Altough manufacturers’ catalogues try to make it 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 expirience to get the selection right.
The reason why it is difficult is because there is always an engineering trade-off between 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.
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 and 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-directional or bi-directional)
- Accuracy of the speed control
- Dynamic response (speed and torque response requiremens)
- Speed regulation requirements with changes in load, temperature, supply voltage
- The duty cycle, including the number of starts and stops per hour
- Overall power factor of the drive system and its effect on the mains supply
- EMI and harmonics in the mains power supply, in the motor and motor cable
- Are EMI filter required??
- Earthing, shielding and surge protection requirements
- Torque pulsations in the rotor shaft
- Control method – manual, automatic, analog, digital, communications
- Control and communications interfaces required for the plant control system
- Indications required
- Reliability requirements, is a dedicated standby unit required
- Protection features, in-built and external features required
- Power and control cable requirements
- Parameter settings, local or remote programming
- Maintenance, spares and repair considerations
- Cost of the alternative systems, taking into consideration the capital cost, performance advantages, energy savings, efficency or process improvements.
- Noise due to the harmonics in the motor
- Mechanical resonance at certain motor speed