Why such high voltage?

[jeffs555]

using LC filters before the motor windings sometimes helps in reducing motor heating. they reduce HF AC current into the winding thus reducing eddy currents.

Doesn't this kind of defeat the purpose of using a high voltage supply? The reason for using a high voltage is to gain more rapid change in current. An LC filter is going to reduce the rate of current change.

[rdpzycho]

Originally Posted by jeffs555 Doesn't this kind of defeat the purpose of using a high voltage supply? The reason for using a high voltage is to gain more rapid change in current. An LC filter is going to reduce the rate of current change.

the LC filter is much smaller compared to the inductance of the winding. Unitrode published an old application note about doing so (I believe it's for UC3717A Stepper Motor Driver).

the LC filter does reduce rate of current change but not that much if sized properly. I've done this to old stepper motors and heat generated while stopped and enabled is significantly less. the filters would be a little pain though if you have to do microstepping.

there are also other ways to make the winding see less HF AC ripple, mostly in the methods of recirculation.

[Eclipze]

Perhaps the LC was for a snubber network, to dissipate the back EMF spikes and prevent them from being absorbed by the motor.

[VVINZ]

Originally Posted by sp1nm0nkey So if I add a shunt resistor to do current sensing and regulate the current within the range of the motor with pwm I should be okay?

I've been using shunt resistors to limit the current to the stepper motors and have had good success. It waste power but it was a quick fix. I would find chopper power supply's from surplus houses most of them were 5 volts at 100 amps so I would find 3 and series them for a total voltage of 15 volts and then get a heavy duty series resistor normally it was wire wound to handle the current and place that in series to limit the max current seen by the stepper motor. My stepper motors were quite large to power my 3hp router head. (www.brokenhammer.com)

I just have not had time to detail my setup but you can see pictures of the control circuitry on brokenhammer.com

good luck and keep on experimenting.

My next project on Brokenhammer will be a cnc plasma cutter or maybe the humvee project . ha

virg

[VVINZ]

Originally Posted by jeffs555 Doesn't this kind of defeat the purpose of using a high voltage supply? The reason for using a high voltage is to gain more rapid change in current. An LC filter is going to reduce the rate of current change.

Sorry just could not help noticing your icon. I competed back in the 70s at an autocross with my formula v that I had designed against an austin healy that had a 327 in it with formula racing tires. Needless to say he captured fastest time of the day. ha www.brokenhammer.com

Virg

[david_geng]

hi guys, wouldn't the higher voltage increase the EMI and heat up the motor due to the higher eddy current?

[VVINZ]

After 2 hours of running the motors would be warm but not hot. It's choosing the right shunt resistor. You want the motors to step gingerly and not stall out and not exceed the specification of the motor. The big fear is damaging the permanent magnets in these old steppers by creating to much magnetic flux. So choose the right shunt resistor. I'm not saying this is right. It's just a poor man's way of experimenting.

I did have a smaller motor for my z axis and just used the same shunt resistor that I had used for the larger x,y motors. It got very hot and I just added heatsinks to the outside of the case. The motor (cross your fingers) is still working today. What I mean by hot (never measured it) was that I could hold my hand on it for about 4-5 seconds before it became uncomfortable. Not very scientific is it. ha

Take care and keep experimenting

I used a cap and large shunting diode at the motor to protect my mosfet drivers.

[rdpzycho]

motor heating may be caused by the eddy currents mainly caused by the ripples. as what Unitrode suggested in their application note, a small LC filter will reduce EMI and lower the ripple seen by the motor (I believe Gecko is using another scheme to lower ripples but not filters) thus reducing heating.

EMI would be caused by higher rate of current rise. filters may help diminish them. the best way to reduce them is using common mode chokes on the wires to the stepper motor. I also put the drivers as near as possible to the motor so that the long wires may not act as antenna.