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Hat and stepper motor matching

Hello

I've purchased 4 DC & stepper motor hat for Raspberry Pi (I have a Raspberry Pi model B+) to run 8 NEMA 17 non captive stepper motors. Motor specs are:
Current per phase 0.84 A
Phase resistance 5.75 Ohms
Phase inductance 9.30 mH
Rated voltage 4.8 V
I'm quite new to stepper motor control, and I've seen too late that the hats have no current limiter, therefore since the phase resistance is low I'm not sure if the stepper motors and the hats are compatible. In case they aren't, is it possible to add a resistor in parallel with the stepper motor to limit current for higher voltages? As I've learnt from other forums, it is a common practice to drive the stepper at an higher voltage to obtain better performances, with the current to be limited accordingly.

Luca

Luca2

Posts: 3
Joined: Thu Jul 11, 2019 4:36 am

Re: Hat and stepper motor matching

Since the current limits for the TB6612 drivers on the HAT are higher (1.2A) than the current ratings of the motor windings (0.84A) you mostly need to be concerned about the motor current. Since there is no current limiting in the HAT, the 'worst-case' current draw is governed by Ohm's Law. I = V/R. A 4.83v supply yields: 4.83v/5.75Ohms = 0.84A.

4.83v is not a standard supply voltage. However, you could probably use a 5v supply, since there will be a small voltage drop through the H-bridge.

Posts: 74650
Joined: Sat Feb 07, 2009 10:11 am

Re: Hat and stepper motor matching

And what about the resistor in parallel? Can it be a solution to apply an higher voltage to the motors?

Luca2

Posts: 3
Joined: Thu Jul 11, 2019 4:36 am

Re: Hat and stepper motor matching

You can do that also. Ohm's Law still applies to the combined resistance of the motor winding and your external resistor. You will need to use a resistor with an appropriate wattage rating to dissipate the excess power as heat.

For example, with a 12v supply, your resistor will need to drop a little over 7v. 7v * 0.84A = 5.88 Watts. So you will need a resistor rated for greater than 5.88 Watts.

Posts: 74650
Joined: Sat Feb 07, 2009 10:11 am

Re: Hat and stepper motor matching

Thank you, that was very clarifier. The last question: the voltage that the hat send to the stepper motor is the one given by the supply, am I right?

Luca2

Posts: 3
Joined: Thu Jul 11, 2019 4:36 am

Re: Hat and stepper motor matching

the voltage that the hat send to the stepper motor is the one given by the supply, am I right?

Yes. It will be very close to the full supply voltage. There will be a fraction of a volt drop through the power FETs in the bridge chip.