Wednesday, January 7, 2015

starters and speed controllers : definite time controller, electronic controllers and summary of starters and speed controllers.

22–10 DEFINITE TIME CONTROLLER

This controller reduces starting resistance at a predetermined rate as the motor accelerates toward the desired speed. The contactors that close to accomplish this are controlled by either a small, motor-driven timer or by one of several types of magnetic timing devices. The schematic diagram, Figure 22–28, shows a definite time controller with two timing relays that accelerate the motor by sequentially cutting out resistors R1 and R2.

Here is how the circuit functions: Closing the start button completes the control circuit from line 1 to point 5, energizing relay coils M and TR1. This starts the motor turning by connecting the armature, through the two series resistors, to the line. Note that the field coil is connected directly to the line and is not affected by the series resistors.

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Meanwhile, the timing relay TR1 pulls the plunger up into the coil at a rate deter- mined by its time escapement mechanism. After a predetermined time, the plunger is pulled up as far as possible, closing the normally open contacts of TR1 (points 1 and 6). This action simultaneously energizes acceleration relay A1 and timing coil TR2. As a result, resistor R1 is cut out of the circuit, allowing the motor to accelerate. After a further time delay, TR2 times out and energizes the second acceleration relay A2.

The escapement mechanism in Figure 22–29, left, controls the time required for the solenoid coil to pull up the plunger. TR1 closes first, energizing relay coil A1. Then after the definite time interval, contactors TR2 close, energizing coil A2. The front and

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right side views in Figure 22–29 show the normally open TR contactors, which act as sealing contactors around the start button. Nearby are the normally closed contacts, which are connected across part of the TR solenoid coil.

The starting overload and running overload protection used with this controller functions practically the same as for the other types of automatic controllers previously described.

22–11 ELECTRONIC CONTROLLERS

In automated machine operations, DC motors are driven by electronically con- trolled rectifiers that get their power directly from an AC line. We have seen, in Section 21–8, how silicon-controlled rectifiers can be employed for speed control by sending triggering pulses to the gate. The implication was that some manual control is provided for the operator to monitor and regulate the performance of the machine. In automated equipment, the driven machine continuously feeds back information to a master control to maintain any reasonable combination of speed and torque desired; see Figure 22–30.

The program control is any system based on punch cards, magnetic tape, or micro- processors that causes the machine to follow a prescribed sequence of operations. These kinds of devices are faster acting and more reliable than electromechanical relays and/or timing devices. Such devices were developed in response to the demands of highly

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specialized, high-speed manufacturing processes. The electronics industry responded with modular solid-state devices.

One such system was known as static control and embodied circuits known as logic gates, such as AND gates, NOR gates, NOT gates, OFF RETURN MEMORY, and J-K flip flops.

With the development of microprocessors and computers, a new product was born: the programmable controller (PC). The PC is a solid-state device designed to perform the logic functions previously accomplished by electromechanical relays, drum switches, mechanical timers, and counters. Internally, there are still logic gates, but they have now been wedded with the graphic display of relay ladder logic, which is understood by all competent electricians.

To achieve such competencies, you will have to continue your studies and expand your knowledge in the field of industrial electronics.

SUMMARY

• DC motors have an extremely high starting current due to low armature resistance and low counter-emf.

• Starting rheostats are not intended for speed control.

• No-voltage release is a safety feature used to prevent the automatic restarting of a motor at the end of a power failure.

• Speed controllers are designed to accelerate a motor to normal speed and to vary the speed.

• For above-normal speeds, resistance is added to the shunt field.

• For below-normal speeds, the starting resistance is reinserted into the armature circuit.

• Series motors require a special type of starting rheostat.

• Series motor starters come with either no-voltage protection or no-load protection.

• Drum controllers are used where the motor is under direct control of an operator and when frequent starting, stopping, reversing, and varying of speeds are necessary.

• Review circuitry and sequence of operations for

a. The counter-emf controller

b. The voltage drop acceleration controller

c. The definite time controller

• Dynamic braking stops a motor by making it act as a generator. It converts its rotational energy to electrical energy and then to heat in a resistor.

• Electrical interlocking is a system for ensuring that one device is disconnected before an interfering or contradictory device is energized.

Achievement Review

1. Show the connections for a three-terminal manual starting rheostat connected to a shunt motor.

2. Give one advantage and one disadvantage of a three-terminal manual starting rheostat.

3. Show the connections for a four-terminal manual starting rheostat connected to a cumulative compound motor. Include a separate field rheostat in the shunt field circuit for speed control.

4. Give one advantage and one disadvantage of a four-terminal manual starting rheostat.

5. A three-terminal manual starting rheostat has a resistance of 5.2 ohms in its starting resistor. The holding coil resistance is 10 ohms. This starting rheostat is connected to a shunt motor. The resistance of the armature is 0.22 ohm; the resistance of the shunt field is 100 ohms. The line voltage for this motor circuit is 220 volts.

a. Determine the starting surge of current taken by the motor.

b. The motor has a full-load current rating of 30 amperes. National Fire Under- writers requires the starting surge of current to be not greater than 150% of a motor’s full-load current rating. Show with computations whether this manual starting rheostat complies with this requirement.

6. Using the data in question 5, determine

a. The current in the holding coil with the movable arm in the run position

b. The counter-electromotive force with the movable arm in the on position if the armature current is 20 amperes

7. Explain the difference between a manual starting rheostat and a manual speed controller.

8. Why does one type of manual starting rheostat used with series motors have no-load protection?

9. State the applications of the drum controller. Why is it desirable in these applications?

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10. Explain why a shunt motor’s direction of rotation does not change if the connections of the two wires are reversed.

11. What is the function of a holding coil in a manual starting rheostat?

12. Explain how to reverse the direction of rotation of

a. A shunt motor

b. A series motor

c. A cumulative compound motor

13. Complete the internal and external connections for the above-normal speed controller and cumulative compound motor shown on page 429.

14. Complete the internal and external connections for the above-and-below-normal speed controller and cumulative compound motor illustrated below.

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15. Draw the graphic symbols (JIC standards, see Figure A–12 in the Appendix) representing the following components:

a. N.O. limit switch

b. N.C. limit switch held open

c. N.O. timer contact (action retarded upon energizing)

d. N.C. pressure switch

e. N.O. float switch (for liquid level)

f. N.C. flow switch (for air or water)

g. N.O. contact with blowout

h. Thermal overload

16. A DC shunt motor is rated at 40 amps, 115 volts, 5 horsepower. Calculate the proper current rating for a thermal overload unit to install in a cemf controller to serve as proper running overload protection. Also, determine the size of the fuses that should be used as starting protection.

17. Complete the diagram below by making all connections necessary to put the motor across the line without a starting resistance. Provide for dynamic breaking when the motor is stopped.

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18. a. Which of the three types of controllers discussed in this chapter is represented by the drawing below?

b. Explain the sequence of operation.

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19. a. Which of the three types of controllers discussed in this chapter is repre- sented by the drawing below?

b. What adjustment could be made to change the acceleration speed of the motor?

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20. a. Which of the three types of controllers discussed in this chapter is repre- sented by the drawing below?

b. Does the voltage drop across the resistors increase or decrease when the motor accelerates? Explain.

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