What is the relationship between the rotational speed of a motor and the number of poles?

The sales department of ATACH Pump factory often receives emails from foreign customers asking how to determine the rotational speed and pole number of the motor/motor installed on the water pump. 

Now let's talk about it in detail:

The number of motor stages is inversely proportional to the rotational speed. 

The more stages there are, the lower the rotational speed. 

The core is determined by the power supply frequency and the number of motor pole pairs. 

The formula can precisely calculate: n=60f/p (n= synchronous rotational speed, f = power supply frequency, p = number of pole pairs). 

A. Core Formulas and Key Concepts 

1.Pole Pair Number (p) : It is half of the number of motor stages. For instance, for a 2-stage motor, the pole pair number p=1; for a 4-stage motor, p=2, and so on. 

2.Power supply frequency (f) : The industrial power standard in China is 50Hz, while in some countries (such as the United States) it is 60Hz. 

When the frequency is fixed, the rotational speed is mainly determined by the number of pole pairs. 

3.Synchronous speed (n) : The theoretical maximum speed of the motor, the speed without mechanical loss and load. The actual speed will be slightly lower (for asynchronous motors). 

B. Common Motor Pole Number and Speed Correspondence Table (Taking 50Hz as an Example) 

Poles          Pole Pair Number (p)                      Synchronous Speed (rpm)                                    Actual Speed of Asynchronous Motor (rpm)                         Typical Application Scenarios 

2 Pole        1                                                                3000,                                                                              2850-2980                                                                                             High-speed Equipment (Centrifuges, Fans) 

4 Pole       2                                                                 1500,                                                                              1400-1480                                                                                              General equipment (water pumps, compressors)

6 Pole       3                                                                 1000,                                                                              950-980                                                                                                   medium-speed equipment (conveyor belts, mixers)

8 Pole       4                                                                 750,                                                                                700-740                                                                                                    low-speed equipment (crushers, mills)

12 Pole     6                                                                 500,                                                                               470-490                                                                                                    ultra-low speed equipment (large ball mill) 

C. The difference between actual speed and synchronous speed 

Asynchronous motors (the most commonly used) have a slip rate (usually 2%-5%), and the actual speed is slightly lower than the synchronous speed. For example, the theoretical speed of a 4-stage asynchronous motor is 1500rpm, but in practice, it is mostly around 1480rpm. 

Synchronous motors (such as permanent magnet synchronous motors) have no slip rate, and their actual rotational speed can approach or be equal to the synchronous speed, making them suitable for scenarios with high requirements for rotational speed accuracy (such as precision machine tools). 

D. Key Points for Application Selection: 

1. Choose the number of stages as needed: Select based on the required rotational speed of the equipment.

   For instance, if a fan needs a high speed, choose level 2; if a conveyor belt needs a medium speed, choose level 4 to 6 to avoid "a big horse pulling a small cart" or mismatched rotational speeds.

2. The number of stages affects the size of the motor: the more stages there are, the larger the motor volume and the greater the torque, making it suitable for low-speed and heavy-load scenarios (for example, an 8-stage motor used in a crusher has a greater torque than a 4-stage one).

3. Supplementary to variable frequency speed regulation: If it is necessary to adjust the speed, a frequency converter can be used. 

For example, a 4-speed motor (1480rpm) can be reduced to 700-1400 RPM through frequency conversion, achieving both flexible speed regulation and energy efficiency.