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When should inverter-duty motors be specified? The Growing Use of Inverters The premature failures of motor insulation systems began to occur as fastswitching, pulse-width-modulated (PWM) VFDs were introduced. The switching rates of modern power semiconductors can lead to voltage overshoots. These voltage spikes can rapidly damage a motor’s insulation system, resulting in premature motor failure. These high voltages can break down the motor’s insulation system. Additionally, the harmonics in a PWM (pulse-width modulation) wave form can lower the motor’s efficiency when compared to operation on a sine wave. Additional heating occurs in the motor, which may also lower the amount of torque provided at low speed. How Inverters Affect Motors Early inverters or VFDs caused significantly higher temperature rise in the motor, and mismatched could easily burn out the motor. As new transistor devices and software attempted to minimize this effect, they introduced other stresses on the motor’s insulation system. It is time to design motors specifically to operate on these new power sources. New IGBT, PWM inverters can output very high switching frequencies, very rapid changes in voltage, and transient voltage spikes that can burn pin holes in the motors insulation causing short circuits and premature motor failure .Insulation systems must be improved to prevent this cause of unscheduled and costly downtime. Other issues are increased motor noise and bearing failure. A VFD driven motor can overheat if it is run too slowly as motors can get hot if they are run slower than their rated speed. Since most motors cool themselves with shaft- mounted fans, slow speeds means less cooling. If the motor overheats bearing and insulation life will be reduced. However, providing an external fan with motor will be a great idea to reduce the issues related to over-heating of motor. High switching rates of modern power semiconductors in VFD lead to rapid changes in voltage in relatively short periods of time, (dV/dt, quantified in units of volts per microsecond). Steep-fronted waves with large dV/dt or very fast rise times lead to voltage overshoots and other power supply problems. Also, when the motor impedance is greater than the impedance of the conductor cable between the motor and the drive, the voltage waveform will reflect at the motor terminals. This creates a standing wave as shown in figure. Standing waves created by reflected voltage. Figure: PWM pulse with reflected voltage Preventive measures can be taken to avoid motor failures caused by voltage spikes. These include using power conditioning equipment (such as dV/dt filters or load reactors) and restricting the distance or cable length between the drive and the motor. Inverter-Duty Motor Most motor manufacturers offer general-purpose, three-phase premium efficiency motors that feature “inverter-friendly” insulation systems. These “inverter-ready” motors are suitable for use with variable torque loads over a wide speed range. An inverter duty motor can withstand the higher voltage spikes produced by all VFDs (amplified at longer cable lengths) and can run at very slow speeds without overheating. This performance comes at a cost: inverter-duty motors can be much more expensive as compared to general purpose motors. Medium-voltage inverter-duty motors with a base rating that exceeds 600 V must be able to withstand a peak voltage equal to 2.04 times the motor’s rated line-to-line voltage. Rise times must equal or exceed 1 microsecond. However, larger inverter-duty motors often have a constant-speed auxiliary blower to provide adequate cooling for motors operating at low speeds For extreme constant-torque speed ranges, inverter-duty motors should be specified. Such inverter-duty motors may have special windings that do not allow operation without an inverter, or they may have separately powered auxiliary cooling fans. Use of inverter-duty motors for variable torque loads is overkill as inverter-ready general-purpose motors are well suited for VFD control with variable torque loads. Conclusion In summary, general purpose motors can be run with drives in many applications ; however inverter-duty motors are designed to handle much lower speeds without overheating and they are capable of withstanding higher voltage spikes without their insulation failing. With the increased performance comes an increase in cost. This additional cost can be worth it if you need greater performance.