Although manufacturers’ catalogues try to make it as easy as possible, there are many variables associated with the selection and rating of the optimum electric motor and AC converter for a variable speed drive (VSD) application. In many cases, it requires considerable experience to get the selection right. The reason why it is difficult is because there is always an engineering trade-off between the following:
• The need to build in a margin of safety into the selection procedure
• The need to keep the initial cost to a minimum, by selecting the optimum type and size of motor and converter for each application.
This chapter covers many of the principles for the correct selection procedure for AC
variable speed drives, which use PWM-type variable voltage variable frequency (VVVF)
converters to control the speed of standard AC squirrel cage induction motors. The following checklist covers most of the factors that need to be considered:
• The nature of the application
• Maximum torque and power requirements and how these change with speed
• Starting torque requirements
• The speed range - minimum and maximum speed
• Acceleration & deceleration requirements (Is braking necessary?)
• Compatibility with the mains supply voltage
• Environmental conditions where the converter and motor are required to operate, ambient temperature, altitude, humidity, water, chemicals, dust, etc
• Ventilation and cooling for the converter and motor
• Direction (uni- or bi-directional)
• Accuracy of the speed control
• Dynamic response (speed and torque response requirements)