Heatcraft Refrigeration Products H-IM-CU Benutzerhandbuch

The electric defrost units are relatively simple and trouble-free in operation:

If the system does not go through its proper sequence , check timer 

operation through a defrost cycle. Check for loose wires or  terminals. Before 

replacing timer, check other components.

To set time of day grasp knob which is in the center of the inner (fail-safe) dial 

and rotate it in a counter-clockwise direction.  This will cause the outer (24 

hour) dial to revolve.  Line up the correct time of day on the outer dial with 

the time pointer.  Do not try to set the time control by grasping the other (24 

hour) dial.  Place pins in the outer dial at the time of day that defrost  

is required.

To set the time, turn the minute hand clockwise until the time of day (and 

AM or PM) on the outer dial is aligned with the triangle marker on the inner 

dial. Do not rotate minute hand counter-clockwise. Move the white tab 

(tripper) on the outer dial outward at each desired initiation time. Each white 

tab (tripper) is a 15 minute interval and provides 15 minutes of defrost. For 

longer defrost duration, move additional tabs (following in time) from the 

initiation tab. For example, if a 45 minute defrost is to start at 7:00 AM, move 

the tabs outward that lie between 7:00 - 7:15, 7:15 - 7:30 and 7:30 - 7:45 on 

the AM side of the dial. The defrost will initiate at 7:00 AM and time terminate 

at 7:45 AM (if temperature termination does not occur first).  For models with 

plastic cover on timer assembly; re-install cover after adjustment.

After correcting faulty condition it is essential that the coil and unit be 

free of ice before placing unit back on automatic operation.

1.  Lockout relays or normally closed switch of auxiliary contact on the 

compressor contactor may be wired to defrost contactor.  Its purpose 

is to prevent energizing of the defrost heaters until the compressor has 

pumped down and stopped, thus keeping power demand to a minimum. 

2.  If the control voltage is to remain energized for any period of time with 

the compressor disabled, remove the defrost clock pins to prevent the 

defrost heaters from energizing.   

3.  A Preventative Maintenance schedule should be set up as soon as 

possible after start-up to maintain equipment integrity.

Power is supplied to the timer at terminals “1” and “N”.
The fan delay and the defrost termination thermostat is closed in the fan  

 

delay position and open in the defrost termination position.  

 

The unit cooler fans run continuously.
The defrost heaters are off.
The room thermostat closes when the temperature rises above the  

 

 

desired setting.
The liquid line solenoid is energized and opens, which allows  

 

 

liquid refrigerant to flow through the unit cooler.  
The low pressure control closes when the suction pressure rises 

 

above the cutin setting of the control.  
On systems with oil pumps, the oil safety control is closed. If the  

 

 

net oil pressure is less than 9 PSIG for more than 120  

 

 

 

seconds, the oil safety opens, thus breaking the circuit  

 

 

 

to the compressor contactor holding coil. The compressor will  

 

 

not operate. This control is reset manually and must be  

 

 

 

reset before the compressor can be restarted.  
The compressor contactor closes. The compressor and condenser 

 

fan start simultaneously.  
The room temperature gradually decreases to the desired temperature.
Once the desired temperature is reached, the thermostat opens and the  

 

liquid line solenoid closes, stopping refrigerant flow through  

 

the evaporator.  
Suction pressure decreases and the compressor contactor opens 

 

when the pressure drops below the cutout setting on the low  

 

 

pressure control. The compressor and condenser fan stop running.
This cycle is repeated as many times as necessary to satisfy the  

 

 

room thermostat.
Frost starts to form on the evaporator coil and continues to form  

 

 

until the defrost cycle is initiated.

The defrost cycle starts automatically by the timer at predetermined    

 

times. Typical settings are two to four defrost cycles per day for freezers.  

 

For heavier frost loads additional settings may be required.
Switch “2” to “4” opens in the timer which breaks the circuit to the room  

 

thermostat, liquid line solenoid, and evaporator fan motors, allowing    

 

the compressor to pump down and shut off.  Simultaneously  

 

 

switch “1” to “3” closes in the timer allowing current to flow to one side    

 

of the  defrost heater contactor. When the compressor  

 

 

 

shuts off, an auxiliary contact will send power to the contactor holding   

 

coil; thus, energizing the defrost heaters.
The heaters raise the temperature of the coil to 32˚F causing the frost to  

 

melt off the coil.
When the coil warms to 45˚F to 55˚F, the defrost termination thermostat  

 

closes, which allows current to the switching solenoid in the timer  

 

 

allowing the refrigeration cycle to begin again.
The evaporator heaters are off.  If the termination thermostat fails to    

 

close, the fail-safe set on the timer will terminate defrost.
The low pressure control closes and the compressor will start.  
When the coil temperature reaches 23˚F to 30˚F, the fan 

 

delay closes. This allows the current to flow to the fan  

 

motors. The fan motors start running.
The system will now operate in the refrigeration cycle until another  

 

 

defrost period is initiated by the timer.

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