9-1-2. Overload protection relay (O.L.P) structure and function
• The overload protection relay cuts the power to protest the motor when the temperature of the compressor rises
abnormally or when the overly high current is sent to the compressor motor.
• Overload protection relay structure is composed of the bi-metal element that cuts the power based on the temperature and
the heater that heats during an over-current. They are attached to the external surface of the compressor and detects the
temperature of the compressor. The composing circuit is connected to the compressor motor in direct connection.
• When the overload protection relay operates, you can here a "tak" sound and the operation contact point of the bi-metal
separates from the fixed contact point. When the heat of the heater is reduced or when the temperature of the compressor
is lowered after 3-5 minutes, the bi-metal connects the circuit with another "tak" sound to operate the compressor.
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The overload protection relay detects and operates by both the temperature and the current. Therefore
even when the power is not connected, it can still operate when the temperature of the surface is high and
it can operate when there is an over-current even with low temperature on the surface. But even though
this will not happen in reality, just make sure that the relay operates by a combination of abnormal
temperature and current. (But the effect from the current is bigger than that from the temperature.)
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After the overload protection relay operates and it recovers after 3-5 minutes when the heat is lowered, it
can still operate again if the surface temperature or current is high. In other words, repeated operation for
several hours can cause a customer claim. At this time, adequately cool the compressor and connect the
power. Also there is a case when the relay operates but immediately recovers to operate again. At this
time, exchange the relay.
9-1-3. Starter (P.T.C) function and operating principle
• The starter is directly connected to the supplementary coil of the
compressor to accelerate the compressor operation. It is
internally installed within the external case of the compressor
like the overload protection relay.
• The starter is generally composed of P.T.C material but in the
past there were ones in relay contact point type.
P.T.C stands for Positive Temperature Coefficient, referring to a
resistance that has a constant value initially (ex 33
Ω
) but when
the power is connected the resistance becomes infinite. To
understand the role of the starter you must understand the
operating principle of the single phase inductive motor and
simple AC circuit.
Characteristics of each part
- 45 -
(Overload relay structure)
Bi-metal
Operation contact point
Fixed contact point
Base (phenol)
Terminal
(a) Internal structure (b) Electric circuit diagram
Caution
AC
CR
P.T.C
(Motor circuit)
Supplementary
coil
Main coil
