Pool pump question for electricians types :)

[markayash]

My pump runs around 305 watts and around 4psi..Well today I closed off one set of returns and just returned to the spa, filter pressure went to around 6 BUT the watts dropped to 286?

My question is why does it do that?
I have an engineering brain and curious. I am relating it to a car going 50 mph and all of a sudden sticking a piece of plywood on the grill, your MPG will drop like a log

 

[ajw22]

Greater restrictions create lower flow which reduces motor load which reduces electrical demand.

@mas985 can feed the engineer side of your brain as to why.

 

[mas985]

The energy draw from a pump motor is related to the impeller load and motor efficiency. In turn, the impeller load is related to the energy transferred to the water which is proportional to the product of head AND flow rate in addition to pump head efficiency.

For residential pool pumps, as pressure/head loss increases, flow rate decreases as described by the pump's head curve. Because of the shape of the head curve, flow rate decreases faster than head loss increase so the net energy delivered to the water also decreases.

Definitions:

• Electrical Horsepower (EHP) = Input Watts / 745.7 = Volts * Amps * Power Factor / 745.7 - Electrical power input delivered to the motor.
• Brake Horsepower (BHP) = EHP * Motor Efficiency - Power delivered by the motor shaft to the impeller. This is not the same as THP or SFHP. BHP is a function of the load on the motor shaft and will change with Head, GPM and RPM.
• Hydraulic HP (HHP) = BHP * Pumping Efficiency = Head (ft) * GPM / 3960 - Power delivered to the water. Sometimes called water HP (WHP) or pumping HP (PHP).
• Motor Power Efficiency = BHP / EHP - I2R, magnetic and mechanical losses in the motor only.
• Pumping Power Efficiency = HHP / BHP - Recirculation and internal friction losses in the wet end only.
• Total Pump Power Efficiency = Motor Efficiency * Pumping Efficiency = HHP / EHP (note this is why total pump efficiency approaches 50%).