Why a 1800RPM instead of a 3600RPM motor?
We get a lot of questions about why we use a 1800 RPM motor instead of a 3600 RPM motor on our variable speed pumps.
The quick answer is that we do this because the 1800 RPM motor will give you more power and will last longer.
The centrifugal pump head runs optimally between 1800-3600 RPM and with the variable speed controller you are likely going to be running the pump around 2700 rpm, maybe a bit slower or a bit faster but never over the 3600 maximum speed. So you want the motor that will run at full HP and torque through that entire operating range.
Let's look a two 1 HP motors.
The 1800 RPM, 1HP motor produces 3 ft. lbs of torque at 1800 RPM.
The 3600 RPM, 1HP motor produces 1.5 ft. lbs. of torque at 3600 RPM.
So if your 1HP pump was operating at 1800 RPM, the 1800 RPM motor would be producing 3 ft. lbs. of torque while the 3600 RPM motor would be producing only 1.5 ft/lbs of torque. When the 1HP pump is operating in the optimal range (approx. 2700 rpm) the 3600 RPM motor is still be producing 1.5 ft/lbs of torque, while the 1800 RPM motor's output will be somewhere between 1.5 and 3 ft/lbs. Finally when the pump is operating at the top speed of 3600 RPM both motors would be putting out the same 1.5 ft. lbs. of torque.
For some pumps there are some other minor considerations such as fan size and motor efficiency, but in general you're better off with the 1800 RPM motor for a centrifugal pump application.