DC motor Control questions

[blakester]

Hello everyone. I'm looking for some information on a pump project Im' thinking about undertaking for my small winery. Mostly it's about DC motor speed control. I know of a couple of ways to run the speed control but I'm not sure what suits my needs best. I'll be running the motor through a stepdown gearbox to go from motor speed to pump speed. In this case I hope to be able to run the motor above the 50% of max current and up for speed control. This way I won't loose too much HP and generate too much heat by trying to go super slowly. That is assuming I went with something like a rheostat to control the current, which I also understand requires brushed motors. The other option is Pulse width modulation. I'm less familiar with this method but it seems straight forward. Pulse current in full to the motor in discreet intervals to vary speed. I think I would like to go with a used motor I have lying around, 5 hp, 16 amp 240V with a rheostat but I'm also unsure what kind of rheostat I might require. Something that can deal with 240V and 16 amps I guess but I have been having hard time finding that part. Any thoughts on a nice low cost way of setting up Variable speed DC motor control.
Cheers,

 

[Redneck in training]

It all depends on the type of motor. If it is serial motor then you can run it on AC also. If it is permanent magnet motor then it is DC only. You can get electronic controller for either type. 5 HP motor is most likely serial. Rheostat for 5HP motor would cost more than electronics not speaking about all the disadvantages such as heat generation.

Look at this one: Dart Controls DC Speed Control, 90/180VDC, 10A, NEMA 4/12 253G-200E | Zoro.com

 

[LD1]

240v is kinda uncommon for DC motors.

More common are 12v, 24v, 36v, 48v, 90v, 180v

Have you considered a 5hp 3-phase and run it off a VFD?

 

[Mendonsy]

I agree about using a 3 phase motor and a VFD. It will be a lot more efficient.
These guys have both types of drives and are reasonable about prices.
AC Drives | DC Drives | Variable Frequency Speed Drives | VFD

 

[buickanddeere]

If it's a shunt motor, only the field current needs to be controlled. Much lower current than the armature current.
I agree that a three phase motor and a VFD is an all around superior solution.

 

[blakester]

Thanks for the info, it seems like after looking around for something in the 5HP 16A range it's a 400$ or more for a VFD. Not sure there is a simpler option based on what you guys are saying really. So maybe that leads me to my second question. I'll be stepping down my 1800-2500 RPM motor down to ABOUT 200-300 Rpm. I wonder if 5HP is way more than I need. Seems like with a 4:1 step down or so I would have a ton of HP to drive a must pump. anyone here have any experience with practical application of HP for a heavy duty pump..

 

[bigdeano]

A rheostat would have to dissipate a large amount of wasted power. I don't think they make them that large.

You can't just control the field voltage. When you reduce the field voltage on a shunt motor, the speed increases, not what he wants.

DC speed controls use either phase control like a light dimmer, or PWM. About all you can do is buy a DC motor speed control. Either type should work fine if it's of good quality.

 

[buickanddeere]

A 5HP motor is still going to produce 5HP on the output side of the 4:1 gear reduction box.

 

[blakester]

so 5hp still on the output, but more available torque? When you step down the speed through a gearbox you get more torque but slower speed right.

 

[LD1]

Yep, gearboxes change the torque and speed (more torque, less speed) but HP remains constant less any frictional losses.

Knowing pressure and flow of the pump would help size the HP you need