Current regulator for electro systems
Current Regulator for switchable magnetic systems
Every power controlled 1Q control system can be used for the operation of electrical systems. As usually only 2Q and 4Q control systems are commercially available and these cannot be applied universally owing to their controlling accuracy, a relatively economy-priced 1Q control system was developed. The current control card was especially developed for the switching of inductive loads, like e.g. of electrically switchable magnetic systems - according to the principle of field displacement - or purely electromagnetic systems. From the point of view of control technique the system in question is the principle of a "low setting circuit".
The current regulator is certified according to the European Conformity law and therefore carries the CE mark. The European standard is described in EN 50081-2 and EN 50082-2 for industry electronics. To reach this conformity it is necessary to use a shielded wiring from the current regulator to the magnetic systems. And additional to that, it is recommended to install a EMC-filter directly behind the mains supply and before the input of the current regulator as it is normally used by electronically driven motors (e.g. brushless motors).
Principle of field displacement (compensation).
This concerns monostable magnetic systems being magnetically neutral in case of current conduction. In order to achieve this field displacement during the current conduction a stabilized current (seen from the temperature) is required so that the heating- up of the coil is irrelevant. A well defined temperature curve considering the heating- up of the permanent magnets would be advantageous. However this is not really required, since this temperature coefficient is clearly smaller.
Electromagnetic system
To prevent the attracting force from being dependent on the coil temperature in a purely electromagnetic system, a stabilized coil current is required here as well. This is all the more urgent, if safety applications are concerned, where a certain minimum holding force has to be guaranteed.
In order to meet this requirement and save power at the same time, i.e. minimize dissipated energy, a current control card was developed which operates according to the principle of pulse width modulation.
1. Functional description
Special emphasis was put on the potential isolation between input and output circuits. The selection is isolated by optocouplers in the input circuit. Due to the broad power supply input the current regulator may be applied to many different sources. The internal placed B6 rectifier bridge allows an operation from AC to DC current whatever can be provided. Therefore it may be used for a lot of other applications for a power supply. The mains supply can be taken from the three phases mains supply from 100 up to 480 VAC as well as DC current from 130 up to 500 VDC. Even one phase or two phase supply is allowed. The output current can be set from 1,5 A up to 15 A. This current is controlled at the output unless the input signal is reset from 24 VDC to zero. The actual current value is admitted via a measurement resistor - with the potential isolated - to the pulse width modulator to be compared to the nominal value. The full isolation of the potential of the selection circuit, control and power circuits results in feedback-free and troublefree operation. If, however, trouble occurs due to inadmissible loads, the 'o. k. message' (24VDC during operation) is reset. This 'o.k. message' is granted 100ms after the input signal is raised. In the case of a short circuit or power fail this signal is at once resetted. This gives the PLC a chance to detect the proper operation.
2. Selection
There are different possibilities of selection. Only the 24 V level is prespectified. The user can arbitrarily perform the selection via a potentialfree relay contact or a potential- connected output of a free programmable control system. In any case a supplementary low- pass filter is provided eliminating interferences with higher frequencies.
2.1 Inputs
All along the time, when the input signal goes up from zero to 24 VDC, the output current is fixed to the predicted value. This predicted current may be set to a certain manually adjusted value between 1,5 up to 15 A. The current conduction is stabilized during the time of selection. To read the actual current setting, it is advised to measure the output current by a current measurement instrument. If it is not necessary to know the exact current, it is possible to measure the voltage at the analogue input because the voltage at this point corresponds to the output current. The analogue input voltage reaches from 1,5 VDC to 10 VDC and this corresponds to an output current of 1,5 up to 15 A.
2.2 Current Setting
The rated value of the current to be stabilized can be set independently of the selection of the inputs. In order to adjust the output current there is provided a pushbutton . Operating the pushbutton selects the trimming potentiometer used to set the rated value of current. An amperemeter can be integrated into the output circuit to display the value of current.
3. Control behaviour and error indication
The current control card includes different control and monitoring functions which all guarantee safe working and safe operations. A more detailed description of the control behaviour and the safety functions follows below.
3.1 Monitoring the Rated Value of Current
The rated value of current is stabilized by the pulse width modulator. The pulse width on the output side is set automatically - depending on the rated value of current -between 5 % and 95 %, which leads to a current stabilization of approx. 1 %. A special resistor inside is responsible for the feed back of the output current to the comparator circuit. This resistor gives additional the signal, whether the current is too high (short circuit) or too low (wire break). Both cases are monitored and registered. If the output current is different to the preset current, the signal for 'current o.k.' is reset at once. This may be monitored by a PLC.
3.2 Short- Circuit Behaviour
The current control card is provided with an electronic short- circuit protection protecting the power stage (transistor) and the subsequent wiring against overloads.
Short-circuit monitoring
The drop in voltage at the measurement resistor is monitored 4000 times a second. If the nominal value is exceeded, the further selection of the output stage is omitted and the signal for proper operation is reset. This signal raises normally 100ms after selecting the input. If this signal is reset from 24 VDC to zero during the normal selection process, there must be a mistake saying ‘error current’. A short circuit or a not allowed overload may trigger this error. Even a short circuit against earth potential is recognized and does no harm to the current controller. The only possibility to damage the controller is a short circuit against earth potential in the positive power supply. This would damage the B6 rectifier. But a possibility is planned to change the rectifier. But this should not be done by the customer.
3.3 Line Break Behaviour or mains failure
Further trouble can occur because of interruptions of the wiring or line break. In order to detect this kind of operational trouble as well, a minimum value of current is interrogated. If this current conduction does not occur within approx. 100 ms after the external selection, the current conducting line must have been interrupted or similar operational trouble must have occurred. The minimum current is firmly set to 1,5A. This slightly reduces the flexibility to control minimum currents as well as maximum ones. Emphasis, however, is put on operational safety and operational monitoring for this current control card. The compromise made here, may be adapted to the special applications case. If operational trouble occurs due to line break or fail of power supply (that would be the same), this malfunction is also detected and the 'current o.k.' signal is reset to zero.
4. Operational Monitoring System
The message 'current o.k.' in monitored at the connection terminal 4 of the controller. This is done in the following way: About 100ms after the external signal has raised, the voltage at the terminal 4 raises from zero to 24 VDC, saying that the predicted current is now reached and regulated. Due to this, it is not necessary to maintain a supplementary current measurement in order to monitor the reached current. In dependence of the inductivity of the electromagnetic system, this time may be shorter, but in normal cases it is convenient to wait 100ms, after having given the input signal, to ask for the proper operation by a PLC. With several current control cards applied this message should be monitored for each card separately in order to know at once, which of all cards produced the failure. Therefore there is the possibility to check each current control card and have a respective message displayed on the monitor, if required (e.g. "magnet column 3: trouble"). Further details concerning the error need not be displayed, since the unit must be put out of operation in case of trouble.
5. Technical values
The power supply can be realised in the range from 100 up to 480 VAC and from 130 up to 500 VDC. In order to achieve a max. bandwidth of operation, it is advised to supply the current controller with nearly double the voltage that is used for the magnetic system. As an example:
If the magnetic systems needs a voltage of about 200 VDC than it is appropriate to supply the current controller by a voltage of 400 VDC or nearly 300 VAC. The B6 rectifier inside converts the 300 VAC into 400 VDC, if you take a three phase mains supply.
The universal power input allows also a power supply of one or two phases and even DC current. Due to this fact this current regulator can be used for a lot of different purposes in order to achieve an output current of 1,5 up to 15 A. Another effect should be mentioned. If you need to increase the time constant of an electromagnetic system, this system may be designed to have an operation voltage of 100 VDC. If you now supply the current regulator with an input voltage 480 VAC you can excite the system nearly 5 time quicker than according to normal time constant.
| Power input and output: | |||
| Supply voltage: | 130 VDC ... 500 VDC | ||
| or 100 VAC ... 480 VAC | |||
| Control voltage: | 24 VDC +- 10 % | ||
| Current of control supply: | ca. 200 mA | ||
| Regulation frequency: | ca. 19 kHz | ||
| Input: | |||
| Input voltage: | 12 bis 24 VDC | ||
| Input current: | 10 mA | ||
| Output: | Type 192 011 | ||
| max. output current:: | 15 A | ||
| min. output current: | 1,5 A | ||
| Control voltage output: system current o.k. | |||
| Control output voltage: | max. 24 VDC | ||
| Current at 24 VDC: | max. 300 mA | ||
| Dimensions: | Lenght | * Width | * Depth(mm) |
| Type 192 011: | 220 | * 143 | * 240 |
Subject to technical changes!
Further current regulators on request!
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