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Richard Hanley
12-15-2005, 06:51 PM
I HAVE A BODINE DC MOTOR CONTROLLER [ 4.2 AMPS ] AND A PM DC MOTOR [ 4.7 AMPS ] WHICH I WOULD LIKE TO USE ON MY 9 “ SOUTH BEND LATHE. DOSE ANYONE KNOW IF I COULD GET AWAY WITH USING THIS CONTROLLER AT 10 % OVER RATING? THANK YOU RICHARD

Fred White
12-15-2005, 07:54 PM
First....please turn off your caps. That is considered shouting.

Second.....probably so. You won't be using the full power of the motor very often so it will be running below it's maximum load. If your controller has a CL ( current limiting pot ) set it at the board max not the motor max. Or put a reduced capacity fuse in the motor power line.

winchman
12-16-2005, 12:20 AM
If the motor controller uses pulse width modulation (PWM), it turns the transistors on and off for short periods of time. The percentage of on-time determines how much power goes to the motor. The transistors will try to supply all the current the motor will draw while they're on, so they will be working beyond their capacity all the time they're on.

Using a fuse to protect the controller won't help much, since the fuse has to stay on and overloaded long enough to blow. It'll protect the motor, but not the controller.

You might do OK by putting a power resistor in the circuit to prevent the motor from drawing more current than the controller is rated for. But, it's better to match the controller to the motor, or vice versa.

Here's an article about PWM motor controllers:
http://www.cpemma.co.uk/pwm.html

Roger

[This message has been edited by winchman (edited 12-16-2005).]

J Tiers
12-16-2005, 12:03 PM
Most older off-line motor controls for DC motors are SCR-based. They are rather similar to a light dimmer, and are outgrowths of the thyratron controls such as found on a Monarch 10EE etc.

Any decent motor controller will have some form of overload protection. SCRs are very tough devices, and will generally take one half-cycle of a current 10 times normal. By then the overcurrent detector should have "discovered" the overcurrent.

Ditto for PWM controllers. They should have even better overcurrent control, because they check current at their (very high) switching rate, and have the ability to turn off current at any time (scrs cannot).

Using slightly over ratings would normally be expected to heat the controller more than normal. It may not have an overtemp shutoff, so a good heatsink is essential even at or near maximum normal power. You are proposing a 12% overload.... not a lot.

There is nothing that should fail at 112% power, even the fuse will probably hold. It will probably work fine.

But you need to realise that you are operating over the spec limits, and not try to push it even farther.

One thing.... it may depend on what the Bodine controller was for. Some usages are very "defined" and not as likely to have overloads.

Controllers for such usage may not have the internal protections that other more general-purpose controllers have. They also may not have overload capability much if at all past the rating.

So you pays your money and you takes your chances.

darryl
12-16-2005, 09:29 PM
You aren't asking for a lot of extra output from it, but you don't know how conservatively the controller was designed, if at all. If me I would take it apart and check to see how the output devices (transistors or scr's, triacs possibly) are mounted to heat sinks. There may be nothing more than small bolt-on heatsinks, or the devices could be mounted to an actual heat sink with proper grease and mounting method. I've seen many cases where the mounting is ok, but no heat conductive grease has been used. I prefer to see a mounting where the output devices are 'pinched' down to the heatsink using a bracket that actually sandwiches the device to the heatsink. That way they operate with less of a temperature rise between the device and the heatsink.

If the controller looks to be well thought out in this regard, chances are it's conservatively rated, and will have no problem handling that small overload. If the heatsinking is shabby, I'd be a little leery, or at least want to improve it somehow. Considerable electronic knowledge would be required to do this properly and safely, and without upsetting the design parameters, so this might not be a good recommendation.

As far as your application being an 'overload', it may not be in actuality. The 4.2 amp rating sounds like it was specified with a certain motor that would normally have been used with that controller. It might well have been spec'd higher for use with another higher power motor (and of course cost more), though it could be the same internally. It might also have the same guts except for higher rated output devices for the higher power motor, and that's an avenue you could explore- replacing just the output devices with higher current rated ones. If me, I would look at those outputs and find the spec's on them and from that decide if there's any options that I could take, or if indeed they were capable of considerably higher output without changing anything.

Richard Hanley
12-18-2005, 04:34 PM
Thank you for all your advise. I have learnt a lot. I have thought about how I use the lathe . Some times when making small parts I run it really fast , which takes a lot of power. I think iI will keep looking at the salvage places for the proper controller . It sure is nice to have your counsel. Richard