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What are the disadvantages of permanent magnet motors?

Permanent magnet motors (PMMs) generate torque through the interaction of stator current with permanent magnets on or within the rotor. It is common for small, low-power motors to be used for surface rotor magnets in IT equipment, commercial machines, and automotive auxiliary equipment. Interior Permanent magnets (IPMs) are common in large machines such as electric vehicles and industrial motors.

In PM motors, the stator may use concentrated (short pitch) windings if torque ripple is not a concern, but distributed windings are common in larger PM motors.

Since permanent magnet motors do not have a mechanical commutator, the inverter is essential to control the winding current. Unlike other types of brushless motors, permanent magnet motors do not require current to support their magnetic field.

So, if small or light, permanent magnet motors provide the most torque and are probably the best choice. No magnetizing current also means higher efficiency at load at the “sweet spot” – i.e. where the motor performs best.

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Furthermore, while permanent magnets bring a performance advantage at low speeds, they are also a technical Achilles’ heel. For example, as the speed of a permanent magnet motor increases, the back EMF approaches the inverter supply voltage, making it impossible to control the winding current.

This defines the base speed of a general permanent magnet motor, and in surface magnet designs typically represents the maximum possible speed for a given supply voltage.

At speeds greater than base speed, the IPM uses active field weakening, in which the stator current is manipulated to deliberately depress the magnetic flux.

The range of speeds that can be reliably implemented is limited to around 4:1. As before, this limit can be achieved by reducing the number of winding turns and accepting greater cost and power loss in the inverter.

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The need for field weakening is speed dependent and has associated losses regardless of torque. This reduces efficiency at high speeds, especially at light loads.

What are the disadvantages of permanent magnet motors?插图1

In electric vehicles driving on the highway, this is very serious. Permanent magnet motors are often favored for EVs, but the efficiency benefits are questionable when real drive cycles are calculated.

Other disadvantages include the fact that it is difficult to manage under fault conditions due to its inherent back EMF. Even if the frequency converter is disconnected, as long as the motor is spinning, current will continue to flow through the faulty winding, causing cogging torque and overheating, both dangerous.

For example, field weakening at high speeds can lead to uncontrolled power generation due to drive shut down, and the DC bus voltage of the inverter can rise to dangerous levels.

Except for those permanent magnet motors that incorporate samarium cobalt magnets, operating temperature is another important limitation. High motor currents due to inverter failure can cause demagnetization.

Maximum speed is limited by mechanical magnet retention. If a permanent magnet motor is damaged, repairing it often requires a return to the factory because safely extracting and handling the rotor is difficult.

Finally, recycling at end-of-life is cumbersome, although the current high value of rare earth materials may make such materials more economically viable.

Despite these drawbacks, permanent magnet motors remain unrivaled in low speed and sweet spot efficiency, and they are useful in situations where size and weight are critical.

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