When using a VFD, it is important to select the proper motor and speed to provide the torque needed to drive the application. This is extremely important with any new installation or modification of an existing system. The motor must be selected based on its construction (open motor, closed motor, low inertia or high inertia, etc.), shaft size, bearing rating, etc. and should be sized to match the driven device.
In some cases, selecting the right motor and speed will not be enough. Additional torque may be needed for the VFD to properly operate the application when the motor speed is reduced. If extra torque is needed, select a higher-horsepower rated motor and/or a higher starting torque.
When selecting a VFD, it is important to always ensure that a properly rated motor is used. If the motor is too small for the application, it may lead to problems when the motor is operated at lower speeds. This could include a higher service factor, a higher duty cycle or a higher starting torque. It is important to select the VFD and motor that provide the necessary torque for the application.
When selecting a VFD, it is also important to calculate the power requirements of the motor. This will depend on the type of motor being used and the application. Once the power requirements are known, the VFD must be selected to be able to safely operate the motor.
Frequency response of the motor is another important factor. VFDs require that the motors torque-speed characteristics properly match the application. This means that the motor must be able to react quickly to changes in speed so that the torque and speed of the driven device remain in balance with the motor. If the motor cannot respond quickly enough to changes in speed, the application may run at an incorrect speed or torque. If a motor has a larger speed range than the VFD then the response time may not be fast enough and the VFD may be unable to provide the necessary torque for the application.
VFDs are an extremely popular choice for variable speed applications and provide a number of advantages. Properly selecting the right motor and VFD is key to ensuring that the system will operate at its best. Selecting the right motor and speed that match the needs of the application as well as taking into account frequency response and power requirements will help ensure that the VFD will be able to provide the correct torque needed for the application.
