how can I use PWM to controll a 24 volt AC power source?

I am building an Antenna rotator to point my directional antennas in a desired direction. What I would like to do is slowly ramp up the motor to help get it moving, and slowly bring the voltage down to slowly stop the motor. I understand how pulse width modulation works, but I don't think I can use PWM to accomplish this. I am going to run my 24 volt ac current through a relay, and I am wondering if I can use PWM on a transistor between the 24volt ac power supply and the relay to slowly bring the voltage up, or is there a simple way to do this that I am totally overlooking?

Hi, PWM will not help directly control the speed of your AC rotator motor.

Is the motor a brush type or a synchronous motor with a phasing capacitor to give direction?

If it is of the phasing capacitor type it will have three wires coming from it and a relay will switch a capacitor between two of the wires to give a directional/2 phase supply, hence forward/reverse.
These motors do not have a good starting torque so they would not take to lower starting currents.
Also if you were to try and change the supply frequency the impedance of the phase capacitor would limit how much current is supplied to the phase shifted winding and it probably not want to start at low speed.

I have seen how a rotator starts and yes on the bigger units it is quite a jolt, so a softstart would be nice.
If the motor is brushed type then you would use SCR control with the arduino controlling the phase angle, as you know your stopping point I can see why you would also program slowdown facility, this would help with braking.
A good use for arduino, if I had the room for a tower and rotator I'd be doing the same.

Tom... :slight_smile:

The motor is a 3 wire type. I took the original controller apart a few weeks ago, and I speant about 2 hours trying to get thisto work again. As of right now, I am using 3 relays , 1 relay for each wire on the motor. THe motor is an ac motor. The 3 wires are layed out as I look at it in front of me, left wire(cw rotation), middle wire(24v input), and right(ccw rotation). So if I power the left wire and the right wire with nothing connected to the middle wire, the motor hums but does not move until I spin the shaft by hand. By doing this, the motorwill turn cw or ccw, depending on which way I spin it. With the propper connection of 24vac to the middle wire, and 24vac to the left, or right wire, the motor turns as it should. There is a capacitor in line witheach of the left and right wires, but I believe that is just to give an extra little push to help get the rotator in motion. I think hard start, and hard stop may be the way I leave it for now.

You could try a solid state relay (which are basically two oppositely-oriented MOSFETs inside) but it might not work.

The best bet is a DC motor.