Speed Control
Speed Control
How a DC Magnetic Motor Control Works
As manufacturing and industrial facilities evolved over the years, so have the ways we power the machinery. By power, I am referring to the controlling or initiating motion to perform a process. The key here is the industrial motors themselves. Whether they're small, medium, or large, motors need to be controlled. They need to be started, stopped, and varied in overall speed for safety and also to properly perform their selected function. A motor rotating at unsafe speeds can be hazardous to personnel and dangerous to the equipment to which they are connected. The motor controller comes into play to do just that. Control of the startup as well as the acceleration to an appropriate speed, then the monitoring of the motor to ensure it is operating within its power rating, and of course the stoppage of the motor. For decades, a magnetic dc motor control was the most efficient way to get the job done. These sometimes complex circuits composed of relays, contactors, timers, and resistors could be found anywhere there was an industrial electric motor. At the time, they were new technology replacing drum controllers which used the human element to control a motor's acceleration. on these controllers, an operator had the responsibility of turning on the motor and bringing it up to its proper speed using a handle attached to a drum of contacts. The faster the operator turned the handle, the faster the motor accelerated. Operating speed for the motor could also be controlled using the handle by stopping at a certain position short of full deflection. Motors could also be reversed using these controls by turning the handle in the opposite direction. Drum controllers relied too much on an operator's gentle touch to be efficient and safe. The dc magnetic controller easily became the accepted method of motor control in its time.
The controlled acceleration of a dc motor and its controlled top speed made these controllers ideal for industrial machinery. The names Cutler Hammer, Westinghouse, Allen Bradley, and General Electric were synonamous with motor control. They all consisted of similar circuitry but various manufacturers had their own improvements and idiocyncrasies. The motor is usually started and stopped from a normally open and normally closed push button assembly. This controls a relay typically labeled CR, for control relay. The control circuit was also interfaced with overload and overtemp contacts for protection of the motor, the machinery, and human personnel. An M contactor indicates a main contactor. These dc contactors are designed with large current carrying contacts because they are responsible for applying and disconnecting the main circuit for the armature. Once the control circuit is energized, the accelerating of the motor is initiated using a series of resistors and contactors. These contactors are typically labeled 1A, 2A, 3A, and so forth. Accelerating contacts are opened and closed based on the armature current draw in some controllers and by timers in others. Another contactor called the FA contactor, or field accelerating contactor, remained closed during the acceleration of the motor. This contact assures that full power is applied to the shunt field of the motor until it is operating at a constant speed. It imay also be called the FF contactor, or full field contactor by some manufacturers. Once the motor has achieved its appropriate speed, the FA or FF contactor would open and speed control of the motor would be handed over to a rheostat. The rheostat would be in series with the shunt field. By varying the current flow through the shunt field, motors could be regulated for speed. Some forms of protection in these motor starters were added in case of motor winding failure or excessive mechanical loading. The FL contactor, or field loss contactor was typically designed with line coil in series with the shunt field. An open in the shunt field circuit would cause the field loss contactor to open and disable the control circuit acting similar to pressing a stop button. The other form of protection would be and overload circuit. The OL contactor or the OLX contactor were used to monitor an overload condition. These contactors also would act as similar to pressing a stop button. an overload typically senses too much current flow through the armature of the motor caused by internal motor winding shorts and opens, motor brush failure, a mechanical problem due to worn motor bearings, or a mechanical failure in the equipment to which the motor is coupled.
Other optional items added to these magnetic motor starters were components like external current meters for personnel to observe. A load meter is a good example of current monitoring modified to display the load on the motor in real time to the operator. There were also reversing options which enabled the direction of the motor to be changed with a switch or by turning a mechanical handle. With a familiarity of magnetic dc motor starters and basic electrical skills, troubleshooting the control circuits of any of these manufacturers becomes easier with experience because the concept and basic schematics from Cutler Hammer, GE, Allen Bradley, and Westinghouse were always similar.
About the Author
Chet is an associate of Obsolete Industrial Parts, an informative site dedicated to locating obsolete parts for industrial equipment including dc magnetic motor controls, servo motors, obsolete plc's, switches, and a host of other parts. Their official website obsoleteindustrial.com is owned and operated by Val Marketing of Carbondale, PA.
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