As production shifts from traditional ICE-powered cars to Electric Vehicles (EV’s), vehicle manufacturers and OEMs face many challenges.  One of these challenges is how to test, characterise and optimise the electric motors used in the vehicles.  This is particularly challenging in Motorsport applications, where motor speeds can approach 30,000rpm.

There are many types of motors used in EV’s, including induction, SPM (Surface-mount Permanent Magnet), and IPM (Interior Permanent Magnet), but the trend is definitely toward IPM motors.  The IPM design is attractive because of high power density, high efficiency, and mechanical robustness, compared to the other motor types.  The only downside is that IPM motors are more challenging to properly control.

Modern inverters that are used to control electric motors use vector control.  They regulate the motor current along two vectors that are 90 degrees apart (generally called Id and Iq).  In a typical SPM motor, the inductance seen along the d-axis is essentially the same as the inductance seen along the q-axis.  However, the topology of the IPM motor creates saliency, which means the inductance along the d-axis is not the same as the q-axis.  Additionally, this inductance will change depending on load, as the iron core of the motor rotor saturates.  In an SPM motor, this saturation tends to affect both d and q axis inductance equally, but this is not the case in an IPM motor.  Finally, because of this saliency, an IPM motor is able to produce both magnetic torque and reluctance torque (while an SPM motor has only magnetic torque).

So to properly and optimally control an IPM motor, the inverter must be able to account for the varying d-axis and q-axis inductance as a function of both speed and torque, and it must be able to adjust the phase angle of its current in order to make optimal use of both magnetic and reluctance torque.  Most generally-purpose inverters are not able to do this. 

Vehicle based inverters are purpose-built and matched to specific IPM motors.  However, Unico offers specialist test stand inverters with multiple features designed to allow them to control a wide range of IPM motors from any manufacturer.  Firstly, Unico offers the ability to operate in direct current control, where the inverter will follow commands from an external controller.  These external commands can be in the form of either id and iq (current vectors), or current magnitude and phase angle.  Next, Unico has built tables into the drive that can be used to store the mapped characteristics of an IPM motor.  Once the map data has been entered, the Unico inverter is able to control the IPM motor just like any other motor, regulating torque, speed, or position as required by the specific application.  The inverter will be able to take advantage of both magnetic and reluctance torque and produce the optimal torque per amp.  Finally, Unico has included automated setup routines that can be used to characterise an unknown IPM motor and generate the map data.

Aside from the challenges inherent in controlling IPM motors, speed can be a big challenge when it comes to testing EV motors.  As speed increases, so to does the electrical frequency of the motors. 

Modern inverters use transistors (typically IGBT’s) to create a PWM voltage and current to the motor.   In order to produce the sinusoidal current required by the motor, the switching frequency of the inverter needs to be more than an order of magnitude faster than the electrical frequency of the motor.  Consider the case of a 6-pole motor rotating at 30,000rpm.  Such a motor will have an electrical frequency of 1500Hz.  For proper control of such a motor, a switching frequency of 18-24kHz would be recommended.  Unico has built inverters with switching frequencies as high as 30kHz,  this can be fine-tuned based on the application and the specific motor being used.

 

UNICO

Unico was founded in 1967 in in Franksville Wisconsin and is a specialist manufacturer of high performance drive systems for many different industries including:

• Automotive Test Stands
• Oil and Gas
• Process Engineering
• Rail

Unico has specialist test stand divisions based in the USA, Canada, United Kingdom, Germany as well as an independent distributor in China.

Over 260 automotive manufacturers, independent test facilities and motorsport teams rely on test equipment powered by Unico drive systems.

For further information please contact:

Andy Gardner - Sales Manager

Unico (UK) Ltd
Garamonde Drive
Wymbush
Milton Keynes MK8 8LF
United Kingdom

Tel:-        +44 1908 260000
DDI:        +44 1908 268209
Mobile:    +44 7850 665940
E-Mail:     [email protected]