Due to the negative effects of switched power electronic drives on electric motor lifetime in certain situations, several industry standards have been developed. These standards set recommended limits on the rise time of the voltage waveform and the peak voltages allowed. Two of the most common standards are the NEMA MG 1-2016 standard and the IEC 60034-25 standard.
Both standards provide similar guidelines on the rise time and peak of the leading edge of the voltage waveform. However, the definitions of rise time are slightly different. Therefore, it is important to look carefully at the standard and how to measure the drive waveforms. The rest of this post will focus on the NEMA standard
NEMA MG 1 Definitions
The NEMA standard defines rise time as 10% to 90% of the steady state voltage. The standard defines the peak voltage Vpeak as the zero to peak line-to-line voltage. Figure 1 shows the rise time and peak voltage of the waveform measured line-line at the motor terminals as defined by the NEMA standard. The plot shows a zoomed in view of the leading edge of a voltage pulse.
The allowable limits for the NEMA standard also change depending on the rated voltage of the system. The limits also depend on whether the motor is a general purpose motor or an inverter-rated motor.
General Purpose Motors
NEMA MG1 Part 30 specifies allowable rise times and peak voltages for general purpose electric motors. These motors are typically connected to sinusoidal line voltages or older thyristor-based drives. The allowable rise time is relatively slow, since some of these motors may not be able to handle the faster rise times of Silicon IGBT drives.
Inverter-Rated Motors
NEMA MG1 Part 31 specifies allowable rise times and peak voltages for inverter-rated motors that are directly connected to Silicon IGBT-based drives. Section 31.4.4.2 specifies allowable rise times that are much shorter than the Part 30 standard. This allows for higher dv/dt than for general purpose motors. The peak voltage also changes based on the system voltage, with different limits for absolute peak voltage, and peak voltage for partial-discharge-free operation.
Are your system waveforms NEMA MG 1 compliant? Head over to the Voltage Waveform Simulation Page on this site to see how the fast rise times of drive waveforms affect overshoot and ringing at the motor terminals. The simulation allows you to test how filters may bring voltage waveforms into compliance with your specifications.