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Controlling high power AC or DC motors.

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jakeypoo:
Hello all,

I have a question for those of you who deal with larger motors.

In terms of simplest control, what is the best choice for large (1-3 hp/ 0.7-2.2kW) electric motors?

I deal with low power motor control most of the time (<200W) where some power MOSFETs and PWM will do the trick. But looking at motors in this larger power class, I've come up with 3 options.

          - Single phase AC motors
          - Three phase AC motors
          - Three phase BLDC motors

It seems like in this power realm (>750W) DC motors scale up very steeply in cost, so AC is preferable.

By control I mean speed control and ideally braking.

I have a rough idea of control for all three options. My question is, for those of you who have been down this path before, what is the least headache inducing path to take (specifically power supply and drivers) to control this with low voltage logic?

Thanks in advance.

IanB:
The traction industry has been down this path. Historically traction motors were all DC. Speed was controlled by varying a resistance in series with the motor and by varying excitation in the field coils. Lately everything has moved over to AC motors with electronic variable frequency drives. Early drives used thyristors but later drives use IGBTs.

I remember an article somewhere that give a typical application map: lower power drives might use MOSFETs, medium power might use SCRs and high power might use IGBTs. I think 1-3 hp would be considered small in an industrial drive context.

Can you buy an off the shelf drive rather than designing one from scratch? I imagine there are plenty of traps for the unwary in designing such a thing.

Gall:
Small IGBTs are usable at low and medium power too. Older high power units might use SCR/triac.

Zero999:

--- Quote from: jakeypoo on September 07, 2011, 01:52:30 pm ---Hello all,

I have a question for those of you who deal with larger motors.

In terms of simplest control, what is the best choice for large (1-3 hp/ 0.7-2.2kW) electric motors?

I deal with low power motor control most of the time (<200W) where some power MOSFETs and PWM will do the trick. But looking at motors in this larger power class, I've come up with 3 options.
--- End quote ---
The best option is to buy a ready made variable frequency drive  (VFD).


--- Quote ---          - Single phase AC motors
--- End quote ---
Don't use them, use a three phase motor running off a VFD powered by single phase.


--- Quote ---          - Three phase AC motors
--- End quote ---
Cab be powered from either single or three phase with the appropriate VFD.

--- Quote ---          - Three phase BLDC motors
--- End quote ---
Basically the same as a three phase motor except they may have other features such as permanent magnets (synchronous), designed to be run off a square wave rather than a sine and have feedback.


--- Quote ---It seems like in this power realm (>750W) DC motors scale up very steeply in cost, so AC is preferable.
--- End quote ---
Yes DC motors are rarely used in industry these days.


--- Quote ---By control I mean speed control and ideally braking.
--- End quote ---
A VFD can do that and if you can buy another inverter to inject power lost through breaking back into the grid, saving you both power and room for a large breaking resistor


--- Quote ---I have a rough idea of control for all three options. My question is, for those of you who have been down this path before, what is the least headache inducing path to take (specifically power supply and drivers) to control this with low voltage logic?
--- End quote ---
A ready made VFD will have all of that built in, will be programmable and can be controlled by standard analogue or digital signals.

jakeypoo:
Thank you,  everyone!

Variable frequency drive using one of the available single chip power modules or IGBTs was where I was heading with this one.
Really needed the reassurance since these kind of powers are uncharted waters for me.   :)


--- Quote from: IanB on September 07, 2011, 03:12:35 pm ---Can you buy an off the shelf drive rather than designing one from scratch? I imagine there are plenty of traps for the unwary in designing such a thing.

--- End quote ---

--- Quote from: Hero999 on September 07, 2011, 05:00:13 pm ---A ready made VFD will have all of that built in, will be programmable and can be controlled by standard analogue or digital signals.

--- End quote ---
This is a one-off rig for some mechanical fatigue testing, and low cost is an issue. Industrial controls are pricey.  Still hoping to find a suitable surplus motor.

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