In this article, we will learn why motor control is required in control systems and the types of motor speed control methods.
All electric motors require a control system. That control may be as simple as an ON/OFF switch, such as for an exhaust fan. Or, the operation may be so complex that a computer must be used in the control system, such as in an automobile assembly plant application.
Both the exhaust fan and assembly plant electric motors are provided with start and stop control by their control systems. But the difference between their control systems is how they provide that control.
In addition to the start/stop control, a motor control system may also provide motor overload protection as well as motor speed and torque regulation.
There is a very comprehensive line of control products such as motor starters, contactors, switches, disconnects, etc. These products are used in applications that include on/off controls with a jog and reverse capabilities for pumps, compressors, fans, conveyors, meat cutters, textile looms, and wood and metalworking machines, just to name a few.
Sometimes these types of control devices (switches, contactors, etc.), are called “across-the-line” starters. This is because full voltage and current are applied directly to the motor.
An AC motor when switched ON like this tends to run at its maximum rated speed and torque. For many applications, this is a perfectly acceptable situation.
But there can be problems:
Because of one or more of these concerns, over the years, various methods have been tried to exert some control over motors.
Some of the types of adjustable speed control that have been used are:
Mechanical methods involve using devices such as brakes, clutches, and gearing to control motor speed. As you can imagine, these methods are not particularly efficient.
Physically the configuration includes a torque converter. This method is not used widely today. It has infinite speed control and moderate cost, but it has complex installation and high maintenance.
A combination of mechanical and electrical controls is used. This method of control has fallen from the current favorite because it is very inefficient. It is simple and moderate cost, easy to maintain.
But it has discrete incremental control only. It uses wound-rotor motors which are usually non-stock, creating higher expenses and more maintenance problems.
A magnetic clutch is used to adjust motor output speed in infinite increments. It is a simple idea, and it works.
However, it is very inefficient and a lot of energy is lost through heat. It is simple, but has poor efficiency and requires cooling, either by water or air.
Direct electronic controls are used most often to control speed. It has the most efficient speed control, low maintenance, and is the most flexible.
It can be expensive at initial purchases but saves money over time. Energy savings and reduced wear and tear on machinery will quickly repay the initial investment.
In this way, we understand why motor control is required in control systems.
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