Stepper Motor Protection

I have a Stepper controller DM556Y connected to a NEMA 23, 4.3 amp Stepper motor, in turn connected to an Arduino feeding the X axis of a Warco Economy mill. All OK so far.

Question. If the whole unit was powered off and the stepper motor rotated by still being connected to the X axis leadscrew, would the current generated by the stepper motor feed back and damage the stepper motor controller ?

A good question! On my power feed I always take out the dog clutch pin when hand feeding to avoid finding out the hard way! Better safe...

Seems to be debatable, some report doing just as you describe without any damage, all I can add is there will be a voltage produced by turning the stepper.

Emgee

Many years ago when I was programming CNC in industry as one of my duties, I observed a CNC mill controller being serviced by an electronics technician from the mill manufacturer. I recall the controller PCB had large diodes on each of the conductors going out from the controller PCB to the 3 stepper motors XYZ. I asked the elec tech what they were for and he said " they keep back EMF from blowing the controller when idiots use the handwheels and it back drives the motors". Just stating what I observed. Diodes on the lines might be worth a try, even large current high voltage diodes are very inexpensive.

Edited By Jeff Dayman on 07/03/2021 00:34:26

Usually there would be protection diodes to protect the electronics against back emf. Sometimes these are inherent in the MOSFET drivers, but effectively still there. If not there, they should be added. The wasp in the jampot is that such diodes, when the motor is driven by external torque, will power up the circuit, possibly leading to undesirable results, especially if the motor is driven fast enough to generate a higher voltage than the circuit is designed for. So ideally the motor should be either electrically or mechanically disconnected when the shaft is being driven by the handwheels. Might not cause any problems in many cases. but no guarantees!

Makes me wonder what might happen if you tow an electric car that has permanent magnet motors. Has the designer thought of that case?

John

Been covered quite well in posts 2018 I think..But John Olsen has it - there ALWAYS are protection diodes - in discrete transistor drive implementations the diodes are separate , in MOSFET/VFET or whatever FET driver designs the diodes is always present in the FET - A power FET cannot be made without this diode and the structure is such that the diode can pass the same current that the FET can in Drain to Source conduction. Be definition it is also a very fast diode, as fast as the FET switching performance, with very good reverse recovery performance.

In integrated drivers, ie, using an integrated circuit with built in power drivers, the diodes are likewise added externally if the drivers are PN junction transistors and inherent if FETS.

I doubt there are drivers on the market today that are PN junction transistor output driver based anymore. FET's rule, as they are simply faster, more efficient and cheaper.

So no need to worry - you will not damage the driver. Furthermore, you would be unable to generate much more than a few tens of volts in the stepper by manually cranking the axis handwheel - even if you manage to wind the axis end to end in a jiffy!

What you may find is that the handwheel feels 'notchy' at low continuous rotation rates, especially if there is gearing ( down) between stepper and leadscrew, as you manually overcome the stepper magnetic indent. The more powerful steppers are worse, obviously.

There are ALWAYS diodes ..but,

High power drivers, sort of the realm of 20 amps up, and 300v potential upwards tend to use IGBT's ( insulated gate bipolar transistor) which is basically a FET driving a PN junction transistor, which does not have this built in body diode. But its is unlikely any of us have such steppers on our machines and if you did, you would be hard pressed to spin it manually fast enough to exceed a few hundred volts, which is what the IGBT can happily withstand..

Joe

Edited By Joseph Noci 1 on 07/03/2021 06:01:06

Thanks Joe - I can sleep easy at nights now ! I can disengage the drive from the leadscrew, but I guess there will always be the time when you forget.

The project has been quite an education in both C+ programming and learning technical French for the supply of parts and stepper motors.

Bob

The diodes in a modern bipolar driver are not for protection, they are commutating diodes which are fundamental to the way the drives works. Basically the drive works as a buck converter turning the volts applied to a winding on and off very quickly with the diode keeping the current flowing. The way they're connected forms a bridge rectifier that will charge the supply rail to some voltage when you drive the motor. I noticed when I accidentally wound the handwheel when developing the x drive with the cover off the electronic box but not powered that various LEDs lit up. I think the problem is that the system is not necessarily designed to cope with this and you don't know quite what will happen. Apart from that it spoils the feel of the handwheel and makes it harder to turn. So I avoid it just in case. My cnc lathe and mill don't have any handwheels and I don't miss them.

from my (little) experience, I have found the good quality drivers (Ebay/chinese) do not seem to object, you may find LED's lighting up but that is all.

However! if you are using small steppers like those on low end hobby 3D printers, the driver board that tend to be sold as suitable for these steppers are NOT well protected, These boards are very small (less than 1 inch square) with a 1/2 in square heat sink. I have had these blow as soon as you turn the stepper manually while others seem to cope with modist rotation.

Best idea is disengage if possible otherwise always drive the stepper electrically as John says.

Indeed! The driver I use on the x-slide power feed is a DRV8825, excellent little IC when used as designed but I couldn't vouch for the protection against back emfs generated by driving the motor.

One thing that makes a difference to he robustness of the system is what power supply you are using. Most modern supplies will let the voltage generated by the motor (and fed through diodes to the supply) to exceed the voltage rating of components in the drive or other circuits, causing damage. One solution is to put a voltage clamp like a zener diode across the supply. For example a 32V zener across a 24V supply. The zener must be large, a 5W one minimum. Another option is a fixed load (resistor) across the supply but this wastes power.

Robert G8RPI