The 4 Types of Chiller Drive Packages in Air Cooled Chillers

There are 4 types of chiller drive packages used in air cooled chillers. 

Cost, efficiency, and reliability are the major factors that play into which is better. 

The first is XL start.

In short, it is a single contactor that pulls in to run the compressor.

Very simple design and minimal components. 

XL stands for Across Line. 

An RTU uses XL start as its primary drive type. 

These do not scale well and have HUGE inrush current. 

Hence very inefficient design. 

The second is Wye-Delta. 

This uses 3 starts and some have a smaller 4th. 

The 3 primary starters go by a variety of names. 

My preference is Start, Run, and Shunt. The fourth is known as a transition contact. 

Basically the starters are used to get the motor rotating with lower current in a Wye winding configuration using the Start & Shunt. Then it transitions to Delta winding configuration by dropping Shunt and only using Start & Run. 

Shunt and Run can not be pulled in together or it go big boom….

This keeps inrush current lower than XL. 

The third is Solid State Soft Starter (SSSS). 

These will have an electromechanical starter like in XL but with SCR added to the circuit in parallel. 

The SCR is used to slowly apply power to the compressor. 

Once the compressor is at speed the starter pulls in as a bypass to the SCR and the SCR are disengaged. 

SCR only packages exist but aren’t usually used in air cooled chillers because the SCR requires a lot of cooling for sustained loads. 

Using them momentarily mounted to a heat sink is sufficient for basic air cooled packages. 

This reduces compressor stress and inrush even further than Wye-Delta. 

The fourth is the industry king VFD. 

A VFD is using a complex assortment of solid state components to have full control of the hertz output going to the compressor. 

The previous drive types have used the grid hertz to operate the compressor. 

A VFD creates its own via PWM technology using inverters. 

This drive will slowly ramp the compressor similar to the SSSS but the drive maintains complete control of the motor speed and is not focused on getting to maximum speed unless needed. 

This not only reduces inrush and stress but also gives great flexibility lowering operating currents to maximize efficiency. 

MTT

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