I know this has been discussed before, but I wanted to clear something up in my mind. I understand the pump impeller loads the electric motor, so a 3/4hp impeller draws 3/4hp shaft hp. The pump's electric motor must provide the 3/4hp to the shaft, and on top of that, absorbs energy in terms of electrical and mechanical losses (electrical resistance, mechanical friction, etc). So, a 1hp name plate motor is drawing more than 1hp in electricity.
Thinking "out loud" here:
The electric motor has it's nameplate information: rated horsepower, service factor, service factor amps, volts, etc. My understanding of the service factor is this: it's a multiplier that states the motor may run at up to X times the name plate horsepower. So a 1hp motor with a service factor of 2 could provide up to 2 hp and would draw the rated service factor amps. The service factor is related to the internal construction of the motor, such as the gauge of copper wire in the motor, the motor wire insulation temperature handling capability, and other factors. Essentially, the service factor tells you how resistant to heat the motor is. I have read that when you operate a motor at it's full service factor hp, you decrease its lifespan. This makes sense: the higher power you provide to the pump, the more heat you generate. Heat is bad. I have read that running a motor at full service factor decreases it's life by half. I think people can debate this, but it's clear more motor heat will decrease the motor's life.
I've also seen pool motor suppliers say that you should always replace a motor with a motor with a higher (hp x SF) value. What is sticking in my brain is that with pumping applications, there is not a lot of load variability or starting torque requirements. If a pump impeller is drawing 1hp, what is the harm in using an uprated and physically smaller 1hp motor? The smaller the motor, the more efficient it technically should be. Yes, you decrease your service factor a bit, but if you are drawing more than 1hp at the shaft there is something wrong mechanically with the motor. The impeller sets the shaft hp load.
I guess the idea that the nameplate horsepower times the service factor equals the "brake horsepower" doesn't fit to what I understand the motor service factor to be.
I would think using an uprated and physically smaller motor for dedicated spa jet applications would make sense since the smaller motor is more efficient and the motor only runs for small portions of its life. Using a full rated motor for filter pumps might make a lot of sense since they run all the time and will be exposed to a lot more heat in its lifetime.
What do you all think?