How do you choose the right electric motor?

By Anders Pedersen Sekanina · 14 November 2019 · ← Back to blog

In this post I give a brief description of the most common types of electric motors.

The electric motor market is very large and complex. Every week there is news of a completely new type of electric motor being invented. However, most are variants of the classic types. Below I describe the many similarities that exist between the classic types as well.

Motors with permanent magnets (PM) vs. without (nPM)

Electric motors can be divided into two groups: with vs. without permanent magnets. The advantage of motors with permanent magnets is that they have higher torque density and in many cases also higher efficiency. The main weakness of motors without permanent magnets is the air gap between rotor and stator. Most of the resistance in the magnetic circuit is in the air gap. For smaller motors the air gap is relatively larger – which is why you rarely see motors without permanent magnets below 1 kW.

The challenges with PM motors are their poor performance at lower speeds, caused by eddy current losses from the magnets, and the high prices of permanent magnets.

Brushed DC motor (DC)

The brushed DC motor is the classic low-power motor familiar from older vacuum cleaners, power tools and many other everyday motors. The advantage is that it is easy to control and only requires a constant DC voltage, for example a battery. The disadvantage is that it does not last as long because the brushes wear out. This type of motor also tends to produce considerable electrical noise.

Brushless DC motor (BLDC)

The brushless DC motor differs from the brushed DC motor in that the phase current is not controlled by mechanical commutation. Instead, power electronics are used to switch the phases on and off. As power electronics have become cheaper and the brushless DC motor has better durability than the brushed DC motor, it is becoming increasingly common and is used today in, among other things, electric vehicles, linear motors, robots, ventilation systems and RC cars.

Stepper motor

The stepper motor is in principle a brushless DC motor with many rotor poles (magnetic pairs on the motor). This means that rotor position can be controlled precisely without either position or current measurement – simply by switching the phases in a predetermined pattern. The stepper motor is known for torque density and position resolution at a low price. On the other hand, efficiency and maximum speed are lower than the brushless DC motor's. Due to its lower efficiency, the stepper motor is only used for smaller motors, since a brushless DC motor with a gearbox is smaller and cheaper in larger applications. The stepper motor is used in industrial robots, printers and other smaller applications where position needs to be controlled.

Switched Reluctance Motor (SRM)

The Switched Reluctance Motor can be described as a brushless DC motor without magnets. Instead of magnets it has a rotor of laminated iron, where the magnetic field runs from the phase's positive pole to negative pole.

The Switched Reluctance Machine was one of the first motors ever built – already in the first half of the 1900s they were first demonstrated. As power electronics did not yet exist, they were driven by mechanical relays mounted on the shaft. This was a rather complex and not particularly stable system, so it was not really developed until power electronics became available in the 1970s.

The Switched Reluctance Motor is one of the simplest types of motor and in most cases has the highest efficiency of motors without permanent magnets. At high speeds in particular, it has the highest efficiency of all motor types. Nevertheless, not very many SRMs are produced today. The only place where the Switched Reluctance Motor has truly won through is in the heavy mining industry.

AC Induction Motor (AC)

The AC induction motor, or asynchronous motor, is truly the workhorse of industry – the vast majority of motors used in conveyor belts, cranes and other industrial machinery are AC induction motors. Unlike other motor types, torque is generated by an induced current in the rotor, allowing the motor to run asynchronously with the input frequency. The two biggest advantages of the induction motor are that it can run without an inverter (motor controller), and that it is inexpensive because it contains no permanent magnets and because many millions of them are produced every year. Disadvantages include lower efficiency than most other motor types, and problems with cooling the rotor, especially at low speeds.

Permanent Magnet Synchronous Motor (PMSM)

The Permanent Magnet Synchronous Motor has the same stator as the AC induction motor but has a rotor with permanent magnets. This means it can directly replace the AC induction motor in many applications, as it uses the same motor controller (inverter) and has the same physical size, while achieving greater efficiency and therefore more torque and power density. Due to its more complex stator compared to the brushless DC motor, the Permanent Magnet Synchronous Motor is more commonly seen in larger applications, such as elevators.

This motor type also exists in a version without permanent magnets – in that case it is called a Synchronous Reluctance Motor, SynRM.

Which is best?

As can be seen above, all types of electric motors have their advantages and disadvantages, so the best choice very much depends on the specific application. Sekanina Electronics can help you find and develop the right motor for your product.

Read more about our services within test, motor control and automation on the Services page.

/Anders Pedersen Sekanina