driving motors at what current?

[900steve]

i'm building a driver circuit with the L297/L298 combo, and i have three 1.4v, 3.9amp steppers, 8 wire. i think they're rather dinky, but i got them at a swap meet for low bucks, and i'm just experimenting right now.

i'm confused as to what power i want to drive these at. i understand that i can over drive the voltage for faster speed, but how how many volts = too many volts?

here is where i'm really confused, what happens to the current? i'm assuming i need to limit the current to these motors? and the simplest, but least efficient way i can think of doing so is with a power resistor.

am i on the right path?

thanks!


-steve

[drawbar]

The L297/L298 combo will give you a "chopper" style stepper driver that does not use current limiting resistors. You can supply the driver with up to 46 volts and the chopper limits the current to whatever level you set it at, up to 4 amps total for the L298. Connect the L297 and L298 together as shown in the datasheets, using the proper sense resistors. Notice that the L297 has a pin labeled Vref. This is where you apply a voltage ranging from 0 to about 3 volts DC to set your output current to the stepper motor. You can use a pot to make this easily adjustable.

Brian

[900steve]

great, thanks Brian! i was weeding through the datasheets, and somehow missed that. but it seems like if i want 4 amps, i need to run two L297's in parallel. no problem though.

does anyone know how many volts over the rated voltage on a stepper is acceptable?

thanks!

[AbSat]

Originally Posted by 900steve does anyone know how many volts over the rated voltage on a stepper is acceptable?

The voltage is no problem, You can use any voltage between 12 and 46 V.
The current control will limit it to the preset value so if your preset is 3 amps the motorcoils will get 3 amps.
To get 4 amps available for the motor you have to switch the two drivers in the L298 parallel and use two of them.
When you have still the choice it would be better to use two L6203 drivers in stead of the L298, The L6203 is a DMOS driver and does not produce so mutch heath as the L298.

AbSat.

[900steve]

i'll look into the L6203 drivers. . . .

but i was more concerned with in general, what's a good rule of thumb as far as the over voltage that a stepper motor will accept before failure or severely reduced efficiency?

thanks for everyones input.

[Ferenczyg]

your motors have 6 wires? If so, they are unipolars, and are the sames that ones I got some time ago. If is the case, discard center taps and use them bipolar (297/298 are for bipolar) then as you doubles the coil length you must feed half current, so 3A9 ->1A95 that is pretty equal to the max the L297/L298 combo can offer (do not forget heatsinking). About the voltage, you can drive the motors with any voltage under 20x nominal. Remember that, basically, a stepper is a current controlled beast so:

- In a stepper, voltage provides speed, and can be any below 20x nominal.
- In a stepper, current provides torque and never must be above nominal.

/F

[900steve]

well, they're 8 wire unipolar. so i can wire both phase A coils in series, and wire the B coil in series, and therefore i halve my current.

i neglected to think about this one, thanks! and the 20x rule seems to jive with what i've found, so i'll roll with that.

thanks all.

-steve

[GrahamIT]

Originally Posted by 900steve well, they're 8 wire unipolar. so i can wire both phase A coils in series, and wire the B coil in series, and therefore i halve my current. i neglected to think about this one, thanks! and the 20x rule seems to jive with what i've found, so i'll roll with that. thanks all. -steve

From experience and only for "series" config, you can go up to 25x V nominal as long as the motors dont go above their usual operating temp (heat kills magnitism so permanent damage to your motors will be the result if you let them get too hot). You don't get something for nothing though. Series config reduces the torque considerably.

[900steve]

ok. sounds like some experimentation is in order. once i finish up my breakout board and drivers, i'll try and post some results.

-steve