What is a back-to-back test bench? A back-to-back test rig is the mechanical and/or electrical coupling of two test items, such as two motors, on a test bed. These two motors are connected. One motor acts as the drive and the other acts as the generator, forming a closed circuit.
A back-to-back electric motor test bench offers several advantages. These include dynamic load testing, power electronics testing, and increased flexibility for powertrain testing and measurements.
Dynamic load testing
When testing different load conditions, a back-to-back test bench offers the advantage of optimal matching of motor and load dynamics, since the motor and load are the same components.
In contrast, on a conventional motor test bench, the rotor inertia of the load is usually greater. Thus, a back-to-back test bench provides a more accurate evaluation of the performance of an engine under different operating conditions.
Power electronics testing
By connecting the two motors on the bench via a common DC link circuit, back-to-back test benches can be used for the testing of power electronics components. A common test might be to check the performance and reliability of converters or DC link capacitors.
A back-to-back test bench offers a high degree of flexibility in performing tests and measurements. By using two motors, it is possible to simulate different operating modes, including forward and reverse operation, different load profiles, and different speed and torque conditions. This makes it possible to test the motor under realistic conditions and make a comprehensive assessment of how it performs.
Overall, a back-to-back electric motor test bench provides the ability to accurately and comprehensively evaluate motor performance, efficiency and reliability. By simulating real-world operating conditions and measuring the flow of energy in both directions, the test bench is able to accurately determine the performance of the motor and evaluate its energy efficiency.
Higher Energy Efficiency ... Because the tester uses two motors working against each other, the energy generated during braking can be recovered and reused, resulting in higher energy efficiency.
Cost Savings ... Significant cost savings can be achieved by eliminating the need for expensive load machines, including controlling them. Only the energy losses need to be supplied.
Time Savings... Since the motor and generator are tested simultaneously, a back-to-back test bench saves considerable time.
Equivalent Component Characteristics... By matching components with characteristics such as power and torque, the drive and load machine always match.
Compact design... A back-to-back test bench can be easily integrated into a climatic chamber.
To determine the operating point of the two test specimens, it is necessary to measure speed and torque.
A technically advanced, yet simple data acquisition solution is to use a wireless telemetric torque and speed measuring system. It is embedded in a compact shaft housing that can be easily mounted on the axle of any test specimen. The transmitter unit is powered by an induction coil.
In addition to the easy mechanical integration into the existing test bench structure, other technical features of this measuring device, called imc Dx, include
- Torque measurement up to 5 kNm
- Speed measurement up to 3000 rpm
- Operating temperature range from -40°C to 125°C
- Strain gauge installation and torque calibration up to 5 kNm
- Easy integration into a test bench automation system
- Receiver unit with CAN and EtherCAT interface
Integrated power measurement
When using the imc Dx telemetry, users also have an innovative complete system for measuring the power of rotating components. In addition to the torque, the speed of the shaft can also be recorded and the power is calculated online from these two variables. The MEMS-based speed sensors are fully integrated into the shaft housing and do not require any additional stator systems.
Back-to-back test bench application example
The illustration shows a back-to-back test bench for electric motors and components that was implemented with imc measurement technology for a German automotive manufacturer.
Author: Rolf Spellmeyer, Business Development E-Mobility