Motor Resolvers Drive High-efficiency EV Motors

Range is a major consideration for potential buyers of electric vehicles. Engineering teams strive to increase range by reducing vehicle mass and maximizing battery size, which are clearly conflicting goals. The way out of this conundrum is to improve electric motor efficiency, and that’s done by using advanced Variable Reluctance (VR) resolvers.

This blog explains what a VR resolver is and how it contributes to electric motor efficiency. It also touches on motor resolver selection criteria and how they compare with alternative sensors.

Achieving Efficient EV Motor Operation

In an internal combustion engine (ICE) vehicle power is produced by burning gasoline or diesel. Maximizing power and efficiency while minimizing emissions requires precise control over the timing and quantity of fuel delivery. This is achieved by the electronic fuel injection system (EFI) standard on every vehicle since the 1980s.

A resolver, working with the motor control unit (MCU), performs a very similar function in electrified vehicles. Rather than managing fuel delivery though, the resolver and MCU control when electricity is delivered to the motor windings with millisecond precision.

Several types of motors are used in EVs, both AC and DC. Regardless of the design details, they all work by sensing a magnetic field in coil windings. This both attracts and repels the armature or rotor at the heart of the motor, making it move from magnetic pole to magnetic pole.

Timing is essential to maintain smooth rotation, by varying the current flowing in the stator windings. (The stator is the part of the motor that does not rotate.) This requires closed loop control of the motor, meaning the MCU needs to know the position of the rotor and its speed and direction at all times.

This is where the Variable Reluctance (VR) Resolver comes in.

Motor Resolver Basics

A resolver is a robust and reliable sensor that a converts mechanical rotary angle into an electric signal. Resolver engineering works by creating changes in a magnetic field as the motor rotates.

Like an electric motor, a resolver has two main components: a rotor and a stator. In a resolver the rotor is made from a ferromagnetic material and has a cammed or lobed profile. As the rotor lobes move inside of the stator, it creates fluctuations in the magnetic field generated by the winding. These fluctuations are converted to valuable data that drives efficient operation of the motor.

The Variable Reluctance (VR) Resolver

Reluctance is the property of resistance to the flow of magnetic flux. It’s analogous to electrical resistance. In a VR resolver the primary coils are supplied with AC, which creates a rotating magnetic field. As the distance between the rotor poles and the stator coils varies the reluctance changes, which causes fluctuations in the secondary voltage. This is output to provide the sine and cosine components of rotor angle.

Competing Technologies

Alternatives to the VR resolver are the Hall sensor and the encoder. While these are effective ways of measuring speed and position in some applications, resolvers are preferred for motor control in EVs. This stems from their accuracy, robustness and immunity to the effects of high temperatures and vibration. Another point in their favor is that they don’t lose position when power is turned off.

Why Choose MinebeaMitsumi for EV Motor Resolvers?

As motor resolvers are used for precise closed-loop motor control, it follows that accuracy and quality are paramount for meeting design goals. MinebeaMitsumi has extensive experience in winding coils for electric motors and performs the same winding for resolvers. Process control expertise honed over many years results in high density, highly uniform coils that provide accurate and repeatable outputs. Contact us to learn more about why you should be using our automotive resolvers in your EV motor designs.