View Full Version : Stepper motor coil resistance. What is an acceptable value?

30-11-2012, 12:43 PM
One of my cnc routers is blowning the mosfets in the stepper drives. One possible problem I am aware of is the coil windings may have too high a resistance to work properly. The coils measure 5.6 ohm at the motor and roughly the same at the end of the steppe cable running to the drive.

Some of my other machine have two different brands of stepper motor and the coil resitance on them varies between 0.9 ohm and 2.3 ohms.

So my question is, is 5.6 ohm too much per coil winding? (Winding is measured across two of the four wires from my stepper.)

Robin Hewitt
30-11-2012, 03:15 PM
It isn't the resistance that blows MOSFETs, it's the inductance. Usually measured in milliHenry's.

It takes time to change the flow of current in a coil. Get it wrong and the Volts go enormous, goodbye MOSFET's.

For an analogy, to stop a car it is better to use the brakes rather than a brick wall.

30-11-2012, 03:37 PM
Thanks Robin,

Does that mean the resistance has no contribution towards causing a voltage spike? What are the common causes of blown MOSFET's.

Robin Hewitt
30-11-2012, 05:53 PM
What are the common causes of blown MOSFET's.

MOSFET's have a break down Voltage which does not want to be exceeded.

The fix for inductance is diodes strategically placed so the current in a coil always has an escape route.

MOSFET's usually have a reverse diode fitted. If not it must an external component and it needs to be fast switching.

Consider this...

A MOSFET says, "I now want the current to flow clockwise through this coil".
The coil replies, "Bad luck mush, my inductance says we are still going counter clockwise, get used to it".
The protection diodes say, "Panic not, we will allow the current to flow".

Basically, both ends of every motor coil can draw power off the ground rail or add power to the voltage rail through the protection diodes.

Now you have to ask yourself:-
- Are the protection diodes fitted?
- Are they fast enough to protect my MOSFET's?
- Can my power supply handle this sudden reversal without going way over Voltage? Can it sink as well as source?