# Measuring current in 3 phase motor with shunt resistor.

#### Nathan Hale

Joined Oct 28, 2011
159
Hello all and hope all is well.

I would like to measure the current draw in a toy 3 phase AC motor.
I am using a 1 mOhms shunt resistor.

To measure the current, I will have the resistor in series with one of the phases and then measure the voltage across the resistor.
Later, using Ohms law, I will figure out the current draw.

Am I on the correct path to perform this test?

Any feedback will be appreciated.

Joined Jul 18, 2013
23,141
At some point before the 3ph, there will be a DC supply to the motor controller, maybe easier just to measure this current, insert a ammeter in the DC line.
Max.

#### Nathan Hale

Joined Oct 28, 2011
159
At some point before the 3ph, there will be a DC supply to the motor controller, maybe easier just to measure this current, insert a ammeter in the DC line.
Max.
Thanks for your reply. Will the reading be accurate enough at that point you mentioned? What percentage of error can I expect, if any?

#### Ian0

Joined Aug 7, 2020
2,276
Will the reading be accurate enough at that point you mentioned?
It depends exactly what you need to know. Are you interested in the power delivered to the motor? The power taken from the power supply? And how accurate do you need?

#### Nathan Hale

Joined Oct 28, 2011
159
It depends exactly what you need to know. Are you interested in the power delivered to the motor? The power taken from the power supply? And how accurate do you need?
Looking to figure out the operational current being drawn by the motor.

#### schmitt trigger

Joined Jul 12, 2010
441
Obviously the most accurate method is to measure the actual AC current drawn by the individual phases.
It is also the most challenging, because it would require isolated supplies for each sampling circuit.
If you used a hall-cell based sensor or a current transformer instead of a resistor, the circuit would be significantly simplified.
Why the resistor requirement?

Joined Jul 18, 2013
23,141
Thanks for your reply. Will the reading be accurate enough at that point you mentioned? What percentage of error can I expect, if any?
Just measure the DC supply with no motor command to get the consumption of the drive control itself, then subtract this from any reading that includes motion control.
Simple DC ammeter.
Max.

#### BobaMosfet

Joined Jul 1, 2009
1,778
Hello all and hope all is well.

I would like to measure the current draw in a toy 3 phase AC motor.
I am using a 1 mOhms shunt resistor.

To measure the current, I will have the resistor in series with one of the phases and then measure the voltage across the resistor.
Later, using Ohms law, I will figure out the current draw.

Am I on the correct path to perform this test?

Any feedback will be appreciated.
If you provide a photo of the motor and any part/mfg information, likely a datasheet can be found.

Joined Jul 18, 2013
23,141
I would like to measure the current draw in a toy 3 phase AC motor.
And of course, the current draw is going to depend on various points of the load.
Max.

#### kaindub

Joined Oct 28, 2019
70
Just measuring the AC current of the motor tells you very little. You need to also measure the AC voltage to the motor and the phase angle between the voltage and current. That requires a hell of a lot of computing power and speed.
As someone suggested the simplest and resonably accurate way is to measure the DC current and DC voltage before the motor controller. INA219 chip will do it

Joined Jul 18, 2013
23,141
Just measuring the AC current of the motor tells you very little. You need to also measure the AC voltage to the motor and the phase angle between the voltage and current. That requires a hell of a lot of computing power and speed.
As someone suggested the simplest and resonably accurate way is to measure the DC current and DC voltage before the motor controller. INA219 chip will do it
Keep in mind this is a toy motor and most likely as in RC versions, it would be a BLDC, not 3ph AC.
Although 3ph AC and BLDC being identical in appearance and construction.
Max.

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#### japj

Joined Sep 17, 2009
3
If measuring ac amps why not just use a clamp on amp meter