Tuesday, January 13, 2015

Control Circuits : Wound-rotor motor control.

WOUND-ROTOR MOTOR CONTROL

An automatic time-operated speed control for a wound-rotor motor is shown in Figure 19–20. In this circuit, the stator of the motor is connected to the line when M load contacts close. Resistors are connected to the rotor of the motor during starting. When the motor is first started, all resistors are connected in series with the rotor. It will be assumed that all timers have been set for a delay of 10 seconds.

When the start button is pushed, a circuit is completed through M coil and TR1 coil. When M contacts close, the stator of the motor is connected to the line, and M auxiliary contact is used as a holding contact. Ten seconds after TR1 coil is energized, TR1 contact closes. This energizes S1 coil and TR2 coil. When S1 coil energizes, S1 contacts close and short out the first set of resistors. This permits the motor to increase to the second step of speed.

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Ten seconds after coil TR2 is energized, contact TR2 closes and energizes coils S2 and TR3 . When S2 energizes, S2 contacts close and short out the second set of resistors. This permits the motor to increase to the third step of speed.

Ten seconds after coil TR3 energizes, contact TR3 closes and energizes coil S3 . This causes S3 contacts to close and short out the remaining resistance and the motor operates at its full speed.

SUMMARY

• The two basic types of control circuits are two-wire and three-wire.

• A schematic diagram shows components in their electrical sequence without respect to physical location.

• A wiring diagram shows components in their physical location with connecting wires.

• Control diagrams are read like a book, from top to bottom and from left to right.

• Control components are always shown in their normal deenergized position.

• Interlocking is used to keep some function or action from happening until some other function or action has taken place.

• Jogging is used to position a machine in some position by starting it with short jabs of power.

• On-delay relays delay changing their contacts when the coil is energized, but change back immediately when the coil is deenergized.

• Off-delay relays change their contacts immediately when they are energized but delay changing them back when they are deenergized.

Achievement Review

1. What are the two basic types of motor control?

2. Define a schematic diagram.

3. Define a wiring diagram.

4. Determine how components for the function of stop are generally wired and connected with the stop button.

5. Determine how components used for the function of start are generally wired and connected with the start button.

6. When reading a schematic diagram, are the components shown in their energized or deenergized position?

7. What does a dashed line drawn between components represent?

8. What is an auxiliary contact?

9. Explain the operation of an on-delay timer.

Explain the operation of an off-delay time