Handling high motor stall currents

[rasen58]

Is there any general advice on how to handle motor stall currents? I'm making a robot with two large motors powering the wheels and the motors take 4A normally but have a 15A stall current each.

Are there certain things I should be doing in either hardware/software to make sure that the stall currents aren't there for prolonged periods of time or if there are, that I protect the circuitry somehow?
For example I think that if I powered the motors at full speed in one direction and then suddenly reversed in the opposite direction, that could create a large emf which could damage my circuitry or motors?

 

[Harald Kapp]

Use a PTC in series. Low resistance at low temperature, high resistance at high temperature. If the motor stalls, current rises leading to an increase in temperature of the PTRC leading to increase n resistance, leading to reduction in current.
Or use a resettable polyfuse, based on the same principle of operation.
Or measure the current and turn off the driver when current is too high for too long.

 

[Tolaneva12]

PTRC have better usability

 

[rasen58]

Thanks. What is PTRC?

 

[rasen58]

Also it looks like the PTC would be great for protecting the motor from over current. But will a PTC also protect my motor driver from back emf current if I suddenly switch off the power going into my motor driver?

This is the simple motor driver circuit I have (two motors and two motor drivers). So should I add a PTC on the OUTB or OUTA pin connecting the driver to the motor?

 

[bertus]

Hello,

From the drivers page:
Output current: 13 A continuous
Active current limiting (chopping) with default threshold of 30 A (can be adjusted lower)
https://www.pololu.com/product/2992

When needed there ia a stronger driver available:
https://www.pololu.com/product/2995
Output current: 21 A continuous
Active current limiting (chopping) with default threshold of 50 A (can be adjusted lower)

Bertus

 

[Harald Kapp]

But will a PTC also protect my motor driver from back emf current if I suddenly switch off the power going into my motor driver?

No, that protection is usually part of the driver.

 

[rasen58]

my driver doesn't explicitly say it protects against back emf current - https://www.pololu.com/product/2992

What is PTRC?

 

[Harald Kapp]

It doesn't have to state that explicitly. This is an inherent function of the MOSFETs' body diode.

 

[rasen58]

Hello,

From the drivers page:
Output current: 13 A continuous
Active current limiting (chopping) with default threshold of 30 A (can be adjusted lower)
https://www.pololu.com/product/2992

When needed there ia a stronger driver available:
https://www.pololu.com/product/2995
Output current: 21 A continuous
Active current limiting (chopping) with default threshold of 50 A (can be adjusted lower)

Bertus

I'm also using this motor driver which doesn't have a current limiting functionality https://www.pololu.com/product/760
Is there anything I can do to be safe when using this one?

 

[ratstar]

If you could measure the velocity of the motor, you need to cut out the battery when the velocity is low.

Would a constant current circuit work for back emf from dc motors? Its actually not resistance load, its actually back generating out from the motor, but would it work anyway?

 

[Harald Kapp]

you need to cut out the battery when the velocity is low.

You will then face the issue of starting the motor. At 0 rpm the battery will be cut out and you need an override circuit - which in turn will need to be protected from a potential high stall current ...

Would a constant current circuit work for back emf from dc motors?

No. Motor current varies with load.