Tuesday, December 2, 2014

Electrical equipment and service

Electrical equipment and service

Description

This chapter is devoted to the electrical aspects of hermetic and semi-hermetic motor compressors and remote-drive motors employed in commercial refrigeration and in domestic refrigerators and freezers for single-phase 240 volt 50 hertz supply.

The smaller hermetic compressors are mainly used for domestic systems. Larger capacity units are to be found in coldrooms, refrigerated display counters and display cases. These compressors are supplied in various sizes and designs; some operate with a reciprocating action, others are rotary types.

The starting devices for these single-phase compressors are described and their operation is explained, with test procedures which are the same for both hermetic and semi-hermetic compressors.

Open-type reciprocating compressors are also employed for the commercial range. The starting device for remote single-phase drive motors is included, since the test procedures are again basically the same.

Drive motors and starter contactors for both single and three-phase supply are dealt with in Chapter 15.

The split phase motors employed can be capacitor start for low starting torque, and capacitor start and run for high starting torque, on a 200/240 volts 50 hertz supply.

Different types of starting device are used: the current relay for low starting torque, and the potential (voltage) relay for high starting torque. A form of solid state device may be used for either. Regardless of type, each will have overload protection. There will be three external terminals to which the compressor controls are connected to complete the circuit to the two motor windings. The three compressor terminals are known as common, start and run (C, S and R).

External overload protectors are both current and temperature sensitive; they are easily replaced. Internal overload protectors tend to be less current

sensitive and provide better motor protection, but cannot be replaced; this means that correct diagnosis of a fault is imperative.

Of the two motor windings, the main or run winding is the larger in both

physical size and cross-section. The other is the start or auxiliary winding. They are both conductors, but current will flow freely through the larger winding and meet resistance in the smaller winding.

Service equipment

To carry out an efficient service and diagnosis, the engineer will require the following instruments: voltmeter, ammeter, ohmmeter (or a multitester), wattmeter, test cord and (not essential) capacitor tester.

The wattmeter is used in conjunction with the manufacturer’s data or that given on the compressor nameplate. Power readings are an aid to diagnosing faults in much the same way as operating head pressures are for open-type compressors. For example, the average running watts are generally provided on the nameplate or model identification plate. Manufacturers’ service manuals will give starting watts, running watts and current consumption under specified conditions for each model.

Current relay

This can best be described as a magnetic switch. It comprises a small solenoid coil around a sleeve and an iron core. Inside the sleeve is a plunger to which the switch contact bridge is attached; the contacts are normally open.

When the coil is energized, a strong magnetic field of force is created because the current will be high during the starting phase. The magnetic force will move the plunger upwards and bridge the switch contacts, completing the circuit to the start winding. The run winding is wired through the relay so that it is always in circuit. Figure 34 shows a typical motor and control arrangement for a current relay, and Figure 35 shows the completed relay circuit.

A high starting current is drawn when the compressor motor starts. The current reduces as the motor gathers speed; the magnetic field through the relay then becomes weaker so that it can no longer hold the contact bridge on to the switch contacts. The plunger then drops down by gravity to open the circuit to the start winding.

It is not uncommon for a start capacitor (see later) to be fitted when a current relay is employed. This is wired in series with the start winding (Figure 36).

 

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Potential (voltage) relay

This type of relay is used with high starting torque motors. It operates in a similar manner to the current relay except that the switch contacts are normally closed. The solenoid coil, once energized, maintains a magnetic force strong enough to open the switch contacts and keep them open whilst the compressor is running.

The relay has a much higher design voltage rating than the supply voltage. As the motor approaches its design speed, the voltage across the coil can sometimes be more than twice that of the supply voltage.

When power is supplied to the circuit, the relay contacts are closed. Both motor windings are energized and starting is achieved. As the motor increases speed, the voltage in the start winding increases to cause an increase in both voltage and current passing through the coil. When the design voltage of the coil is reached, the current creates a strong magnetic force to pull in the plunger and contact bridge to open the start circuit, but allows the compressor to operate on the run winding.

When the relay contacts open, the voltage and current across the coil will decrease but will maintain a magnetic force strong enough to keep the contacts open until power is disconnected. The contacts will then return to the closed position ready for a restart.

This relay is wired in parallel, unlike the current type which is wired in series. A voltage relay circuit is shown in Figure 37.

Solid state starting devices

Semiconductor starting devices are used on domestic and commercial units with fractional horsepower motors. Some versions are not recommended for permanent installation but are used rather as emergency replacements, because they may not always be suitable for the motor design characteristics.

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Positive temperature coefficient (PTC) device

Many commercial motor compressors have this type of starting device. In most cases the motor is designed with an internal overload protector; this gives better protection against overload conditions since it is more sensitive to the temperature of the motor windings. It is less current sensitive than the external overload protector.

When the compressor starts, resistance in the semiconductor is low and

current can pass freely to the start winding. As the current flows it heats the semiconductor (in approximately 2 to 3 seconds). This causes an increase in resistance through the semiconductor, thereby reducing the flow of current; by this time the motor is up to speed. The reduced current flow is sufficient

to maintain the heat in the semiconductor and prevent high current flow to the start winding, allowing the compressor to operate on the run winding.

In the event of a starting failure the increased current drawn will also cause a rise in temperature of the motor windings and actuate the overload protector. Obviously current will not flow to the start winding after a starting failure. The semiconductor has to cool before another starting attempt can be made, and there is therefore greater motor protection.

A further advantage of this type of starting device is that it does not have any moving parts, unlike current and voltage relays. This lessens the risk of failure due to wear and corrosion of contacts. It also makes it more acceptable where stringent demands are made for low noise levels; arcing across switch

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contacts can affect tape recorders, videos and other domestic and industrial electronic equipment.

The normal cooling period before a restart is 3 to 5 minutes. The protector cut-out temperature is about 140°C or 285°F, and the cut-in temperature is about 105°C or 220°F. At 140°C the cooling period before the compressor can restart may be as long as 45 minutes, depending upon the ambient temperature.

A basic PTC circuit and wiring diagram is shown in Figure 38.

IC-22 relay

This is another form of solid state relay for non-capacitor and capacitor start motors. It can be used as a replacement for the conventional types.

The normal operating temperature of the semiconductor is approximately 76°C or 170°F. Therefore care must be taken when handling if the compressor is running.

Installation for non-capacitor start motors is as follows:

1 Remove the defective relay but leave the overload protector in place if it is serviceable.

2 Connect one lead of the IC relay to the compressor start terminal.

3 Connect temperature/pressure controls, fans etc. to the run lead of the compressor using the quick connector provided.

For capacitor start motors, follow steps 1, 2 and 3, then adapt the compressor wiring to connect the capacitor in series.

Note that if the overload protector is an integral part of the defective relay which has been removed, there will be no overload protection for the motor. A suitable overload must be selected and fitted.

Figure 39 shows the electrical connections for an IC-22 relay.

Capacitors

Those normally used on smaller units are of the electrolytic type. They can be considered as being an electrochemical component employed to improve the phase angle relationship between the motor windings when the motor starts and runs.

They may be installed in a series or parallel with the motor windings. If a specific capacitance is not available as a direct replacement, two or more capacitors may be used. The capacitance is stamped on to the casing of a capacitor, and its value is given in microfarads.

imageA method of selecting and connecting capacitors for making up specific capacitances is given in Figure 40.

Centrifugal switch (open-type motors)

This form of starting device is used on open drive motors. It comprises three major components: the switch contacts, the moving contact arm and the governor assembly (see Figure 41).

The switch contacts are wired in series with the supply and the start winding.

The governor assembly is mounted on the motor shaft. It consists of a spring loaded weight on a small rod. The spring loaded contact arm has a circular inclined plane extending around the shaft; this is commonly known as the skillet plate.

When the motor is idle, the switch contacts are closed. When the motor starts, the shaft rotates, gathers speed and sets up a strong centrifugal force which causes the weight to move outwards against the spring tension. This

imagepermits the contact arm to move and open the switch contacts, thus disconnecting the start circuit.

As the supply is disconnected to the run winding by the action of the temperature control, the motor speed reduces and the centrifugal force weakens. This allows the governor to move in towards the shaft, contacting and depressing the skillet plate and closing the switch contacts ready for a restart.

Motor protection

The most common cause of motor failure is overheating. This condition is created when a motor exceeds its normal operating current flow. The result can be either a breakdown of the motor winding insulation and a short circuit, or a winding burn-out. For this reason, overload protection is provided in the form of a current and temperature sensitive control which will open the circuit before any damage to the motor can occur.

The following types of control are used: fuses, circuit breakers, bimetal switches and thermistors. Fuses and circuit breakers are located remotely and are normally required to protect the circuit conductors.

Bimetal switches are accepted as an overload protection for most hermetic and semi-hermetic motor compressors (Figures 42 and 43).

A thermistor is a solid state semiconductor which heats up as current is passed through it. As the temperature of the material of which it is made

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imageincreases, a greater resistance to current flow is created and under overload conditions the current flow very quickly almost ceases, thereby stopping the compressor. Most thermistors are made of lithium chloride or coated barium titanate. The resistance changes approximately 6 per cent of each degree C.

Thermistors that have a PTC are connected in series with the windings and prevent current flow when the temperature of the motor increases.

The negative temperature coefficient (NTC) device is a small module which

is embedded in the motor windings. Internal overload protectors perform the same function as the external and thermistor types (Figure 44).

Motor overheating has various causes. Motor compressors used for refrigeration duty are designed to start when there is an equalizing of pressures within the compressor. An increased starting load such as a high head pressure can cause overheating and excess current to be drawn. A shortage of refrigerant can also result in overheating of the motor windings, since the motors rely on returning suction vapour to assist in the cooling of the windings.

Electrical test procedures
Supply and control switch

1 Check the electrical supply at the power point, the fuse and the fuse rating.

2 Ensure that the control switch is set to the on position, and check the electrical supply to the control. If there is supply to the input side of the

imagecontrol, bridge out the control contacts with a jumper wire or join the two control wires together; this will effectively take the control out of circuit (Figure 45).

3 If the compressor fails to start, check the supply voltage at the compressor terminals or at the relay terminal board.

Overload protector

If the compressor is cold to the touch it is evident that it has been idle for some time. The overload protector should be checked for continuity or by bridging the two contacts as described for the control switch (Figure 46). Try to start the unit.

Windings

Isolate the unit electrically and remove the electrical leads from the compressor terminals, or the relay if it is a push-on type.

Check the compressor windings for continuity and resistance values (Figure 47). The highest resistance value between any two terminals is the

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imagesum of two winding resistances (start and run terminals). The next highest resistance value between two terminals is the value of the start winding (start and common terminals). The lowest resistance value is that of the run winding (run and common terminals).

Test cord

If the check shows that the continuity and resistances are satisfactory, connect a test cord (see Figure 48). With the test cord connected, switch on the power supply. At this stage the compressor will be trying to start on the run winding only and a humming sound should be audible.

clip_image031Depress the test cord bias switch to bring the start winding in circuit, and hold for 2 3 seconds. The compressor should start. If the compressor starts when the start winding is in circuit but stops when the bias switch is released, the run winding is defective. Failure to start at all means that the start winding is defective. In either case a replacement compressor will be required.

A maximum deflection recorded on the ohmmeter indicates that the compressor motor is down to earth, and will probably blow fuses when the windings are energized.

imageWhen capacitors are included in the circuit they should be connected in series if a test cord is used; Figure 48 shows a start capacitor in series with the start winding.

The larger hermetic and semi-hermetic 220/240 volt a.c. 50 hertz capacitor- start/capacitor-run motor compressors draw considerable starting currents and it is advisable to use a heavy duty test cord similar to the type shown in Figure 49 when testing a compressor operation. The connection procedure is as follows:

image1 With all switches in the ‘off’ position, attach the three motor leads of the test cord to the compressor terminals.

2 Attach a known good capacitor of the correct microfarad rating to the ‘start capacitor’ leads of the test cord. (If two start capacitors are fitted, connect them in parallel.)

3 Attach the appropriate capacitor to the ‘run capacitor’ leads of the test cord.

4 Move the ‘run capacitor’ switch to the ‘on’ position.

5 Ensure that the earth lead of the test cord locates to the compressor screw and makes good contact.

6 Connect the test cord to the mains supply.

7 Hold the start switch at the ‘on’ position and move the main switch to the ‘on’ position.

8 Release the start switch when the motor is up to speed.

If the compressor fails to operate it must be replaced. If the compressor runs on the test cord, then the fault may be in the overload protector or starting relay.

Capacitors

If the unit fails to start with the test cord it could also mean that the capacitor is unserviceable. It should be tested with a capacitor tester if available.

Another means of testing a capacitor is to connect it across a 50 Hz supply in series with an ammeter and several lamps to provide a resistive load. The voltmeter should be connected across the capacitor terminals as shown in Figure 50.

clip_image036Capacitance in microfarads (p,F) is given by (amperes/volts) ð 3200. For example, suppose a capacitor rating is 80 105. If the mains supply is 240 V 50 Hz and the current drawn is 7.5 A, then (7.5/240) ð 3200 D 100 p,F. This shows that the capacitor is functioning within the design rating. It must be emphasized that this method must not be used to select a capacitor.

imageShould a capacitor show any signs of leakage or damage to the outer casing, it should be changed.

Potential relay

First ensure that no capacitor in the circuit is defective or shows signs of leakage or damage.

1 Check for continuity by removing the relay and measuring the resistance across the relay coil (see Figure 51). In this case the reading is taken between terminals 2 and 5, but this may differ according to the relay type. A high resistance should be recorded. If a zero reading is recorded then the coil is open circuit.

2 Replace the relay and switch on the unit. If the contacts are stuck open, a humming sound should be heard; the compressor is trying to start on the run winding only. After 15 20 seconds the motor will trip out on the overload.

3 Isolate the unit and install a jumper wire across the switch terminals, in

this case between terminals 1 and 2. Switch on the unit. The jumper wire should include a bias switch for safety.

4 Hold the bias switch in circuit for approximately 5 seconds to allow the

compressor to reach its design speed. Then release the bias switch; the compressor should continue to run. This means that the relay contacts are stuck open.

This test will only be effective if the capacitors are serviceable, and is similar to using a test cord.

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