3hp motor with 1.5hp impeller?

[poolguy03]

I am trying to understand how HP of the motor and HP of the impeller relate. I understand that if you use a lower rated motor than the rating of the impeller the motor will overwork and eventually burn out. However, if I was to use a 3hp motor with a 1.5hp impeller instead of a 1.5hp motor, what would happen to flow and energy usage? I assume that since the 1.5hp impeller would still be spinning at 3450 rpm with either motor, both motors would have produce the same flow rate. However, would the 3hp use more, less, or the same power as the 1.5hp motor?

My guess is less power since the 3hp motor would be working less?

 

[poolguy03]

After some research it seems I found my answer. What determines amp draw is the wet end as that is what is doing the work. I also found that most single speed motors used in pool pumps are nearly 90% efficient at 50% load, so in theory the 3hp motor will be more efficient with a 1.5hp impeller than a 1.5hp motor at 100% load.

 

[Toxophilite]

The phrase that comes to play is "all else being equal". Here's a good article explaining the process: How To Downsize a Pool Pump Impeller - INYOPools.com

 

[1poolman1]

After some research it seems I found my answer. What determines amp draw is the wet end as that is what is doing the work. I also found that most single speed motors used in pool pumps are nearly 90% efficient at 50% load, so in theory the 3hp motor will be more efficient with a 1.5hp impeller than a 1.5hp motor at 100% load.

Of all the pumps I "downsized" as you are planning, I typically saw a 40% reduction in amp draw.

 

[mas985]

After some research it seems I found my answer. What determines amp draw is the wet end as that is what is doing the work. I also found that most single speed motors used in pool pumps are nearly 90% efficient at 50% load, so in theory the 3hp motor will be more efficient with a 1.5hp impeller than a 1.5hp motor at 100% load.

That is correct in that the wet end and more specifically, the impeller determines the load on a motor and thus the energy use. For very large induction motors, the efficiency curve is fairly flat from full load down to 50% load. However, most induction motors for pool pumps are well below 90% efficiency (65-85%). VS motors, however, are closer to 90% efficiency. Going below 50% loading starts to significantly effect power factor which is why the efficiency curve drops off. But even with an efficiency hit, the smaller impeller will have much less energy draw.

Typical induction motor efficiency curves (note y-axis is efficiency/full load efficiency and NOT absolute efficiency):

 

[JamesW]

However, if I was to use a 3hp motor with a 1.5hp impeller instead of a 1.5hp motor, what would happen to flow and energy usage?

All motors you install now should be variable speed.

For a 3 hp vs motor, a 3 hp impeller is best and then you adjust the speed to produce the flow you need.

 

[Poolbreh]

Keep in mind some diffusers differ in size depending on make model. Usually 3hp being that threshold of different size.