Part 5

Performance

What are the key performance criteria of medium voltage drives?

Substantially improved process control is a significant additional benefit obtained by using variable speed drives.

A control system offers comprehensive, accurate control of speed and torque. Whenever co-ordination is important, the control system regulates the process and ensures that the relative speeds of motors will always be optimized.

There are several main performance related factors:

Harmonics

Users should be sure they comply with the limits or regulations which are imposed by local electricity authorities or utilities such as IEEE519 in the USA or G5/3 for the UK. Reputable drive suppliers will be able to advise on precisely where their products stand in relation to these authorities requirements.

Some suppliers have also developed harmonic analysis tools which enable rapid and efficient harmonic calculations to be undertaken.

Torque Pulsations

Some medium voltage drives produce problematic torque pulsations and the negative effects of these need to be avoided otherwise they can damage the weakest link in the chain such as the gearbox or even the load.

For drives which do produce unacceptable torque pulsations, suppliers should advise on problematic torque pulsations and a decision should be made as to whether a full torsional vibration analysis is needed. Reputable drives suppliers should be able to provide this service.

Motor Control

Innovative motor control platforms like Direct Torque Control, provide outstanding speed and torque control performance. A properly controlled motor contributes to a well controlled process which means better and consistent product quality.

Voltage Fluctuations

Wherever there is a weak network or unreliable power supply then voltage fluctuations pose a potential problem for medium voltage motors. A motor running on such a network needs to be able to ride through these transient voltage dips. Medium voltage drives should be able to handle normal fluctuations without tripping.

Medium voltage motors are also limited to the number of starts they can undergo per hour without damage occurring from temperature rise. Frequent trips can mean long process downtime for motors which are connected direct-on-line. However, variable speed drives overcome any problems by providing a soft start. Thus, unlimited number of starts can be achieved with no inrush currents.

Noise

Compared to conventional frequency drives with modulators, new motor control technologies like Direct Torque Control, do not use fixed carrier frequencies and also incorporate techniques like flux optimization - both of which provide a low motor noise. Furthermore, the use of components such as low noise fans help alleviate unnecessary high sound levels.