Matthew Conville, MBA, PE
EASA Technical Support Specialist

In today’s global economy, it is quite possible to get electric motors from all over the world. This gives us plenty of options to choose from when selecting an electric motor. Most electric motors that are 500 hp (373 kW) or less will conform to one of two basic manufacturing standards; NEMA or IEC. Both have specific clauses which define the voltage stress limits an induction machine can be subjected to in its application, which is powered by Variable Frequency Drives (VFDs)/converters. The intent of this article is to focus on IEC standards for managing voltage stress in VFD applications as it pertains to the motor’s windings. If more detail is needed on NEMA applications, please refer to NEMA MG-1 Part 31.

IEC 60034-1, Clause 7.2.1.2, states that when machines are operated with static converters, which will have Harmonic Voltage Factor (HVF) greater than that permitted in normal operation, the requirements of IEC 60034-25 apply. HVF is the permissible harmonic content of the supply voltage of a squirrel cage induction machine. IEC does not provide guidance on deration of machines for operation with static converters other than the application guidance within IEC 60034-25 and insulation requirements of IEC 60034-18-41 and IEC 60034-18-42. IEC 60034-25 provides useful information on the application and specification of electrical machines with converters, explaining how the complete drive system comprised of the supply system, converter, cabling, electrical machine, grounding system, filters (if installed), mechanical shafting and control equipment affect the performance and operation of machines. This includes guidance on machines specifically designed for use with converters as well as standard machines to IEC 60034-12 used with a converter. Insulation systems for IEC machines operated with stat ic converters must be qualified to:

• IEC 60034-18-41 (Type 1), for machines with rated voltage 700V or less where partial discharge activity is not present in normal operation (these typically will have random wire wound stators).
  
  Or to:
   
• IEC 60034-18-42 (Type 2), for machines with rated voltage greater than 700V where partial discharge activity may be present on normal operation (these typically will have formed coil windings).

The motor manufacturer should be able to supply the impulse voltage insulation class (IVIC ‘X’) for the insulation system. This informs the system integrator what the limits of the impulse rise time and peak-to-peak voltage can be experienced at the motor terminals within the capability of the insulation system. The peak-to-peak voltage can be experienced as a phase-to-phase or phase-to-ground voltage stress and the rate of change of voltage in the voltage impulse rise time can introduce turn-to-turn stress, which the insulation system must withstand. These peak voltages are a function of the incoming AC voltage to the drive system and the DC link voltage present in the converter section. Typically, we can say that the peak voltages will be 1.35 to 1.51 times the incoming AC voltage to the drive system at the output terminals of the drive. For example, a 480VAC input voltage will have a potential peak voltage and the output terminals of the drive system of approximately 650 to 725V. Figure 1 shows the impulse pattern of each Pulse Width Modulated (PWM) pulse that occurs at the specified carrier frequency of the VFD.

Ground wall insulation in the slot and phase-to-phase insulation is significantly affected by the peak-to-peak voltages at fundamental frequency and at the PWM rate. The turn-to-turn insulation is significantly affected by the impulse rise time and jump voltage. Jump voltage is the change in voltage at the terminals of a machine occurring at the start of each impulse when fed from a converter and the impulse rise time. IEC 60034-18-41 Annex C requires machine manufacturers to specify in their documentation and on their rating plate the limits for reliable performance under converter-fed conditions (impulse voltage insulation class). This is done by use of the letters IVIC ”X,” where “X” is the IVIC class in Table C-1 of the standard. Please see Table 1 for maximum allowable voltages for 500V insulation systems. To determine the actual peaks, take the values listed and multiply them by 500 (e.g., IVIC A phase to ground. 500V x 2.3 = 1,150V).

IEC 60034-18-41 Clause 7 specifies four overshoot stress categories for Type 1 insulation systems and uses a default rise time of 0.3 μs. When application of the machine is unknown, a stress category C is recommended. The benign level (A) of overshoot factor, ≥1.1 (Table 2), relates to a converter directly coupled to the machine or through a very short cable. The extreme level (D) of overshoot factor, >2.0 - ≥2.5, may occur with long cable length and when regenerative breaking can occur or in specific crane applications. The band from overshoot factor 1-1 to 2.0 is split into two equal bands to create bands B–Moderate and C–Severe. IEC TS 60034-18-41 Annex B provides an example of the maximum overshoot factors for a 500V rated winding fed from a 2-level converter.

If the IVIC stress category used is less than required for the VFD application, it is quite possible that the electric machine will not last very long in service before there is a fault in the winding. Typically, we see a turn-to-turn short as a result. (See Figure 2.)

If you want to learn more about the voltage stress, refer to Voltage Stress: Not as Simple as it Sounds, EASA Currents August 2007 and EASA’s Principles of Medium and Large AC Motors, IEC Edition. Some things to remember before we procure our next electric motor with a winding rated for VFD operation:

• Not all VFD-rated systems are created equal from a standards perspective. NEMA and IEC provide different guidelines
• NEMA MG-1 Part 31
• IEC 60034-25
  • Ask questions about application such as:
  • What voltage is supplying the VFD?
  • How far away from the VFD will the motor be in service?
  • Are there any types of output filters on the VFD?
  • Is regeneration or dynamic braking required?
  • If all application details are unknown, specify an IVIC C at a minimum if procuring an IEC machine.

Categories: Technical topics, AC motors, Drives/controls

Tags: Variable frequency drives IEC

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