# What is phase current of stepper motor?

#### nandishJasani

Joined Jul 4, 2023
2
Hi,

I have stepper motor with me without its part number i.e. i dont know the make and part number of this motor. I have following parameters with it. I have A5976 stepper motor driver controller interfacing with FPGA. In A5976 i have to set the Vref which is corresponding to winding current. So anyone can help from below parameters of stepper motor that what is the winding current and step degree of this motor?

Its Stepper motor Nema 23 (57mm)
Its length is 76mm.
Its two phase motor.
Its winding resistance is 7.8ohms. (Measured with multimeter)
Its winding inductance is as follows (Measured with LCR Meter)
Measuring frequency 100Hz - 48mH.
Measuring frequency 1KHz - 35mH.
Measuring frequency 10KHz - 20mH.

Thanks
Nandish Jasani.

Joined Jul 18, 2013
28,045
A common step angle is 1.8° =200 steps/rev.
You can measure this by pulsing one winding with a 4v DC source, one pulse at time.
The typical Va for a NEMA 23 would be between 7 and 10 Va. with 76mm putting it at ~9Va to 10Va.
Estimated.

.

Last edited:

#### BobTPH

Joined Jun 5, 2013
8,104
The parameters should include the phase current. It is not something you can determine.

The voltage required to achieve that current depends on how fast it is running. When stopped (holding) it is simply the current times the winding resistance.

#### MisterBill2

Joined Jan 23, 2018
16,566
Consider that the resistance is what determines the winding current in a stepper, that is used as a stepper motor, while the inductance determines the applied voltage and series resistor to deliver the needed maximum speed, although this system probably uses PWM to control current and torque.
The step angle is easily found by turning the motor by hand and counting the number of resistance torque steps in one revolution. Then the step angle is 350 degrees divided by the steps per revolution. Stamped case steppers vary in the number of steps per revolution.

#### Ian0

Joined Aug 7, 2020
8,944
You can find the step angle by shorting out one winding, then turning thr shaft by hand until it completes a full turn. Each click is four steps. 50 clicks indicates it is a 1.8° motor.
Phase current can be determined by calculating I^2.R for a similarly sized motor to determine its maximum power dissipation then working back to give Imax for your motor. It will only be an approximation, as you do not know whether the maximum insulation temperatures are the same.

#### panic mode

Joined Oct 10, 2011
2,524
one way to determine number steps is to run it... send pulses until it completes revolution. about current that motor will handle - who knows. you need part number and datasheet. if unsure run it at lower voltage. increase voltage as practical. of just get new motor. no-name parts are not worth it unless you are willing to sacrifice them - or use them conservatively.

#### Ian0

Joined Aug 7, 2020
8,944
If you don’t have stepper motor drive circuitry to hand, you can run it as a capacitor run motor from a low voltage transformer.
A 1.8° motor will go round once a second on a 50Hz supply.

Joined Jul 18, 2013
28,045
I still have a few stepper manuf. literature from decades ago and they show the max temp of the centre of the frame should not exceed 100° C. for any stepper.
Most drives now use a translator circuit/IC to maintain the motor rated current throughout the RPM range.
e.g. L297 etc.

#### MisterBill2

Joined Jan 23, 2018
16,566
Max is correct. The series resistor scheme was common bck in the early 1980's era. Now PWM control does a better job and is vastly more efficient.

#### nandishJasani

Joined Jul 4, 2023
2
Thank you so much all
This discussion is helping me and i gave 200 pulse and motor rotates 360 degrees i.e. it has 1.8 Degree resolution. And for current i limit it to 1Amp and it works fine.

Regards,

#### MisterBill2

Joined Jan 23, 2018
16,566
The final check for the current limit setting is to feel the motor after it has been in use a half hour. If it is too hot to be touched then the current limit is too high. Usually this is a simple and easy check.