World's most powerful gearbox with 47 MNm output torque

Model-based dimensioning and design of multi-motor drives

The size of an electric motor is primarily determined by its torque requirements. To make motors more compact, two main strategies are employed: increasing the motor speed to reduce the diameter and dividing the torque among multiple motors. Higher speeds lead to smaller, slimmer motors, although physical limitations such as machine dynamics and critical bending speeds restrict maximum length, further promoting compactness.

As motor speed increases, the rotor surface area decreases, which impacts torque generation. Due to electromagnetic constraints, the specific air gap torque remains limited, resulting in a reduction of rated power at higher speeds. To achieve high power outputs, such as 34.8 MW, multiple motors are combined, with the number of units increasing as the rated speed rises.

In multi-motor drive systems, several electric motors drive a common output via a shared gear and pinion, distributing torque evenly and reducing gear size. Adjusting the gear ratio further influences the torque reduction and overall dimensions. The design considerations include the outer diameter of the collective gearbox and the system length, both affected by the number of motors and gear ratio. In the application considered here, a configuration with 12 motors offers a compact diameter and a minimized volume and thus represents an efficient solution in terms of installation space and mechanical design.
 

Compensating the low natural frequency

When used as a wind turbine test rig drive, a multi-motor drive must meet different requirements than in applications like ships or mill drives. Its primary goal is to realistically simulate field conditions, including turbulent wind and dynamic load scenarios. Additionally, it must replicate the resonant frequency of the wind turbine’s drivetrain without the rotor, which is crucial for accurate testing. Simulating emergency stops and low-voltage ride-through (LVRT) events with DFIG systems, where rapid torque changes occur, presents further challenges, demanding highly dynamic drive responses and sign changes in torque.

The control bandwidth, determined by the torque actuation time and the drivetrain’s resonance, is critical for dynamic control. Comparing direct drive and multi-motor setups on a wind nacelle test rig, the natural frequency of the multi-motor system is at 9.5 Hz, lower than the 12.5 Hz of the direct drive. This lower natural frequency results in reduced bandwidth and dynamics, which can be mitigated using a state space controller. This advanced control approach processes multiple state variables—nacelle speed, torque, and motor speed—allowing for effective damping and closer operation near resonance.

Using pole placement and coefficient comparison, the controller ensures robust damping, enabling accurate load control up to 3 Hz. This is essential for nacelle testing, where load amplitudes are significant within this frequency range. The state space control significantly reduces deviations compared to conventional controllers, ensuring precise simulation of real-world conditions for wind turbine testing.
 

Passing through the gear backlash

A key difference between a direct-drive drivetrain and a geared solution like a multi-motor drive is gear backlash. When used in a test bench, torque changes can occur, requiring the gear clearance to be overcome. Sign changes in torque cause gear teeth to engage different flanks, disrupting the proportional relationship between load and motor torque. Applying a torque set point can lead to gear acceleration which results in noise and torque overshoots.

To mitigate this, it is essential to prevent acceleration within the gear backlash. As described earlier, a state space control approach uses feedback from motor and load speeds, enabling the limitation of differential speed during gear clearance traversal. Detecting when the system is in gear backlash is achieved by monitoring load torque; if the torque exceeds a threshold or remains low for a period, the system recognizes that it is outside backlash and deactivates the differential speed limitation.

This approach was investigated by simulation and it was proven that the differential speed limitation can reduce the torque overshoot caused by the gear backlash by up to 78 %.
 

Conclusion

In summary, multi-motor drive systems enable compact, high-power solutions by distributing torque across multiple motors. Advanced control strategies, such as state space control and the differential speed limitation, effectively mitigate disadvantages like gear backlash and low torque transmission dynamics, compared to a direct drive. This approach enhances dynamic response and reduces torque overshoot, ensuring precise simulation of real-world conditions.