# Phase current calculation

#### electronicsLearner77

Jul 2, 2015
277
I have seen a 24V PMSM motor specifications and it shows the line - line resistance of 0.57 ohm and L-L inductance of 0.64, suppose i apply an algorithm (sinusoidal etc) and a voltage of 11V RMS at the inverter side, the current that would be flowing is 11/0.57 = 19.29 Amps. It is a small motor i don't think that much current will flow. What mistake i am doing in my calculation? Please advise. The motor is star connected.

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,182
What mistake i am doing in my calculation?
The current is not defined by the resistance of the coil in Ohm alone.
You need to consider also:
- reactance of the windings (inductive resistance) which increases with the frequency of the driving signal.
- back emf of the motor which depends on the rotational speed (link)

#### electronicsLearner77

Jul 2, 2015
277
Thank you very much the reply, as per the specification i can use the below parameter for the back emf calculation based on the rotor speed i think.

#### electronicsLearner77

Jul 2, 2015
277
Now i understand it more better the maximum RPM of the motor is 4000, so the back emf generated will be approximately
4k*6.2 = 24.8Volts. But i have one doubt there is only one parameter for inductance

How do i get Ld, Lq separately?

#### Bluejets

Oct 5, 2014
6,329
I recommend using Ohm's law to calculate the phase current of a circuit as it is a simple and reliable method.
Op is using AC where impedance rules.

#### electronicsLearner77

Jul 2, 2015
277
The current is not defined by the resistance of the coil in Ohm alone.
You need to consider also:
- reactance of the windings (inductive resistance) which increases with the frequency of the driving signal.
- back emf of the motor which depends on the rotational speed (link)
I was going through the simple foc website open loop there is the below statement
"This is very inefficient, as for different motors with different phase resistances the same voltage values can produce wildly different currents. For gimbal motor, you can run it in the open loop with the voltage limits of 5-10 Volts and it will reach the currents of 0.5-2 amps as it has the pahse resistance from 5-15 Ohms. For drone motors, the voltage limits should stay very low, under 1 volt. Because they have pahse resisatnce of 0.05 to 0.2 Ohms."
Here he is not considering the back emf, please help me to understand why it is different?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,182
It seems teh library used on that website calculates back-emf internally:
The best way to use this control strategy would be to provide both phase resistance value and KV rating of your motor. The the library would be able to calculate he back-EMF voltage and much more precisely estimate the consumed current. And with the current and the back-EMF current the library can set much more appropriate voltage to the motor.

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