Full-voltage, Single-speed Motor Starters

Single-speed squirrel-cage motors have starters that fall into two categories: full-voltage or across-the-line starters; and reduced-voltage starters.

Full-voltage starters (manual and magnetic) apply full voltage directly to motor terminals. Two other types, combination and reversing starters, consist of a starter, usually magnetic, with added functions.

Reduce-voltage, Single-speed Motor Starters

Some machines or loads may require a gentle start and smooth acceleration up to full speed. In addition to load demands, power company regulations may limit the current surge or voltage fluctuation that can be imposed on the power supply during motor starting.

Many starters apply reduced voltage to motor windings; primary resistor, primary reactor, autotransformer, and solid state. Part winding and wye-delta starters can also provide reduced-voltage starting, although technically they are not reduced-voltage starters.

Multispeed Motor Starters

Motor windings in multispeed squirrel-cage motors may require special starters.

Starters for separate-winding two-speed motors consist of two standard three-pole starter units that are electrically and mechanically interlocked and mounted in a single enclosure. Additional units can be used for each speed. Although these are always electrically interlocked, it may not be practical to provide mechanical interlocks on more than two starters.

The starter for a consequent-pole two-speed motor requires a three-pole unit and a five-pole unit. The design of the particular motor winding determines whether the fast or slow-speed connection is made by the five-pole unit.

For three-speed consequent-pole motors, a three-pole starter is used for the single-speed winding; a five-pole starter and a second three-pole starter handle the reconnectable winding. A four-speed consequent-pole motor requires two sets of three and five-pole starters.

Different power circuits are needed for delta-type multispeed motors, because currents circulate within the inactive or unconnected winding. A pair of four-pole starters is required for a two-speed motor with separate open-delta windings. Another four-pole starter is required for each speed. Thus, three and four-speed motors with open-delta windings require very complex starters.

Specific winding information is used to select the motor controls. Torque characteristics also deserve special attention to ensure selection of the proper control. Constant-horsepower motors require larger starters than either constant-torque or variable-torque motors of equal horsepower. Reversing and reduced-voltage operations can be incorporated in a multispeed motor starter.

Synchronous Motor Controls

Controllers for synchronous motors have four components: a three-pole starter for the ac stator circuit, a contactor for the dc field circuit, an automatic synchronizing device to control the dc field contactor, and a cage-winding protective relay to open the ac circuit if the motor operates too long without synchronizing.

Synchronous motors require ac power during both starting and running, thus the main contactor is closed when the motor is operating. The dc winding of the rotor is energized by the field contactor as the motor approaches synchronous speed. Two normally open poles on the field contactor make the connection for dc excitation, and one normally closed pole permits dissipation of induced field current (through a resistor) during any period of nonsynchronous operation.

For smooth synchronization, two conditions determine the instant that the dc field is energized: the rotor must be turning at the proper speed — usually 93 to 98% of synchronous speed, and the rotor poles must be lagging slightly behind stator poles of opposite polarity. Several synchronizing devices apply dc to the rotor field. Special relay systems can automatically close the dc contactor when the rotor reaches the proper speed and the rotor and stator poles are in proper relationship.

These relays will open if the motor falls out of step because of a momentary overload or voltage dip. Synchronous operation is automatically restored when voltage returns to normal or the overload is removed. In some cases, resynchronization may not be desirable from a safety standpoint. In such cases, the controller can be designed to disconnect power from the stator.

Out-of-step protection is provided by a conventional thermal-overload device (OSP). This device is energized whenever the motor is running without rotor field excitation. If, during the start, the motor does not synchronize within a given time period (usually 15 to 20 sec) the OSP relay opens the main contactor. If the motor pulls out of step while running and does not resynchronize within the specified time, the relay will disconnect the motor. This protection is necessary because the cage winding has limited thermal capacity and will overheat in a short time at subsynchronous speed. Synchronous motors can be stopped quickly by methods similar to those used with induction motors.

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