Before you condem your walkin cooler or freezer compressor try the USR

USR

Universal Starting Relay with Capacitor for Commercial Refrigeration

Features / Benefits

• For compressors from ½ hp thru 5 hp

• Pre-wired for easy installation

• Potential Relay Technology

THE BENEFITS OF A/C HARD STARTS

Benifits of using a hard start

®’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 

“normal” conditions.

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 

clear.

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.

Figure 1

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.

Figure 2

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 wire