B&G Centrifugal Pump Motor Speed Facts

Mon, June 06, 2018

Understanding electric motor speed is a vital part of the proper selection and application of B&G Centrifugal Pumps.  The following is an explanation of how motors are designed to operate at a certain speed and how motors speed varies when installed to drive a centrifugal pump. “No Load” Motor RPM Every AC Induction motor has magnetic poles. These poles are created by bundles of magnet wire wound together in the slots of the stationary part of the motor (the stator core). Look inside an electric motor, and you can count the number of poles or windings. The number and alignment of these bundles of wires creates magnetic poles, and the number of poles in the motor determines the motor’s speed, stated in revolutions per minute. This motor speed is commonly referred to as the no-load RPM as it is a factor of the number of motor poles and power frequency (hertz). With that knowledge in mind, you may surmise that every four-pole AC induction motor runs at the same speed. This is in fact the case, and you can determine this speed by using the following formula:

• For 60 hertz electrical systems that are common in North America: 7,200 divided by the number of poles in the motor gives you the no load RPM.  Thus, the following are common motor RPMs on 60 hertz are:
  • 2 pole motor design – No Load Speed 3600 RPM (7200 divided by 2)
  • 4 pole motor design – No Load Speed 1800 RPM (7200 divided by 4)
  • 6 pole motor design – No Load Speed 1200 RPM (7200 divided by 6)
• For 50 hertz systems: 6,000 divided by the number of poles gives you the no load RPM. In North American, motors run on the 60 hertz system.

“Under Load” Motor RPM Another important element to note is that under load, the rotating parts of the motor fall behind or “slip” the magnetic speed. As shown above, a four-pole motor operating under no-load conditions will run at 1800 RPM (7200 divided by four poles). Loaded, the motor will slip to between 1600 and 1750 RPM.  At full rated load, slip typically varies from more than 5% for small or special purpose motors to less than 1% for large motors. Similarly, a two-pole motor will operate at 3600 RPM unloaded and between 3000 and 3450 RPM under load.  Six-pole motors run at 1200 RPM unloaded and between 1050 and 1175 RPM loaded. B&G Pump Performance Curves are published at Under Load Motor RPM B&G’s Centrifugal Pump Performance Curves are published in curve booklets at Under Load Speeds to accurately depict pump performance when installed in a building owner’s system.  These same Under Load Speeds are also used for pump selections in B&G Pump Selection Software (ESP-Systemwize) as well. The following is an example of a B&G Series e-1510 End Suction Base Mounted Pump Curve with Four Pole (1800 RPM) motor with the performance depicted at the Under Load Speed of 1750 RPM. Understanding the difference between No Load Motor RPM and Under Load Motor RPM is critical to explaining Centrifugal Pump Performance to your customers. Thank you to Jerry Wiegele, Centrifugal Pumps Product Specialist at Xylem, for contributing this post to our blog.

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Tags: bell & gossett , centrifugal pumps , engineering , HVAC , motors , pump curves