Zhejiang dynamometer manufacturing factory: Telling you about the stages of development of dynamometers

What are the stages of development of dynamometers? Electric dynamometers not only can achieve energy feedback but can also be used to measure the power of motors, which is not available in other dynamometers. This is especially essential in electric shutdown test benches, thus electric dynamometers are increasingly valued. Currently, most electric dynamometers use DC motors because of their good speed regulation performance and simple control. However, due to the influence of the commutator, DC motors cannot be used for high-speed operation. Therefore, in high-speed environments, mechanical reduction devices are often used, making the system complex and increasing noise.

AC electric dynamometers do not have the commutator issue, thus they have a simple structure and high reliability. With the development of power electronics technology, the performance of AC drive systems has significantly improved in both dynamic and static states. By adopting a synchronous scheme, energy feedback occurs simultaneously during power measurement. However, when the dynamometer operates in electric mode, the higher speed in the measurement area is still limited by the synchronous speed. The dynamometer uses an AC motor with a variable frequency scheme to achieve bidirectional energy flow.

Dynamic and static performance has greatly improved compared to the synchronous scheme, but it adopts a dual control system. The motor uses vector control, making the control system complex and sensitive to changes in motor parameters. This paper proposes a new dynamic electric dynamometer scheme that uses direct torque control technology to perform torque and speed control on the asynchronous motor of the electric dynamometer, improving its dynamic performance. Direct torque control has a low dependency on motor model parameters. The three-phase PWM rectification technology generates a DC bus voltage, which can also feed the generated electrical energy back to the grid during power measurement, while keeping the grid-side current sinusoidal and the power factor close to 1, reducing harmonic disturbances to the grid and saving energy.

This dynamometer can operate in both electric and generating modes. When the test machine is a passive device (such as a fan, water pump, generator, etc.), it may be used for starting, cold running, and measuring friction power of the prime mover. At this time, the electric dynamometer will operate in electric mode. The converter on the grid side, under the control of the DSP control board, uses three-phase PWM rectification technology to bring the power factor close to 1, converting the three-phase AC power from the grid into a DC bus voltage u. This voltage is then converted back into the required speed or torque to drive the test machine through the converter on the motor side under direct torque control of the asynchronous motor.

When the test machine is a prime mover (such as an electric motor or internal combustion engine), the prime mover drives the dynamometer's asynchronous motor, causing its speed to exceed the set synchronous speed. At this time, the slip rate is less than 0. Compared to the electric operating state, the electromotive force and current direction in the rotor bars are opposite, and the electromagnetic torque acting on the rotor is also reversed, changing into a braking torque, causing the dynamometer to operate in generating mode. Under the control of the DSP control board, the converter on the motor side rectifies the generated electrical power of any frequency into DC voltage. This DC voltage is then inverted by the converter on the grid side into AC power with the same frequency and phase as the grid, supplying it to the grid, and the power factor can be controlled to be close to unity. The automatic switching between electric and generating states achieves bidirectional energy flow, realizing the full functionality of the dynamometer while addressing the energy consumption issues of other types of dynamometers, thus saving energy.