How to choose a medium voltage VFD:
Motor compatibility
VFD – motor compatibility is an essential precondition for successful operation of power drive system. There is a large amount of publications underlying this topic. This article aims to explain the matter without requiring the reader to have expert knowledge in this subject.
There are several VFD topologies ([1]). Some of them provide high quality output waveform close to a sinusoidal supply. Others have demanding chopped waveforms and require specially designed motors.
Voltage source inverters (VSI) chop the almost constant dc voltage (neglecting the ripple) and send it as voltage pulses to the motor. Figure 1 shows simplified waveforms of a 3-level inverter, 5-level inverter and inverter with integrated sine filter. To improve the voltage waveform two basic principles established. The first one is the use of multi-level inverters. The more intermediate levels the closer the voltage waveform to the sine wave (Fig. 1a). It still has a stair type waveform, but the distortion is lower. The current harmonics are reduced and voltage steps are inherently smaller meaning lower insulation stress. The other approach is to use inverter with less levels in combination with an output sine filter (Fig. 1b). VFDs with sine filter have very clean waveform comparable with sinusoidal supply from the grid, sometimes even better (since the grid can also be polluted and voltages may not perfectly sinusoidal). Another relevant parameter is the voltage rate of change, commonly called ‘dv/dt’. The higher this value the more demanding it might be for the insulation system. Modern power semiconductors are very fast with dv/dt of 10 kV/μs or even higher. Without any mitigation the motor insulation would be unnecessarily stressed and there would likely be high frequency oscillations in the inverter-cable-motor system. To eliminate such issues each VSI drive shall have proper dv/dt filters at its output.
Above principles used in VSI drives help to increase the motor friendliness and are suitable for many general purpose applications like pumps, fans etc (typically 50/60 Hz rated frequency and quadratic torque type of load). Other applications such as rolling mill drives or marine propulsion drives require specially designed motors, typically low speed or very low speed. Since these motors are anyway special the high level of motor friendliness is typically not required. Instead, the motors can be built for inverter duty considering the harmonic content, insulation stress and other requirements.
If the motors will be newly built one or the other strategy can be applied: motor friendly inverter with more ‘standard’ motor design or less friendly inverter and special (inverter duty) motor. For very high dynamic applications the inverter duty motors are the only solution as the motor friendly VFD topologies are generally not suitable. For a compressor drive with motor to be certified for hazardous area a motor friendly inverter makes usually sense. For retrofit of existing machine originally not designed for VFD operation a motor friendly inverter waveform is mandatory. Several checks shall be made to confirm the feasibility of operating under inverter duty. Motor insulation, harmonic heating, effectiveness of cooling at variable speed or bearing design are just few items to be named. The compatibility becomes even more important if the motor is certified for hazardous area and such cases shall be checked very carefully.
Current source inverters (CSI) are nowadays quite seldom with exception of very high power where the load-commutated inverter (LCI) is still very popular due to its robustness and scalability (for comparison of LCI and VSI technology refer to [4]). High power LCI drives with continuous duty typically require very specifically designed motors (overexcited machine with capacitive power factor, dual star stator windings with 30° electrical phase shift). In contrast, LCI as soft starter can be used to start practically any synchronous machine. CSI drives in combination with induction motors are very rare and actively marketed by only one manufacturer. In general, CSI drives are very insulation friendly. Current harmonic distortion and its thermal effects shall be checked on project base.
VFD – motor compatibility areas
There are two fundamental areas when checking VFD – motor compatibility. One is the motor insulation system (dielectric concern) and voltage waveform with high dv/dt, the other is the thermal aspect of additional heating due to current harmonic distortion.
Motor dielectric insulation system
– Peak voltage phase to ground
– Voltage rise time dv/dt
– Repetition rate, i.e. switching frequency
Motor thermal insulation system / temperature rise
– Additional heating due to non-sinusoidal stator current
Sound VFD manufacturers (e.g. [1]) can guide you through the compatibility matrix of inverter and motor types. Many of them also have some sort of retrofit guidelines that help you in case you want to convert existing direct on-line motor into variable speed mode or to retrofit variable speed drive system by keeping the existing motor and purchasing a new VFD.
Summary
VFD – motor compatibility of specific inverter and motor shall always be checked. When buying a motor-drive package from one manufacturer this compatibility shall be automatically guaranteed. Combining VFD and motor of two different manufacturers is generally also possible, but simply needs a closer look. When investigating the compatibility the motor specialists usually ask for peak voltages phase to phase and phase to ground, dv/dt values, common mode voltage, current harmonic distortion and harmonic spectrum, and torque spectrum with corresponding ripple.
References
[1] ABB medium voltage AC drives portfolio, https://new.abb.com/drives/medium-voltage-ac-drives
[2] ABB Motors and Generators, https://new.abb.com/motors-generators
[3] IEC 60034-18-42:2017, Rotating electrical machines – Partial discharge resistant electrical insulation system (Type II)
[4] Comparison of LCI and VSI drive technology, https://mb-drive-services.com/category/lci-versus-vsi/
