 2xpixfxC)

EC is the no load auxiliary winding voltage with motor running on main winding only
V voltage of the motor
Io no load current of the motor running on main winding only

Do the following measurements to find out Ec and Io

1.Disconnect the capacitor which is to be replaced
2 Give supply V volts to main winding only. do not give supply to auxiliary winding
Rotate the shaft with hand so that motor rotates almost at full speed
3. Measure current drawn by the motor . This is Io.
4. Measure voltage across the auxiliary winding under the above condition. This is Ec

5. Now calculate Xc by formula (1)
6 Calculate capacitor value as under

C = 1000,000/(2xXcxpix f ) in micro farads the voltage rating of capacitir should be more than
2xV

example
Let V= 110v f= 60hz
let Io = 2 amps Ec = 145 volts
Then Xc = 145x145/110/2 =96 ohms
C = 1000,000/(2x96x3.14x60) =28 Mfd say 30 Mfd
The voltage rating of capacitor should be more than 220 v say 230V