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GOALS:
- Discuss cautions that must be observed in reversing circuits.
- Explain how to reverse a three-phase motor.
- Discuss interlocking methods.
- Connect a forward-reverse motor control circuit.
The direction of rotation of any three-phase motor can be reversed by changing any two motor T leads (Ill. 1). Since the motor is connected to the power line regardless of which direction it operates, a separate contactor is needed for each direction. If the reversing starters adhere to NEMA standards, T leads 1 and 3 will be changed (Ill. 2). Since only one motor is in operation, however, only one overload relay is needed to protect the motor. True reversing controllers contain two separate contactors and one over load relay. Some reversing starters will use one separate contactor and a starter with a built-in overload relay.
Others use two separate contactors and a separate over load relay. A vertical reversing starter with overload re lay is shown in Ill. 3, and a horizontal reversing starter without overload relay is shown in Ill. 4.
Ill. 1 The direction of rotation of any three-phase motor can be changed
by reversing connection to any two motor T leads.
Interlocking
Interlocking prevents some action from taking place until some other action has been performed. In the case of reversing starters, interlocking is used to prevent both contactors from being energized at the same time.
This would result in two of the three phase lines being shorted together. Interlocking forces one contactor to be de-energized before the other one can be energized.
There are three methods that can be employed to assure interlocking. Many reversing controls use all three.
Mechanical Interlocking
Most reversing controllers contain mechanical interlocks as well as electrical interlocks. Mechanical interlocking is accomplished by using the contactors to operate a mechanical lever that prevents the other contactor from closing while one is energized. Mechanical interlocks are supplied by the manufacturer and are built into reversing starters. In a schematic diagram, mechanical interlocks are shown as dashed lines from each coil joining at a solid line (Ill. 5).
Electrical Interlocking
Two methods of electrical interlocking are avail able. One method is accomplished with the use of double acting push buttons (Ill. 6). The dashed lines drawn between the push buttons indicate that they are mechanically connected. Both push buttons will pushed at the same time. The normally closed part of the FORWARD push button is connected in series with R coil, and the normally closed part of the REVERSE push button is connected in series with F coil. If the motor should be running in the forward direction and the REVERSE push button is pressed, the normally closed part of the push button will open and disconnect F coil from the line before the normally open part closes to energize R coil. The normally closed section of either push button has the same effect on the circuit as pressing the STOP button.
The second method of electrical interlocking is accomplished by connecting the normally closed auxiliary contacts on one contactor in series with the coil of the other contactor (Ill. 7). Assume that the FORWARD push button is pressed and F coil energizes. This causes all F contacts to change position.
The three F load contacts close and connect the motor to the line. The normally open F auxiliary contact closes to maintain the circuit when the FORWARD push button is released, and the normally closed F auxiliary contact connected in series with R coil opens (Ill. 8).
If the opposite direction of rotation is desired, the STOP button must be pressed first. If the REVERSE push button were to be pressed first, the now open F auxiliary contact connected in series with R coil would prevent a complete circuit from being established.
Once the STOP button has been pressed, however, F coil de-energizes and all F contacts return to their normal position. The REVERSE push button can now be pressed to energize R coil (Ill. 9). When R coil energizes, all R contacts change position. The three R load contacts close and connect the motor to the line. Notice, however, that two of the motor T leads are connected to different lines. The normally closed R auxiliary contact opens to prevent the possibility of F coil being energized until R coil is de-energized.
Ill. 2 Magnetic reversing starters generally change T leads 1 and 3 to
reverse the motor.
Developing a Wiring Diagram
The same basic procedure is used to develop a wiring diagram from the schematic as was followed in the previous sections. The components needed to construct this circuit are shown in Ill. 10. In this example, assume that two contactors and a separate three-phase overload relay are to be used.
The first step is to place wire numbers on the schematic diagram. A suggested numbering sequence is shown in Ill. 11. The next step is to place the wire numbers beside the corresponding components of the wiring diagram (Ill. 12).
Reversing Single-Phase Split-Phase Motors
To reverse the direction of rotation of a single-phase split-phase motor, either the starting winding leads or running winding leads, but not both, are inter changed. A schematic diagram of a forward-reverse control for a single-phase split-phase motor is shown in Ill. 13. Notice that the control section is the same as that used for reversing three-phase motors. In this example, run winding lead T1 will always be connected to L1, and T4 will always be connected to L2.
The start winding leads, however, will be changed.
When the forward contactor is energized, start winding lead T5 will be connected to L1, and T8 will be connected to L2. When the reverse contactor is energized, start winding lead T5 will be connected to L2, and T8 will be connected to L1.
Ill. 3 Vertical reversing starter with overload relay.
Ill. 4 Horizontal reversing starter.
Ill. 5 Mechanical interlocks are indicated by dashed lines extending
from each coil.
Ill. 6 Interlocking with double acting push buttons.
Ill. 7 Electrical interlocking is also accomplished with normally closed
auxiliary contacts.
Ill. 8 Motor operating in the forward direction.
Ill. 9 Motor operating in the reverse direction.
Ill. 10 Components needed to construct a reversing control.
Ill. 11 Placing numbers on the schematic.
Ill. 12 Components needed to construct a reversing control circuit.
Ill. 13 Reversing a single-phase split-phase motor.
QUIZ:
1. How can the direction of rotation of a three-phase motor be changed?
2. What is interlocking?
3. Referring to the schematic shown in Ill. 7, how would the circuit operate if the normally closed R contact connected in series with F coil were connected normally open?
4. What would be the danger, if any, if the circuit were wired as stated in question 3?
5. How would the circuit operate if the normally closed auxiliary contacts were connected so that F contact was connected in series with F coil, and R contact was connected in series with R coil, Ill. 7?
6. Assume that the circuit shown in Ill. 7 were to be connected as shown in Ill. 14. In what way would the operation of the circuit be different, if at all?
Ill. 14 The position of the holding contacts has been changed from that
in Ill. 7.