The Inner Workings And Applications Of AC Motor Speed Control Devices
Your standard AC motor speed control device takes the form of an Adjustable speed drive (an ASD) or a variable-speed drive (a VSD). These are important to a great many industries that require equipment to control the speed of their machinery. Obviously, the vast majority of industrial mechanisms, such as the machinery surrounding assembly lines, has to work at differing speeds when making different products. In addition, AC motor speed control can help to save energy in cases where the adjustment of the flow from a fan or pump is required, often much more so than other techniques used in flow control.
Adjustable speed drives can of course utilize any of numerous means of motion actuation, be it electromechanical, purely mechanical, electronic, hydraulic (fluid powered) or pneumatic (gas powered). In cases where it’s possible to select speeds from a limited number of pre-determined settings, you’ll find that drives are referred as ‘adjustable’, whereas if a device’s speed or force output can be altered by means of an adjustment that doesn’t skip points in a range, the drive can correctly be referred to as ‘variable speed’.
There are several varieties of electric speed control drive, among them eddy current drives, DC motor drives, and ac motor speed control drives. The term ‘electric drive’ should be taken to encompass both a speed control system or unit and the electric motor it controls, though of course the term is often used to refer just to the controller and not the motor. Back in the day, electromechanical control systems were standard, but these were displaced by vacuum tube controllers, which in turn were supplanted by the superior accuracy and size of solid state electronic components.
Now, given the fact that DC motor speeds are inversely proportional to field current while being directly proportional to armature voltage, varying one of these factors is the ideal way to control motor speed.
AC motor speed control, on the other hand, was, in the past, often achieved by means of a slip controlled drive. Such drives work by either decreasing the voltage of electricity sent to the motor, or raising the resistance provided by the rotor windings. These drives are, nowadays, only used in certain situations, having lost favor with engineers due to their poor efficiency when compared to other drives.
Rather, the most popular AC motor speed controller these days is an Adjustable Frequency Drive, or AFD, also sometimes referred to as a Variable Frequency Drive, or VFD. These devices provide their control over synchronous or induction motors by way of increasing or decreasing the frequency of power provided to the motor.
When increasing or decreasing the frequency of the electrical power being channeled into an AC motor, typically speaking an AC motor speed control will maintain the ratio of the applied voltage to the applied frequency at a constant value, which will obviously fall somewhere between the minimum and maximum operational frequencies of the device. This is because operation at a constant voltage above a given frequency reduces torque and constant power capability above that frequency. This is why the devices will constantly vary the applied voltage to maintain this golden ratio.
For those looking to build their own ac motor speed control system, you’ll be happy to know that AC motor speed controller kits offering high torque, even at low RPM, can come as cheap as forty dollars (check out electronickits.com).
