Electronics for DC motor reversing for milling machine

[bryan1]

Hi,
I recently purchased a turrent mill and the power feed doesn't work. After looking at the circuit board which drives a 4 wire servo motor the board looks like it came from the ark. I can buy a 300 watt DC motor with a flange mount face so I can modify it to fit in the mill but the hard part is working out a circuit to drive it. Idealy it will need a pot for variable speed also have rapid traverse and be reversable. As the machine is 3 phase I was thinking of using one of the many 12 volt transformers I have laying around.
Any suggestions will be greatly appreciated.

Cheers Bryan

 

[Oznog]

You can get 90V DC motors as surplus, those are pretty common.

Reversing a DC motor is a matter of switching the wires to reverse the DC voltage. You can do this with an H-bridge or a relay.

If you have true DC supply, you'd control speed with PWM. If you've got a low voltage motor you might do this. You're probably going to have an AC supply though so phase control, like with an SCR, is required.

There is one low-tech option- a variac. That can feed a transformer or go directly into a rectifier then the motor if the voltage is already appropriate. It may be an expensive part but it is simple.

I've seen some motor speed controls for sale cheap too- the specified use was for plugging in a router- which might do the job.

I suspect the 4-wire servo didn't have a lot of gearing, did it? See, a DC motor is going to be like 10x faster, and slowing it down will only kill the torque. The servo's benefit was also that it was probably set up so that it wouldn't slow down no matter how much you load it. These other DC motor solutions could lead to something that slows down when it hits a hard spot and surges ahead when it has a lighter load.

 

[bryan1]

Thanks for the quick reply Oznog I dont think that heavy cutting will slow the motor as I forgot to mention before the motor drives a worm drive of which ratio I'll check tomorrow. As far as the surplus motors and using a variac go, over here in Adelaide anything worthwhile secondhand goes for bout 90% of the retail price. I've looked at quite a few alternatives and I reckon making my own electronic drive is the cheapest option. I'm just starting to get into pic microcontrollers but a project such as this is a bit beyond my knowledge at this stage. By the way I'm a machinist by trade and I've been learning electronics for just over 12 months.

Cheers Bryan