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

 

[blakester]

I probably am looking for 50 psi and 300L per hour ish....

 

[blakester]

3000 L/ph I mean or 750Gph

 

[bigdeano]

50 psi and 750gph figures out to .36 hp. I don't know what kind of pump you're using, but 3/4 hp would probably be enough power.

 

[LD1]

Yea, 5hp is major overkill

 

[damartin]

I read that many people needing a variable speed motor will buy a used treadmill and use the motor and controls off of it.
I thing many of the treadmills are about 2 hp and they are easy to find on Craigslist. Good luck with your project.

Dwight

 

[Handloader109]

Still, look at a vfd if you run a 3ph motor. It will both allow you to power with single phase, and allow you to vary the speed of the motor. Now a gearbox may be better, as slowing down the motor reduces the torque, hence the output.... And evilbay can be your friend for vfds

 

[jenkinsph]

3000 L/ph I mean or 750Gph

For 50 psi at 750 gallons per hour, and what are you pumping? That can make a difference too, does it need to be food grade? What is the viscosity of the liquid?

Your flow and pressure may not require much for a low viscosity liquid and a small simple pump for $150 may be all you need.

 

[evo803]

Now a gearbox may be better, as slowing down the motor reduces the torque, hence the output

I thought that an electric motor has maximum torque output at zero RPM ? I thought that is what Elon Musk said in an interview about why his Tesla P90d was so quick .

 

[Mendonsy]

[/B]

I thought that an electric motor has maximum torque output at zero RPM ? I thought that is what Elon Musk said in an interview about why his Tesla P90d was so quick .

It depends on how the motor is made, some do, some don't.

 

[Dave M7040]

Why not try and accomplish the flows and pressures you want on the fluid side.
Restricting the flow on a centrifugal pump reduces the load since there is no flow.
A globe valve on the output would give you a reasonable throttling control. Don't use a ball valve as fine tuning it is difficult.
You can also tee off some of the output and send it back to the reservoir using a valve to control how much .
Dave M7040