Constant current, variable speed motors

[martinmfg]

I'm a big out of my specialty here, but vaguely remember something from my sophomore year that leads me to think that what I want is fairly simple and doable.

I want to run 2 DC motors in such a way that one assists the other in moving a liquid. the first motor gets a certain volume of liquid moving, the second motor speeds that liquid a bit more. I want one control to operate both motors (variable current or voltage input), and at a given current, I want each motor to find it's own speed based on the torque.

In this way, both motors could be the same size, neither one would wear out much sooner than the other one, and operation should be fairly smooth and vibration free.

What kind of motors, and how to wire them together? Low current draw, and the ability to accelerate the volume of material moved by adjustment of the input current or voltage. I think 36 or 48vdc would be what I need to use.

Thanks in advance.

 

[Skogsgurra]

I do not get it. You seem to describe a solution rather than what you want to do.

Can you step back a little and describe, in layman terms, what your plant is supposed to do. Without assuming anything about constant current and so forth?

Gunnar Englund

http://www.gke.org

 

[martinmfg]

It is for a personal problem. I want to move water thru a tube, using 2 motors with a prop on each. I prefer not to disclose too much. I want to have the same current in both motors, but each motor should output the same torque (same current). I will either vary the prop pitch on the two motors to keep the RPM's the same, or I will allow one prop to turn double speed if that's the best way.

Just off the top of my head, I THINK I remember an instructor talking about using series motors, with the rotors of the motors wired in series with each other. I think I would prefer a compound motor though, but I am not sure.

 

[powerjunx]

Hi martinmfg,
I hardly get your point,

let me clarify:

1. do you mean two motors in tandem - driving a load together?
2. or you mean a closed loop control having a master unit (first motor) and a slave unit (2nd motor), that is; where the slave backs up the master unit for any change of flow.

 

[martinmfg]

motors with propellers in series. Like motor 1 speeds the water up from 0-X, the other one takes it from X-2X(+/-).

Reqmts:
Motors should speed up at a given input votage based on reduced load, or slow down based on high load. Current should remain constant.

Motors should be wired together so that each one operates at the same torque output (not speed)for a given input voltage.

 

[Skogsgurra]

Propellers in an open vessel? Would be difficult to arrange what you say in a tube since the water would behave like an entity that can not have different speeds.

What you describe are two motors running off constant current sources with an upper voltage limit. As simple as that. Any dual power supply (or two single supplies) with adjustable current limit can be used together with suitable DC motors.

Your idea of putting the motors in series will save one power supply. But the speed of one motor will be dependent on the other. So if one is lightly loaded, it will speed up - and that will make the other motor slow down and vice versa. Is that, perhaps what you want to achieve?



Gunnar Englund

http://www.gke.org

 

[martinmfg]

The second one would be more lightly loaded than the first one, so it would speed up, unless I put a higher pitch prop on it. I was thinking that rather than have electronic controls, if I wire them in series, I would get the same current in both, thus the same torque output, one would not need more current than the other, so I could use the same motor in both places. One would "push" with the same force that the other one "pulls", even though they operate at the same RPM or with different props.