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I need to control a 90v dc motor, the thing is that I dont have a way to get a 90v suply so I just rectified the ac voltage and got 170 volts, without any load and filter caps, as far as I know this voltage will drop once there is a load but it is stil high, is it ok to use pwm at this voltage to power the motor, obviously with about 60% duty cycle.

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    \$\begingroup\$ Stop what you're doing before you hurt yourself. \$\endgroup\$ – Matt Young Oct 31 '17 at 2:59
  • \$\begingroup\$ so it is not ok? \$\endgroup\$ – chal7401 Oct 31 '17 at 3:01
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    \$\begingroup\$ use a regular 110v light bulb (maybe 60W) as a test load and see what the output voltage is when loaded. the worst that will happen is a burned-out lightbulb (try 2 in series if necessary) ... use 100W for a heavier load \$\endgroup\$ – jsotola Oct 31 '17 at 4:33
  • \$\begingroup\$ You might get away with it when using a heater, although that still would be dangerous. Don't do this with a motor. Many things can go wrong: 1.9 times the voltage could be 3.6 times the power; sparks could damage the motor; current spikes could damage the motor; inductive flyback currents could be out of control; and so on. \$\endgroup\$ – Jot Oct 31 '17 at 4:53
  • \$\begingroup\$ any suggestions to reduce the voltage apart from using a transformer? \$\endgroup\$ – chal7401 Oct 31 '17 at 5:01
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No, it's not very OK. During any software glitch, you might be running your motor at 125% of rated RPM. (DC motor speed is proportional to drive voltage.) Instead, use a single 400V power diode rather than a bridge rectifier. That should run your motor at roughly 65% of full speed, at most.

90VDC motors are intended for use with motor speed controls: a specialized triac dimmer followed by a full-wave bridge rectifier. ($40 to $100 on eBay!) The little KB Inc. or Dart motor controllers typically adjust from 0% to 75% of full AC output.

Your 170V reading is probably wrong; you're seeing the peak volts rather than unfiltered true-RMS average.

To reduce the motor voltage, you could try using a standard incandescent lighting dimmer, plus a bridge rectifier. Choose a dimmer-wattage rating appropriate to the horsepower rating for your motor.

If you want to play with opto-isolated, voltage-signal AC controllers, VELLEMAN KITS has a $30 unit, a lighting dimmer pcb kit with 0V-10Vdc control input.

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