In standard system design, electric motors are intended to operate at a fixed speed. This speed is determined by the frequency of power supplied by the utility and motor design (number of poles). The shaft load on the motor is determined by the product of shaft speed and torque. With a fixed speed, motor
power is determined by the torque of the load. With a change in speed, motor load will not only benefit from the speed reduction, but also any reduction in torque with speed. Two types of motor loads exist; constant torque and variable torque.
Positive displacement compressors (e.g., screw, reciprocating, rotary vane) are constant torque devices. That is, the twisting force required to turn the shaft is constant, regardless of speed. Therefore, the shaft power is determined by operating conditions (pressures) and method of capacity control, which both effect torque. In general, a reduction in 50% speed would provide a proportional 50% reduction in shaft power.
General drive control
Using frequency converters to control cold storage capacity provide improved control and efficiency, whether for compressors, fans or pumps.
There are several incentives for using speed control on screw compressors:
- Drive control will reduce the power penalty associated with slide valve, poppet valve, or throttling capacity control. On compressors with no capacity control, speed control will eliminate other poor control strategies.
- Drive control will reduce wear and tear associated with slide valve action.
- Drive control allows a precise suction pressure to be maintained. With slide valve, a broad dead band is often maintained to avoid excessive wear.
- Drive speed control provides compressor size reduction with the same system capacity demand.
- Drive operation of screw compressor
Almost every rotary screw compressor uses a slide valve for unloading. The slide valve moves along the length of the rotors, reducing the compression
length within the rotors. Although this control method is infinitely adjustable and provides reasonable suction pressure control, there can be a substantial power penalty associated with slide valve control. As the compressor unloads, there is not a proportional reduction in power.
In general, part-load performance degrades with deeper suction or higher discharge pressure. Also, economised compressors typically lose economiser
operation at approximately 75% slide position. Below this position, the compressor operates non economised. Most screw compressors can operate
down to 50% speed, as rated by the factory. Below 50% speed, the slide valve must be used for further capacity reduction.