Considerations for using VFDs with standard motors
Motors that meet the requirements of NEMA: MG1 Part 31 are designed for use with variable-frequency drives (VFDs). Motors that meet the requirements of NEMA: MG1 Part 30 may be suitable for inverter duty if appropriate measures are taken such as line conditioning. End users desiring speed and/or torque control often procure and install VFDs to modify existing applications where a standard-induction motor is in place. Frequently, they try to control costs by using the existing motor. There are a few areas of concern involving misapplication of a standard induction motor.
Motors meeting the requirements of NEMA: MG 1 Part 31 have defined speed-torque characteristics which is shown in Figure 1. Figure 2 shows a typical speed-torque curve for an induction motor with fixed voltage applied to the machine terminals that results in acceleration, according to the machine dynamics. Point 3 in Figure 2 represents the speed at rated or full-load torque and corresponds to Point 3 in Figure 1. Using a standard induction motor with a VFD without proper evaluation to determine Points 1, 2, and 4 from Figure 1 introduces the potential for overheating in the lower speed range (below Point 3) and mechanical damage from over speeding (beyond Point 3).
Shaft currents are another major concern. The high-switching frequency associated with inverter operation produces a capacitive coupling between the rotor and stator, which can lead to shaft currents that damage the bearings and lubricant. Motors designed for this type of operation are often constructed with insulated bearings and shaft-grounding brushes. These modifications can often be made to standard motors.
Standard-induction motor stator windings usually are not insulated for use in VFD applications. Most machines designed for inverter duty use a modified magnet wire. The ground insulation may also be enhanced, and more robust coil bracing is common.
It’s important to establish a low-impedance, common ground between the motor drive and electrical system. Cable manufacturers have designed products specifically for this purpose (see Figure 3).
Service centers can modify existing machines to address potential issues with bearing insulation and stator-winding insulation. However, defining a speed-torque curve to a standard motor, as shown in Figure 1, isn’t an easy task. Variable-torque loads such as fans and centrifugal pumps,are less risky candidates, providing the maximum operating speed doesn’t exceed the motor’s base speed. Constant-torque loads like conveyor belts would be more susceptible to overheating in the low-speed range. The most conservative approach is to procure an inverter-duty motor that’s appropriate for the application. If the goal is just to limit starting current, a simpler option is a variable-voltage, fixed-frequency soft starter.
-Mike Howell is a technical support specialist at the Electrical Apparatus Service Association (EASA). EASA is a CFE Media content partner.