AC to DC motor speed controler

[JTknives]

Well this is my first post here so i hope it is ok for me to ask a question like this. See im a knife maker and i have a 130v 2.5hp dc motor that i would like to run off ac like from my wall. now if i use a full wave rectifier i can get the dc voltage i want but the motor runs at top speed. so waht i was thinking was to use a TRIAC Dimmer switch on the ac side of the full wave rectifier so i can control the ac wave pulses to the full wave rectifier so that i would get a time delayed dc pulse to the motor so as to control the speed. will this work and do you think i would need a filter cap acros the dc side. I am making a hevy duty disk grinder and need to control the speed. if this would not work is there another way i could do it that would be cheep. thanks so much for your help with this project.

 

[stevez]

I don't want to make this too complicated but consider this -

A. If you vary the voltage to a DC motor the resulting speed will vary if the load remains relatively constant.

B. If you vary the load on a DC motor while maintaining a constant voltage the speed will vary.

It sounds like you want to make a grinder or similar machine. The load on the DC motor is not likely to be constant so if you supply a constant voltage (constant over the short period that you are grinding) so your motor speed will vary - dropping as the load increases.

Now, you said you want to "control" the speed - does that mean you want to adjust the speed AND once the speed is set, have the speed remain relatively constant as you grind?? If so, you will need to not only vary the voltage (by some means) and also provide a system to constant monitor the speed of the motor and adjust the voltage to maintain the constant speed.

You might be satisfied by making a relatively simple variable DC source or by using PWM - and just live with the resulting variation.

A freind of mine went thru a knifemaking appreticeship and has built his own belt grinder. It sounds like he gets much better control when his grinder runs at a constant speed regardless of load. He cheated and simply took a 2 HP DC motor off of a treadmill - along with the speed control system.

 

[JTknives]

well that is where i got mo motor but it is rated at least 2.5hp. but i dont have the speed control. so if i kept the same voltage but vared the current that would very the speed, i could use 100w lamps in eather series or pareial to change the resitince on the total circirt so that it pulls less amps. if some one has plans for a simple dc speed control that would be great. O and if i can find a tredmill speed controller how would i wire it so the the speed control runs off a pot and not the digital display on the tredmill?

 

[stevez]

The net result of lamps will be to reduce the voltage to the motor - because considerable voltage drop will be taken across the lamps. A single 120 volt, 100 watt bulb will present considerable resistance, as compared to the motor so most of the voltage drop will happen across the bulb -leaving little for the motor.

If you think of the 2.5 hp motor as a 2,000 watt bulb - then a 2,000 watt bulb in series with the motor would put 1/2 the voltage at the motor. All of this assumes the motor is fully loaded and won't work out exactly this way - but it illustrates the order of magnitude.

My knifemaking pal took the motor and controller off the treadmill and kept the wiring as it was.

Unless I am mistaken you are essentially starting with a 20 amp or so 130 vdc supply. Then you need to vary either the voltage - or shut the 130 vdc on/off at a fast rate so that you can control the average amount of power going to the motor. Pulse width modulation is one simple way of doing that.

A crude way to accomplish this might be to purchase a 20 amp variac and rectify/crudely filter the voltage. There are certainly better ways to do this but at this power and voltage the project is far from simple.

 

[Hero999]

You need to convert the AC to a steady DC supply, do you have a 3 phase supply available, it's helpful as you don't need big filter capacitors?

What's the AC supply voltage in your area as you'll probably need a transformer?

Or do you already have a 130V supply and you want to make a speed controller?

By the way using a resistor or variac isn't the best way of controling the speed as reducing the voltage reduces the torque, PWM doesn't have this problem.