# Single phase induction motor

#### Ubaid Ansari

Joined Oct 10, 2022
7
Hi All, i need technical clarification regarding the voltage across the 20mfd capacitor of running 1hp water pump motor is around 348VAC against expected 325VAC. Since supply rms voltage is around 230VAC so the peak will be 325VAC. But measured voltage is 23V more than this value. How is this happening. Motor is having both starting and running winding. Motor type is PSC induction motor. Your valuable suggestions and explanation is highly appreciated. Thanks.

#### LowQCab

Joined Nov 6, 2012
3,957
The only way to see what is actually going on inside
that Capacitor is with an Oscilloscope,
( WITH NO GROUND CONNECTIONS ).

A DMM is not going to provide any useful information.
.
.
.

#### crutschow

Joined Mar 14, 2008
34,075
The start and run capacitors are in series with their respective windings forming a resonant circuit, and the voltages across the elements in a resonant circuit can be larger than the applied voltage.

#### nsaspook

Joined Aug 27, 2009
12,852
You are reading a reactive voltage on a phase shift circuit. The single phase motor is actually a two phase motor with the second phase winding voltage phase shifted due to the start/run capacitor to generate rotational flux.

https://www.yourelectricalguide.com/2017/01/capacitor-start-capacitor-run-motor.html

The voltage across the capacitor is a complex number (because of the reactive (magnetic winding coil and electric capacitor) elements of the electrical energy) that the meter reads as a 'real' value.
What's one way to solve the single phase problem?

Build a 2-phase motor, deriving 2-phase power from single phase.

#### MisterBill2

Joined Jan 23, 2018
17,885
When the motor is running the voltage across the phase shift capacitor is not large. And if the motor starts correctly and does not over-heat what is the complaint???The voltage during the starting time will be greater.
AND most of the capacitor motors that I have used do not have that centrifugal starting switch.

Joined Jul 18, 2013
28,533
Motor is having both starting and running winding. Motor type is PSC induction motor. Your valuable suggestions and explanation is highly appreciated. Thanks.
OP states it is a PSC motor, which implies it has permanent start connected winding only, i.e. not a dual start winding motor.
In a common PSC motor, the two windings are identical.

Last edited:

Joined Jul 18, 2013
28,533
AND most of the capacitor motors that I have used do not have that centrifugal starting switch.
You can't have come across many motor over time, as run and start winding motors are and were quite common?? These a generally motors over 1HP.
i.e. Posses Centrifugal SW for e.g..
Otherwise, PSC.

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#### MisterBill2

Joined Jan 23, 2018
17,885
You can't have come across many motor over time, as run and start winding motors are and were quite common?? These a generally motors over 1HP.
i.e. Posses Centrifugal SW for e.g..
Otherwise, PSC.
OK, I was talking about more recent production and smaller motors (FHP types)