motor speed

[DMKMM]

Hey folks, I know forum users hate it when people join just to as a question... but hey its Friday afternoon, and I’m having a serious brain f**t, and the boss keeps asking me how its going Disclaimer, i do not play with electronics for a living, and this is like the 3rd circuit I’ve ever tried to make. I didn’t even know what a PWM ment until this morning, but i know what it does, not how it works...

So please bare with.

Goals:
1. variable speed motor
2. be able to change motor direction
3. be able to change direction without having to adjust speed
4. have a override "fast" mode where speed adjustment can be bypassed

Questions:

1. Will this work?
2. Do I need the diode?
3. Should I implement some relays?
4. Is there a better -> cheaper way to do it?
5. Is the PWM gunna shoot 20A at the motor all the time? or will the motor just draw what it needs?
6. The motor is only rated at 16A, but this is very intermittent use...

[see attached]

[DMKMM]

nothing?

[pebe]

No-one else has replied so I'll have a shot at it.

[quote]1. Will this work?
2. Do I need the diode?
3. Should I implement some relays?
4. Is there a better -> cheaper way to do it?
5. Is the PWM gunna shoot 20A at the motor all the time? or will the motor just draw what it needs?
6. The motor is only rated at 16A, but this is very intermittent use...[/quote]

1. Yes - but see my last comment
2. I don't think you need the diode - it serves no useful purpose.
3. I don't think relays would help.
4. I cannot think of a cheaper way.
5. The motor will only take what it needs. If 16A is the rated current for full load then that will be the maximum required from the power supply. At lesser loads, the motor will take less current.
6. See 5.

The thing I am not sure about is what will happen if you reverse the motor when it is running at maximum speed. Normally, the motor when running generates a back-EMF, roughly proportional to its speed, that opposes the current input. That is what limits the input current. Without that, the current would only depend on the resistance of the windings and a high current would flow.

If you reverse the motor at high speed then that back-EMF is reversed and actually adds to the input voltage. You could end up with a very high current flow until the motor has got up to speed in the reverse direction.

My supposition is only based on theory - not experience. So it would be useful if some other member made a contribution to the thread.