WO1990007787A1

WO1990007787A1 - Digitally controlled contactor and method for controlling a contactor

Info

Publication number: WO1990007787A1
Application number: PCT/FI1990/000003
Authority: WO
Inventors: Petri Solanti; Hannu Tenhunen; Esko Kiiskinen
Original assignee: Abb Strömberg Kojeet Oy
Priority date: 1988-12-30
Filing date: 1990-01-02
Publication date: 1990-07-12
Also published as: FI81925B; AU4816890A; EP0402455A1; FI81925C

Abstract

The present invention concerns a contactor (2) for the mutual connection and disconnection of plants and loads as well as a method for controlling the contactor. The contactor (2) comprises a frame (8) of the contactor; an at least two-part magnetic circuit (5) attached to the frame (8) of the contactor; a coil (3) attached to the magnetic circuit (5), which coil makes it possible to close the magnetic circuit (5); coil connectors (4) adapted to the frame (8) and capable of feeding current to the coil (3); load-switching contacts (7) adapted to the frame (8) and capable of switching power at connected plants or loads; and a contactor assembly tie (6) attached to the magnetic circuit (5) and capable of providing the desired connection of the load-switching contacts (7). In accordance with the invention, housed in the frame (8) or in a separate enclosure, which is attachable to the frame (8), is arranged a digital control device (1) of the coil, into which the supply voltage is routed from the coil connectors (4) and which device comprises a measuring unit (30) of the coil control voltage, capable of measuring the voltage imposed over the coil connectors (4); a modulator unit (31), capable of providing the modulation of the current of the coil (3) on the basis voltage values measured by means of the voltage measuring unit (30); an I/O unit, which controls and monitors the functions of the control device and communicates with the external environment; and a timing unit (32), which generates the timing signals required by the control voltage measuring unit (30), the modulator unit (31) and the I/O unit (33).

Description

Digitally controlled contactor and method for controlling^t contactor

The present invention relates to an "intelligent" contact in accordance with the preamble of claim 1 for the connecti and disconnection of power at plants and loads .

The invention also concerns a method for controlling t connector.

In conventional connectors the coil circuit is dimensioned operate over a very narrow range of voltage, and differe coil designs are needed for DC and AC operation, respe tively. Over a hundred various coil designs are required f a single contactor model.

In addition, the DC coils of larger contactors a complemented with auxiliary contacts used together with external series resistor in order to reduce the curre through the coil during the hold, thus achieving a lower temperature rise of the coil.

The mating surfaces of the - magnetic circuit in a direct actuated contactor are subjected to an unnecessarily hi stress, because during the closing stage the input power the coil is at a constant level, whereby the contact assemb tie reaches a relatively high final speed prior to t closure of the magnetic circuit. Since the power requir during the closing stage is higher than the power requir for holding the magnetic circuit closed, the closing co current must be designed to exceed the necessary holdi current by up to several ten-fold, depending on the contact model. Due to the overdimensioning, an increase of both pow consumption and heat load of the coil results. Aiming to overcome the aforementioned drawbacks, separat control units based on analog technology are used fo controlling the coil current. The operating voltage range i the conventional control units is appreciably narrower tha that achievable with the apparatus disclosed herein, sinc analog technology is incapable of providing a sufficientl wide range of control. In addition, the prior art devices ar expensive and large in size, while their implementatio technology limits the possibilities of increasing the numbe of functions available.

The aim of this invention is to overcome the disadvantages o prior-art methods described above and to achieve a novel typ of contactor and a method for controlling a contactor.

A control circuit in the context of this disclosure refers t a device, which is packaged into a single monolithic hybrid circuit, or onto a single printed-circuit board, a which performs the necessary control and steering functio in the coil circuit of a contactor.

The invention is based on a control unit, which implemented in the digital technology, and attached to t coil circuit of the contactor, operates with pulse-wid modulation, having an integral power stage for the control the coil current. The reference value of the coil current formed by way of a voltage signal obtained by measuring t control voltage of the coil.

The coil voltage is rectified and slightly iltered prior modulation, thereby allowing the apparatus to be used f both DC and AC control of a magnetic circuit designed for control, thus avoiding- the need for separate shading rin for the smoothing of the magnetic field. More specifically, the contactor in accordance with t invention is characterized by what is stated in t characterizing part of claim 1.

Furthermore, the method in accordance with the invention i characterized by what is stated in the characterizing part o claim 8.

The invention provides outstanding technical and economica benefits over prior art technology.

The implementation in accordance with the invention achieve an accurate control of a contactor according to any prevalen conditions. The magnetic circuit surfaces are relieved fro excessive closing stresses, and heat loads are reduced by th limitation of the hold power. By virtue of the digital imple mentation, the control circuit is easily complemented b integrating additional functions into it as to be describe later.

The invention is next examined in detail with the help o exemplifying embodiments illustrated in the attache drawings.

Figure 1 shows a partially sectioned side view of a contacto in accordance with the invention.

Figure 2 shows in a graph the power required by the contacto coil as a function of time during the different stages of th closing of the contactor.

Figure 3 shows a block diagram of a possible embodiment fo the functional implementation of the control circuit. Figure 4 shows the block diagram of a voltage measuring un in accordance with the invention.

Figure 5 shows the block diagram of pulse-width modulat unit and power unit in accordance with the invention.

Figure 6 shows in a graph the adaptation of the pull force the contactor coil to the contact forces and the restorin spring forces as a function of force and opening gap of t magnetic circuit.

In accordance with Fig. 1, a digital control and steeri circuit 1 is connected between a coil circuit 3 of contactor 2 and a controlling line voltage 4. The princip function of the control circuit is to control the curre passing through the coil 3 of the contactor 2 during t different stages of the contact closure (refer to Fig. 2) such a manner that unnecessary heating of the coil 3 avoided, yet assuring the closure of a magnetic circuit even in arduous environmental conditions such as stro vibration, for instance.

At the closure of the magnetic circuit 5, a contact assem tie 6 is closed and poles 7 are short-circuited by contactor 2. The position of the tie 6 is monitored by me of a sensor 9. The voltage of the controlling line 4 can v freely in the range 24 120 V or 120 450 V, while the f of the voltage can be DC or AC. The control circuit receives the information necessary for controlling the c current from the voltage of the line 4 and the contac control information wired into the control circuit 1. extend the applications of use, the circuit is provided w an I/O unit (not shown) , through which the communications the control circuit with the external environment ta place. The I/O unit will later be described in detail. The design of the control circuit 1 pays special attention the operating conditions of contactors. The circuit subjected to operation over extremely varying temperature heavy vibration and strong magnetic ields. In order eliminate magnetic and electrical interference, the contr circuit is provided with several monitoring and resetti facilities that are aimed to minimize the duration of a possible malfunction. Due to its susceptibility interference, a microcontroller in the control circuit h been discarded in the favour of discrete logic circuits which are divided into independently operating function blocks that are reset by a reset pulse obtained at se intervals from a timing unit. The block' construction a timed resetting of the control circuit results in appreciably shorter recovery time than that obtainable with microcontroller-based structure, allowing in a favourabl case the malfunction to be confined to only a single bloc whereby the system function may remain unaffected.

According to Fig. 2, three different current levels ar necessary for the contactor operation:

During the initial phase a of the operate stage, sufficien power is fed to the coil 3 to make the contacts close, ye not completing the closure of the magnetic circuit. Thi phase requires power only for overcoming the force of t restoring spring.

During the second phase b, the energy stored in the coil 3 i increased to assure the closure of the magnetic circuit 5 i all possible ambient conditions.

The third phase called the hold phase c is extended up to th next deactivation of the control signal. The current of th coil 3 is limited during the hold phase to a level sufficie for maintaining the closure of the magnetic circuit, y avoiding the heating of the coil.

The minimization of the difference of the pulling a restraining forces in accordance with Fig. 2 results in lower acceleration of the contact assembly tie, there reducing the mechanical stresses during the operate stag The methods of conventional technology aim to adapt the pu force curve to the restraining force curve (Fig. 6) by way different lever constructions. The implementation accordance with the invention adapts the pull force curve electronic means, thereby avoiding the need for mechanic damping structures.

The functions of the control circuit 1 take into account so disturbance conditions, which can be cleared by a conve tional contactor only through the use of external component One of most frequent situations is the quick reclosure of t supply after a short outage. The control circuit 1 has state memory with a backup time of about one second from t loss of coil control voltage. If the contactor 2 w energized at the loss of coil voltage, the circuit initiates a start sequence in case the supply volta recovers within the backup time of the memory. After a long outage, the control circuit is latched to the "releas state, requiring the switch-off of all control signals pri to the next "operate" control signal. The quick reclosu function can be disabled, whereafter even a momentary bre in the coil control voltage results in the resetting of t control circuit, and the contactor will stay nonenergiz until the next "operate" control signal. Consequently, t implementation of quick reclosure is possible without the u of external RC circuits that are required in convention systems. To prevent the welding of contacts, the control circuit 1 incorporated with a delay for the control signals th inhibits the next "operate" control signal until the conta assembly tie can reach its upper position after the precedi "release" control signal. The delay varies for differe types of contactors, but a typical value is less than 200 m The circuit accepts the next control signal, but initiat the start sequence only after the set delay. Th characteristic is also valid for the quick reclosure. prior art systems the delay is implemented by way auxiliary contactors.

In accordance with a standard concerning contactors, t contactor must release during an undervoltage condition the range 20...75 % of nominal voltage in AC controll operation and in the range 10...75 % of nominal voltage in controlled operation. The undervoltage monitoring function implemented in a voltage measuring unit 30, which issues t modulator unit an enabling "run" signal if the coil contr voltage is within allowed range limits. On the basis of t coil voltage measured during the start sequence, the limit an undervoltage condition is set to within the ran expressed in the standard.

In an overvoltage condition the control is disabled when t control voltage exceeds by 15 % the upper limit (138 V f the voltage range 24...120 V and 517 V for the voltage ran 120...450 V) of allowable control voltage range of t control circuit. For overvoltage monitoring the volta measuring unit is provided with two separate systems, which one detects transient voltage changes and the oth monitors the change of control voltage average value over longer span of time.- Tripping by under- and overvolta resets the control circuit, and recovery of voltages within allowable range does not energize the contactor prio to a new control signal.

Figure 3 illustrates the division of ASIC circuitry co taining the intelligence of the control system into fo functional blocks. All information transfer between t blocks is asynchronous to eliminate the effect disturbances. The inter aces and signals communicati externally from the blocks are to be defined later.

The functional blocks of the control circuit are:

- voltage measuring unit 30

- modulator 31

- timing unit 32

- I/O unit 33.

According to Fig. 4 the measurement of the control voltage implemented with an A/D converter 40, based on sigma-del modulation and connected after a voltage divider circuit 4 feeding a digital averaging low-pass filter 41. An alte native to the averaging filter is an integrating filte which could deliver an RMS value of higher accuracy, but th solution would consume more space on the silicon substra and the averaging function is suf iciently accurate in rel tion to the ambient conditions. Using a sufficiently hi sampling rate above the minimum, the S/D modulator (n shown) of the A/D converter 40 is capable of filtering t high- and medium-frequency interference from the measur signal, resulting in a 7-bit binary value, which after fi tration through the digital low-pass filter 41 represents t average value of the measured voltage. The voltage inform tion is taken at appropriate intervals to the coding logic order to obtain a new set value for the modulation signa The over- and undervoltage monitoring is implemented in detector 42 after the filtration, whereby possible distur ance transients in the supply line do not trigger an ove voltage trip. Due to the delay caused by the A/D conversi and filtration, the system is provided with a separate anal overvoltage monitoring, which is implemented with a volta reference 47 and a comparator 43. In order to eliminate t effect of disturbance transients, the output of the compara tor is routed via a delay line block 44, which prevents a overvoltage trip caused by fast transients (having a duratio of approx. 1...2 s) . The delay line block issues the over voltage signal to the detector unit only when the comparato output has continuously been high for the duration set by th delay (approx. 1...2 ms) . The detector unit will discontinu the "run" signal when it receives an "overvoltage" signal 4 from the delay line block, disregarding whether the averagin voltage measurement would indicate the line voltage to b within allowable limits. The overvoltage limit is set accord ing to the maximum peak voltage U_RM^_^.. * 1.42 of the contro circuit in order to avoid tripping within an acceptabl voltage range.

When the control circuit is connected to the coil suppl voltage, the voltage measuring unit first measures the coi circuit voltage via a measurement input 37 for about 20 ms i order to compute a definite value for the average voltage after which it sets the undervoltage limit and issues a "run enable signal to the modulator 31. If the coil supply voltag (coil control circuit voltage) is lower than a permanentl set- lower limit or higher than the overvoltage trip limit the "run" signal will not be issued. According to Fig. 5 the modulator unit comprises two majo blocks: a non- eedback digital pulse width modulato

(NFBDPWM) 50 and an ROM circuit 51, which contains the store reference values of modulation for different types o contactors and voltages, complemented with the addres decoding logic of the memory. The memory can be implemente using PROM, EPROM, EEPROM Flash-EEPROM or other memor circuit technology in an external block or designed integra with the circuitry.

The pulse-width modulator unit 31 is an independent unit wit internally synchronized function, whose only control signal are received from the voltage measuring unit 30 as t voltage information signal and the "run" signal, whi informs the modulator unit to set its operating state. On t basis of precomputed pulse-width data stored into the memo 51, the pulse-width modulator 50 modulates the control pul signal routed to a drive unit 52 of the power switches. Th construction removes the instability and feedback-relat control errors typical to analog pulse-width modulators. T modulator 50 resets itself after each control pulse and rea a new reference value of modulation from its latch circui The latch circuit has a dual-part construction, whi facilitates its asynchronous setting and reading. T modulator unit 31 is capable of controlling the pulse du cycle at 0.4 % accuracy over the range 0.4...98.4 %. Betwe each subsequent pulse remains a short gap, during which t new modulation data is read and the modulator is rese thereby making it impossible to use a full 100 % duty cyc of modulation. In comparison with corresponding anal implementations, the NFBDPWM approach offers an appreciab wide range of pulse-width duty cycle.

Pulse-width duty cycle values computed from tests for t hold power of contacts are programmed into the memory 51 a fetched therefrom by addresses decoded on the basis contactor control voltage and contactor type data, aft which they are latched into the latch circuit of t modulator. The coils of different voltage ranges a dimensioned so as to avoid the need for separate memo blocks for them. The power levels of the start sequence a obtained by adding a constant value determined by t contactor type to the value stored in the memory. The memo can be organized, for instance, into 8 rows that a addressed by a 3-bit contactor type data 56, and into 1 columns that are addressed by 7-bit voltage information 5 Each memory cell contains 8-bit data of modulati information.

The actuator element of the control system is a switchi regulator implemented using a FET power switch that utiliz the contactor coil as its inductance element. The FET swit is steered according to the contactor-type specif modulation information by the afore-described non-feedba digital pulse-width modulator (NFBDPWM) 50, which controlled by the voltage information signal obtained fr the voltage measuring unit 30. The operating frequency of t switching regulator is above 20 kHz, whereby audib vibrations in the magnetic circuit 5 of the contactor 3 a avoided.

The modulator unit 31 does not incorporate a driver circu 52 of the FET switch, because the integration of the driv onto the same silicon chip with the logic control circuits impossible due to the difference in their operating volta ranges. The FET switch is parallelled with an SA "snubbe circuit 54, whose function is to filter away the switchi transients imposed on the FET switch 53 and to reduce t switching losses of the FET. The coil 3 is parallelled with protection diode 55, which absorbs the voltage spikes induc by the coil itself. When using high-power FET switches, problem arises from their relatively high gate capacitanc that slows down the operating speed of the FET. The curren output capability of CMOS circuits is relatively low, and t 5 V supply voltage of the control logic circuits i insufficient for the drive voltage of the FET switch 53 whose gate voltage normally is in excess of 10 V. In order t obtain a sufficient drive voltage level and protection fo the control logic, a driver/isolator block is designe between the power switch block and the control circuit.

Illustrated in Fig. 3 is a timing unit 32, which delivers al timing signals required for the control circuit 1. The clo generator of the timing unit 32 comprises a free-oscillati 5 MHz ring oscillator, whose output frequency is used as t basic frequency of the modulator. The other timing signa are obtained by dividing the basic frequency and generati different sequences therefrom. The different frequencies a required for, e.g., the voltage measuring unit 30 and the I units 33. The timing unit 32 also generates the resetti signals required during the start sequence in a correct timed order for the different blocks. Furthermore, the un incorporates a passive timer for the delaying of the contr signals and a short starting delay for duration of the clo generator startup.

An I/O unit 33 controls the operation of the control unit

(incorporating the pulse-width modulator with auxilia blocks in accordance with Fig. 5) and provides the commun cations interface of the system to the external environmen The digital implementation of the I/O unit makes it feasib to integrate into the unit such functions that are possib in the conventional technology only through the use discrete equipment. The I/O unit 33 concentrates all functions required for t control, communications, adjustments and internal diagnosti of the contactor. The unit incorporates, for instance, thr different logic control signal connections, an interface f the serial-bus control, a programmable timer for setting t operate and release delays, a starting control logic f generating the commands to be delivered to the differe blocks during the start sequence in a correctly timed orde and an error-logging logic for collecting all reportab events occurring within the control circuit (such as malfunction, operation of thermal trip, etc.) and indicati these events by an LED indicator lamp and via an opt isolated open-collector output. All communications of the I unit to the other units takes place via a separate contr bus 36. The number of interchanged messages is minimized designing the units for maximum independence up to the internal diagnostics. The only external connections n passing through the I/O unit are the measurement input of t voltage measuring unit and the outputs (2 pcs.) of the puls width modulator. The connections of the I/O unit are lat designated by reference numbers 33a...33h.

The control at the logic signal level (5...24 V DC/AC) served by an opto-isolated interface 33a (insulation lev 4 kV) . The contactor operates at the rising edge of t control signal and releases at the falling edge. If t control signal is high when the coil voltage is applied, t contactor will not operate before the control signal is on taken low. The automatic quick reclosure after a short outa maintains the state of the contactor prior to the break the memory and is activated when the control signal is high

A contact-closure signal input 33b has similar function wi the programmable-logic control input 33a; it is activate however, by a contact closure signal, rather than by voltage control signal. This input can be controlled by relay, switch or open-collector logic output. The contac closure control input is in parallel with the other input thereby allowing an active signal at any of the inputs operate the contactor.

A pulse-signal control input 33c is intended for mot controls and other similar applications, where the sta switch can be a pushbutton. The contactor operates at a sho input pulse, after which it retains its state up to the ne voltage break or a reset pulse issued by a momentary cut-o of the PTC control signal.

The I/O unit has two resistance-measuring circuits 33d, whi can be used for connecting the PTC resistors applied to t temperature monitoring of the motor windings and bearing When the external sensor resistance grows above a set lim value, the control circuit is reset and will not resta before the sensor resistance falls below the set limit val and all control signals are momentarily cut off. T activation of the thermal trip is alerted via the I/O unit a designated signal. The error message is retained in t memory until the next cut-off of coil voltage.

The I/O unit has a separate direct input 33e for electronic thermal trip. This input connection is activat by a contact-closure signal, which disables the contact operation as long as the thermal trip contacts are open. T connection can be used for direct routing of the thermal tr signal to the contactor, and the activation of the ther trip can be relayed via the contactor's error mess interface to an automatic control system. The thermal t input can be disabled at will. The I/O unit of the control circuit incorporates an interfa 33f for optical gap sensors that are used for monitoring t position and status of the contact assembly tie. Altern tively, magnetic sensors or microswitches, for instance, c be used as position sensors. The position monitoring of t contact assembly serves for assuring a reliable conta closure and detecting a possible welding of the contacts contactor release.

When starting from a normal condition in which the conta assembly tie is in its upper position, the circuit initiat a start sequence. If the tie does not reach its low position at the end of the sequence, the circuit cuts off t coil voltage, waits for the return of the tie to its upp position and repeats the start attempt twice more. If ev the third attempt is unsuccessful, the I/O unit sets t error message flag and issues an alarm signal. The err message flag prevents the control circuit from functioni until its control voltage is cut off. If the contact assemb tie is not in its upper position at the beginning of t start sequence, the error message flag will be s immediately, thus preventing the initiation of the sta sequence.

The I/O unit has an integral timer, which can be used f delaying the operate time of the contactor after the issuan of the "operate" signal. The operate delay is settable in t range 0.1...18 s or, alternatively, 1...180 s (±20 %) . T release delay has an independent timer settable in the ran 0.1...18 s. The release delay is functional only conjunction with external control signals.

The control circuit 1 further includes a function, whi allows the heating of motor windings in humid environment f preventing condensation. The control circuit in this ca carries on with the modulator function even after receivi the "release" control signal, but the modulator signal outp is routed to a designated output which feeds a separatel attached FET power switch with driver circuitry. T necessary heating power is precomputed, coded according the contactor type and stored in the memory of the modulat unit.

The internal diagnostics monitors the function of a internal circuit blocks of the control circuit and aims rectifying singular malfunctions. The diagnostics al monitors the status of the power elements, and for the ca of power switch failure, the control circuit incorporates electronic fuse, which in a malfunction disconnects the co circuit.

Claims

WHAT IS CLAIMED IS:

1. A contactor (2) for the connection and disconnection power at plants and loads, said contactor (2) comprising

- a frame (8) of the contactor,

- an at least two-part magnetic circuit (5) attac to the frame (8) of the contactor,

- a coil (3) attached to the magnetic circuit ( which coil makes it possible to close the magne circuit (5) ,

- coil connectors (4) adapted to the frame ( through which current can fed to the coil (3) ,

- load-switching contacts (7) adapted to the fr (8) , through which power can be connected at pla or loads, and

- a contactor assembly tie (6) attached to the m netic circuit (5) , capable of providing the desi connection of the load-switching contacts (7) ,

c h a r a c t e r i z e d in that

- housed in the frame (8) or in a separate enclosu which is attachable to the frame (8) , is arrange digital control device (1) of the coil, into wh the supply voltage is routed from the coil connect (4) and which device comprises

- a measuring unit (30) of the coil control volta capable of measuring the voltage imposed over coil connectors (4) ,

- a modulator unit (31) , capable of providing t modulation of the current of the coil (3) on t basis^" of voltage values measured by means of t voltage measuring unit (30) ,

- an I/O unit, which controls and monitors t functions of the control device and communicates wi the external environment, and

- a timing unit (32) , which generates the timi signals required by the control voltage measuri unit (30) , the modulator unit (31) and the I/O un (33) .

2. A contactor (2) in accordance with claim 1, c h a r a c t e r i z e d in that the signal path of the volta measuring unit (30) comprises a voltage divider (46) , an A converter (40) and a low-pass filter (41) .

3. A contactor (2) in accordance with claim 1 or 2, c h a r a c t e r i z e d in that signal path of the volta measuring unit (30) comprises a voltage divider network (46 followed by an A/D converter (40) based on the sigma-del modulation, next along the signal path followed by a digit low-pass filter (41) .

4. A contactor (2) in accordance with claim 1, 2 or 3 c h a r a c t e r i z e d in that the modulator unit ( comprises a non-feedback digital pulse-width modulator ( and a memory (51) , which stores the reference values modulation for different contactor types and cont voltages. _

5. A contactor (2) in accordance with any of the foregoi claims, c h a r a c t e r i z e d in that the I/O un (33) incorporates connections for logic-level control (33a) contact-closure signal control (33b) , pulse signal contr (33c) , a resistance measurement connection (33d) temperature sensors, a connection (33e) for an electron temperature trip, a position monitoring connection of t contact assembly tie (6), a storage register for collecti the quantity information of contactor operate cycles, alarm output (33g) for reporting the malfunctions of t contactor (2) , and a connection (33h) for attaching external unit using a serial bus communications protocol.

6. A contactor (2) in accordance with any of the foregoi claims, c h a r a c t e r i z e d in that the modulat unit (31) comprises a separate output, which through attached power control stage is capable of feeding a heati current to a load comprised of a motor winding in order prevent condensation when the motor-controlling contactor controlled to the "open" state.

7. A contactor (2) in accordance with any of the foregoi claims, c h a r a c t e r i z e d in that the contr device (1) has an integral logic for the position-monitori of the contact assembly tie (6) , said logic receiving i control information from sensors (9) adapted to the frame ( of the contactor.

8. A method for controlling such a contactor (2) th comprises a frame (8) of the contactor, an at least two-pa magnetic circuit (5) adapted to the frame (8) of t contactor, a coil (3) adapted to the magnetic circuit (5) f closing the magnetic circuit, coil connectors (4) adapted the frame (8) , load-switching contacts (7) adapted to t frame (8) for switching power at plants and loads, and contact assembly tie (6) adapted to the magnetic circuit ( for closing the load-switching contacts (7) , in which metho

- the coil voltage is measured and

- the current fed to the coil (3) is controlled modulating the current to the coil,

c h a r a c t e r i z e d in that

- the coil voltage is measured by way of an analo digital converter, connected to a filter and detect unit (Fig. 4) and

- the current of the coil (3) is controlled with non-feedback digital pulse-width modulator (50 which further is controlled by data stored in memory (51) so that each voltage value and contact type is matched by a programmed value of pulse-wid information.

9. A method in accordance with claim 8, c h a r a c t e r i z e d in that complementing the coil is connected digital, IC-technology based control system, which perfor the control function described in claim 8 and additionally

- filters away interference from the measureme signal by means of a digital filter,

- performs internal monitoring of system functio using integral self-diagnostics for, e.g registering the operate cycles of the contactor a monitoring the condition of the power-switchi stage, and - contains an I/O unit, which performs centraliz handling of external communications and controls t unctions of the measuring and controlling units.

10. A method in accordance with claim 8 or 9, c h a r a c t e r i z e d in that the function of the control device controlled and monitored by an integral I/O unit (33) of t device that processes external control signals, handles err messages in a centralized manner, generates internal star up delays and synchronizes the functions of other units the control device via a control signal bus (36) .

11. A method in accordance with any of the foregoing meth claims, c h a r a c t e r i z e d in that the contr circuit device (1) monitors the position of the conta assembly tie (6) by means of sensors.

12. A method in accordance with any of the foregoing meth claims, c h a r a c t e r i z e d in that in conjuncti with the control of contactors, a small heating current fed from a separate power control output to a winding of motor during periods of motor power off in order to preve condensation in electric motors operating in humi environment.