CH-06 ACS 5000A Excitation Unit

Objective:

After completing this module, you will be able to describe: The DCS800 power converter used as a field exciter in ACS 5000 drives for synchronous motors. The main circuit diagram of the ACS 5000 Excitation Units with DCS800, both direct and brushless. The control and protection equipment of the ACS 5000 Excitation Unit.

Synchronous Motor:

The synchronous motor has a three-phase stator winding and a rotor with alternating north and south magnetic poles. The magnetic attraction between the poles of the stator and the poles of opposite polarity of the rotor will “lock” the rotor and stator poles together. Consequently, the rotor will turn with the same speed as the rotating stator field, which is the so-called “synchronous speed”. The rotor can be equipped either with permanent magnets or with windings supplied with DC in order to create the magnetic poles. In an application with ACS 5000 controlling a synchronous motor with a wound rotor, the Excitation Unit EXU will supply the rotor winding with DC. Two excitation principles are used for the excitation supply in case of ACS 5000, either direct excitation, where the Excitation Unit supplies directly the rotor winding with controlled DC by the mean of two slip rings and brushes, or AC brushless excitation, where the Excitation Unit supplies the stator of an auxiliary electrical machine with controlled AC. This auxiliary machine is called the exciter machine and is installed on the shaft of the synchronous motor. A DC brushless excitation method is also possible, but it is not used with ACS 5000, but mainly with synchronous generators.

Direct Excitation:

In a motor with direct excitation, the DC exciter supplies the excitation current directly to the rotor windings through brushes and slip rings installed on the main shaft of the motor. The excitation current can be controlled and measured directly. The control of this type of excitation is fast and accurate. But it has the disadvantage of using brushes and slip rings, which require costly repair and maintenance works. Sparks may appear between brushes and slip rings. For this reason, the direct excitation is not suitable for environments with risk of explosion, as in oil and gas applications.

Brushless Excitation:

The brushless excitation consists of an auxiliary electrical machine, a small 3-phase AC generator, called further in the presentation exciter machine, and a 3-phase diode bridge. Both of them are installed on the main shaft of the synchronous motor. In the brushless system there is no electrical contact between the rotor and the fixed part of the motor. A 3-phase controlled AC current is supplied to the stator of the exciter machine, which induces an AC current in the rotor windings of the exciter. The AC rotor current is rectified by the 3-phase diode bridge and applied to the winding of the motor excitation. The rotating field in the stator of the exciter machine and the rotor of the synchronous motor must turn in opposite directions. If the rotor changes the direction, the phase rotation of the stator of the exciter machine must be also changed. The brushless excitation involves less maintenance and repair costs, and it is suitable for applications in a harsh environment or with risk of explosion, as, for instance, marine or oil and gas.

DCS800 Generalities:

The power converter DCS800 is an air-cooled DC drive designed to be used in a broad field of industrial applications such as metal, pulp and paper, battery chargers, ski lifts, etc. As an armature drive for DC motors, DCS800 is available in seven different sizes, differentiated by the voltage and current range and marked D1 to D7, with D1 the smallest and D7 the largest drive. The power converter DCS800 includes both the power stage and the control electronics in the same module, uses DriveWindow as a PC tool and can be provided with a local control panel.

Types of Excitation Unit:

As part of the Excitation Unit for ACS 5000, only versions D4 and D5 of the DCS800 are used. Size D4 is used for both direct and brushless excitation while D5 is used only for direct excitation.

Type Code:

The EXU type supplied with the ACS 5000 depends on the excitation method and the required output current. The excitation unit type code includes all this information. In the type of code, the first letter E means excitation unit. The second letter is the excitation principle, where D is direct excitation, and B means brushless excitation. The next letter shows the maximum supply voltage which can be used for DCS800. 5 means the input voltage must be less then 525 V and 7 means it must be less then 690 V. The last letter indicates the maximum output current which can be generated by this module. V means up to 900 A DC, Y means up to1400 A DC current and R or S means up to 750 A AC.

DCS800 Excitation System:

The DCS800 excitation system is available for both excitation methods, direct and AC brushless. With the direct excitation method, the DCS800 converter installed in the Excitation Unit supplies the field winding of the synchronous motor directly with DC, by way of brushes and slip rings. In the AC brushless excitation system, the synchronous motor is equipped with a small exciter machine installed on the main shaft. The Excitation Unit with DCS800 supplies the stator of the exciter machine with controlled AC. The current induced in the rotor of the exciter is rectified by a diode bridge installed on the same main shaft and applied to the field winding of the motor. The direct excitation version is based on the standard DCS800 system equipment, which is a 6-pulse, single bridge thyristor converter and the overvoltage protection unit DCF506. The brushless excitation is based on the same DCS800 converter, but the 6-pulse bridge was mechanically modified in order to position the six thyristors in groups of two by two in anti-parallel on each phase.

Direct Excitation: Description

The air-cooled direct excitation uses standard DCS800 equipment: a 3-phase single bridge DC power converter with an overvoltage protection device of the type DCF506. The main power diagram of the Excitation Unit includes also a circuit breaker, an electronic overload relay, and fuses. The number and location of the fuses depends on the DC converter size. The Excitation Unit with DCS800 size D4 has three
external fuses on the AC input. In the Excitation Unit with DCS800 size D5, there are six fuses included in the power stage, one fuse for each thyristor branch. An earth fault detection relay is available as an option. The Excitation Unit is supplied by a separate 3-phase transformer, 50 or 60 Hz. The voltage range is 230 to 525 VAC for size D4 and 230 to 690 VAC for size D5.

Direct Excitation: Protection:

The excitation transformer has to fulfill both following conditions: It must be equipped with a grounded screen winding and the secondary winding must have the same vector group as the primary. If one of these conditions is not fulfilled, the external main circuit breaker of the Excitation Unit must be provided with two feedback contacts: “Status ON”, normally open, and “Open command”, normally closed. The internal breaker has to open first; therefore the open command contact must open 80 ms before the external circuit breaker opens.

Direct EXU with DCS800: Configuration:

All the control boards are included in the DCS800 enclosure. The main part of the unit is the power converter DCS800. In the direct Excitation Unit there is also the overvoltage protection device DCF506. The other main components of the unit are the input circuit breaker and the electronic overload relay. Size D4 has also three fuses on the AC input. The earth fault detection relay is optional. The cabinet temperature is monitored by a thermostat. A differential air pressure switch supervises the air flow in the unit. The terminal block compartment houses not only the customer terminals, but also the control and auxiliary power circuit breakers, interposing relays and contactors for the control of the DCS800 and the fans.

Brushless Excitation Description:

The air-cooled brushless excitation uses the single bridge DCS800 power converter size D4 only. The bridge was mechanically modified. The six thyristors are installed in three groups, two by two in anti-parallel. The main power diagram of the Excitation Unit includes also a circuit breaker, an electronic overload relay, and three fuses on the AC side. An optional earth fault detection relay is also available. The phase rotation is changed by the means of the speed reversing switch. The reversing switch is an option. Another option available with the brushless Excitation Unit is the output disconnector. The speed reversing switch and the output disconnector are offered in the same option package.

Brushless EXU with DCS800 : Configuration:

The brushless Excitation Unit contains mainly the same components as the direct EXU except for the overvoltage protection device. For type S, the option package “speed reversal switch and output disconnector” is available.

EXU D4 Types: ED5V, EB5R, EB5S: Main Components:

Here is the overview of the D4 size Excitation Unit. The main components are: The input fuses the earth fault detection device, the overload relay, the input circuit breaker, the D4 size DCS800 module with the optional control panel, the overvoltage protection, the air pressure switch, the cooling fans, and the air exhaust hood.

DCS800 D4 Unit:

The DCS800 module size D4 has a plastic cover which can be removed to get access to the control boards and the power stage. The control board SDCS-CON-4 is protected by a gray cover. The terminal blocks X6
and X7 of the control board can be accessed without removing this cover. The SDCS-COM-8 is used for the fiber optic communication. The unit has also an interface board, SDCS-PIN-4, including the 24 V power supply, current and voltage measurements and the pulse transformers for the firing of the thyristors. The power stage consists of three thyristor modules, each module includes two 2 thyristors. They are connected either in a 6-pulse bridge for direct excitation or two by two in anti-parallel for brushless excitation. Two or three CT’s are installed for the current feedback.

EXU D5 Type ED7Y:

Here is the overview of the D5 size Excitation Unit. The main components are: The D5 size DCS800 module, the air pressure switch, the cooling fans, the overvoltage protection, the earth fault detection device, the input circuit breaker, and the air exhaust hood.

DCS800: D5 Unit Control Board:

DCS800 module size D5 is located in a metal enclosure with a swing door at the front. The control boards are installed on the door, with the main control board SDCS-CON-4 and the communication board SDCS-COM-8 in an ESD enclosure. The front cover must be removed in order to gain access to the terminal blocks X6 and X7 or to the fiber optic connectors of SDCS-COM-8. Size D5 has two interface boards: SDCS-PIN-51, the current and voltage measuring interface, and SDCS-PIN-48, the pulse transformer interface. It has also a separate power supply board, SDCS-POW-4. Behind the door there is the power stage.

DCS800:D5 Power Stage:

The power stage consists of six individual disk type thyristors and two CTs for the current feedback. There are six fuses, one for each thyristor branch.

DCS800 Converter Configuration:

The following control boards are the same for both direct and brushless excitation. SDCS-CON-4 is the main control board, containing the firmware and the application parameters. An optional DCS control panel can be connected at X33. If the control panel is not present, a 7-segment display, showing the status of the control board, will be visible. SDCS-CON-4 has several terminal blocks, for analog, digital and encoder I/O’s. Only terminal blocks X6 and X7 are used in ACS 5000 Excitation Units. SDCS-COM-8: Fiber optic communication board. SDCS-PIN-4: Interface board for size D4. The 24 V control power supply is integrated in this board. SDCS-PIN-48 and SDCS-PIN-51: Interface boards. SDCS-POW-4: 24 V control power supply only for the DCS800 size D5.

DCS800 Control Board: SDCS-CON-4: I/O’s used with EXU:

The main control board of the DCS800, the SDCS-CON-4, is equipped with a Flash PROM containing the
firmware and the application parameters. When replacing the board, the firmware and the parameters must be restored to the new board. The jumpers S1 to S4 are dedicated to speed measurement and analog I/O’s. They are not used for the field exciter application. The jumper S5 is in position 1-2 for step 1 of the firmware download and in position 3-4 for step 2 of the firmware download and normal operation. The board has several connectors: X6: Standard Digital Inputs, X7: Standard Digital Outputs, X37: DC power supply connector, X12 and X13: Connected to interface board, and X11 is used for the communication board SDCS-COM-8. The other terminals are not used in our case.

DCS800 Control Board: SDCS-CON-4: Seven Segment Display

If the control panel is not installed, a 7-segment display with decimal point will give information about the status of the control board SDCS-CON-4. The messages are coded. There are four categories of messages: general messages, power up errors, alarms and faults. The general messages are the following: Decimal point only: program running, no alarms, no faults. Digit “8”: firmware not running. Horizontal bar: loading
firmware, step 1. Letter “d”: loading firmware, step 2. Letter “u”: formatting the flash. Do not switch off the
board during this step. In case of an alarm, fault or error the display will change alternatively, showing the
fault or alarm code. For example: A 1 0 4, which means alarm 104, converter overtemperature. The complete list of alarm, fault and error codes can be found in the converter firmware manual.

DCS800 Control Boards: SDCS-COM-8:

Generally, the communication board SDCS-COM-8 provides fiber optic communication with options as
overriding system, DriveWindow, Master/Follower and extension I/O cards via four channels. In the Excitation Unit application, only channels 0 and 3 are used. Channel 0 for the overriding system, in our case the drive control board AMC, and channel 3 for DriveWindow.

DCS800 Control Board: SDCS-PIN-4:

For the module size D4, the interface between the control board and the power stage is done by the interface board SDCS-PIN4. SDCS-PIN4 has normally three functions, but only two are used for the field exciter application: One of them is to act as a DC power supply for all the electronic boards . Secondly, it is used for power stage measurements and pulse interface to the thyristors. The third function is applicable for DC motor control only.

DCS800 Control Board: SDCS PIN51 + PIN46/48:

The interface between the control and the power stage is done in the size D5 by two boards: SDCS-PIN51, the measuring board, and SDCS-PIN46 or 48, the pulse transformer board.

DCS Control Board: SDCS POW-4:

SDCS-POW-4 supplies the DC voltages for the electronic boards in D5 modules. The input voltage is
automatically detected. The jumpers select the voltage for the speed encoder; they are not used in our case. On this board is also installed a standard digital output of the drive, Digital Output 8, configurable in SDCS-CON-4.

Overvolt Protection Circuit DCF506:

If the DCS800 is used as a field exciter, the overvoltage protection module DCF506 is required in case of
direct excitation. The protection activates a free-wheeling circuit between the field terminals F+ and F- if an overvoltage is detected. The free-wheeling function will last until the DC current is smaller than 0.5 A. A relay indicates to the control board at Digital Input 7 the overvoltage situation. The relay stays closed as long as the free-wheeling function is active.

Overvolt Relay:

The excitation system with DCS800 is equipped with an electronic overload protection relay installed on the AC input. The protection includes also the three CT’s measuring the AC current to the power converter. The overload relay has an output contact wired to Digital Input 4 of the SDCS-CON4.

Differential Pressure Switch:

The differential pressure switch monitors the air flow in the drive by measuring the air pressure in the air ducts versus the atmospheric pressure. If the air flow drops below a certain level, the switch sends a signal to the control board Digital Input 1 of the control board.

Earth Fault Supervision:

The earth fault detection device is used to detect earth faults in the excitation circuit. It has a relay connected to the control board at Digital Input 3.