The speed of a three-phase asynchronous motor is graded and determined by the number of poles.
The "number of poles" in a three-phase asynchronous motor refers to the number of magnetic poles in the stator magnetic field. Different connection methods of the stator windings can result in different numbers of poles in the stator magnetic field. The selection of the number of poles of the motor is determined by the speed required by the load. The number of poles of the motor directly affects the speed of the motor, while the current of the motor is only related to the voltage and power of the motor.
Motor speed = 60 x motor frequency / number of motor pole pairs
1. Classification
1. The number of poles reflects the synchronous speed of the motor.
The synchronous speed for 2-pole synchro is 3000 r/min, for 4-pole synchro it is 1500 r/min, for 6-pole synchro it is 1000 r/min, and for 8-pole synchro it is 750 r/min.
The windings must move back and forth to form a circuit, which is called the number of magnetic pole pairs. They appear in pairs. A pole is a magnetic pole. When current flows through these windings, they will generate a magnetic field, and there will be corresponding magnetic poles.
Each coil of a three-phase AC motor generates N and S magnetic poles. The number of magnetic poles in each phase of each motor is called the pole number. Since magnetic poles appear in pairs, motors are classified as having 2, 4, 6, 8, etc.
2. If the frequency of the three-phase alternating current is 50Hz, then the synchronous speed of the combined magnetic field is 50r/s, or 3000r/min.
If the rotating magnetic field of the electric motor has more than one pair of poles, further analysis can yield the relationship between the synchronous speed n and the number of magnetic pole pairs p: n = 60f/p, where f is the frequency in Hz and n is in r/min.
There is a strict relationship between ns and the frequency (f) of the AC power supply and the number of pole pairs (P) of the motor: ns = f/P.
In China, the power frequency is 50 Hz, so the synchronous speed of a two-pole motor is 3000 rpm, and the synchronous speed of a four-pole motor is 1500 rpm, and so on. The rotor speed of an asynchronous motor is always lower or higher than the speed of its rotating magnetic field, hence the name "asynchronous." The difference between the rotor speed and the rotating magnetic field speed of an asynchronous motor (called slip) is usually within 10%.
Therefore, it can be seen that the speed of an AC motor (whether synchronous or asynchronous) is limited by the power supply frequency. Consequently, speed regulation of AC motors is relatively difficult; the best solution is to change the power supply frequency, which has historically been quite complex.
Therefore, before the 1970s, DC motors were mostly used in applications requiring speed regulation. With the development of power electronics technology, variable frequency speed control technology for AC motors has begun to be put into practical use.
3. The number of poles of an AC three-phase asynchronous motor is the total number of coil groups divided by three.
4. The speed of a synchronous motor = 60 * frequency / number of pole pairs (the power frequency in my country is 50Hz).
Asynchronous motor speed = (60 * frequency / number of pole pairs) × (1 - s)
s: Slip, a physical quantity used to represent the degree of difference between the rotor speed n and the magnetic field speed n0.
In addition, for motors of the same power, the higher the speed, the lower the output torque.
5. Number of poles of a synchronous motor
High-capacity synchronous motors are all pole-reversing type, meaning the rotor is the magnetic pole, generated by direct current flowing through the excitation winding or by permanent magnets on the rotor. The number of pole pairs in a synchronous motor is the number of rotor magnetic pole pairs. An eight-pole motor means the rotor has 8 magnetic poles, 2p=8, meaning this motor has 4 pairs of magnetic poles (Public Account: Pump Manager).
Steam turbine generators are generally salient-pole motors with very few pole pairs, usually only 1 or 2 pairs. Since n=60f/p, their speed is very high, reaching up to 3000 rpm (power frequency). In contrast, hydro turbine generators have a large number of poles and a salient-pole rotor structure, making the manufacturing process more complex. Because of their large number of poles, their speed is very low, possibly only a few revolutions per second.
Identification method:
1. Look at the speed. For example, 1430 r/min is actually 1500 rpm. The speed formula is: speed = time (60 seconds) × frequency (50 Hz) divided by the number of magnetic pole pairs. One magnetic pole pair is 2 poles. Therefore, we can calculate that 3000 ÷ 1500 = 2 magnetic pole pairs, which is a 4-pole motor.
2. Checking the model number is even more direct:
For example, the motor model is Y132M-4
Y → Three-phase asynchronous motor. Among them, the product name codes for three-phase asynchronous motors are: YR for wound-rotor asynchronous motor; YB for explosion-proof asynchronous motor; YQ for high starting torque asynchronous motor; YX3 for high efficiency asynchronous motor; and YE3 for ultra-high efficiency asynchronous motor.
132 → Center height of the frame (mm). Generally, once the power and number of stages are determined, the center height and frame length are also determined.
M → Base length code
4 → Number of magnetic poles
3. Asynchronous motors are indicated by a prefix "Y": YR for squirrel-cage type and YA for increased safety type, followed by the center height and number of poles. For example, YR400-45606KV indicates an asynchronous squirrel-cage motor with a center height of 400mm, 4 poles, a rated power of 560KW, and a rated voltage of 6KV.
Selection Method
The power of the motor is approximately equal to the power of the water pump divided by the pump efficiency divided by the motor efficiency. Motor efficiency is typically 0.85. You provided the corresponding motor powers of 5.5KW and 15KW, with a speed of 2900 rpm. Therefore, the motors are 5.5KW-2P and 15KW-2P respectively. The motor speed = frequency × 60s ÷ (number of poles ÷ 2).
