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Refer to the circuit shown in Figure 10–6. Assume that when the thermostat contact closes the compressor does not start, but the condenser fan and evaporator fan do start. When troubleshooting the circuit, a voltmeter connected across the compressor contactor coil CC indicates 0 volts. A voltmeter connected across the high-pressure switch indicates 0 volts, and a voltmeter placed across the thermostat contact indicates 0 volts. When the voltmeter is connected across the low-pressure switch it indicates 24 volts. Which of the following conditions could cause this problem? a. The refrigerant pressure is too high. b. The compressor contactor coil is open. c. The refrigerant pressure is too low. d. The thermostat contact is open. Fig. 10–6 Pressure switches are often used in air-conditioning circuits.

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Electric Motor Control

10th Edition
Herman
Publisher: CENGAGE L
ISBN: 9781133702818

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Electric Motor Control

10th Edition
Herman
Publisher: CENGAGE L
ISBN: 9781133702818
Chapter 10, Problem 9SQ
Textbook Problem
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Refer to the circuit shown in Figure 10–6. Assume that when the thermostat contact closes the compressor does not start, but the condenser fan and evaporator fan do start. When troubleshooting the circuit, a voltmeter connected across the compressor contactor coil CC indicates 0 volts. A voltmeter connected across the high-pressure switch indicates 0 volts, and a voltmeter placed across the thermostat contact indicates 0 volts. When the voltmeter is connected across the low-pressure switch it indicates 24 volts. Which of the following conditions could cause this problem?

a. The refrigerant pressure is too high.

b. The compressor contactor coil is open.

c. The refrigerant pressure is too low.

d. The thermostat contact is open.

Chapter 10, Problem 9SQ, Refer to the circuit shown in Figure 106. Assume that when the thermostat contact closes the

Fig. 10–6 Pressure switches are often used in air-conditioning circuits.

To determine

Find the option which is the cause for the given problem.

Explanation of Solution

Description:

Refer to Figure 10-6 in the textbook for the air conditioning circuits.

The thermostat is connected to the secondary coil of the control transformer, followed by three parallel-connected coils such as CC, CF, EF and three series-connected normally closed overload relay contactors such as CCOL, CFOL, and EFOL. The low pressure and high pressure switches are connected in series along with coil CC, which are used to protect the compressor when the pressure is greater or lesser than normal pressure.

Option a:

If the pressure of the refrigerant is too high, the high pressure switch will get opened and the control circuit will get opened. Due to this condition, there is no further current flow in the circuit. Whenever a switch is opened, there will be a potential difference (24 V) across it, and the current and potential difference in all the other components will be zero.

However, in the required condition, there is no voltage indication in the voltmeter, which is connected across the high pressure switch. Hence, ‘option a’ is incorrect.

Option b:

If the coil of the compressor contactor gets open, there is no further current in the control circuit. The voltage across the CC coil is 24 V and the voltage across all the other components is zero.

Therefore, the voltage across the high pressure switch is 24 V and the voltage across all the other components is zero...

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