# calculating inductance from the power factor of a 240vac motor

Thread Starter

#### denison

Joined Oct 13, 2018
200
What is the formula for the inductance if the power factor for a 240vac motor is known? I don't know if its even possible to do this. Does any body know?

#### crutschow

Joined Mar 14, 2008
27,169
A motor PF varies with load.
Under what conditions is the power factor measured?

Thread Starter

#### denison

Joined Oct 13, 2018
200
A motor PF varies with load.
Under what conditions is the power factor measured?

Thread Starter

#### denison

Joined Oct 13, 2018
200
no load on the motor. I can get the power factor from a 'reduction revolution' power meter. So then is there a formula for the inductance of the motor? I can also measure the resistance with a multimeter and the current.
So then it must be possible to calculate the inductance from a formula.
There are inductance meters but they are quite expensive.

Thread Starter

#### denison

Joined Oct 13, 2018
200
Hang on there is a load. the motor is attached to a fan. but how to calculate this load is another matter.

#### crutschow

Joined Mar 14, 2008
27,169
You'd have to know the motor current and voltage when the PF was measured.
From that you can calculate the inductance as if it were a series RL circuit.

#### Dodgydave

Joined Jun 22, 2012
9,951
When you find it's inductance, what will you do with it??

#### Alec_t

Joined Sep 17, 2013
12,060
Isn't the inductance going to vary cyclically as rotor poles move relative to stator poles?

#### MisterBill2

Joined Jan 23, 2018
8,709
When you find it's inductance, what will you do with it??
I was wondering the same thing. What application requires knowing the inductance of a motor? Knowing the current and power factor versus load and RPM provides the needed information.

• Dodgydave
Thread Starter

#### denison

Joined Oct 13, 2018
200
You'd have to know the motor current and voltage when the PF was measured.
From that you can calculate the inductance as if it were a series RL circuit.

Thread Starter

#### denison

Joined Oct 13, 2018
200
do you have the equation for that Mr Crutschow? Other people on this forum are asking why I need the inductance. It is because I want to model a 240vac motor on Ltspice to try to solve spike problems I am having at switching the motor.

Thread Starter

#### denison

Joined Oct 13, 2018
200
Thread Starter

#### denison

Joined Oct 13, 2018
200
I will simulate the motor on Lt spice

#### crutschow

Joined Mar 14, 2008
27,169
do you have the equation for that Mr Crutschow?
Do you understand phasor calculations?
Here's a discussion of an RL series circuit, which is what a motor looks like.
Given the voltage, current and phase angle (power factor) you can calculate L.

#### Alec_t

Joined Sep 17, 2013
12,060
Thread Starter

#### denison

Joined Oct 13, 2018
200
Do you understand phasor calculations?
Here's a discussion of an RL series circuit, which is what a motor looks like.
Given the voltage, current and phase angle (power factor) you can calculate L.

Thread Starter

#### denison

Joined Oct 13, 2018
200
I have done phasor calculations in the past but now forget much of it. I have been looking for an online calculator to do the calculations for me. Putting in voltage, current and power factor.
Unfortunately I cannot find one. There are some great maths and electronic calculators now in the cloud which saves pages of hand calculations.
Can one of you guys point to an online inductance calculator using V,I and PF?