Bryant A.C. Refrigeration equipment

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150

RefrigerationEquipment

2 The current rating of a cartridge fuse is 30 A and the fusing factor is 1.75;

the fuse will rupture at 30 • 1.75 = 52.5 A.

3 The current rating of an HRC fuse is 20 A and the fusing factor is 1.25;

the fuse will rupture at 20 x 1.25 = 25 A.

It must therefore be realized that a fuse is rated at the amount of current it

can carry, and not the amount at which it will rupture. Rewireable fuses have

fusing factors of approximately 1.8; cartridge fuses of between 1.25 and 1.75;

HRC fuses of up to 1.25 maximum; and motor cartridge fuses of 1.75.

Circuit breakers

A fuse element provides protection by destroying itself and must be replaced.

It cannot be tested without destruction; therefore the result of a test will not

apply to the replacement.

The circuit breaker (CB) is an automatic switch that will open in the event

of excess current and can be closed again when a fault is rectified. The switch

contacts are closed against spring pressure, and held closed by a form of latch

arrangement. A slight movement of the latch will release the contacts quickly

under the spring pressure to open the circuit; only excessive currents will

operate it.

Two types exist: thermal and magnetic.

Thermal

The load current is passed through a small heater, the temperature of which

depends upon the current it carries. The heater will warm up a bimetal strip.

When excessive current flows the bimetal strip will warp to trip the latch

mechanism.

Some delay occurs owing to the transfer of heat produced by the load

current to the bimetal strip. Thermal trips are suitable only for small overloads

of long duration. Excessive heat caused by heavy overload can buckle and

distort the bimetal strip.

Magnetic

The principle used in this type is the magnetic force of attraction set up by

the magnetic field of a coil carrying the load current. At normal currents the

Electrical circuit protection 151

Figure 101

Typical magnetic circuit breaker

magnetic field is not strong enough to attract the latch. Overload currents will

increase the force of attraction and operate the latch to trip the main contacts.

A typical magnetic circuit breaker is shown in Figure 101.

Drive motors

Before installing or replacing a drive motor, it must be established that the

voltage and frequency of the motor (as stated on the nameplate) agree with

those of the supply.

A motor must not be connected to a supply outside the range stated on the

nameplate. If connected to a low voltage supply it may not start. If it does

start it may not run to its full speed; the windings will then overheat, and

may burn out if continued starting attempts are made by the action of the

overcurrent protector.

It is therefore important that the overload protector is of the correct rating

if a replacement is made. Those fitted to open drive motors should be rated

as near as possible to 125 per cent of the nameplate current.

Installation details

Earthing

The motor frame must always be connected to the local earth circuit. It is

most important to ensure that the flexible earthing strip between the motor

frame and the cradle is not broken and that the correct connection is made.

When water cooled systems are involved, under no circumstances must the

water supply tubing be used for earthing.

Lubrication

Motor bearings should be lubricated at the time of installation or replacement.

For sleeve bearings use high grade light mineral oil; for ball bearings use pure

mineral grease.

Do not apply an excess of oil or grease, because excess lubricant can damage

motor windings and switch mechanisms. Do not allow oil to contact the

resilient rubber motor mounting pads. A small drop of oil should be applied

to the swivel pivot.

Drive motors 153

Bearings

Worn bearings will probably be evident in the form of a hum or rattle from

the motor.

The end play should not exceed 0.25 mm. If shims are used to correct the

clearance or take up wear, they must not be fitted at one bearing only. This

will result in the rotor being out of centre with the stator and will cause

humming.

Thermal overloads

These are usually self-resetting after cooling. They are set to cut out when the

temperature of the motor windings reaches approximately 90~ (194 ~

Nuisance tripping of a motor may be the result of poor ventilation of the

motor or the load on the compressor. This should be checked before any

replacement is made.

Direction of rotation

To reverse the rotation of a capacitor start motor, the direction of current

flow in either the start or the run winding must be reversed (but not both

windings). This entails reversing the internal connections of a winding, as

shown in Figure 102. For three phase motors it is only necessary to reverse

any two electrical leads to the motor terminals.

Red

Black ",

~~ ;~ Black , A1 'I

Reverse either red or black

wires

Figure

102 Reversing direction of rotation

Thermostat

Crompton Parkinson

154

Refrigeration Equipment

.f t,

..... T T

SEM

l Hoover mark 1

l I ,, ,

Hoover mark 2

Figure 103

Drive motor connections

Drive motors 155

Capacitor intemally connected

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T .....

Figure 103

(continued)

AC Delco

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Terminal board connections

Suppliers of drive motors are numerous and the terminal identification is not

standard. A variety of connections for motors in common use is provided in

Figure 103.

Three phase electrical connections

Smaller compressor motors incorporate overload protectors, either internally

or externally mounted, and use various small starting devices. By contrast

the three phase motor, being larger, requires more sophisticated methods of

automatic starting and overload protection, provided by contactors or starters.

Contactors and starters

A contactor is a power operated switch suitable for frequent making and

breaking of an electrical circuit. A starter is a power operated switch with

inbuilt overcurrent features, employed for starting a motor and protecting it

during running. Both controls enable large currents to be switched by means

of a system control (thermostat) with low current rating.

156

Refrigeration Equipment

41--- Start

Load

Stop

Figure 104

Electrical connections for a single phase starter

Figure 105

Type of three phase starter

Suppliers of these controls are numerous. Each has its own options with

regard to overload protection, single phasing protection and accessibility

for component replacement. Wiring diagrams for the controls are normally

supplied or are available on demand. These should be referred to before any

attempt is made to install, make electrical connections to or diagnose faults on

Drive motors 157

the controls. Because of the different types of control, only typical examples

are dealt with here. Figure 104 shows connections for single phase starting,

and Figure 105 shows a particular three phase starter.

Direct-on-line starter

With the standard squirrel cage induction motor, it is advantageous to have

DOL starting. This allows a cheaper motor starter and control gear to be used,

and also imposes less strain on motors during starting.

............... ,. .... .i._

~(~) ~0 ~0 C), X

Reset

To control circuit

To compressor motor

L1 L IL~,~

' ~~o,

"--: Start

Figure 106 Electrical connections for three phase DOL starters

158

Refrigeration Equipment

.J_

Fuses

Test

4---Reset

Coil and

contact breakers

To control circuit

Figure 106

(continued)

However, electrical supply authorities will normally restrict starting

currents. DOL starters should only be specified for motors within the supply

authority limits.

Figure 106 shows the electrical connections for three phase DOL starters.

Commz'ssioning of

refrzgerat ng systems

Commissioning can be defined as the advancement of an installation from the

stage of static completion to full working order to specific requirements.

A number of different contractors may have been involved: refrigeration

fitters, electricians, plumbers, builders, carpenters and shop fitters. Close

cooperation among all concerned is essential. It is therefore desirable that

the commissioning of a plant be carried out under the guidance of a single

authority; that usually means the service or installation engineer.

The customary safety procedures must be observed at all times. During

commissioning the following will prevail:

1 Use only the specified refrigerant (manufacturer's data).

2 Substitute refrigerants should only be used if approved by the equipment

manufacturer.

3 Do not re-use refrigerant which might be contaminated.

Contaminants

During installation the system should be kept as clean and dry as possible,

with the least exposure to air. Avoid the entry of foreign matter such as solder

fluxes, solvents, metal and dirt particles, and carbon deposits; the last are the

outcome of soldering joints without passing a neutral atmosphere through the

pipework.

Failure to take precautions may result in corrosion caused by air and mois-

ture, or by an oxidizer under high temperature conditions. Other problems

include:

Copper plating due to contaminated oil. It forms on bearings and planed

surfaces in high temperature areas. Moisture in the system can also be the

cause.