When To Use An Inverter Duty Motor vs Standard Motor

The term “inverter-duty” refers to a motor whose speed is controlled by an inverter or VFD (variable frequency drive). While both inverter-duty motors and standard motors are capable of operating under VFDs, the main differences lie in construction and capability.

The more demanding the application is, the more specifications the motor must have to accommodate it. You can determine which motor would best optimize your operational efficiency by answering these 3 key application questions:

1 – Is the Application Constant Torque or Variable Torque?

The first factor to consider when determining whether to use an inverter duty motor vs standard motor is the torque capability. Variable torque applications, like fan and pump applications, require more torque as the speed increases. Standard motors generally will not overheat based on reduced torque and current requirements at lower speeds. Therefore, variable torque applications are typically paired with standard motors.

Constant torque applications, like conveyors or general machine applications, require the same amount of torque throughout the speed range. Because inverter duty motors are designed to operate at low speeds and not overheat, inverter-duty motors can produce a wider constant torque speed range than a standard motor.

2 – What is the Maximum and Minimum RPM Speed the Motor Will Run At?

Due to the motors’ unique capabilities, specifying the application’s range of speed can often point you in the right direction when considering an inverter-duty motor vs standard motor.

Depending on the size (HP) and type, a standard, totally enclosed fan-cooled motor is proficient for a 4:1 up to a 10:1 constant torque speed range. If the application requires a speed range beyond what is noted on the standard duty motor nameplate, an inverter duty motor should be considered. Inverter duty motors can accommodate speed ranges up to 1000:1 or more and are capable of providing full-rated torque at zero speed, as well as operating over its base speed.

3 – What is the Duty Cycle and Dynamic Performance Required for the Application?    

A motor’s duty cycle, or the number of times per hour the motor starts/stops, and dynamic performance, how quickly the motor is required to get the load up to speed, are two key indicators for determining the type of motor performance required.

In applications where speed synchronization is critical for optimum production quality, like electronic line shafting, inverter duty motors with encoders are preferred for their ability to provide rotor speed and position information. When needed, inverter duty motors can offer wider speed ranges, encoder kits and motor cooling designs for slow speed operation.