Patent GB-1440790-A (GB)
Title: Method and apparatus for controlling a power supply for a direct current motor
IPC: B60L15/04
Filed: 1973-09-19
Abstract: 1440790 Control of D.C. motors AUTOMOBILES CITROEN SA 19 Sept 1973 [21 Sept 1972] 44048/73 Heading H2J A D.C. series motor 3 is controlled by a thyristor chopper 10 and by an electro-chemical generator having two groups 1, 2 of elements which may be connected in series or parallel by operation of ganged switches 5a, 6a, 7a; 5b, 6b, 7b, the arrangement being such that the power required by the motor is monitored and the resultant signal is used to control the switches so that, regardless of the speed of the motor, the groups 1, 2 are connected in parallel in all cases where the power required can be supplied by the parallel connection. The invention is primarily applicable to electro-chemical generators of the fuel cell type, e.g. metal-air, and the energy loss in a cell of this type is analysed to show that, for q groups each having a large number of elements, the ratio W 2 (S)/W 2 (P) is approximately equal to q<SP>2</SP>, where W 2 (S) is the insulation current loss for series-connection of the groups and W 2 (P) is the loss for parallel connection. Hence with the power/speed characteristics P and S shown for the motor in Fig. 2, respectively corresponding to parallel and series connection of the groups 1, 2, the energy loss in the groups is reduced by operating the motor in the zone D 2  below the characteristic P whenever possible and operating in the zone D 3  between characteristics P and S and point A when the parallel connection cannot supply the required power. Zone D 2  may be limited by the line E corresponding to constant motor torque. Switching between series and parallel connections is effected by the winding of an electromagnet 8 whose movable linkage l simultaneously operates the switches 5a ... 7b, the winding being controlled by a flip-flop 9 whose inputs 9a, 9b respectively receive signals to connect the groups in series or in parallel, the appearance or disappearance of one signal automatically removing or restoring the other. This may be achieved by controlling both inputs or arranging for the non-controlled input to adopt the inverse state to the controlled input. One or more functions may be monitored to derive a signal representative of the required power and this signal is compared with a threshold signal corresponding to the maximum value of the function for parallel connection of the groups under the prevailing operating conditions, the result of the comparison applying a signal to input 9a or 9b according to whether the prevailing operating conditions are respectively parallel or series connection. When the latter condition obtains, the input 9b is controlled by the circuit shown in Fig. 4, a contact 20 being closed for the series connection (contact 20 may take the form of an AND gate) and a comparator 19 producing an output to input 9b when a signal at input 19b, representing the average voltage U M  at the motor measured by circuit 16, is less than a signal at input 19a, representing the maximum value of voltage at the motor for parallel connection, given by U 0  - r/2I M  where U 0  is the open circuit voltage of each group 1, 2, r the internal resistance of each group and I M  the average motor current sensed by members 18, 18a. A differential amplifier 17 derives the signal U 0  - r/2I M . Alternatively it may derive a signal U M +r/2I M , the comparator 19 constituting a Schmidt trigger with input 19b receiving the signal U 0  and producing an output at 19c when U M +r/2I M  is less than U 0 , Fig. 7 (not shown). Other modifications compare the motor current I M  with the maximum value 2(U 0  - U M )/r, with I M  being represented if desired by the current I C  set for the chopper 10 by members 11, 12, 13. Fig. 5 (which may replace or augment Fig. 4) shows a circuit to control the input 9a when a parallel connection exists, changeover to the series connection occurring when contact 26d is closed (groups 1 and 2 in parallel), there is total conduction of chopper 10 and I M  is less than I C . Control pulses 21 to make the chopper conductive pass through calibrating circuit 22 and integrator 23 to a comparator 26, which produces an output when the frequency of pulses 21 is zero, this output being passed to an AND gate 27 also fed with a voltage input U IM  representative of motor current, to prevent changeover to series connection when the motor is stopped and the frequency of the pulses 21 is zero. A comparator 29 receives the signals U C  and U IM  representative of set I C  and actual motor current I M  respectively and produces an output when I M  is less than I C . If an AND gate 28 receives signals from gate 27 and comparator 29, it passes a signal to input 9a to effect series connection. This signal can be adjusted to produce the characteristic Q in Fig. 2 and so avoid continual switching at the limit of zones D 2  and D 3 . Instead of using the conduction-initiating pulses 21, one of the following parameters may be used: (1) the trailing edge (d 2 ) of the triangular waveform of motor current corresponding to offperiods of the chopper, Fig. 6 (not shown); (2) pulses initiating non-conductive periods of the chopper; (3) the D.C. voltage across the free wheel diode 15; or (4) the A.C. component of the motor voltage. In a final modification to change to the series connection, Fig. 8 (not shown and again replacing or augmenting Fig. 4), the power required signal is the difference between the set current I C  for the chopper 10 and and the average motor current I M , this difference being obtained from a differential amplifier (30) and passed via an RC circuit (31, 32) to a Schmidt trigger (33), which produces an output signal when (I C  - I M ) is greater than a predetermined value. In addition to controlling the flip-flop 9, the signal (I C  - I M ) is also applied to threshold comparator (34) emitting signals to switch the chopper 10 on and off. This modification may also include the total conduction criterion as a safety measure. Switches 5a, 6a, 7a and 5b, 6b, 7b may be replaced by electronic switches. The value I M  may be deduced from the average motor torque.