Alternating Current Motor Control
AC motors use alternating current to generate a huge range of mechanical output power, from fractional horsepower all the way to hundreds of kW. They are the most widely varied electric motor, as they are simple in design and operation, and are applicable to many different uses. However, they cannot produce these results without some form of control; AC motor controllers solve this problem by electrically regulating the speed of these useful machines. Their adjustable output allows the motor speed to be precisely controlled which is often difficult with most AC motors, and this article will investigate these controllers, how they work, what types are available, and what are their benefits and drawbacks.
What are AC Motor Controllers?
The operation of AC motors is explained thoroughly in our article on induction motors and is a good review on how AC motors convert electromagnetism into useful mechanical energy.
An important note is that the speed of an AC motor is dictated by its AC supply frequency, as the alternating current in its stator coils generates a rotating magnetic field (RMF) which electromagnetically induces an opposing magnetic field in the rotor, and produces output rotation. Also note that in asynchronous models, the phenomenon of slip exists, which is a discrepancy between the input frequency and the output rotational frequency. It makes intuitive sense that, if some kind of device can either control the AC signal frequency, or change the slip between the rotor and the AC frequency, then speed control can be attained.
An important note is that the speed of an AC motor is dictated by its AC supply frequency, as the alternating current in its stator coils generates a rotating magnetic field (RMF) which electromagnetically induces an opposing magnetic field in the rotor, and produces output rotation. Also note that in asynchronous models, the phenomenon of slip exists, which is a discrepancy between the input frequency and the output rotational frequency. It makes intuitive sense that, if some kind of device can either control the AC signal frequency, or change the slip between the rotor and the AC frequency, then speed control can be attained.
How do AC Motor Controllers work?
As previously stated, AC controllers convert an input AC frequency into a DC current that simulates an AC frequency, which can then be set by the user, thus controlling the speed of the motor. This is accomplished using three main components: a rectifier, an inverter, and a DC circuit that connects the two.
The rectifier is directly connected to the AC electrical supply and is the component which switches, or “rectifies”, the AC current to DC current. The connecting DC circuit then stores this energy (usually via high power capacitors) for the inverter component to use. The inverter is powered by the connecting DC circuit and modulates the rectified frequency to the motor, effectively changing the speed of the motor.
There are also methods for increasing the slip of an AC motor, which will change the speed of rotation in relation to the AC frequency, thus controlling speed. Increasing the slip can be done by either varying the resistance through the motor coils (variable series resistance, see our article wound rotor motors), or varying the voltage to the coils (variable voltage transformers); however, AC drives are more commonplace, as they are more modular and have been engineered to give the user precise digital control over their motor without using integrated components such as resistors or rheostats in the motor windings.