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I have an 8 wire stepper motor and an unipolar motor driver. I wired the motor as unipolar connection. On motor 3.15V and 4.2A is written. unipolar motor connection

My connection is above. When I connect V to 24V 14A source to V, at 1 ms step, the motor (has 200 steps) rotates at 3 Hz, but I expect 5 Hz. When I set step to 10ms or 100ms, the motor rotates very absurdly, like rotating 20' to left and waits then rotating right to 40' and lots of unexpected rotations.

When I connect V to 5V, it rotates at 1 ms step (5Hz expected) at 2Hz but vibrates too much. When I set 10 ms (0.5 Hz rotation is expected), the motor rotates at 0.45Hz; and vibration is very low. At 100ms step it works as expected.

My conclusions are

  • High voltage is necessary for fast switching
  • High voltage does not work at lower frequencies (long step sizes like 10ms or 100ms).
  • Low voltage does not work at high frequencies (short step sizes like 1ms)
  • Low voltage works pretty well at low frequencies.

My questions are

  • Is my wiring schematic correct for unipolar scheme?
  • Why did high voltage experiment not work for low frequencies?
  • Why did low voltage experiment not work for high frequencies?
  • Which voltage should I give to V at the schematic to rotate the motor at both high and low frequencies?
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"On motor 3.15V and 4.2A is written."

This means that each coil has a resistance of 3.15 / 4.2 , or about 0.75 ohms.

You do not describe your driver in detail. Is it a constant-current type? If so, what current have you set it to? Note that I doubt that it is a constant-current unit. If not, what is the maximum current each phase can handle?

I suspect that, at 24 volts you are wildly overdriving your motor (the power supply current limited and you got 7 amps through each energized coil), and I'm a little surprised it still works. Plus, I'm also surprised that your driver has not been damaged.

Given what you did to it at 24 volts, and since at low speeds it seems to work properly, I'd suggest that one or more phases have been damaged, and/or one or more driver channels has been damaged. The low speed performance is caused by the damaged phase(s) working "more or less" while at higher speeds more or less is not good enough.

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  • \$\begingroup\$ +1 From the Stepper Bee + driver link provided by the OP, I see no means of setting current so I would assume the user is responsible for providing the proper voltage or series resistor to limit current. It does appear that the drive is over-current protected. \$\endgroup\$
    – Tut
    Commented Dec 15, 2014 at 19:42
  • \$\begingroup\$ @WhatRoughBeast high voltages are used to decrease rising time and explained at. here How am I sure that the motor is broken or not? \$\endgroup\$
    – electro103
    Commented Dec 18, 2014 at 6:15
  • \$\begingroup\$ First, you check the driver. Provide dummy loads in place of the motor - for your power supply, 5 ohms @ 100 watts. Provide proper cooling. Use all 4, since you need them to limit the current when you drive the motor. Connect the resistors to 24 volts in place of the motor, and check the driver voltages - at any time 2 voltages should be less than ~ 0.5 volts. If all 4 channels check good, connect the motor common to +24, with the resistors in series with the 4 phases to the driver. Now check operation. If you don't get smooth operation, I'd suspect demagnetization of at least 1 motor phase. \$\endgroup\$
    – WhatRoughBeast
    Commented Dec 18, 2014 at 21:47
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On stepper motors exist something called "mid-band resonance". This is due to the fact that at this speed the stepper behaves like a Spring Mass System. I found a good explanation about that written by @zebonaut at this question

Have you tried increasing the speed to see if the motor rotates smoothly again?

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