Understand the motor and drive design process

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The purpose of the motor is to create torque and ultimately the rotational motion of other equipment. The torque depends on a flux wave in the air gap and the relative speed of the rotor to this flux wave. The strength of the flux wave depends on the voltage and frequency applied across the stator terminals, as shown in Equation 1:

Ø = V/f

A load applied to a motor reduces the speed of the rotor (increased slip). The speed becomes constant when the motor torque matches the load torque but the rotor stalls if the load exceeds the maximum torque (breakdown torque).

A VFD controls both voltage and frequency to produce a constant flux, thereby maintaining maximum torque at various speeds. The VFD rectifies the AC power in the inverter module, then converts it back to AC power by wave modulation.

In this process, harmonic waves and short duration insulation erosion voltage spikes are also generated in amplitudes depending on the carrier frequency of the VFD and its circuitry, as well as any included filtering, if necessary. The output voltage of a VFD is critical information for the motor manufacturer. The motor manufacturer should consider the effects of the VFD in terms of size, cost, and best motor-VFD performance. Here are the key points to consider when designing a custom motor to run on a VFD.

Couple

The flux and current density create a current concentration at the top of each rotor bar (skin effect). This effect is responsible for generating the starting torque (locked rotor torque) and keeping the starting current (locked rotor current) below a target value during acceleration of the mains-powered motor.

When it comes to a VFD, the skin effect is no longer relevant to allowing a motor to accelerate and maintain low starting current. Nevertheless, it is necessary to limit the time harmonic generated by the VFD due to its ability to create heat losses. However, the breakdown torque is critical and must match the needs of the application for fast speed response, loading, and achieving the desired speed in the targeted constant power region.

Proper sizing of the rotor is essential. Increasing the number of bars of the rotor or its length, thus also changing the dimensions of the stator, are examples of design adjustments to increase torque. The VFD must supply enough current to generate the required torque during normal and overload operation, but a motor specially designed to match a VFD can limit the current to avoid applying a larger and more expensive VFD. This action results in a lower system acquisition cost.

An output wave of a 6 kV five-level VFD (Images courtesy of TMEIC)

Torque pulses

When mains power is supplied to a motor, slot magnetic leakage saturation, rotor static and dynamic eccentricities, and number of rotor and stator slots, among others, can generate harmonics at the fundamental frequency. The results are mechanical losses and stresses, which the harmonics introduced by the VFD increase in quantity. These harmonics create parasitic radial torque, leading to additional vibration and heat if not taken into account in the motor design.

Efficiency

The electricity consumption of industrial electric motors is increasingly becoming a critical criterion in motor design. Many countries have regulations for small AC induction motors (below 375kW) because they represent, by far, the largest amount of induction motors in operation and therefore have the most impact on electricity consumption.

Although there are no regulations for large machines up to several megawatts, efficiency is relevant as they are responsible for 23% of electricity consumption.¹

Efficiency is the ratio between the power delivered to the shaft and the input power at the motor terminals. The International Electrotechnical Commission (IEC) 60034-2-1 and the Institute of Electrical and Electronics Engineers (IEEE) 112 have helped industries develop test procedures to determine the efficiency of motors by testing or calculating losses. The efficiency test includes the determination of motor losses called:

  • I²R stator
  • I²R-rotor
  • friction and drift
  • core losses
  • parasitic load losses

Parasitic load losses, which occur on stator and rotor cores and other metal parts, eddy current loss in rotor bars and stator windings are difficult to predict and measure. Considering the harmonics generated by the VFD complicates this challenge.

Cooling

The heat dissipation capacity is a determining factor in the size of the machine and the respect of its temperature rise class. The motor manufacturer should design an efficient internal airflow path and provide independent external air cooling for constant torque applications.

Vibration

Standard bipolar motors have a critical speed below their rated speed, which resonates amplifying vibration to the point of equipment damage if the drive operates the motor at or near critical speed. The motor manufacturer will need to know if the operating speed range will cross the critical speed to provide a solution which may be vibration damping, modification of the motor critical speed or programming the VFD to avoid the critical speed region .

Insulation

The voltage waveform provided by the VFDs consists of pulses with rapidly rising peaks, behaving like surges in the connected motor winding. These differ from regular sine waves. The impedance mismatch between the power leads and the stator will increase the voltage by reflection. Over time, the stator insulation will deteriorate and fail if these fast growing spikes are not addressed.

An actual waveform representing total harmonic distortion is valuable information but not always provided, requiring the motor manufacturer to make assumptions. Overestimating or underestimating the harmonics leads to an additional cost for the user. In the absence of such data, a manufacturer may decide not to offer an engine due to the high level of risk.

Motors and drives

Sourcing a motor and variable frequency drive (VFD) from the same manufacturer can offer advantages. Medium voltage motors over 375 kilowatts (kW), up to several megawatts, are made-to-order, custom-designed, and expensive. Combining a motor with a VFD system designed and produced by the same manufacturer can offer:

  • maximum system longevity
  • reliable and continuous operation
  • reduced electricity consumption

If the motor and the VFD are from separate sources, an efficient exchange of information between the equipment suppliers and the system integrator is necessary to match the specified system application results.

The decision factors above do not cover all aspects when designing an electric motor or integrating a VFD, but are provided to demonstrate the complexity of the custom motor design process, which can encourage the integrator or end user to purchase the equipment from the same manufacturer.

The references

Induction Machine Design Handbook – 2nd Edition Ion Boldea/Syed A. Nasar

IEC 60034-2-1 Rotating electrical machines – Standard methods for determining losses and efficiency from tests.

IEEE std 112 – Standard test procedures for polyphase induction motors and generators.

NEMA MG1 – 2016 National Association of Electrical Manufacturers

U.S. Department of Energy – High Efficiency Motor Selection and Application Guide.

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