AC Motor Control

[Frosty_47]

Hello, I would like to build a circuit to control the speed of a 120 VAC aquarium pump. The pump is ratted at 40Watts. The pump's flow rate is too big for my application so I would like to have the convenience of an adjustable speed control. Please provide working schematics.

P.S. I am not interested in purchasing a premade system, so please avoid such posts.

 

[dknguyen]

Is the pump running off AC or DC? (Not what plugs into the pump, what the pump is actually running off of).

*crosses fingers and hopes it's DC*

 

[Cry_Wolf]

i quite sure that an aquarium pump will have an AC motor. And really never saw a design for controlling speed of an AC motor (except for my ceiling fan lol )

 

[jpanhalt]

Controls for shaded pole and "universal" work fine. Presumably and hopefully the OP's is shaded pole. We need his reply. John

 

[KMoffett]

jpanhalt:

Shaded pole motors are induction motors, and therefore are frequency controlled for speed. Attempts to reduce the voltage (Variac or dimmer) to reduce their speed results in speed reduction, but "dramatically" reduces the torque to the point of being useless.

If it were a universal motor, voltage control would work. But, these are brushed motors, and not likely to ever be used in a 120VAC aquarium pump.

Frosty_47,

Have you considered using output restriction to control the flow? Ya' know like turning the faucet from a gush...to medium flow...to trickle. A constant head-pressure is what's required, and your pump should do that.

Ken

 

[Frosty_47]

Thank you for your replies. Here is the pump I've got:
http://www.aquariumguys.com/maxijet1200.html
I made a mistake in its power consumption ratting in previous post. The power consumption of this pump is 20W. I am very sure that this pump runs of brush less induction motor. A voltage control is not a good way to control the speed because it will result in dramatic torque loss (and I really don't want that). I found one schematic for controlling the speed of induction motors but don’t make any sense to me. Here is the link for it:
**broken link removed**

Other than that, I have not found anything.

P.S. I will not limit the flow rate through “flow restriction” as that builds up pressure and puts a lot of stress on the motor.

 

[jpanhalt]

jpanhalt:

Shaded pole motors are induction motors, and therefore are frequency controlled for speed. Attempts to reduce the voltage (Variac or dimmer) to reduce their speed results in speed reduction, but "dramatically" reduces the torque to the point of being useless.

Ken

I don't disagree that one loses some torque, but that wasn't the question. It is a pump and a little loss in torque may work.

Fans, magnetic stirrers, and various appliances with shaded-pole motors work fine with such controls and provide adequate power. Now, you can't run a lathe, air compressor, or belt sander with one, but you don't often see shaded pole motors in those applications.

Here are three reputable vendors:

http://www.bardac.com/pages/products/ac_optidrive7.html
http://www.controlres.com/nimbus.php
http://www.electricmotorwarehouse.com/kbwc.htm

The cheap KB control (about $20) is very similar to light dimmer. I agree that a Variac will not work and never meant to suggest using one for a shaded-pole motor. John

 

[jpanhalt]

I found one schematic for controlli... [url]http://en.wikipedia.org/wiki/Dimmer

The controls I have used for shaded pole motors did not have a rectifier bridge and were similar to this phase control:

**broken link removed**

Phase control might work in your case. I know you don't want to buy the controller, so my suggestion would be to look up phase control and see whether you want to try to build one. You will find circuits that use an SCR and others that use a triac for motor control and light dimming. I don't have the experience to know whether one is better than the other.

However, the big question is what type of motor is in your pump? I suspect you don't want to tear it apart to find out. At the same time, if you make a phase control yourself and it doesn't work, you won't know whether it is the wrong technology or the way you made it that is the problem. I suggest buying a cheap one like shown in the links I provided. If it works, then you can experiment with making your own. If it doesn't work, you can keep the controller for something else, and still experiment on ways to control the motor in your pump.

John

 

[Frosty_47]

Thank you for your help and suggestions. Phase control sounds very interesting so I will definitely try to find one at my local electronics store.

I was wondering if anyone has a practical circuit for a variable frequency drive. By practical I don't mean a 555 timer that changes dc in pulsating AC.

Thank you for your time and suggestions!

 

[dknguyen]

You can't limit current without controlling the voltage. Current limiting is actually controlling the voltage based on current feedback. Motors have their current increase with torque output. Depending on the type of motor, it's speed is also influenced by the frequency, voltage, or both. It's a 4-way relationship. Voltage/Frequency, current, torque, and speed all influence each other. Nothing is independent from the other.

If you limit the current, you limit torque which will slow the motor down. THis is true of DC motors and induction motors (but the relationship is not as straightforward with induction motors).

This isn't true of synchronous AC motors. THey will draw however much current they need to run at a fixed speed based upon the input frequency. And if they get out of synch, they will stop running.

 

[Amjadskt]

About the pump, i doubted it contain a motor. Does it runs silently or makes some beepy sound. May be its an electromegnet!

 

[Frosty_47]

About the pump, i doubted it contain a motor. Does it runs silently or makes some beepy sound. May be its an electromegnet!

What are u talking about?

A rotating electro magnet is a motor. Just like a motor is an electro magnet.

Most induction motors can run relatively silent. I have a 1/2HP commercial garage door opener motor and it runs extremely silent.

I think that the best way to control the speed of my pump/motor is to vary the frequency via variable frequency drive. The speed of induction motor is found by this formula/relation:

120 * F/n

Where: F is frequency
n is the number of poles in a stator

Thus if I can lower the frequency, the speed of the motor should decrease while the torque should remain unchanged (assuming voltage and current remain the same).

 

[Hero999]

It's probably a shaded pole motor.

The coils are probably potted and the permanent magnet spins round inside it.

Some shaded pole motors, e.g. ceiling fans can be run off lamp dimmers or even maybe a variac but that's because they're designed for it. I wouldn't think a phase controller would work for all motors.

I think your best bet is to buy a propper controller.

 

[jpanhalt]

It's probably a shaded pole motor.

The coils are probably potted and the permanent magnet spins round inside it.

Permanent magnet??? See: https://en.wikipedia.org/wiki/Shaded-pole_motor

John

 

[Hero999]

Yes, I'm sure there is such thing as a shaded pole synchronous motor.

 

[jpanhalt]

Yes, I'm sure there is such thing as a shaded pole synchronous motor.

If that is in response to my comment, the question raised was whether shaded pole motors have permanent magnets as the rotating piece. Can you give an example of one? Thanks. John

 

[Hero999]

I can't give a definite example but I'm pretty sure mechanicla mains clocks us shaded pole synchronous motors.

 

[480threephase]

not really an electrical solution, but have you considered installing a relief valve to regulate the flow rate instead of the motor speed. much simpler! just adjust to the flow rate you want and the excess flow will dump to whatever tank you are pumping from w/o an increase in pressure

 

[BeeBop]

FWIW,
most air pumps have a motor, but it isn't a rotational motor, rather, it is a sort of linear one, in which a magnet moves back and forth between two coils, moving the bellows as it goes.

 

[JimB]

Similar to 480threephases' suggestion, dont look for an electrical solution, but look for an hydraulic solution.

I assume that you dont want to be constantly varying the flowrate, so just install a bypass valve around the pump. ie a tee piece on the input to the pump connects to a valve which in turn connects to a tee piece in the output line of the pump.

When the bypass valve is fully closed the pump will deliver full flowrate, when the valve is fully open, the pump will deliver very little, it will just be recycling.

JimB