PART NO. APPLICATION VOLTAGE CAPACITOR (330 V) REPLACES 5-2-1
THE BENEFITS OF A/C HARD STARTS
®’s introduction of the 2-wire hard start kit in the late 1970’s, there has been
a proliferation of different technologies offering various reasons and solutions to the same
problems that spurred their initial development; an air conditioner that just will not start under
We will explain the need for hard starts in the first place, along with the intrinsic benefits of
installing one in an air conditioning system. With several options available, guidance to
choosing the right hard start for the application will be outlined. With this knowledge in hand,
making the right decision for inclusion of a hard start in an air conditioning system will become
Why Do We Need a Hard Start, Anyway?
Most single phase air conditioners and heat pumps use non-bleed thermostatic expansion
valves (TXV’s) to control refrigerant. A problem with TXV’s occurs when a reciprocating
compressor shuts off, refrigerant pressures don’t fully equalize. Pressures do equalize in scroll
compressors; which, typically do not need hard start kits for this condition. In a reciprocating
compressor, the discharge pressure will drop to about 150 psig and the suction pressure will
rise only to about 100 psig. When the compressor tries to start, there’s too much load for the
starting motor torque to overcome. This is especially true if the supply voltage is low.
To increase starting torque, a start assist device can be used. When a TXV is used, a potential
relay hard start kit is employed. This will increase starting torque by a minimum of 300 %
over using just a run capacitor. Figure one (1) shows the wiring diagram for the OEM style
3-wire hard start.
3 Wire Connection
The potential relay opens at manufacturer’s specified voltage across the
start winding of the motor, effectively removing the start capacitor from
the circuit. A third wire is necessary to connect to the common wire.
A more convenient method for providing increased torque to the compressor is the 2-wire
potential relay hard start device. In this case, the device can add as much starting torque as
a 3-wire hard start, but installation is made simpler and cost is usually lower. Figure two (2)
shows the wiring diagram for a 2-wire hard start.
2 Wire Connection
Two wire hard start kits are connected in parallel with the run capacitor for
Permanent Split Capacitor (PSC) type compressors; and connected to the
Start and Run terminals on the compressor for Capacitor Start Induction
Run(CSIR) type compressors. There is no need for a third connection
to common wirr