Synchronous and asynchronous motors : What are the differences?

🔗 Mechanical transmission

Date of publication : 17/12/2023

Reading time : 3 Minutes


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Electric motors play a vital and ubiquitous role in the invisible infrastructure that powers our modern lives. Whether in the gentle sound of a ceiling fan or the deafening roar of sophisticated industrial machinery, these ingenious devices are the silent artisans of our daily lives.

However, the diversity of their functions remains a mystery to many and it can be difficult to distinguish between different types of electric motors. The aim of this article is to lift the veil on this complexity by considering two fundamental categories : synchronous and asynchronous motors. These two protagonists of electrical engineering have different roles and applications, providing specific services adapted to specific situations.

Asynchronous motor

Asynchronous motors connected to frequency converters are the most commonly used type of motor in applications where the speed and movement of a load must be controlled.

Motor drive systems are ideal for applications such as elevators, as they require excellent precision, both in terms of speed (ease of use) and accuracy of car position in relation to stops.

The popularity of asynchronous motors is based on their low maintenance, simple construction, standardization and robustness.

The operating principle of asynchronous motors is based on:

  • On the one hand, it is based on the generation of electric current induced in a conductor located in a rotating magnetic field. The conductor in question is one of the squirrel cage rotor rods underneath that forms the motor rotor. Current induction only occurs when the conductors are shorted (because the two side rings connect all the bars).
  • On the one hand, it involves the generation of a driving force on the conductor considered (carried by an electric current and placed in a rotating or fluctuating magnetic field), the meaning of which is given by the following rules: Three fingers on the right hand.

Three-phase asynchronous motor

Three-phase asynchronous motors are the most commonly used type of electric motor in industry. In fact, it is estimated that 80% of motors used worldwide are asynchronous motors. Pumps, compressors, freight elevators, fans, etc. generally use three-phase asynchronous motors. If reduction is necessary, a gearbox mounted on the motor shaft can be used to adjust the speed according to the workload.

Asynchronous motors have a simple construction, robustness and low maintenance, which makes them very reliable and widely used machines, especially in the industrial sector. A controversial topic for this type of electric motor is the price.

Like other types of electric motors, asynchronous motors convert electrical energy into mechanical energy. A three-phase asynchronous motor also consists of two main parts: the stator and the rotor.

The stators

The stator of a three-phase motor, which is the most common in terms of medium and high power, is the static component of the entire asynchronous motor. It mainly consists of:

  • of the carcass,
  • bearings,
  • bearing flanges,
  • of the fan cooling the engine,
  • the cover protecting the fan.

Inside the stator, we mainly find:

  • a laminated iron core so as to channel the magnetic flux,
  • the windings (or copper winding) of the three phases housed in the notches of the core.

In a three-phase motor, the windings are arranged in a minimum of three, offset from each other by 120 degrees, as shown in the diagram below. The rotor, for its part, represents the moving part of the asynchronous motor.

The rotor

The rotor is the moving part of the asynchronous motor. Mechanically coupled to an elevator winch for example, it will create a motor torque capable of providing work for raising and lowering the elevator car. It essentially consists of:

  • A stack of thin discs insulated from each other and keyed to the rotor shaft in order to channel and facilitate the passage of the magnetic flux.
  • A cast aluminum squirrel cage whose bars are trapezoidal in shape for standard asynchronous motors and closed laterally by two conductive “flanges”.

Single-phase asynchronous motor

Single-phase asynchronous motors operate on single-phase voltage, so they are widely used in most common mixers.It uses a squirrel cage principle with one winding on the rotor and two windings on the stator, with both ends shorted in a closed loop.

The presence of a cylindrical capacitor in a single phase motor serves to provide load or current input to the motor itself.In other words, it allows the current in one of the two windings to be shifted relative to the other in order to obtain a rotating magnetic field during start-up.Therefore, if high starting torque is required, a single-phase motor with two capacitors is used.

What is a synchronous motor ?

Formerly used as alternating current generators, the development of power electronics and the widespread use of magnets as inductors now make it possible to use synchronous machines as motors over a wide power range.

In most cases, synchronous machines are used in three phases. Works as a reversible electromagnetic energy converter: the can be used as a motor or generator.

Thus, in a synchronous motor, the speed of the rotor is proportional to the frequency of the three-phase current supplied to the stator. Depending on the voltage, a synchronous motor must operate as a “motor” or as a “generator” or “alternator”.

What are the advantages and disadvantages of synchronous motors?

Asynchronous motors and synchronous motors are distinguished by their rotors. In a synchronous motor, the motor is made up of magnets or electromagnets. This rotor therefore rotates at a synchronous speed and the synchronous motor rotates at the same speed as the magnetic field.

Good points :

  • Thanks to this device, there is no loss of speed and the engine efficiency is excellent. The motor speed remains constant regardless of load, making it very practical in certain applications.

The negative points of the synchronous motor:

  • Risk of stalling if the maximum torque is exceeded and direct starting is impossible.

Synchronous VS asynchronous motors

Although synchronous motors are appreciated for their efficiency, asynchronous motors remain the most commonly used in industry because they are:

  • less expensive
  • less bulky
  • easier to maintain and highly durable
  • compatible with a large majority of industrial machines,

equipped with a efficient speed variator and overall simpler to set up than on a synchronous motor

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