Solid-state circuitry for air-conditioning units has been in use for some time. The following is an illustration of how some of the circuitry has been incorporated into the protection of compressor motors. This module is manufactured by Robertshaw Controls Co. of Milford, Connecticut.
Solid-state motor protection prevents motor damage caused by excessive temperature in the stator windings.
These solid-state devices provide excellent phase-leg protection by means of separate sensors for each phase winding. The principal advantage of this solid-state sys- tem is its speed and sensitivity to motor temperature and its automatic reset provision.
There are two major components to the protection system:
1. The protector sensors are embedded in the motor windings at the time the motor is manufactured.
2. The control module is a sealed enclosure containing a transformer and switch. Figure 3-47 shows two models.
Operation
Leads from the internal motor sensors are connected to the compressor terminals as shown in Fig. 3-48. Leads from the compressor terminals to the control module are connected as shown in Fig. 3-49. Figure 3-49A shows the older model and Fig. 3-49B the newer model. While the exact internal circuitry is quite com- plicated, basically the modules sense resistance change through the sensors as the result of motor-temperature changes in the motor windings. This resistance change triggers, the action of the control circuit relay at prede- termined opening and closing settings, which causes the line voltage circuit to the compressor to be broken and completed, respectively.
The modules are available for either 208/240- or 120-V circuits. The module is plainly marked as to the input voltage. The sensors operate at any of the stated because an internal transformer provides the proper power for the solid-state components.
The two terminals on the module marked “power supply” (T1 and T2) are connected to a power source of the proper voltage, normally the line terminals on the compressor motor contact, or the control-circuit transformer as required.
Fig. 3-46 A thermal actuator.
2. Connect a jumper wire across the control-circuit ter- minals on the terminal board. See Fig. 3-49. This will bypass the relay in the module. If the com- pressor will not operate with the jumper installed, then the problem is external to the solid-state protec- tion system. If the compressor operates with the module bypassed, but will not operate when the jumper wire is removed, then the control-circuit relay is open.
3. If, after allowing time for motor cooling, the protec- tor still remains open, the motor sensors may be checked as follows:
• Remove the wiring connections from the sensor and common terminals on the compressor board.
See Figs. 3-48 and 3-49.
• Warning. Use an ohmmeter with a 3-V maximum battery power supply. The sensors are sensitive and easily damaged, and no attempt should be made to check continuity through them. Any ex- ternal voltage or current applied to the sensors may cause damage, necessitating compressor re- placement.
• Measure the resistance from each sensor terminal to the common terminal. The resistance should be in the following range: 75 (cold) to 125 Ω(hot).
Resistance readings in this range indicate the sen- sors are good. A resistance approaching zero indi- cates a short. A resistance approaching infinity indicates an open connection. If the sensors are damaged, they cannot be repaired or replaced in the field, and the compressor must be replaced to restore motor protection.
Troubleshooting the Control
The solid-state module cannot be repaired in the field, and if the cover is opened or the module physically damaged, the warranty on the module is voided. No at- tempt should be made to adjust or repair this module, and if it becomes defective, it must be returned in- tact for replacement. This is the usual procedure for most solid-state units. However, if the unit becomes defective, you should be able to recognize that fact and replace it.
If the compressor motor is inoperable or is not op- erating properly, the solid-state control circuit may be checked as follows:
1. If the compressor has been operating and has tripped on the protector, allow the compressor to cool for at least 1 h before checking to allow time for the motor to cool and the control circuit to reset.
Electronic Compressor Motor Protection 89 (a) OLD
MP13 CONTROL MODULE
MP23 CONTROL MODULE
(b) NEW
Fig. 3-47 Solid-state control modules. (A) Older unit. (B) Newer unit.(Robertshaw)
S1 S2 S3 S4
Fig. 3-48 Compressor terminal board. (Robertshaw)
S1 S1
MOTOR SENSOR 1 SENSOR 2 SENSOR 3 JUMPER FOR AUTOMATIC
PUSH BUTTON FOR MANUAL RESET (OPTIONAL)
S2 S3
V. A.C S2
EMBEDDED IN WINDING
POWER SUPPLY SENSORS
CONTROL CIRCUIT
SENSOR COMMON
MANUAL RESET
CONTACTOR
(a)
M1 M2 C J1 J2
Fig. 3-49 (A) Solid-state control modules (Older unit wiring details). (Robertshaw) (B) Con- tinuing schematic for control modules (Newer unit wiring details). (Robertshaw)
SENSOR 1 MOTOR ORANGE LEADS
BLACK LEAD
T1 J2 J1 J2 M1M2
SENSOR 2 SENSOR 3
EMBEDDED IN WINDING
CONTACTOR
MODULE VOLTAGE SUPPLY
CONTROL CIRCUIT VOLTAGE SUPPLY
Note: Control is automatic reset when terminals J1 and J2 are not included. The control is manual reset when terminals J1 and J2 are included.
MOTOR VOLTAGE SUPPLY
(b)
degree of protection, it does provide a means of contin- uing compressor operation with a reasonable degree of safety.
REVIEW QUESTIONS
1. What is Ohm’s law?
2. Describe a parallel circuit.
3. What is the formula for finding resistance in a par- allel circuit?
4. What is the basic unit of measurement for electrical power?
5. What is a capacitor?
6. What is a dielectric?
7. What three factors determine the capacitance of a capacitor?
8. What is a microfarad?
9. What makes an electrolytic capacitor different from a standard paper-type?
10. What is the unit of measurement for inductance?
11. What is the symbol for an audio-frequency inductor?
12. What is inductive reactance?
13. What effect does the turns ratio have on the output voltage of a transformer?
If the sensors have proper resistance and the com- pressor will run with the control circuit bypassed, but will not run when connected properly, the solid-state module is defective and must be replaced. The replace- ment module must be the same voltage and made by the same manufacturer as the original module on the compressor.
Restoring Service
In the unlikely event that one sensor is damaged and has an open circuit, the control module will prevent compressor operation even though the motor may be in perfect condition. If such a situation should be encoun- tered in the field, as an emergency means of operating the compressor until such time as a replacement can be made, a properly sized resistor can be added between the terminal of the open sensor and the common sensor terminal in the compressor terminal box. See Figs. 3-48 and 3-50. This, then indicates to the control module an acceptable resistance in the damaged sensor circuit, and compressor operation can be restored. The emer- gency resistor should be a 2 W, 82-Ω, wire wound with a tolerance of ±5 percent.
In effect, the compressor will continue operation with two-leg protection rather than three-leg protection.
While this obviously does not provide the same high
Review Questions 91 POWER
SUPPLY
SENSORS
SENSORS SENSORS
CONTROL CIRCUIT
SENSOR COMMON
POWER SUPPLY
SENSORS CONTROL CIRCUIT
OPEN SENSOR CIRCUIT OPEN SENSOR CIRCUIT
82-Ω RESISTOR
(a) (b)
Fig. 3-50 Adding a resistor to compensate for an open sensor.(Robertshaw)
14. What is a zener diode?
15. What do the letters SCR stand for?
16. What is a bridge circuit?
17. What are the three parts of electronic controllers?
18. What is a thermal actuator?
19. What does the word semiconductor mean?
20. What are the two materials used for semiconductor devices?
21. What is a diode?
22. What is the PN junction diode used for?
23. What are the uses for diodes?
24. What are the two main uses for transistors?
25. What is an SCR? Where is it used?
26. What are the two main uses for a thermister?
27. What is a PNP transistor?
28. What is an integrated circuit?
29. What does CAB stand for in a humidity circuit?
30. What is a bridge circuit?
31. How do balanced and unbalanced bridge circuits differ?
32. What is a sensor?
33. How is a sensing bridge connected?
34. What is an actuator?
35. What is a differential amplifier used for?
CHAPTER 4
Solenoids and Valves
Copyright © 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.
PERFORMANCE OBJECTIVES
After studying this chapter, you should:
1. Know the basis of magnetic induction.
2. Know how electromagnets are made.
3. Know how solenoids differ from relays.
4. Know the primary purpose of an electrically-operated solenoid valve.
5. Know the difference between NC and NO.
6. Know what happens when a valve leaks in a hot-gas defrost system.
7. Know the usual voltage ratings of solenoid valves.
8. Know what VA stands for and why you need to know it.
A solenoid, where the length is greater than the diameter is one of the most common types of coil construction used in electricity and electronics. The field intensity is the highest at the center in an iron-core solenoid. At the ends of the air-core coil, the field strength falls to a lower value.
A solenoid that is long, compared to the diameter, has a field intensity at the ends approximately one-half of that at the center. If the solenoid has a ferromagnetic core, the magnetic lines pass uniformly through the core.
Mechanical motion can be produced by the action of a solenoid or it can generate a voltage that is a result of some mechanical movement. The term solenoid has commonly come to mean a coil of wire with a moving iron core that can center itself lengthwise within the coil when current is applied to the coil. Then if a ferro- magnetic core is properly suspended and under suit- able tension, it can be moved in and out of a solenoid coil form with the application of coil current. This is the operating basis of some relays and a number of other electromechanical devices. If an outside force is used to move the ferromagnetic core physically, it is possible to induce a voltage in the solenoid coil.
There is a tendency in a solenoid for the core to move so that it encloses a maximum number of magnetic lines of force. Each line of force has the shortest possible length (Fig. 4-1). In the illustration the core is outside the coil. Because it is a ferromagnetic material, the coil pre- sents a low-reluctance path to the magnetic lines of force at the north end of the coil. These lines of force concen- trate on the soft-iron core and then complete their paths back to the south pole of the electromagnet.
Electromagnetic lines of force that pass through the core magnetize it. This means that the induced magnetic field in the core has a south pole near the coil’s north pole. Inasmuch as unlike poles attract, the
core is attracted toward the hole in the solenoid coil.
This attraction tends to pull the core into the coil. As the iron core is pulled into the coil, the magnetic field becomes increasingly shorter and the magnetic lines of force travel the shortest possible distance when the core centers itself in the coil.
By attaching a spring to the core, it is possible to have the core return to its outside position once the power is interrupted to the coil. When the power is then turned on again, it pulls the core back into the coil. It is this type of movement that is utilized in the construc- tion of industrial solenoids that operate switch contacts in relays and motor starters, and valves in gas, air, and liquid lines of various types.