turn a singlespeed pump into a 2 speed one?

[Crey]

Ideally, one would want to actually replace a singlespeed pump with a multispeed one but, is there anyway to turn a singlespeed pump into a multispeed one? Much like someone may put a dimmer switch on a lightbulb, is there something similar that can be put on a pump motor?

Thanks

 

[The Mermaid Queen]

I have no idea, but welcome to TFP!

 

[hodag]

You can adjust the speed of a motor, within certain limits, by controlling the voltage applied to the windings. Since the components of most motors are designed only for a specific amount of electricity that means you can slow a motor down but you can't make it go faster since applying more juice would overload it (not safe).

Most dimmer switches are designed to dim a lightbulb or chandelier fixture, which is maybe a 100-150 watt fixture (max). A pool pump needs a lot more juice; my pump has a maximum draw of 13.8 amps at 115V, or about 1,600 watts. I am sure there are rheostats available that can handle bigger loads, but it is not something you are likely to find at your local hardware store or home center. I suspect if you want a multi-speed pump you should just buy one designed for the purpose.

 

[mas985]

You can adjust the speed of a motor, within certain limits, by controlling the voltage applied to the windings. Since the components of most motors are designed only for a specific amount of electricity that means you can slow a motor down but you can't make it go faster since applying more juice would overload it (not safe).

Most dimmer switches are designed to dim a lightbulb or chandelier fixture, which is maybe a 100-150 watt fixture (max). A pool pump needs a lot more juice; my pump has a maximum draw of 13.8 amps at 115V, or about 1,600 watts. I am sure there are rheostats available that can handle bigger loads, but it is not something you are likely to find at your local

Not True! Do not try.

Unfortunately, you cannot control the speed of an AC induction motor with line voltage. Only DC Motors speed can be controlled with voltage. AC induction motors run at speed proportional to the line frequency so most single speed pool pumps will always run at a speed of around 3450 RPM. If the voltage is reduced, the current must increase to keep the same power so it will end up burning out the motor.

The only way to control the speed of an AC induction motor is to change the line frequency. This can be done with a very expensive VFD (variable frequency drive) control unit. This is basically what the variable speed pumps use and why they are so expensive. There are add on VFD units for single phase AC induction motors but usually cost more than if you just bought an Intelliflo.

However, you can replace the single speed motor with a two speed motor which has a second set of windings for low speed. If you go this route, just make sure that the braking HP (service factor * label HP) is matched to the wet end of the pump.

 

[gumarogarc]

The speed of an induction motor (AC) is Speed=120f/# poles

If a motor has 4 poles, and with 60 hz, the speed is 120*60/4=1800 rpm; with two poles=3600 rpm. That is the sincronic speed and there is no torque produced, that why the speed is a little bit lower (3450).

Then, you just can change the frecuency or the poles. As mas985 says, for frecuency you need an expensive power control circuit. the other way is less expensive, but really you have two windings in one enclosure+rotor. Is like two motors in one, so the price is higher than a single spped/winding motor.

Gumaro

 

[polyvue]

Gumaro, thanks for detailing the RPM equation... I had been wondering why all of my pumps run at the same speed

However, you can replace the single speed motor with a two speed motor which has a second set of windings for low speed. If you go this route, just make sure that the braking HP (service factor * label HP) is matched to the wet end of the pump.

Mark, could you elaborate on what you mean about matching BHP to the "wet end" of the pump? If I want to replace my 1.85 BHP single-speed pump with a 2-speed model for purposes of energy conservaton, how do I apply this advice?

Thanks :!:


Greg

 

[mas985]

The impeller in a pump is matched to the full horsepower (BHP) of the motor. So unless, you change the impeller, you have match the motor to the impeller. Therefore, you want to pick a motor that has a BHP of at least 1.85 but no lower. Higher is ok although you will start to lose efficiency because the motor will be under loaded. However, if you choose a motor which has a lower BHP, the motor will be over loaded and will eventually fail.

BTW, the actual equation for rotor speed in an AC Induction motor is 120 * Line Frequency / # Poles * (1 - slip). Most pumps are designed for full load which is around 4% slip. So this is why the speed is usually rated at 3450 RPM.

 

[Crey]

Thanks for the info everyone. i like how you guys gave the reason why, other than a "No". I'm glad to have joined this community, many well educated and experienced folks on here.

I've always wondered why the multispeed pumps were so expensive and now I know why.