Analysis of advantages and disadvantages of permanent magnet motors

Update:09 Nov 2020

For permanent magnet motors, concentrated windings can […]

For permanent magnet motors, concentrated windings can be used for the stator if torque ripple is not taken into account, but the windings are usually distributed over larger permanent magnet motors.

Since permanent magnet motors do not have a mechanical rectifier, an inverter is essential to control the winding current.

Unlike other types of brushless motors, permanent magnet motors do not require an electric current to support a magnetic field. Therefore, if the volume is small or small, the permanent magnet motor can provide maximum torque and may be the best choice. The lack of field current also means that it is more efficient at "sweet spot" loads (motors perform best).

In addition, while permanent magnets have performance advantages at low speeds, they are also technically a "fatal weakness". For example, as the speed of a permanent magnet motor increases, the counter electromotive force approaches the power supply voltage of the inverter, and the winding current becomes uncontrollable. This defines the basic speed of a typical permanent magnet motor and usually represents the maximum possible speed of a particular supply voltage in a surface magnet design.

At speeds faster than the base speed, IPM uses active magnetic field weakening. In this weakening, the stator current is manipulated to intentionally suppress the magnetic flux. The speed range that can be reliably achieved is limited to approximately 4: 1. As mentioned earlier, this limitation can be achieved by reducing the number of winding turns and increasing the cost and power consumption of the inverter.

The need to weaken the magnetic field is related to velocity, and no matter how much torque there is, there is an associated loss. This reduces efficiency at high speeds, especially at light loads. This is very serious for electric vehicles on the highway. Permanent magnet motors are generally preferred for electric vehicles, but the benefits of efficiency are questionable when calculating the actual driving cycle. Interestingly, at least one well-known electric vehicle manufacturer has converted a permanent magnet motor into an induction motor.

Other drawbacks include the difficulty of managing the inherent back electromotive force in a fault condition. Even if the drive is disconnected, as long as the motor is rotating, current will continue to flow due to winding failure, causing cogging torque and overheating, which is dangerous. For example, if the fast magnetic field is weakened because the inverter is off, uncontrollable power generation can occur and the inverter's DC bus voltage can rise to dangerous levels.

In addition to permanent magnet motors with cobalt magnets, operating temperature is another important limitation. If the motor current is high due to an inverter failure, the holding force of the mechanical magnet limits the maximum demagnetization rate. If the permanent magnet motor is damaged, it is difficult to safely remove and dispose of the rotor, so repairs usually require a return to the factory. Finally, the high value of current rare earth materials may make this material more economically viable, but it is cumbersome to recycle when it is discarded.

Despite these drawbacks, permanent magnet motors are unmatched in terms of low speed and excellent efficiency, which is very useful when size and weight are important.