The headline rule from ANSI/NEMA MG 1-2021: a Part 31 "definite-purpose inverter-fed polyphase" motor is built and tested for VFD operation from day one, while a Part 30 general-purpose motor is only conditionally usable on a VFD once reflected-wave voltage, harmonic heating, and reduced-speed cooling have been addressed [S1][S3].
Selection usually comes down to three questions: cable distance from drive to motor, required speed range under load, and whether across-the-line bypass is needed [S4]. On a 480 V system with a short lead (under roughly 7.5 m / 25 ft) the practical risk of a reflected-wave insulation failure is much lower, which is why Part 30 retrofits remain common; on longer leads or 600 V-class systems the insulation rule diverges sharply between the two parts [S4].
Scope and Definition: What Each Part Actually Covers
MG1 Part 30, titled "Application Considerations for Constant Speed Motors Used on a Sinusoidal Bus with Harmonic Content and General Purpose Motors Used with Adjustable-Voltage or Adjustable-Frequency Controls or Both," treats the motor as a general-purpose machine that may be retasked to a VFD under stated application guidance [S4]. MG1 Part 31, titled "Definite-Purpose Inverter-Fed Polyphase Motors," is a product standard: the motor is designed, insulated, and temperature-rated for inverter-fed service from the factory [S1][S3][S4].
The two parts split on who is responsible for the inverter-compatibility problem. Under Part 30, the specifier carries the burden of derating, filtering, and operating-window selection; under Part 31, the manufacturer has already engineered winding stress, lamination losses, and cooling for the duty cycle [S1][S3]. On retrofit work where the motor nameplate simply reads "D.O.L. only (not suitable for V.F.D.)" (Direct-On-Line, meaning across-the-line starting only) the Part 30/31 question is moot, and using that motor on a VFD in a listed configuration is treated as a violation of the listing under NEC 110.3(B) [S5].
Insulation Withstand: The 1 kV vs 3.1×Vrated Rule
The cleanest quantitative split is in peak-voltage withstand. MG1 Part 30.2.2.8 requires that motors with a nominal insulation rating of 600 V or higher have a peak voltage withstand of at least 1 kV at a rise time of 2 microseconds or longer [S4]. MG1 Part 31.4.4.2 requires the peak voltage withstand to be 3.1 times the rated voltage at a rise time of 0.1 microseconds or longer, which on a 460 V nameplate motor (or a 480 V nominal bus) works out to roughly 1,426 V or 1,488 V depending on how Vrated is read [S4].
In practice that 40% to 50% headroom is what keeps a Part 31 winding alive when the IGBT output stage of a VFD produces a reflected-wave transient at the motor terminals. A Part 30 motor, meeting only the 1 kV threshold, depends on either a cable run short enough that the reflection does not fully develop (about 25 ft or less on 480 V) or on mitigation hardware (dv/dt filters, sine-wave output filters, or reactor at the drive) to hold the peak below 1 kV at the motor [S4].
Operating Speed Range and Thermal Limits
Speed range is the second decision gate. Part 30 machines are assumed to run near nameplate speed with limited deviation; Part 31 machines are designed to handle constant-torque operation down to low speeds, including extended low-speed duty where the shaft-mounted fan no longer moves enough air to cool the frame [S3][S4]. For variable-torque loads (fans, centrifugal pumps) the cooling penalty at low speed is small because the load torque also drops, and a Part 30 motor is frequently acceptable [S4].
For constant-torque loads at low speed, the issue is thermally bounded: the motor has to shed full I²R loss heat at reduced airflow, and only Part 31 (or a separately forced-ventilated Part 30) is rated for that envelope [S3][S4]. So-called "vector-duty" motors extend the constant-torque range further but are typically NEMA Design A (unrestricted locked-rotor current) and are not suitable for across-the-line bypass, because the inrush can trip upstream protection [S4].
Decision Matrix: Part 30 vs Part 31 vs Neither
Use the following selection rules, all grounded in MG1-2021 language from the cited material:
1. New build, 480 V or 600 V, constant-torque, full speed range, or any lead over 25 ft: specify Part 31. The insulation (3.1×Vrated at 0.1 microseconds) and the low-speed thermal rating are already in the design, eliminating the need for filters or special commissioning rules [S3][S4].
2. Retrofit, short lead (under about 25 ft on 380 V or higher), 230 V or 380 V system, variable-torque load: a Part 30 motor can be retained, ideally with a reactor or dv/dt filter at the VFD output and a baseline insulation check at 1 kV per Part 30.2.2.8 [S4].
3. Hazardous (classified) location, VFD-driven, no Part 31 option available: this is a code problem before it is a spec problem. The motor nameplate and listing drive the answer, and "D.O.L. only" marked motors in a listed assembly should not be placed on a VFD regardless of Part 30/Part 31 status [S5].
Reflected Waves, Cable Length, and the 230 V Exception
On 230 V systems the reflected-wave peak is typically clamped to between roughly 650 V and 1,000 V, which is below the standard 600 V insulation rating margin and rarely produces insulation damage even on Part 30 motors [S4]. The risk profile climbs with bus voltage, with IGBT rise time (modern drives trend to faster, sharper edges that worsen the transient), and with cable length, because a long lead lets the reflection develop fully before the next IGBT edge arrives [S4].
For systems of 380 V or higher where the lead cannot be kept short, mitigation options are: (a) a dv/dt reactor or output filter at the VFD, (b) a sine-wave filter that reconstructs a near-sinusoidal waveform at the motor terminals, or (c) replacing the motor with a Part 31 unit built for the 3.1×Vrated envelope [S4]. The "across-the-line bypass" question matters here: if the driven process must be able to start the motor directly from the line when the VFD is out of service, the motor must be capable of DOL starting, which is a constraint on Part 31 "vector-duty" machines in particular [S4].
Failure Modes, Standards, and Sourcing
Two failure modes dominate the field. The first is turn-to-turn insulation breakdown from peak reflected-wave voltage at the motor terminals, which presents as a winding short, often on the first phase, often shortly after commissioning a longer-cable retrofit [S4]. The second is stator winding overheating at low speed under constant-torque load, which presents as a thermal trip or, in severe cases, winding discoloration during a teardown [S3][S4].
Authoritative sourcing for these rules is the published NEMA MG 1-2021 standard itself, with ABB and EASA white papers (e.g. the EmpoweringPumps-hosted ABB comparison) reproducing the relevant clauses on Part 30.2.2.8 and Part 31.4.4.2 [S3][S6]. EASA's member note echoes Plant Engineering's caution: Part 30 motors may be used on VFDs "if appropriate measures are taken such as line conditioning" [S1][S6]. For applications that also need short-circuit and thermal coordination upstream, see the related guide on ANSI 27/46 protection functions in motor protection relay undervoltage and voltage unbalance settings, which covers the same MG1-driven machine from the protection-relay side. Inverter-rated motor selection for hazardous-area service, particularly fractional-HP explosion-proof frames, remains a thin part of the catalog and should be confirmed with the manufacturer before commitment, not at commissioning [S5].
Component reference pages worth checking: vfd duty motor.