Refrigeration systems are designed for full-load conditions. Most of the time, though, the load is not peak, but average, and full motor capacity is not required. In these average conditions, motors in traditionally designed systems (without VFDs) are either constantly running at a higher speed than necessary, or they are cycling on and off frequently. Producing more capacity than needed wastes considerable costly energy, and frequent cycling on and off accelerates wear and shortens the useful lifetime of motors, contactors, and other components. Starting and stopping motors frequently, and always accelerating them to full speed, eliminates many opportunities for reducing energy cost. VFDs can help in both of these areas, in addition to providing better product environments.
Although maximum load conditions—high ambient temperature, high humidity, and fully loaded store fixtures and storage boxes existed as little as 4% of running time, refrigeration systems must be designed to cope with those conditions. But it is just as desirable for systems to provide as close as possible to the capacity required for the part load conditions that are the norm. Unfortunately, by providing capacity to meet peak demands, considerable energy is wasted when part load conditions exist if machines are simply switched on and off.
In the 1980s, not really that long ago in the history of mechanical refrigeration, many supermarket installations still had one condensing unit for each circuit, each condensing unit having a single compressor. Control was provided by a low pressure switch of the type still found today (often as backup for a microprocessor control). There were two major problems with these systems: a lot of cycling and a circuit without refrigeration when there was a compressor breakdown.
Since the days when single-compressor systems were the norm, strategies first improved with twin-compressor systems. These gave full or half capacity, and there were fewer, but still significant cycling problems under light load conditions. Product losses due to failed compressors were considerably lessened.
With the advent of three-compressor racks, compressors were most often of equal capacity (for instance, three ten-horsepower compressors), giving four capacities deliverable to the served circuits (often multiple circuits were served by one rack). The four capacities, with three tens, would be 0, 10, 20, or 30 horsepower. Designers also built racks with unequal-sized compressors, using for instance one ten and two twenty horsepower units, able to deliver, in various combinations, 0, 10, 20, 30, 40, or 50 horsepower. Quad racks followed with four compressors that could offer ten capacities or more, depending on the size of compressors available.
Control of uneven parallel racks presented a complex engineering challenge that was met starting in the 1980s by the application of microprocessor controllers with software that has continued to grow in sophistication and capability to the present day. Stable, reliable sensors constantly provide control system software with temperature and pressure information for accurate, repeatable control.
The larger the number of capacity steps available to a refrigeration system, the more nearly the system can match the changing load. Using only the capacity needed saves considerable energy. The application of a single variable frequency drive on a refrigeration rack can vary the speed, and hence the capacity, of a compressor from zero (off) to as much as 150% of nominal speed, because the drive can effectively supply up to 90 Hz (150% of rated current frequency). The capacity supplied by a compressor so driven, then, is very much greater than nominal capacity . It is important in these applications to use inverter rated motors.
In today’s supermarkets, variable frequency drives are also commonly applied to regulate the operation of remote condenser fans, and to a lesser but growing extent, air handler motors and HVAC fans.
In addition to constantly providing load-matching capacity and providing energy savings by eliminating over-capacity running, variable speed operation can easily result in substantial maintenance savings and enhanced compressor and fan motor lifetimes. In the case of condenser fans, variable speed has also solved problems in noise abatement
It is important in specifying variable speed installations that inverter rated motors be used. In retrofitting variable frequency drives to existing installations, it is likewise important to work with a qualified vendor who can fit appropriate filtering to existing motors.
How variable frequency drives work
The compressors and condenser motors used in supermarkets are three-phase ac synchronous induction motors. The speed of such a motor depends upon the number of motor poles and on the frequency of the applied electric current. These variables are related by the formula
RPM = (120*v)/p
where
RPM is motor speed in revolutions per minute
ν is the frequency of the current in Hertz
p is the number of pole pairs in the motor
When a variable speed system starts, the VFD outputs ac voltage at low frequency to the controlled motor. For a refrigeration compressor, the startup frequency can be less than 5 Hz, at a correspondingly low voltage. This low speed start avoids the high startup current (as much as 300% of rated current) of systems that cycle on at full speed, as the motor can develop as much as 150% of its rated starting torque while only drawing 50% of rated current. This is in stark contrast to the large inrush current draw and relatively low output of a motor switched on at full power.
When used to vary the speed of a refrigeration compressor and networked to an appropriate controller (for example, the Danfoss AK2-SC 255), a variable frequency drive offers a virtually infinite range of capacities from the refrigeration rack. Neither a contactor nor a motor starter is required for compressors or fans controlled by variable frequency drives. With initial current frequency at, say, 30 Hz, the motor speed is slowly ramped up to supply the capacity required. Then, as load changes, motor speed is varied constantly to supply the correct capacity. This means that rack capacity can be controlled to exactly match the load on the refrigeration system. There is considerably less on-off cycling of compressors.
Without the use of a VFD, the only way to vary the rack's capacity is to turn compressors on in different combinations; but each time a compressor is started, considerable energy (high inrush current) is expended to overcome inertia and bring the motor up to speed. With a VFD on one of the rack's compressors, and with the rack sized properly, the speed variation on that one compressor will provide all the capacity change necessary to meet 90% of the load variations the rack has to deal with. There is much less cycling, and considerable energy is saved. Wear on the compressors is also lessened, extending their useful life and reducing service calls.