How to Pick a Centrifugal Pump Part 8: Motor Speed Considerations
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By Chad Edmondson
How does RPM (revolutions per minute) impact pump operation? And what RPM is best suited for hydronic applications?
A pump’s RPM or “synchronous speed” is determined by the number of magnetic poles in the motor winding as they interact with alternating current power. Magnetic fields are induced in the stator or the part of the motor with copper windings. The rotor or the part with the shaft has an opposite magnetic field. As the magnetic field is changed in the stator, the rotor moves. Think of playing with two magnets of opposite polarity. You can get one to move by moving the other without having them touch each other. The number of “magnetic” poles in the rotor will determine its speed.
Pumps used for hydronic HVAC or plumbing applications include the following RPMs:
- Two pole, 60 Hz motors that run at 3600 rpm
- Four pole, 60 Hz motors that run at 1800 rpm
- Six pole, 60 Hz that run at 1200 rpm
Perhaps you’ve noticed that the speed listed on the nameplate of the motor is slightly less speed than stated above. The industry may call a motor 1800 RPM or 1750 RPM and the nameplate might read 1770 RPM or 1765 RPM. These are all the same in our world. The reason can be stated simply or made very complex. The simple reason is that the magnets in the motor spin faster than the shaft due to bearing friction and torque or load. This difference is called “slip.”
In the past, motors had lower efficiencies than the premium efficiency motors of today, so you will see older motors and older pump curves at slower speeds such as 1750 or 1735, while today the motor may be run at 1770 or 1780 RPM. The EPAct Energy Policy Act premium efficiency regulation increased the minimum efficiency which reduced the friction in the motor and increased the rated speed of the shaft. Different people in the HVAC world may refer to a motor as 3500, 3600, 3550, or 3565 RPM but for practical purposes they are all the same.
Electronically commutatedmotors (ECM) for HVAC centrifugal pumps are also an option for smaller pumps. ECM motors are variable speed and extremely efficient. Typically, the larger motors we normally use in the industry are induction motors that use an alternating current (AC) to produce a magnetic field as described above. This process produces heat which reduces efficiency. However, ECM motors utilize permanent magnets and since we are not having to produce a magnetic field, the efficiency of the ECM motor is higher than an induction motor.
The challenge with ECM motors is size. The maximum ECM horsepower available today is around 5 HP. For this reason, ECM motors are normally only available on smaller pumps for smaller applications. They are very prevalent in fractional horsepower pumps and fans. Unless we are willing to install and operate multiple pumps in series and in parallel we are limited to using induction motors for larger applications.
