Issue 5/2004
09/02/04
Fascinating DC technology: ride quality unrivalled!
Mechatronics for Lift Engineers and Drive Technicians
Dipl.-Ing. Götz Benczek
Many readers have asked us to pick up the story of the “revamping of old DC gearless machines” again. In the meantime the experience gathered with machines with brushgear has grown considerably. The modern IGTB-based drive technology combined with old DC machines provides an excellent ride quality at maximum efficiency.
Category: Issue 5/2004
Posted by: Editor
But a few things need to be taken into account if you want the system to operate with the usual reliability for many years to come. Especially the often very low armature voltage and the electric strength of all windings against earth must be taken into consideration. There are only a few products able to stand the intermediate circuit voltage or high dU/dt of modern converters.
When the armature voltage is distinctly below 300 V, an isolating transformer is usually required upstream of the converter which also assures the electric strength against earth. High-quality contactors need to be provided in the armature circuit which in emergencies (i.e. high DC current and high DC voltage) can interrupt the supply of energy to the armature circuit. But the contactors no longer need to be dimensioned to provide a steadily recurring isolation under load. The power varistor of the DC motor armature is a particularly important component as it prevents the collector from being damaged when the safety circuit is opened. Depending on the performance of the motor, the varistor has a tablet size between 40 and 80 and a voltage matching the effective intermediate circuit. A converter with a 3 AC 230 V network therefore requires a 300 V varistor at the armature winding (e.g. type SIOV B60K300).
Fig. 147 shows the switching proposal for the power circuit (R3 = 2k? 0.5 W carbon film)
Fig. 147
Note: there must be no contactor in the field circuit. There are only two contactors in the armature circuit, the three pairs of contacts of which are each connected in series in the above example.
Fig. 148 shows that the typical parameters and variables are not much different from conventional “brushless” drives (asynchronous or synchronous).
Fig. 148
Parameters “F1” and “F2” as well as variables “0E6A” and “0E6E” are new:
“F1” (Nnom – Imax) is the speed up to which the operation is effected with full armature current. From “F1” onwards the armature current is limited by the commutating limit “0E6E” up to the “F2” speed.
“F2” (Nmax – Inom) is the speed up to which the commutating limit “0E6E” is supposed to run. After “F2” the field suppression is starting (the field being reduced to half as a maximum).
“0E6A” (field current) indicates the field winding value in “mA”. The field as well as the armature currents are controlled so that the magnetic field does not depend on the temperature.
“0E6E” (commutating limit) is indicated as a nominal current percentage. Inbetween the “F1” and “F2” speeds the current is e.g. limited to 90 % of the nominal current.
Fig. 149 again displays the connection between “F1”, “F2” and “0E6E”.
Fig. 149
Up to the”F1” speed the full DC converter dynamics are put at disposal. From the “F1” speed up to the “F2” speed it is possible to limit the armature current in order to protect the commutator. Cell “0E6E” indicates the percentage (referred to the motor’s Imax rating plate) by which the current has to be reduced.
From “F2” onwards, up to 2 x “F2” are linearly run into the field suppression. In this particular case it must however be taken into account that both functions are not always parallelly available. One can either select a field suppression or adjust the black band:
Example 1 (black band as from 135 rpm up to the end speed of 159 rpm):
F1 = 135
F2 = 159
F2 = 159
0E6E = 70 %
In example 1 the “F2” speed corresponds to the maximum operating speed of the machine (e.g. for 2.5 m/s).
Example 2 (field suppression as from 110 rpm up to e.g. 220 rpm with a 1 to 2 ratio):
F1 = 109
F2 = 110
F2 = 110
0E6E = 90 %
In example 2 the motor also has to run up to 159 rpm but with a field that is suppressed by a factor 1.34 (the field current must e.g. drop from 8 A to 6 A). With this setting the field current is reduced to 50 % from “F1” = 110 rpm up to 2 x “F2” = 220 rpm. For 159 rpm this results in 75 %. On the other hand the value in “0E6E” has practically no effect since “F1” and “F2” inevitably are very close.
Just like the brushless gearless machines, the DC machines with brushgear are equipped with incremental transducers (instead of the old tachometers) which need to provide 2048 pulses (with four 1 Vss levels) per motor revolution. Fig. 150 and Fig. 151 show the typical ride curves of a “Dover” DC machine (empty up, empty down):
Fig. 150 clearly shows how the influence of the rope weight on the armature current grows.
Fig. 151 shows the maximum torque demand when the cab leaves the top landing.
Fig. 152 shows a Siemens machine managing without a transformer.
Fig. 153 shows how a rotary transducer is attached to a Dover DC machine.
Fig. 154 shows the armature’s bevel groove of the Dover machine (with dismantled field winding).
Fig. 155 shows the internal drum brake of an Otis DC machine.
Fig. 156 shows two of the four carbon brushes on the collector inside an Otis DC machine.
Fig. 157 shows two of the four field windings with one of the four commutating windings (Otis).
Fig. 158 shows how the rope is guided in a double wrap on an Otis DC machine.
1) Refer also to earlier articles in LR 1/2001, p. 50 – LR 2/2001, p. 43 – LR 3/2001, p. 38 – LR 4/2001, p. 52 – LR 5/2001, p. 82 – LR 6/2001, p. 94 – LR 1/2002, p. 58 – LR 2/2002, p. 34 – LR 3/2002, p. 46 – LR 4/2002, p. 67 – LR 5/2002, p. 50. – LR 6/2002, p. 42 – LR 2/2003, p. 18 – LR 5/2003, p. 136 – LR 6/2003, p. 22 – LR 1/2004, p. 26 – LR 3/2004, p. 20 – LR 4/2004, p. 50.
5/2004
