# One stepper motor of two getting really hot

I'm using two stepper motor 28BYG-48 rated for 12V. Those are unipolar motor with 5 wires. 1 Common wire and 4 wire to control the 2 phases. The motor that is getting hot has around 76-77ohm the common pin and the other pins, while the motor that is cool has around 78-79 ohms.

Both motor are wired the same way through uln2003 driver. For my current tests, I was running the motor very slowly. Almost 1 step per second, I'd expect both motors to be cool as the current rarely switch direction.

Is it possible that one motor is getting more power than the other which would explain why one is getting hot.

When I switch motor positions, the motor that was cool is getting hot and the motor that was hot stay cool. My guess is it's a problem with the uln2003 chips.

One other thing, the uln2003 connected to the hot motor is quite cool compared to the uln2003 connected to the cool motor. When there is no activity (no motor switching direction), one of the two motor gets hot progressively.

How can I fix this? What could be actually causing one motor to get excessively hot?

Note:

After taking pictures, I realized I mixed up the namings, those are ULN2003 chips. I have l293d chips but they're for other motors.

## bumped to the homepage by Community♦2 days ago

This question has answers that may be good or bad; the system has marked it active so that they can be reviewed.

• Measure their power consumption. – PlasmaHH Sep 26 '17 at 12:56
• Since the heat stays with the driver, either your driver wiring is messed up or there may be an issue with the driver itself. – Trevor_G Sep 26 '17 at 12:57
• @LoïcFaure-Lacroix Visual inspection for shorts and bad connections is your first thing to try. Use your ohm-meter to see if things are connected the way you expect. I don't know what test equipment you have. A scope would help, but failing that you need to measure applied voltages and currents to see what is different between the two drivers. Note: Do a bunch of testing with the motors disconnected first in case you fry the drivers. You should know how it SHOULD work! Study the data sheet. – Trevor_G Sep 26 '17 at 13:19
• Your apparent belief that heat would only be generated when current "swtiiches direction" is fundamentally mistaken. If your control software deactivates (or PWM reduces) the driver once the motor is in position, that would indeed reduce heating - but it would also mean no (or at least lesser) holding torque. – Chris Stratton Sep 26 '17 at 20:37
• @Trevor I checked for shorts already, maybe it was the connection on the breadboard that wasn't right. I ended up converting the motor into bi-polar motors and using the l293d I had instead of the uln2003 chips. As a result, I can run those motors at 15v without them getting hot. Those l293d aren't getting hot either so I believe I might have damaged one of the uln2003 in a way the motor worked fine but thought my ohm-meter couldn't detect any short. – Loïc Faure-Lacroix Oct 2 '17 at 18:46

The L293D is only a driver that switches the supplied voltage to the load (motor). It has no current control, like chopper,... So basically when the motor is at standstill the current is limited only by its winding resistance. If it is spinning then the current is lower, since the back EMF voltage builds up: $$V_{supp} = I\cdot R+V_{BEMF}$$ $$I=\dfrac{V_{supp}-V_{BEMF}}{R}$$ $$V_{BEMF}=k_e\cdot N_{RPM}$$
$$I=\dfrac{V_{supp}}{R}$$