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Why a 1800RPM instead of a 3600RPM motor?

Posted by Donald Byres on

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.

If you start with a 1 HP 1800 RPM motor you will have 1 HP and the full 3 ft/lbs of torque at 1800 rpm and as you speed it up to 3600RPM the HP will stay at 1 HP all the way through the speed range.  If you start with a 1 HP 3600 RPM motor you will still get 1HP and 1 ½ ft/lbs of torque at 3600RPM but as you slow down the motor your HP and torque will be reduced.

Example:


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.
 
So in summary, in the range of 1800-3600 RPM the 1800 RPM motor will provide you with more torque to operate your pump!

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.

 


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