CONTROLLER CAPABILITIES Each controller must be capable of starting and stopping the motor which it controls and, for an alternating-current motor, it must be capable of interrupting the stalled-rotor current of the motor.

Horsepower Ratings The controller must have a horsepower rating not lower than the horsepower rating of the motor.

Exceptions are indicated below. For a stationary motor rated at 1/ 8 horsepower or less, normally left running and so constructed that it cannot be damaged by overload or failure to start (such as clock motors), the branch-circuit overcurrent device may serve as the controller. For a stationary motor rated at 2 horsepower or less and 300 volts or less, the controller may be a general-use switch with an ampere rating of at least twice the full-load current rating of the motor. For a portable motor rated at 1/ 3 horsepower or less, the controller may be an attachment plug connector and receptacle. A branch-circuit circuit breaker, rated in amperes only, may be used as a controller. Branch-circuit conductors must have an amperage capacity (ampacity) not less than 125 percent of the motor full-load current rating.

Single Controller Serving a Group of Motors Each motor must have an individual controller, except for motors of 600 volts or less; a single controller can serve a group of motors under any one of the following conditions:

A number of motors drive several parts of a single machine or piece of apparatus, such as a metal and woodworking machine, crane, hoist, and similar apparatus.

A group of motors is under the protection of one overcurrent device. A group of motors is located in a single room within sight of the controller location.

Conductors supplying two or more motors must have an ampacity equal to the sum of the full-load current rating of all motors plus 25 percent of the highest rated motor in the group.

CONTROLLER MARKINGS Controllers are marked with the maker's name or identification, the voltage, the current or horsepower rating, and other data as may be needed to properly indicate the motors for which it is suitable. A controller that includes motor running overcurrent protection or is suitable for group motor application is marked with the motor running overcurrent protection and the maximum branch-circuit overcurrent protection for such applications. Be extremely careful about installing unmarked controllers into any circuit. Controllers should be properly marked.

CONTROLLER CIRCUITRY Before you condemn a motor, make sure that the fault does not lie within the controller. The only way to be sure the fault is not in the controller is to understand the circuitry of the controller. As previously mentioned, there are two general types of motor controllers: manual and magnetic.

Manual Controllers Manual controllers (motor starters) are available up to 7 1/ 2 horsepower at 600 volts (three-phase) and to 3 horsepower at 220 volts (single-phase). 

TOGGLE SWITCHES OR CIRCUIT BREAKERS. -A toggle switch or circuit breaker can serve as a controller, provided its ampere rating is at least twice the full-load current rating of the motor and the motor rating is 2 horsepower or less. It must be connected in a branch circuit with an overcurrent device that opens all ungrounded conductors to the switch or circuit breaker. These switches or circuit breakers may be air-brake devices operable directly by applying the hand to a lever or handle. An oil switch can be used on a circuit with a rating which does not exceed 600 volts or 100 amperes, or on a circuit exceeding this capacity, under expert supervision and by permission. A single-phase motor requires a one-element overload device,

Figure 7-50.- Across-the-line manual controller. while a polyphase motor requires a two-element overload device (fig. 7-50).

DISCONNECTS.- Disconnects may be used as controllers on motors rated up to 3 horsepower at 220 volts. They must be located within sight of the motor or be able to lock in the open position. A distance of more than 50 feet is considered "out of sight." Double-throw disconnects may be used for reversing three-phase motors if they conform to these requirements.

DRUM CONTROL.- The drum control is a lever-operated, three-position switch. The center position is usually the OFF position with the right and left positions FORWARD and REVERSE, respectively. Normally, it is used to direct the rotation of a three-phase motor. Oil-immersed drum switches are used wherever the air can become charged with corrosive gases or highly flammable dust or lint.

Magnetic Full-Voltage Starters Magnetic starters are made to handle motors from 2 to 50 horsepower. They can be controlled by a start-stop station located locally or remotely. The starter has two different circuits: the control circuit and the load circuit.

CONTROL CIRCUIT.- The control circuit receives its power from the incoming leads to the starter.

It is a series circuit (fig. 7-51) going through the start-stop station, the magnetic coil, the overload contacts, and returning to another phase. However, it may return to the ground, depending on the voltage rating of the coil. 

LOAD CIRCUIT.- The current flowing through the coil activates a mechanical lever and closes the main line contacts. This closing develops the load circuit and applies power to the motor. The fourth set of contacts provides a shunt around the start button, known as the holding circuit.

Figure 7-51.- Magnetic starter circuit.