Test-Bench for Ultra-High-Speed Electrical Drives (Test-Bench #8)
Capability of the Test-Bench
P = 38 kW, T = 30 Nm, n = 12.100 min-1 / nmax = 140.000 min-1
Principle Design of the Test-Bench
The design of the test-bench is composed of a high-speed inverter-driven induction motor as a load, a gear-set to reach an ultra high-speed, and a torquemeter specially designed for this speed level. In addition, a controlled cooling system is applied to guarantee certain operating points during the measurements.
The principle set-up is shown in the following block-diagram:
Components of the Test-Bench
a) Load
The load of this test-bench is composed of a 4-quadrant power electronics supplying the high-speed induction motor. The inverter is operated with a switching frequency of 16 kHz and a maximum power rating of 52 kVA. The induction motor has a maximum speed of 25.000 1/min, with a field weakening range of 2:1. The maximum torque is 30 Nm. The induction motor itself is supplied by a frequency inverter and control unit. As a consequence the motor may be operated with field oriented control (FOC) of just by using a voltage-frequency-characteristic. The closed-loop control region (torque or speed control can be selected) is limited to a maximum speed of 16.000 1/min. In the region between 16.000 1/min and 25.000 1/min just open-loop control is possible. The reference value input may be realized by setting the system parameters via "Combivis", by analogue input (e.g. potentiometer), or by digital input (e.g. keyboard or bus). The bus used in our set-up is the CAN-bus.
b) Gear-Set
The gear-set, which was especially designed and built for this test-bench, offers different, selectable ratios: 1:1, 1:3, and 1:9. In addition, the maximum speed of this gear-set is limited to 180.000 1/min. This limitation comes from the fact that the high-speed bearing at the output of the gear-set is not capable to reach higher speeds. Moreover, this high-speed bearing requires an oil-pressure supply.
c) Torquemeter
Measuring torques at that high speed is a very delicat business. In this test-bench a measuring unit for torque and speed has been introduced, that is capable of a maximum speed of 140.000 1/min. In addition to this limitation, which limits the maximum speed of the entire test-bench, even the bearings of this component have to be supplied by an external oil-reservoir. This oil-supply is a different one to that used for the high-speed bearing of the gear-set. The software driver to read out the torque values has been realized with the software package LabView.
The torque-speed-characteristics of the different parts of the entire test-bench (load motor, gear-set, torquemeter) are shown in the following figure:
d) Safety means
Testing electrical motors at ultra high-speed means that in the rotating masses an extremely high energy is stored. Even in the case of malfunction, the use of the test-bench must be safe for the people operating this set-up. To guarantee this, a box has been built covering all rotating parts. This box is made of a sandwich structure with hard and soft material. The soft material is used for energy absorbing means, the hard material guarantees that no particles will get out of this box.
e) Controlled cooling
The thermal behaviour of electrical drives critically determines the performance characteristics. Therefore, the knowledge of the thermal state of the drive is an essential prerequisite for a detailed analysis (e.g. efficiency measurements). This test-bench is equipped with a means for controlled liquid cooling of the test objects. The parameters are: Maximum cooling power Pmax-cool = 13 kW, controlled flow Φmax = 60 liters/min, and controlled temperature of the cooling liquid θ = -10 ... +90 °C.
f) Linear amplifier
The entire test-bench is completed by a linear amplifier of maximum power Smax = 5.25 kVA in the frequency region of f = 45 Hz ... 5 kHz. This linear amplifier is used to drive the test motors in addition to an inverter supply. From a comparison the influence of the inverter supply on the test drive can be measured. Such a comparison is of special interest in the ultra-high-speed region (e.g. because of possible additional iron losses as a consequence of the inverter supply).
g) Data acquisition
A very sophisticated data acquisition system is required for such a complex test-bench. To have a single system for operating the test-bench as well as for data acquisition, the software package LabView is used. Measuring devices like a Yokogawa Precision Power Meter or the Torquemeter (see section c)) are included into the set-up. The user interface realized with LabView is as simple as possible to guarantee high concentration of the people operating the test-bench: just the values of speed, torque and power are displayed, together with the temperatures at some critical spots. The measured data can be stored directly to an excel-file.