Across the Line Starting

Objectives

After studying this unit, the student will be able to:

• Describe across-the-line starting for small de motors

• State why a current-limiting resistor may be used in the starting circuit for a de motor

• Connect across-the-line starters used with small de motors

• Recommend troubleshooting solutions for across-the-line starters

• Draw diagrams for three motor starter control circuits

Small de motors can be connected directly across the line for starting because a small amount of friction and inertia is overcome quickly in gain ing full speed and developing a counter emf. Frac tional horsepower manual starters (discussed in unit 12) or magnetic contactors and starters (unit

13) are used for across-the-line starting of small de motors, figure 44-1.

Magnetic across-the-line control of small de motors is similar to ac control or to two- or three wire control. Some de across-the-line starter coils have dual windings because of the added load of multiple break contacts and the fact that the decircuit lacks the inductive reactance which is present with ac electromagnets. Both windings are used to lift and close the contacts, but only one winding remains in the holding position. The starting (or lifting) winding of the coil is designed for momentary duty only. In figure 44-2, assume that coil M is energized momentarily by the start button. When the starter is closed, it maintains itself through the normally open maintaining con tact (M) and the upper winding of the coil since the normally closed contact (M) is now open. Power contacts M close, and the motor starts across the full line voltage. The double-break power contacts are designed to minimize the effects of arcing. (De arcs are greater than those due to alternating current.)

Figure 44-3 shows another control method used to start a de motor. In this method, a current-limiting resistor is provided to prevent coil burnout. It is used to limit a continuous duty cur rent flow to some coils or when the coils are over heating.

The coil first receives the maximum current required to close the starter. It then receives the minimum current necessary to hold in the contacts and for continuous duty through the current-lim iting resistor.