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The start relay has normally closed contacts, so when the compressor starts, both the run
and start capacitors are connected to the start terminal. This causes a very high current to
go through the start winding when power is first applied. This high start current increases
the starting torque of the compressor motor enough that the motor will start even though the
refrigerant pressures haven’t equalized, or in an “under-voltage” condition.
Once the compressor begins running, the voltage across the start winding increases. This
occurs because the motor acts partly like a generator and partly like a transformer. The start
relay coil is connected in parallel with the start winding. When the voltage across the start
winding increases above the pick-up rating on the start relay coil, the start relay contacts
open. The start capacitor is then out of the circuit.
Systems with capillary tubes or fixed restrictors usually don’t need a full hard start kit, unless
the compressor bearings are tight. In such cases, the compressor is probably near the end of
its useful life anyway.
These types of systems usually need only a start assist device that includes a PTC (positive
temperature coefficient) relay wired in series with a start capacitor. These start assist de-
vices are wired in parallel with the run capacitor and use only two wires. The PTC adds current
to the start winding. When current passes through the PTC, it gets hot. The resistance of the
PTC goes up as it gets hotter. This increases the heat output so the resistance goes up even
more. The effect is that the PTC is super boosted and taken out of the circuit in a fraction of a
second. The PTC needs to cool down before the start assist device can be engaged again.
There are two reasons the start capacitor can’t stay in the circuit full time.
1. The start winding of the compressor can’t carry such a heavy current continuously
without overheating and burning out.
2. The start capacitors are made very compact and would overheat in a short while
because they aren’t big enough to dissipate heat as rapidly as it’s generated.
The plastic casing on the start capacitor also plays a role in its tendency to overheat.
When a start capacitor does overheat, the little putty filled hole in the top of the
capacitor blows and all the fluid inside runs out, causing the capacitor to fail.
History shows that air conditioning manufacturers agree that a start assist device is a neces-
sary part of the equipment; all units included a starting device. They have been eliminated
through the years as manufacturers reduce costs. In applications where starting every time is
paramount, like refrigeration systems, start assist devices are included with each compressor.
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