Selection process retrofitting to existing equipment:Motor suitability and de-rating

Justification

There are approximately 20 times as many pumps in service as are supplied new every year. It is therefore apparent that a major opportunity exists for modifying installed systems to make them more energy efficient.

Most system designers allow a contingency on the system head required. It is estimated that 75% of pump systems are oversized, usually by more than 20% . It follows therefore that retrofitting variable speed drives could match pumps to actual system requirements more accurately and save considerable amounts of energy.

Also many existing systems use control valves and bypasses, all of which absorb energy not required to satisfy system demands. The fitting of variable speed drives and the removal of control valves and bypasses will save energy and often the payback for the modifications is short. Just consider that a speed reduction of 10% with a rotodynamic pump will save approximately 30% of the electrical energy absorbed .

Pump manufacturers should be contacted to ensure pumps can be run at slower speeds with no detrimental effects, and the motor manufacturers to ensure motors are suitable for use with variable speed drives. There may also be other alternatives ; for example if a pump is oversized but operates at a single duty, the impeller diameter can be permanently reduced, which may achieve the same energy reduction as a VSD but at a small cost.

Motor suitability and de-rating

When considering adding a variable speed drive to an existing motor, care should be taken to match the electrical characteristics of the motor and frequency converter, otherwise the risk of premature failure is introduced into the system.

The following parameters vary from one converter design to another

and will affect the drive system performance:

• Total harmonic distortion

• Peak voltage

• Maximum rate of change of voltage

• Switching frequency

• Cable length between the motor and inverter

Early frequency converters produced outputs with a very high harmonic content in the waveform. This resulted in substantial additional heating of motor windings, and therefore motors were de-rated for inverter use. A modern inverter output causes relatively small levels of harmonic current distortion in the motor windings and therefore little de-rating is normally required. Whilst this de-rating will be minimal, it will vary from one motor design to another. It is also dependent on the type of inverter used.

Motor de-rating may be required to compensate for the reduced cooling at lower speeds when the motor shaft mounted fan is not generating the airflow achieved at normal synchronous speed. Motor manufacturers’ expertise must be used to determine how much de-rating is required and their recommendations applied. There are no definitive rules that can be applied to all motors.

In general, little or no de-rating is required with most rotodynamic pump drives, however PD pumps with a constant torque characteristic are more likely to require de-rating of the motor for reduced speed operation, although this is usually a lot less than the starting torque.

Typically, motors with a separately driven, fixed-speed, cooling fan, can deliver a constant torque continuously over a wider speed range. Above normal base speed, the drive provides a constant voltage, resulting in a weakening field. In this area the motor torque capability decreases and the motor can at best deliver a constant power. At frequencies well above base speed, the design characteristics of the motor may introduce further restrictions and specific information for the motor should be obtained from the manufacturer.

Figure 10.2 shows the effect on torque capabilities of varying the frequency (speed) of a typical motor, with a power rating based on Class B (80 Kelvin temperature rise).

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