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.
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.