How to Determine Speed for an Ac Induction Motor

September 30, 2000
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Illustration of a six-pole stator.


One way an hvacr service tech can determine that s/he is making good motor replacement decisions in the field is to understand the concept of motor speed so that the speed of one ac induction motor can be matched to another.

At the same time, techs also need to become familiar with the concept of poles, since poles represent one key to a successful replacement.

Let’s begin the discussion by describing what causes an ac induction motor to run at a particular speed.



Poles and Speed

Every ac induction motor has poles, just like a magnet. However, unlike a simple magnet, these poles are formed by bundles of magnet wire (windings) wound together in slots of the stator core.

In most cases, you can look inside the motor and count the number of poles in the winding; they are distinct bundles of wire evenly spaced around the stator core.

The number of poles, combined with the ac line frequency (Hertz, Hz), are all that determine the no-load revolutions per minute (rpm) of the motor. So, all four-pole motors will run at the same speed under no-load conditions, all six-pole motors will run at the same speed, and so on.

The mathematical formula to remember in helping make this calculation is the number of cycles (Hz) times 60 (for seconds in a minute) times two (for the positive and negative pulses in the cycle) divided by the number of poles.

Therefore, for a 60-Hz system, the formula would be:

60 x 60 x 2 = 7,200 no-load rpm ÷ number of poles.

For a 50-Hz system, the formula would be:

50 x 60 x 2 = 6,000 no-load rpm ÷ number of poles.

Using this formula, you can see that a four-pole motor operating on the bench under no-load conditions runs at 1,800 rpm (7,200 ÷ 4 poles). Note that when an ac motor is loaded, the spinning magnetic field in the stator does not change speed. Instead, the rotor or moving part of the motor is restrained by the load from “catching up” to the field speed.

The difference between the field speed of 1,800 rpm in this example and the rotor speed of approximately 1,725 rpm is called the “slip.” Slip varies with the load over a narrow operating range for each motor design.



Motor Speeds, Both Loaded and Unloaded

Our spinning four-pole motor, then, operates at 1,800 rpm in this example under no-load conditions and approximately 1,725 rpm under load. Motors of this speed are commonly found in belted applications such as blowers, fans, air-handling equipment, compressors, and some conveyors.

A two-pole motor operates at 3,600 rpm (7,200 rpm ÷ 2) unloaded, and approximately 3,450 under load. Two-pole motors often are found in pump applications, such as sump pumps, swimming pool pumps, and water recirculating equipment.

One thing for the service technician to keep in mind in the field is that the higher the rpm, the noisier a motor may sound to the untrained ear. It is beneficial to become aware of the different speed-related sounds motors make.

Six-pole motors run at 1,200 rpm unloaded (7,200 ÷ 6) and between 1,050 and 1,175 rpm loaded. They are often used for air-handling equipment, direct-drive applications, window fans, furnace blowers, room air conditioners, heat pumps, and other equipment where the relatively slower motor speed makes for quieter operation. All can come in either totally open, totally enclosed, or combination models, adding to their versatility.

To satisfy consumers’ desires for quieter motors, manufacturers have developed eight-pole motors. These operate at 900 rpm (unloaded) and approximately 800 rpm under load. They are being used in applications where customers expect quieter operation, such as room air conditioners and outdoor heat pump applications.

Less-common pole configurations include 12-pole motors (600 rpm) that are used in applications requiring slow speeds, such as washing machines, and 16-pole motors (450 rpm unloaded), often found in ceiling fans.

Simon is with A.O. Smith Corp., Milwaukee, WI; 414-359-4104,(phone) 414-359-2064 (fax))

Publication date: 10/02/2000

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