Replacement Motor Size Confusion

[chenss2000]

I currently have a 3/4 hp whisperflo. It started to make noise due to the bearings, so I called home warranty to repair it since I just bought the house couple months ago. They came with a 2 hp motor and convinced me it would work since my impellers are for the smaller motor. Good thing is it would not fit in the space so they are bringing a smaller motor on Monday.

Is going with a larger motor a good idea? Will it use more power even with the smaller impeller? I am trying to confirm the size of the smaller motor, but really I just want the same size since everything works (including the spa). I definitely don't want to use more power than I need to.

Thanks

 

[PoolGuyNJ]

A larger motor with the same impeller will use slightly more current.

Scott

 

[JasonLion]

Welcome to TFP!

The larger motor will work, if it fits. Unfortunately it probably won't be a high efficiency motor, like the ones that come with most WhisperFlo pumps. Power usage will be similar to the 3/4 HP motor, but higher, no where near as high as a standard 2 HP pump.

This is one of the big problems with nearly all of the home warranty companies. They replace things that break with something that works, but they aren't required to match the quality level of what was there originally. The WhisperFlo pump is a particularly nice high efficiency pump matched with a high efficiency motor. It is very unlikely that the home warranty people will use parts of that quality/efficiency.

 

[mas985]

A slightly larger motor can actually end up being slightly more efficient than a correct sized motor because larger motors tend to use larger gauge windings to handle the current and the I2R losses are usually slightly less for a larger motor than a smaller motor. For a line of motors, the larger the motor, the more efficient it is. However, if the new motor is standard efficiency while the old motor was a high efficiency, the power usage might end up being the same but if they are both standard efficiency motors, then the new motor might end up drawing less power than the old motor.

Also, when measuring current, the power factor must be taken into consideration. The larger motor may have a lower power factor and so the measured current might be higher than the old motor but real power usage could end up being less.

Power Usage = V * I * power factor

If the power factor gets too low, then efficiency will start to suffer so there is such a thing as a motor being too large. But generally speaking, underloading a motor by less than 2:1 should maintain fairly good efficiency and even 3:1 is not too bad.

 

[chenss2000]

Thanks everyone! The pool repair contractor offered me a brand new motor for $93 extra. It is the exact same one I had before and now it comes with a one year warranty. Home warranties are definitely shady. I would the $93 extra any day for a brand new motor that will last me many years.

I am also glad the hp rating is the same and I won't be consuming more power. Also I was wondering, do variable speed pumps really reduce power usage by 50%?

 

[JasonLion]

Yes, in most cases a variable speed pump can lower total power usage by around 50% compared to a single speed pump. You may or may not ever make up the additional cost, depending on what your electric rates are.

Pumps are more efficient at lower speeds. You run the variable speed at a lower speed for a longer time and it saves so much electricity at the lower speed that it more than makes up for the longer run time.

It doesn't work out that way if you really need to run the pump at full speed, for example running a spa, or waterfall, or in-floor cleaning jets. In those cases there is still a small savings, but it is nothing like what you get when you can run on low speed.