Varying speed of submersible pump

NT

what sort of motor is it?

Wouldn't *by far* the simplest solution be to put it on a time switch so it runs say 1 or 2 hours a day?

Why do you need to keep the filter "ticking over"?

What sort of filter is it? does algae grow in it or something?

The time switch seems the simplest, or even a cyclic timer if you want a continuous even duty cycle.

AB

It's not a good idea to vary the speed of the pump, it relies on water for cooling. The heat generated is not linearly related to the water pumped, it may overheat.

As a zero cost option, partially close off one of the water valves so reducing the flow or fit a cheap timer and run the pump part time. One of the plugin ons would be OK.

I run my koi pump only by day when the PV panels are operating

It depends on the type of motor and the design of the pump. I'm not familiar with koi pond apparatus. What's the power rating, and any other rating plate info it gives? A picture of it might make it obvious what the type of motor is. It will almost certainly be an asynchronous induction motor of some type.

If it's designed to operate with a small slip (i.e. near synchronous with mains frequency at max torque), then slowing it really needs a variable frequency drive (VFD) supply, which is not going to be cheap. Slowing it by using a simple series impedance or phase control (light dimmer) is like using a motor which isn't powerful enough to get to its designed operating speed, and you risk overheating it, whilst also not saving much energy. This type of motor is usually of cylindrical build and needs a capacitor which may be a separate smaller cylinder or a bulge in the casing of the motor.

If it's a low power shaded pole induction motor, it probably isn't operating anywhere near the synchronous frequency anyway, and slowing it down (increasing the slip) using a simple series impedance or phase control (light dimmer), within reason, won't have a bad effect on it. The larger submersible pumps I've seen are not of this type though, as shaded pole motors are too inefficient at the power ratings normally involved (50W is typical max rating for a shaded pole motor, and they can't be built for anything much over 100W as they would overheat in normal use).

As harry pointed out, when slowing a motor, you may also find it no longer provides sufficient cooling. This is an issue where forced air cooling is driven from its own shaft. If the heat conduction is directly to the water, that has a higher heat capacity it may not be such a problem.

Are less powerful models of the pump available?

Are these fish edible?

Bill

Put the pump on a timer that gives (say) one hour on the the next off or whatever ratio you want.

AWEM

I don't know about the OP's filter, but when I had a fish tank, turning the filter pump off was a big no-no, as it causes the concentrated debris in the filter to start drifting out back into the tank water.