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How To: Differentiate Synchronous and Asynchronous Motors

There is a long-standing confusion about the difference between synchronous and asynchronous motors. What do these terms mean? How do you tell the difference between the motor types?

Differences between Synchronous and Asynchronous AC MotorsIn a typical AC motor, a rotating magnetic field is produced in the stator. The speed of this rotating field is called the synchronous speed and is determined only by the frequency of the power supply and the number of poles of the machine. A synchronous motor is one in which the rotor rotates at the same speed as the rotating magnetic field in the stator. An asynchronous motor is one in which the rotor rotates at a speed slower than the synchronous speed.

When people refer to asynchronous motors, they are generally referring to AC induction motors. A typical “squirrel cage” AC induction motor has a stationary winding called a stator and a rotor that is made from electrical steel and conductive bars of aluminum or copper which are shorted on each end. In an induction motor, the input current creates a rotating magnetic field in the stator. This changing magnetic field induces currents in the rotor’s conductive bars which results in an attraction between the rotating magnetic field of the stator and the induced magnetic field of the rotor. Because a changing magnetic field is required to induce currents in the rotor, the rotor will always rotate slower than the synchronous speed of the magnetic field in the stator. The difference between these two speeds is called “slip” and is usually given as a percentage of the synchronous speed.

There are many varieties of synchronous motors but what they all have in common is that they are all AC motors and all their rotors rotate at the synchronous speed of the stator. Brushless motors, variable reluctance motors, switched reluctance and hysteresis motors are all synchronous motors. Brushless motors are built with a stator similar to an induction motor but the rotor is made of permanent magnets rather than conductive bars. In general, motor companies have classified brushless motors into two groups based on the shape of their back-emf’s. Motors that have a trapezoidal back-emf and are driven with a trapezoidal (“six-step”) control are generally referred to as brushless DC (BLDC) motors. Motors that have a sinusoidal back-emf and are driven with a sinusoidal control are generally referred to as brushless AC (BLAC) motors. Some motor manufacturers refer to BLAC motors as servo motors or permanent magnet synchronous motors (PMSM). Some motor manufacturers don’t make a distinction between the two types of motors and call both of them brushless permanent magnet (BPM) motors. In practice, due to physical, design, and manufacturing limitations, there is no way to achieve a perfectly trapezoidal or sinusoidal back-emf, so all brushless motors fall somewhere in between the two ideals.

As we’ve seen, asynchronous motors have slip and synchronous motors don’t. Furthermore, the amount of slip is dependent on the load. What this means is that asynchronous motors will not be able to maintain a constant speed under variable load torque applications. However, this disadvantage can be overcome using a variable frequency drive (VFD) or a vector drive.

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