Controlling dc pump motor speed/flowrate

[JHZR2]

Hi,

I'm running some tests that have a cooling loop. The loop uses a 24vdc small pump, looks like an aquarium pump. It is fed by a variable power supply. I can limit current or set voltage. Essentially, the pump is cooling too much, so I'd like to reduce flow.

The tests I'm doing are in a radiation controlled area, so I can't get close to adjust a bunch of valves and bypass. I'd like to throttle by limiting current or reducing voltage. Which is the better way to go?

The pump is 24vdc, 1.6A and draws 1.0-1.1A at 24v steady state.

Recommendations?

Thanks!

 

[Eddie]

DC motor controllers that I am familiar with use a rheostats so voltage control is my guess.

 

[river_rat]

If it is a permananent magnet field type, the torque is constant. Armature voltage would be used to set speed.

 

[JHZR2]

I'm assuming that is the type. It is about 1.5" in diameter with integrated pump and motor. Looks like an aquarium pump.

 

[PandaBear]

Do you have any feedback sensor and controller? Usually the "right" way to do it must have some sort of feed back like position, flow, speed, etc so you can control the variable that increase or reduce the speed, voltage, or current of the pump. These variables are not constant, so depends on the resistance or load it needs to be increase or decrease at run time.

Normally, for the right controller these are limited by PWM which turns on for certain amount of time then off for certain amount of time, at a very high frequency, that to the winding of the motor feels like a constant current.

 

[JHZR2]

No I don't. Really what I'm trying to do is minimize water flow as much as possible on a small pump without stalling it (which it does in my system at about 6V.

My hope was to do it all electrically to avoid bypass valves and other nuisance items since I'm running tests in a synchrotron that has a lot of interlocks and lacking access near my test.

I need a certain amount of pressure yet really low flow. May not have the ultimate pump for the job, may need to rethink this further.

 

[tom slick]

What about restricting the return?

 

[PandaBear]

Originally Posted By: JHZR2
No I don't. Really what I'm trying to do is minimize water flow as much as possible on a small pump without stalling it (which it does in my system at about 6V.


I've seen some control engineers using back EMF to check if the motor stalled, but that's for a large inertia payload (hard drive spindle) rather than a fluid pump that is running at a bare minimum speed to move fluid slowly.

I think you can do it by experiment and go a bit on the safe side; it would be hard to minimize flow safely without feedback.

If it is a brush type motor can you check the ripple of the voltage or current when the motor brush switch "phase" as a feedback? N number of phase shift / ripple indicate 1 revolution.

 

[river_rat]

Originally Posted By: JHZR2
I'm assuming that is the type. It is about 1.5" in diameter with integrated pump and motor. Looks like an aquarium pump.

Probably is. If it is then regulating the DC to it is probably going to work fine. Even a rheostat of sufficient wattage should do fairly well here.

 

[brianl703]

Pulse width modulate it.

 

[PandaBear]

The problem I think is not whether it is controlled by PWM, rheostats, or etc but how to tell if the motor is stalled or not.

I'm not sure how critical it is to keep the flow minimal vs stalling, and how "radiated" the environment is (whether it is just a gamma ray that stopped or particles with radiation in the contaminated water that remains. If it is possible to add either some sort of water flow sensor or buy a synchronous AC motor that is wired to a transformer and current sensor to detect stalling, etc.

If low cost is the key, I think the cheapest way to do it is to buy the smallest pump that has the high power for the job, but the maximum flow is not too high that it will exceed the maximum flow the system can tolerate. No control is needed, no need to worry about mechanical design or electrical sensor or feedback.

Or with your existing aquarium pump, find the right voltage / current in the worst case scenario by trial and error, and keep a healthy safety margin.

 

[brianl703]

The motor is MUCH less likely to stall if it is PWM controlled than if it is voltage controlled.

Using PWM control allows the motor to develop full torque at low speeds.

With voltage control, the torque will drop off as the speed does, which means it is more likely to stall at slower speeds.

That's why any motor speed control that's worth anything is done with PWM. Look at radio control cars for an example.

http://www.myo-p.com/PWM Motor Controller.pdf

Quote:
Typically when most of us think about controlling the speed of a DC motor we think of
varying the voltage to the motor. This is normally done with a variable resistor and
provides a limited useful range of operation. The operational range is limited for most
applications primarily because torque drops off faster than the voltage drops. Most DC
motors cannot effectively operate with a very low voltage. This method also causes
overheating of the coils and eventual failure of the motor if operated too slowly.


http://store.qkits.com/moreinfo.cfm/KTA-246