Behind a simple number lies a world of difference in motor performance.

    "Which pole motor should this device be equipped with?" " In the factory workshop, Old Zhang, the equipment supervisor, and Xiao Wang, the technician, were discussing a newly installed crusher.
    Based on experience, a 6-pole or 8-pole one should be used. The lower the rotational speed, the greater the torque, and it is suitable to directly drive the crushing roller. Xiao Wang replied hesitantly.
    Experience cannot be the standard. If the wrong number of poles is selected, the motor will either burn out or fail to drive, resulting in huge losses! Old Zhang said seriously.
    In the world of motors, the seemingly simple parameter of the number of poles directly determines the core performance and application scenarios of the motor. Behind the numbers 2, 4, 6 and 8 lie significant differences in key characteristics such as rotational speed, torque and efficiency.

What is the number of poles in a motor?

    When we talk about the number of poles of a motor, we refer to the number of magnetic poles in the stator magnetic field of the motor. In simple terms, it refers to the number of magnetic poles formed when the stator winding is energized. Because magnetic poles always appear in pairs (N poles and S poles), motors have even poles such as 2, 4, 6, and 8.
    The number of poles of a three-phase asynchronous motor is determined by the connection method of the stator winding. When each group of coils is energized, it will generate two magnetic poles, N and S. The number of magnetic poles contained in each phase of the motor is the number of poles. The number of poles of the motor selected depends on the rotational speed required by the load.
    How does the number of poles determine the motor speed?
    The relationship between the motor speed and the number of poles is determined by a basic formula:
    Synchronous speed n₀ = 60f/p
    Among them, f represents the power supply frequency (50Hz in China), and p represents the number of magnetic pole pairs (pole number divided by 2).
    According to this formula, we can calculate the synchronous speed of motors with different pole numbers under a 50Hz power supply:

    It is worth noting that the actual rotational speed of an asynchronous motor is slightly lower than that of a synchronous motor, which is precisely where the name "asynchronous" comes from. The passing rate is usually between 2% and 5%, which is a normal phenomenon.

The same power, different number of poles: A major revelation of core differences

 1. The "seesaw relationship" between rotational speed and torque

    Under the same power, the more poles there are, the lower the rotational speed, but the greater the torque. This is the most crucial difference. According to the power formula P = T × n / 9550 (where P represents power in kW, T represents torque in N·m, and n represents rotational speed in r/min), when the power P is fixed, the lower the rotational speed n, the greater the torque T must be.
     Specifically,
    The torque ratio of a 6-pole motor is as high as 4, approximately 50%
    The torque ratio of an 8-pole motor is extremely high, approximately 100%

2. A significant difference in volume and weight

    The more poles a motor has, the larger its volume and weight will be. The reason is that low-speed motors (with a high number of poles) require more pairs of magnetic poles to generate a rotating magnetic field, the core needs to accommodate more windings, and to ensure output torque, the magnetic circuit area needs to be increased.

Compare motors with the same power but different pole numbers:

    A 2-pole motor is 30% to 50% smaller in size than an 8-pole motor
    A 2-pole motor is 20% to 40% lighter than an 8-pole motor

 3. Comparison of efficiency and power factor

In terms of efficiency:

    The efficiency of 2-pole or 4-pole motors is usually higher (with more mature designs and better loss balance).
    When the number of poles is too large, the core losses (eddy current and hysteresis losses) and winding copper losses increase, and the efficiency may slightly decrease (especially under light load)

In terms of power factor:

    High-pole number motors (such as 8-pole ones) have a lower power factor because their winding inductance is larger and the proportion of reactive current is higher
    In industrial applications, high-pole-count motors often require additional capacitor compensation

4. Startup performance and operational smoothness

Starting torque

    High-pole number motors have a larger starting torque (due to the lower speed, the "slip rate" during starting is higher, and the electromagnetic torque is easier to establish).
    - Suitable for direct start-up of heavy-duty equipment (such as crushers, mixers)

Operational smoothness

    The more poles there are, the smoother the operation will be
    Motors with more poles have a smaller "step size" of the rotating magnetic field, and usually have lower vibration and noise during operation (for example, a 6-pole motor runs more smoothly than a 2-pole motor).
    Application scenarios of motors with different pole numbers

1. 2Extreme motor (high speed, low torque)

    Rotational speed range: 2800-3000 revolutions per minute
    Features: Smallest torque, smallest volume
    Applicable equipment: Water pumps, fans, centrifuges and other devices with small load torque and requiring high-speed operation

2. 4-pole motor (medium speed, torque)

    Rotational speed range: 1400-1500 revolutions per minute
    Features: Medium torque, moderate size, and the best efficiency balance
    Applicable equipment: Conveyor belts, compressors, machine tool spindles and most other general machinery (the most widely used)

 3. 6-pole motor (low speed, high torque)

    Rotational speed range: 900-1000 revolutions per minute
    Features: Low speed with high torque, smooth operation
    Applicable equipment: Medium-load equipment such as presses, lifting mechanisms, etc

4. 8-pole and above motors (ultra-low speed, ultra-high torque)

    Rotational speed range: Below 750 revolutions per minute
    Features: Maximum torque, capable of directly driving high-torque loads
    Applicable equipment: Crushers, ball mills, cranes and other heavy-duty equipment (no need for additional reducers, reducing transmission chain losses)

How to identify the number of poles in a motor?

1. The method of observing rotational speed

    The number of poles can be determined by the rotational speed value on the motor nameplate
    Around 1430r/min → Synchronous speed 1500r/min → 4-pole motor
    Approximately 960r/min → Synchronous speed 1000r/min → 6-pole motor
    Calculation formula: Synchronous speed = (Actual speed ÷ 0.95-0.98)

2. The model method
The motor model directly indicates the number of poles

    case：Y 132 M - 4

     Y: Three-phase asynchronous motor
    132: Center height of the frame (mm)
    M: Frame length code
    4: Number of magnetic poles (4 poles

 3. Multimeter measurement method (without nameplate)

Steps

    1. Remove the power cord and wiring
    2. Use the R×100Ω range of a multimeter to locate the two leads of any phase winding
    3. Set the multimeter to the μA current range and clamp the above-mentioned leads
    4. Rotate the motor shaft at a constant speed for one full circle
    5. Observe that the number of pointer swings = the number of magnetic pole pairs
    Swing once → 1-pole pair → 2-pole motor
    Swing twice → 2 pairs of poles → 4-pole motor

The Golden Rule for Selection

Selecting the number of poles of a motor is essentially about matching the speed and torque requirements of the load. Follow the following principles:

    1. Select the number of poles according to the rated speed of the load
    Select 2 poles at 2900r/min
    At 1450r/min, select 4 poles; at 970r/min, select 6 poles
    2. For heavy-load starting, select high pole count: High-pole count motors have a large starting torque and can directly drive heavy-load equipment, eliminating the need for a reduction device
    3. When space is limited, choose a lower number of poles: If the installation space is small, select a 2-pole or 4-pole motor with a smaller volume
    4. For long-term operation and high energy efficiency, choose 4-pole motors: 4-pole motors achieve the best balance among efficiency, power factor and volume