Passenger Lifts with regenerative power supply

The manufacturers of lifts are coming under increasing pressure to design mains friendly systems, i.e. with the lowest possible mains pollution. Furthermore, the necessary rectifier units should require less maintenance and be more robust to mains faults than conventional equipment. Energy costs must be reduced by feeding back the braking energy that arises regularly with lift operation. Schindler Aufzüge AG commissioned IAM GmbH to develop the corresponding equipment that would take these criteria into account. In the meantime the resulting solution is being incorporated in the most modern lifts of Schindler.

These requirements led to the development of a digitally controlled rectifier unit that is much better with regard to mains pollution than even the most stringent standards. The control is virtually maintenance-free because digitisation ensures that the controlling parameters remain stable once they have been adjusted, i.e. independent of ambient temperature or component ageing. Moreover, the considerable braking energy that is generated is returned to the mains. The control is extremely robust in regard of all types of mains faults. The production costs for the control cards are very favourable due to the fact that the control card for the field-orientated controlled lift motor is of almost identical design. The control cards have an identical layout and differ only slightly by their assembly.
Figure 1 shows the configuration of this feed-back capable driving unit.


Fig. 1: Configuration of the drive systemt

Complete control of all kinds of mains faults

In Europe voltage changes are limited to maximum 10%. In the far East, however, there are many weak networks with voltage changes that greatly exceed this value. The step-up converter presented here can compensate such voltage fluctuations and supply the necessary voltage for the driving motor.

Furthermore, brief overvoltages and voltage dips are quite common. They are usually in the order of a few milliseconds, differ significantly from the rated value and their rise time is very short. Such faults are no problem for uncontrolled diode bridges. However, it is not so simple with controlled power converters. Therefore, it has been stipulated that it must remain stable for at least 10 ms (half a mains period duration) before switching off.

Changeover from the mains to the emergency power pack and back again can result in considerable sudden phase changes which do not cause any instability or trigger overcurrents in the rectifier unit. Furthermore, phase failures and asymmetrical phase voltages are not a problem for the rectifier unit.

Lowest mains pollution fulfils all standards

Sine-shaped current control reduces the mains pollution of the unit to such an extent that the voltage waveform of the given mains remains virtually unchanged. When under load the rectifier unit behaves in the mains like a three-phase symmetrical ohmic resistance. Contrary to the uncontrolled bridge rectifiers still used in many units and which level out the voltage peaks in the mains (harmonic content increases) because they can only draw current from the mains at maximum voltage levels, the rectifier unit withdraws current continuously throughout the entire voltage characteristic, thereby only causing harmonic waves that are below 1% in their entire spectrum. This means that the rectifier unit fulfils even the most stringent standards. The power factor is so close to 1 that, for practical purposes, it can be equated with 1. No quality difference regarding the current waveform was found between rectifier and feed-back. As a result of the adjustable phase shift in conjunction with the possibility to predetermine the reactive-current amplitude, a reactive power can be adjusted which makes the unit a phase modifier. In this manner a consumer can compensate its tapped reactive power at the mains connection so as to draw energy from the mains at a favourable cost.

The control process

Figure 2 shows that, as a control system, the rectifier unit only has to control the inductivity of the compensating reactor with the mains voltage as the influencing variable. A phase-locked loop (PLL) records the rotating angle g g of the mains voltage UL. The currents are measured and displayed, by way of input transformation, as a co-ordinates system rotating with the mains voltage (Fig. 3). By correcting the angle by p/2, the current IGd equals the reactive current, and IGq the active current of the rectifier unit with regard to the mains (Fig. 2). These currents can be impressed by two independent current controllers (Fig. 3), and thus the phase angle j between the current IG and the mains voltage UL. Normally, the reactive current IGd is maintained at zero with the result that the current indicator IG falls together with the mains voltage UL (j = 0). The intermediate circuit voltage is only controlled by way of the reactive current IGq (Fig. 3). The voltage controller provides, accordingly, the target value of the cross current IGq for the intermediate circuit. The intermediate circuit voltage is maintained at a value of 800 V.


Fig. 2: Voltages and currents at the rectifier unit

The special feature of the control structure shown in Fig. 3 is that, as a result of transforming the co-ordinates with the mains angle in stationary state, the current and voltage controllers only have to control the zero-frequency variables. Precontrol of the measured mains voltages at the outputs of the current controllers means that the current controllers only have to control the voltage drop by way of the compensating reactors. This permits low-level limitation of the current controller outputs which is desirable for safety reasons. In the event of a brief mains voltage failure due to a fault, then precontrol of this voltage also fails so that there is no unwanted current increase in the compensating reactor.


Fig. 3: Mains angle oriented control of power supply

Implementation

The control algorithms were implemented in the dSMC® signal processor developed by sci-worx - Industrial Systems. The language of the signal processor can be adapted to the requirements of electrical drive control. The control is operated with a switching frequency of 10 kHz and a scanning frequency of 20 kHz. The mains voltage measuring channels are alternatively used for the tracks of a sinusoidal transmitter so that an asynchronous motor can be controlled with the same control card.

A detailed description of this development, combined with extensive documentation of the fault behaviour and harmonics, is given in the subsequent publication, which was jointly produced with Schindler Aufzüge AG:

Ehrenberg, J; Drossmann, J.; Fan, Z.; Eugster, R.:
Aufzugsteuerung vollständig digitalisiert. Elektronik 1997, No. 12, pages 72, 93...100