How a DC Magnetic Motor Driver Works – Basic Troubleshooting of an Older DC Motor Starter
As industrial and manufacturing facilities have evolved over the years, so have the ways in which we power machinery. By power, I mean the control or initiation movement to perform a process. The key here is the industrial engines themselves. Whether they are small, medium or large motors, it is necessary to control them. They must be started, stopped, and vary the overall speed for safety and also to properly perform the selected function. A motor rotating at unsafe speeds can be dangerous to personnel and the equipment to which it is connected. The motor controller comes into play to do just that. Starting control as well as acceleration to a suitable speed, then monitoring the engine to ensure that it is operating within its nominal power and of course stopping the engine.
For decades, a magnetic DC motor control was the most efficient way to get the job done. These sometimes complex circuits made up of relays, contactors, timers, and resistors can be found anywhere there is an industrial electric motor. At the time, they were a new technology that replaced drum controllers that used the human element to control the acceleration of a motor. in these controllers, an operator had the responsibility of starting 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 engine revved. The running speed of the motor could also be controlled using the handle by stopping at a certain position before full deflection. The motors can also be reversed using these controls by turning the handle in the opposite direction. Drum controllers relied too heavily on the gentle touch of an operator to be efficient and safe. The DC magnetic drive easily became the accepted method of motor control in its day.
The controlled acceleration of a DC motor and its controlled maximum speed made these controllers ideal for industrial machinery. The names Cutler Hammer, Westinghouse, Allen Bradley, and General Electric were synonymous with motor control. They all consisted of similar circuitry, but various manufacturers had their own improvements and idiosyncrasies. 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 over-temperature contacts for protection of the motor, machinery, and human personnel. A contactor M indicates a main contactor. These DC contactors are designed with large current-carrying contacts because they are responsible for making and breaking the main armature circuit. Once the control circuit is energized, acceleration of the motor is initiated using a series of resistors and contactors. These contactors are usually labeled 1A, 2A, 3A, etc. Acceleration contacts open and close based on armature current draw in some controllers and by timers in others.
Another contactor called the FA contactor, or field throttle contactor, remained closed during motor acceleration. This contact ensures that full power is applied to the bypass field of the motor until it is running at a constant speed. Some manufacturers may also call it an FF contactor or a full field contactor. Once the motor has reached its proper speed, either the FA or FF contactor would open and the speed control of the motor would pass to a rheostat. The rheostat would be in series with the shunt field. By varying the current flow through the shunt field, the motors could be speed regulated. Some forms of protection have been added in these motor starters in case of motor winding failure or excessive mechanical load. The FL contactor, or loss-of-field contactor, was typically designed with a line coil in series with the bypass field. An open circuit in the bypass field 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 an overload circuit. The OL contactor or OLX contactor was used to monitor an overload condition. These contactors would also act similar to pressing a stop button. An overload typically detects too much current flow through the motor armature caused by shorts and opens in the motor’s internal winding, motor brush failure, a mechanical problem due to worn motor bearings, or a mechanical failure in the equipment. to which the motor is attached.
Other optional items added to these magnetic motor starters were components such as 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 that allowed the direction of the motor to be changed with a switch or by turning a mechanical handle. With a familiarity with magnetic DC motor starters and basic electrical skills, troubleshooting control circuits from any of these manufacturers becomes easier with experience because the basic concept and schematics of Cutler Hammer, GE, Allen Bradley and Westinghouse were always alike.