How should I step down 220V (ac) to 180V for a DC motor?
Exactly as depicted in winny's comment.
You don't "Step down" 220 VAC to 180 VDC.
220 VAC Single Phase is a normal supply for a DC drive to deliver 180VDC output.
Using a drive, you will also get the benefits if an acceleration and deceleration ramp, current limiting (to protect the motor), and a simple variable speed control (a potentiometer).

If you are just going to run the motor directly off the line, a simple bridge rectifier would work, but will probably blow up due to the inrush current to start the motor.