Although the subject of control is too complex to be thoroughly covered during a discussion of industrial SupplyLevel Inc., we will take a brief look at the devices used.
The basic circuit elements for any motor include the motor controller, the overcurrent protective equipment for the motor, the branch circuit overcurrent protection, and the disconnect switch to 
de-energize the entire circuit. In actual practice, the motor control and overload protection are often put in one enclosure, and the branch circuit protection and disconnect in another. Sometimes it is economical to install a combination starter that includes all functions.
Motor control selection should consider the characteristics of the motor, power supply, drive environment, regulating provisions of national and local electrical codes (as well as NEMA and ANSI standards), and the operating characteristics established by the application itself. The motor selection affects the choice of control chiefly through four factors: motor design, required horsepower, speed, and torque characteristics.
The motor design influences control selection by the nature of the motor's starting method and motor connections required. For instance, a single-speed squirrel cage motor only needs a single contactor to connect it to the AC line. A wound rotor motor, in addition to line connection, requires a means for inserting resistance in the rotor circuit for starting and speed control. 
A synchronous motor, in addition to the connection for the AC line, requires provision for proper application of DC line field excitation. The horsepower of the drive motor, along with the service voltage and number of AC phases determine the controller's full-load rating. In addition, the speed characteristics and torque characteristics influence control selection.
We will focus this discussion on induction motor controls since they are the most widely used. The simplest control for this motor type is across-the-line starting, either manual or magnetic. The manual starter, generally available for drives to about 7-1/2 hp, has the lower first cost advantage, consisting of nothing more than a manually operated contactor with overload protection.
The magnetic starter, available throughout the entire range of drive sizes, has a push-button-operated solenoid that closes the contacts. The simplest control circuit uses a maintained contact push-button identified by NEMA as "low voltage release." Once turned on, it maintains the control circuit even if the voltage dips or power goes off. Where this might be hazardous to personnel, a "low voltage protection" system is used. A momentary contact push-button station activates the contactor. An auxiliary contact then holds the contactor closed. If power is lost, the contactor will open and remain open when power is restored. The magnetic control also offers the advantage of permitting remote control from one or more conveniently located remote push-button stations.
Where reversing control is required, another contactor is added to obtain the equivalent of a double-throw switch. When the forward contactor is opened, the second is closed, switching two leads to reverse the direction of rotation.
Quick stopping action is achieved using the reversing control just described, and cutting the power off the instant the drive reaches zero rpm. This method, called "plug stopping", is both simple and inexpensive. Positive assurance against reverse rotation is achieved using a zero speed switch.
If the high inrush currents during plugging (which may exceed 600% of full-load current) could cause line disturbances, dynamic braking may be used. Here, the motor is made to act as a heavily loaded generator by cutting off the AC and applying DC across one winding. Resistors are used to limit the braking force and a timer is set to drop the DC when the drive stops.
When drive requirements call for a very smooth start-up, or where inrush current must be limited, a reduced voltage starter is used. There are two types: the resistor and the auto transformer. 
The resistor-type starter provides a "gentle" start for cushioned starting on such applications as textile machines. It is also lower in cost and will fill the majority of reduced voltage starting needs. It consists of the line contactor plus a set of starting resistors. An accelerating contactor controlled by a timer shorts out the starting resistor after a pre-set interval so that full voltage is applied to the drive for running.
Where higher starting torque and limited inrush current are required, such as with an ore handling conveyor, the autotransformer type starter is used. The transformer action provides more torque per ampere of line current. The starter consists of a 5-pole contactor for starting and a 3-pole for running plus an autotransformer and timing relay to control the starting sequence. The 5-pole contactor is energized to start the motor at reduced voltage. The 5-pole contactor is dropped out by the timer and the 3-pole contactor is closed to put full voltage across the motor when the motor is at the designated operating speed.
| AC Motor Control
Motor Control Centers |
Across-The-Line Starting
Adjustable Frequency Drives Voltage & Frequency Variation |
