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