[Daily Question 20090402] Is the no-load speed of a DC motor also inversely proportional to the number of magnetic pole pairs?

The speed of the motor is related to the number of magnetic poles and the frequency of power supply. The rotation speed of the magnetic field is also called synchronous speed, which is related to the frequency of the three-phase current and the number of magnetic pole pairs p. If the stator winding has only one pair of magnetic poles at any time (number of pole pairs p=1), that is, there are only two magnetic poles. For a rotating magnetic field with only one pair of magnetic poles, the three-phase current changes for one cycle, and the synthetic magnetic field also rotates for one cycle. If it is 50hz alternating current, the synchronous speed of the rotating magnetic field is 50 rpm or 3000 rpm. In engineering technology, rpm (r/min) is commonly used to express the rotation speed. If the magnetic field formed by the stator winding has two pairs of magnetic poles (number of magnetic pole pairs p=2), that is, there are four magnetic poles, it can be proved that when the current changes for one period, the resultant magnetic field rotates 180 degrees in space, which can be generalized to: The synchronous rotation speed per minute of the rotating magnetic field of the magnetic pole is n=60f/p. Therefore, the principles of AC motors and DC motors are the same. When the number of magnetic pole pairs is constant, if the frequency of alternating current is changed, the synchronous speed of the rotating magnetic field can be changed. This is the basic principle of frequency conversion speed regulation.

This can be said under certain conditions, but it depends on many other factors. The synchronous speed of the AC motor is only related to the number of pole pairs. Within one cycle of the alternating current change, the rotating magnetic field of the AC motor rotates through a pair of magnetic poles. Therefore, the greater the number of magnetic pole pairs, the slower the synchronous speed of the motor. That is, the synchronous speed is inversely proportional to the number of magnetic pole pairs. The no-load speed of a DC motor is related to many factors such as armature power supply voltage, number of magnetic pole pairs, air gap flux per pole, armature winding type and number of turns. These factors are all subject to large-scale changes and selections during motor design, unlike AC motors which have a clear relationship. Therefore, the AC motor can clearly estimate the motor speed based on the power frequency and the number of magnetic pole pairs, but the DC motor cannot estimate the speed based solely on the number of magnetic pole pairs.

The rated speed of the motor is selected based on the speed requirements of the production machinery. When the power supply frequency is determined (the frequency of my country's power grid is 50Hz, the synchronous speed of an AC motor is inversely proportional to the number of poles. The corresponding relationship between the number of poles and the synchronous speed of commonly used AC motors is as follows:: ┌─────-----──┬─---─┬--──┬─--─┬─-─┬─--─┐ │Number of poles│ 2 │ 4 │ 6 │ 8 │ 10 │ ├────---─-─-─┼-----─┼─--─┼--──┼─-─┼-──┤ │Synchronous speed/r/min │3000│1500│1000│750 │600 │ └─────-----──┴─--─┴─---─┴─--─┴─-─┴----──┘ The rated speed of the induction motor is slightly lower than the corresponding synchronous speed. There is no strict relationship between the number of poles and the speed of a DC motor like that of an AC motor. The rated speeds of basic series DC motors for general purposes are also the five speed values ​​in the above table. Generally speaking, the higher the speed of a motor of the same power level, the smaller the size and weight, the lower the price, and the smaller its flywheel torque GD2 is generally. The flywheel torque GD2 of the motor has a great influence on the dynamic performance of the motor (such as starting, speed regulation performance, etc.), so attention should be paid when selecting the motor.