Although all three-phase motors operate with a similar power supply, their internal construction can vary significantly, especially that of their rotor. To understand how motor construction varies, consider that they have a visible rotating shaft and an invisible rotating magnetic field, and they can be classified into two broad types based on how their field and rotor interact:
In both cases, the speed of the rotating magnetic field is called the synchronous speed, and it can be calculated based on the voltage supply frequency (in Hertz) and the number of poles in the motor’s magnetic field. The formula is very simple:
Therefore, a 4-pole motor connected to a 60-Hertz power supply will have a magnetic field rotating at 1,800 rpm, while a 6-pole motor connected to a 50-Hz supply will have a synchronous speed of 1,000 rpm. Regardless of rotor construction, the synchronous speed of motors is defined by the number of poles and the electric supply frequency.
Synchronous motors operate with a very simple principle. If a magnet is attached to a shaft and placed in a rotating magnetic field, it will rotate at the same speed. The magnet itself can be permanent, or an electromagnet connected to direct-current power supply. In both cases, the shaft follows the motor’s magnetic field at the same speed.
The same physical construction is used for a synchronous generator, which is supplied with mechanical power at the shaft and provides electric power at its terminals. These are the types of generators used in power plants, since their direct relationship between shaft speed and magnetic field speed allows control of the electric frequency produced.
An induction motor has similarities with a transformer, which produces a voltage in its secondary winding when power is supplied to the primary winding. As power is supplied to the stator of an induction motor, the rotating magnetic field induces voltage in the rotor, which spins thanks to the interaction between both magnetic fields. However, there is a slight lag between the magnetic field speed and the actual rotating speed of the rotor; for example, you may notice that 4-pole motor connected to a 60-Hz voltage will not rotate at the synchronous speed of 1,800 rpm, but rather a lower value such as 1,750 rpm.
The speed difference between the magnetic field and the rotor is described with a concept called slip, which is calculated as follows:
For example, the 1,750 rpm motor described above would have the following slip value:
It is important to note that the rated speed of 1,750 rpm applies when the motor is fully loaded. If you connect this motor while unloaded it will rotate faster, possibly above 1,790 rpm but without reaching 1,800 rpm. An induction motor would only reach synchronous speed with zero load, which is impossible to achieve because there are always frictional losses.
There are two main types of asynchronous motors. Their rotor construction varies but both are based on electromagnetic induction.
A squirrel-cage induction motor has conductive bars embedded in its rotor (shown below), which are connected to each other with rings at their ends to allow current flow. #usr783995
A wound-rotor induction motor has windings in its rotor just like in the stator, but the principle is the same - as voltage and current are induced in them, the rotor generates a magnetic field that follows that of the stator.