Shortcomings of permanent magnet motors

16/10/2019

Permanent magnet motors (PMM) generate torque by the interaction of stator currents with permanent magnets on the rotor or within the rotor. Small, low-power motors are common for surface rotor magnets in IT equipment, commercial machines, and automotive auxiliary equipment. Internal magnets (IPM) are common in large machines such as electric vehicles and industrial motors.

In permanent magnet machines, if the torque ripple is not considered, the stator may use concentrated (short pitch) windings, but it is common to distribute the windings in larger permanent magnet motors.

Since permanent magnet motors do not have mechanical commutators, the inverter is critical to controlling the winding current. Unlike other types of brushless motors, permanent magnet motors do not require current to support their magnetic fields.
Therefore, if the volume is small or light, the permanent magnet motor can provide the maximum torque and may be the best choice. The absence of magnetizing current also means higher efficiency at the "best point" load - where the motor performance is optimal.

In addition, although permanent magnets offer performance advantages at low speeds, they are also technically "Achilles heel." For example, as the speed of the permanent magnet motor increases, the back electromotive force approaches the inverter supply voltage, making it impossible to control the winding current. This defines the basic speed of a universal permanent magnet machine and typically represents the maximum possible speed of a given supply voltage in a surface magnet design.

At speeds greater than the base speed, the IPM uses an active magnetic field weakening in which the stator current is manipulated to deliberately depress the magnetic flux. The speed range that can be reliably implemented is limited to around 4:1. As before, this limitation can be achieved by reducing the number of winding turns and accepting greater cost and power loss in the inverter.

The need to weaken the magnetic field is speed dependent and produces associated losses regardless of torque. This will reduce efficiency at high speeds, especially at light loads.

This is very serious in electric vehicles driving on highways. Permanent magnet motors are often favored by electric vehicles, but the benefits of efficiency are questionable when calculating the actual driving cycle. Interestingly, at least one well-known electric car manufacturer has switched from PM to induction motors.

Other disadvantages include the fact that its inherent back EMF is difficult to manage under fault conditions. Even if the drive is disconnected, as long as the motor rotates, current will continue to flow through the winding fault, causing cogging torque and overheating, and are dangerous.

For example, due to the frequency converter shutdown, the weakening of the magnetic field at high speeds can result in uncontrolled power generation and the DC link voltage of the inverter can rise to dangerous levels.

In addition to permanent magnet motors with samarium-cobalt magnets, operating temperatures are another important limitation. High motor currents due to inverter failure can cause demagnetization.

The maximum speed is limited by the holding force of the mechanical magnet. If the permanent magnet motor is damaged, repairing it usually requires returning to the factory because it is difficult to safely extract and handle the rotor. Finally, recycling at the time of scrapping is also cumbersome, although the current high value of rare earth materials may make this material more economically viable.

Despite these shortcomings, permanent magnet motors remain unbeatable in terms of low speed and dessert efficiency, and they are very useful in situations where size and weight are critical.