Objectives
After studying this unit, the student will be able to:
• Explain the purpose of the various interlocking methods
• Read and interpret wiring and line diagrams of reversing controls
• Read and interpret wiring and line diagrams of interlocking controls
• Wire and troubleshoot reversing and interlocking controls
The direction of rotation of three-phase mo tors can be reversed by interchanging any two mo tor leads to the line. If magnetic control devices are to be used, then reversing starters accomplish the reversal of the motor direction, figure 39-IA. Reversing starters wired to NEMA standards in terchange lines Ll and L3, figure 39-IB. To do this, two contactors for the starter assembly are required-one for the forward direction and one for the reverse direction, figure 39-IC. A tech nique called interlocking is used to prevent the con tactors from being energized simultaneously or closing together and causing a short circuit. There are three basic methods of interlocking.
MECHANICAL INTERLOCK
A mechanical interlocking device is assem bled at the factory between the forward and re verse contactors. This interlock locks out one con tactor at the beginning of the stroke of either contactor to prevent short circuits and burnouts.
The mechanical interlock between the contac tors is represented in the elementary diagram of figure 39-2 by the broken line between the coils. The broken line indicates that coils F and R can not close contacts simultaneously because of the mechanical interlocking action of the device.
When the forward contactor coil (F) is ener gized and closed through the forward push but ton, the mechanical interlock prevents the acci dental closing of coil R. Starter F is blocked by coil R in the same manner. The first coil to close moves a lever to a position that prevents the other coil from closing its contacts when it is energized. If an oversight allows the second coil to remain energized without closing its contacts, the excess current in the coil due to the lack of the proper
inductive reactance will damage the coil.
Note in the elementary diagram of figure 39-2 that the stop button must be pushed before the motor can be reversed.
Reversing starters are available in horizontal and vertical construction. A vertical starter is shown in figure 39-3A.
jority of reversing starters in addition to the use of one or both of the following electrical methods: push-button interlock and auxiliary contact inter lock.
PUSH-BUTTON INTERLOCK
Push-button interlocking is an electrical method of preventing both starter coils from being energized simultaneously.
When the forward button in figure 39-3B is pressed, coil F is energized and the normally open (NO) contact F closes to hold in the forward con tactor. Because the normally closed (NC) contacts are used in the forward and reverse push-button units, there is no need to press the stop button before changing the direction of rotation. If the reverse button is pressed while the motor is run ning in the forward direction, the forward control circuit is de-energized and the reverse contactor is energized and held closed.
Repeated reversals of the direction of motor rotation are not recommended. Such reversals may cause the overload relays and starting fuses to over heat; this disconnects the motor from the circuit. The driven machine may be damaged also. It may be necessary to wait until the motor has coasted to a standstill.
NEMA specifications call for a starter to be derated. That is, the next size larger starter must be selected when it is to be used for “plugging” to stop, or “reversing” at a rate of more than five times per minute.
Reversing starters consisting of mechanical and electrical interlocked devices are preferred for maximum safety.
AUXILIARY CONTACT INTERLOCK
Another method of electrical interlock con sists of normally closed auxiliary contacts on the forward and reverse contactors of a reversing starter, figure 39-4.
When the motor is running forward, an NC contact (F) on the forward contactor opens and prevents the reverse contactor from being ener gized by mistake and by closing. The same oper ation occurs if the motor is running in reverse.
The term, interlocking, is also used generally when referring to motor controllers and control stations that are interconnected to provide control of production operations.
To reverse the direction of rotation of single phase motors, either the starting or running wind ing motor leads are interchanged, but not both. Figure 39-SA completes the wiring diagram for the single-phase, four-wire, split phase induction motor; figure 39-SB is a wiring diagram for a sin gle-phase vertical starter; and figure 39-SC is a line diagram of the connections.
REVIEW QUESTIONS
1. How is a change in the direction of rotation of a three-phase motor accomplished?
2. What is the purpose of interlocking?
3. What will happen if both start buttons are pushed in a control with push button interlocking? Why?
4. How is auxiliary contact interlocking obtained on a reversing starter?
5. When the forward coil is energized, in what position is the forward inter lock (F)?
6. If a mechanical interlock is the only means of interlocking used, describe the operation that must be followed to reverse the direction of rotation of the motor while running.
7. If pilot lights are to indicate the direction of rotation of a motor, where should the devices be connected so as not to add any contacts?
8. What is the sequence of the operations if limit switches are used in figure 39-4?
9. What will happen in figure 39-4 if limit switches are installed and the jumpers from terminals 6 and 7 to the coils are not removed?
10. In place of the push buttons in figure 39-2, draw a selector switch for forward reverse-stop control. Show the target table for this selector switch.
11. From the elementary drawing in figure 39-6, determine the number and terminal identification of the wiring in each conduit in the conduit layout. Indicate your solutions in the same manner as the example given below the disconnect switch.
12. Convert the control circuit only, figure 39-7, from the wiring diagram to an elementary diagram. Include the limit switches (RLS, FLS) as operating in the control circuit.