Selecting the Right Electric Motors and Drives Is Key to Vehicle Electrification

Selecting the Right Electric Motors and Drives Is Key to Vehicle Electrification

This piece was originally published in the May/June 2019 issue of electroindustry.

Mike Logsdon, Vice President of Technology, Regal Beloit

Mike Logsdon has more than 30 years of industry experience in electric motors and power transmission equipment.

The transportation sector’s systems are rapidly moving away from internal combustion engines and toward electrification. Engineers now have many design and performance options thanks to significant advances in electric motor and control technologies. They can prioritize performance requirements and their understanding of design tradeoffs to make the best choice. For a successful electrification program, here are a few main considerations:

First, engineers must choose the form factor required for the electric motor. These form factors include radial with internal rotor (traditional motor), radial with external rotor (commonly used in motorized fans), axial (thin, flat form factor), or conical (a mix between radial and axial). Engineers make these decisions mostly based on size and weight requirements:

  • Radial machines are best when smaller diameters are required and axial length is not as
  • Axial machines are best when minimum axial length is critical and diameter is flexible. Axial machines are also well suited for high torque and high-efficiency direct drive applications due to their larger diameter and inherently high pole count.

Once form factor is determined, engineers must next consider motor electromagnetics. Options include induction, permanent magnet (PMAC, PMDC, or brushed dc), and reluctance type (switched reluctance or synchronous reluctance). Many factors go into selecting the right type of motor, including efficiency, torque, speed, power density, battery life, reliability, gearing, and total system cost.

In parallel with motor selection, engineers must consider control topology and location of the control. The “control” typically consists of the power electronics that provide voltage and current to drive the motor as well as electronics that interpret input signals and control the power electronics. Variable frequency drives or inverters that use pulse wave modulation that vary the speed of the motor typically drive induction motors and most permanent magnet motors. Characteristic transportation applications may use centralized controls that drive multiple motors or a configuration of motors with their integrated controls. The physical layout of the vehicle and application parameters such as ingress protection, temperature, vibration, performance, reliability, and system cost usually determines the selection.

Relative comparison of motor type for given performance factors in electrified transportation systems

The good news is that there are many options for a system engineer to choose from when designing an electrified vehicle. Options exist with today’s technology that can easily meet—or even far exceed— the performance of existing internal combustion engine designs. Perhaps the biggest challenge is energy storage onboard the vehicle; this can be accomplished by battery power alone or by also using a small internal combustion engine in a hybrid approach.

System efficiency is paramount in selecting the right design. While most motor and control manufacturers have a very broad portfolio of ac motors and controls, they have developed few options for mobile applications with dc inputs. Custom designs may be attractive from a form factor and performance standpoint but cost more until manufacturers achieve significant production volumes.

Motor and control manufacturers are rapidly developing cost-effective, high-performance standardized platforms for vehicle electrification. Overall, the future is bright for electrification in  the transportation sector, and we can expect to see electrified vehicles of all types in the near future. ei


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