How to Optimize a powertrain with multiple electric motors and controllers?

Integrating electric motors with inverters is a well-known engineering challenge, particularly when components are not designed as a unified system. This is often the case in applications where off-the-shelf motor controllers are used, requiring maximum flexibility to ensure compatibility and performance.

One particularly demanding scenario arises in high-power applications, such as electric buses and trucks, where a single motor may be driven by multiple motor controllers. Coordinating these inverters efficiently is key to ensuring reliable and synchronized system behavior.

 

 

Multiple motor controllers operating on a single electric motor

At NX, we have developed a practical and robust solution: a primary-secondary motor controller configuration that allows multiple inverters to operate on a single electric motor.

• The primary motor controller  supplies the position sensor, executes the main control algorithm (speed/torque), and transmits synchronization signals.
• The secondary motor controller receives required information and operates in coordination with the primary unit, ensuring optimal performance and precise torque or speed control.

This approach significantly simplifies system architecture while maintaining high performance. It is ideal for complex and high-power electric motors.

By allowing primary-secondary unit mode, NX delivers greater flexibility and scalability in power electronics applications. It is a valuable solution in scenarios where high power and high current demand smarter and coordinated motor controller strategies.

The diagram below aims to depict a setup for a double winding or multiphase electric motor driven by 2 NX’s controllers. Here the 3 synchronization levels are shown: position sensor level, inverters must share the same HW; PWM level to switch synchronically; and current control level to optimise phase and current sharing through the winding sets.

 

 

 

Electric motor applications

This solution opens the door to new levels of controllability in various advanced applications, including:

• Electric motors with dual isolated windings
• Multiphase machines
• Stacked motors using a single position sensor
• Back-to-back configurations with shared position feedback

The following video demonstrates this functionality using a small test bench, where the primary inverter controls a motor in speed mode and the secondary unit acts as a load by braking and regenerating energy.

 

 

How Primary-Secondary unit Mode Works

The system relies on synchronization at three levels separately:

1. Motor controller PWM synchronization

Synchronizing Pulse Width Modulation across inverters is not strictly necessary, but it helps reduce DC link ripple and switching interference. Without synchronization, PWM timing might drift between devices and impact performance and efficiency.

2. Electric machine position sensing

In a system with single position sensor and multiple inverters, the sensor needs to be read by all devices in order to have the same position measured within the controller. This ensures identical current injection in both transformed axis, direct axis and quadrature axis.

3. Electric motor current control coordination

Each electric motor controller will execute its current control on its own, being the setpoints from the same source, assuming windings are electrically identical. Nevertheless, control synchronization can be achieves as well from torque control level, having in this way a separate motor electrical model for each winding set.

In the case of a 6-phase or 9-phase electric motor, synchronization at the current control level involves incorporating appropriate phase shifts between the multiple phases. This coordination is essential to ensure a continuous and smooth torque output, minimizing pulsations and enhancing the overall performance of the machine

As a remark, in multiphase or multiwinding electric motorss this method will only work out off-the-shelf, if the electric motors’s windings do not share an end connection. 

To end with, this primary-secondary configuration provides on different levels a practical and scalable way to control advanced electric motors, ideal for high power demanding applications. It enables high performance with fewer components.

Learn more about NX’s motor controllers here.