Table 7-1. - Power and Control Connections for Across-the-Line Motor Controllers/Starters
|DIRECT CURRENT||SINGLE PHASE||THREE PHASE|
|Line markings for...........||L1 & L2||L1 & L2||L1, L2, & L3|
|Overload relay heaters in............||L1||L1||T1, T2, & T3|
|Contactor coil connected to ...........||L2||L2||L2|
|Overload relay contacts in .............||L2||L2||L2|
|Control circuits connected to .............||L1 & L2||L1 & L2||L1 & L2|
|Control circuit switching connected to......||L1||L1||L1|
|Reversing interchange lines..............||N/A||N/A||L1 & L3|
|Requiring grounding....................||L1 is always ungrounded||L1 is always ungrounded||L2|
Figure 7-15. - Two-wire control circuit.
THREE-WIRE CONTROL. - The three-wire control involves the use of a maintaining circuit. This method eliminates the need for the operator to press continuously on the push button to keep the coil energized. Refer to the elementary control circuit diagram in figure 7-16. When the START button is pressed, coil M is energized across L1 and L2. This action closes contact M to place a shunt circuit around terminals 2 and 3. the START button. A parallel circuit is formed with one circuit through push-button terminals 2 and 3 and one circuit through contact M. As a result. current will flow through the M coil. If pressure is removed from the START button, terminals 2 and 3 open. The other circuit through contacts M remains closed. supplying current to coil M and maintaining a started-closed position. Such a circuit is called a maintaining circuit: a sealing circuit, or a holding circuit.
The phrases no-voltage protection and three- wire control should indicate to the electrician that the most common means of providing this type of control is a start-stop push-button station.
The main distinction between the two types of control is that in no-voltage release (two-wire control), the coil circuit is maintained through the pilot-switch contacts; in no-voltage protection (three-wire control), the circuit is maintained through a stop contact on the push-button station and an auxiliary (maintaining) contact on the starter.
Figure 7-16. - Three-wire control circuit.Continue Reading