Theory Of Operation 6.1 Hardware - GE 269Plus Instruction Manual

6 THEORY OF OPERATION HARDWARE
269 MOTOR MANAGEMENT RELAY 6 THEORY OF OPERATION 6.1 HARDWARE
All relay functions are controlled by an 80C32 8-bit microcomputer. The microprocessor contains internal RAM
and timers, but all firmware and display messages are stored in an external EPROM. A 12-key keypad and a 2
row by 24-character display are used to enter relay setpoints and display all values and messages. A hardware
block diagram is shown in Figure 6–1: HARDWARE BLOCK DIAGRAM on page 6–2.
The power supply uses a dual primary / triple secondary transformer for connection to a 120/240 V AC source.
A 24 / 48 / 125 / 250 V DC input switching power supply is also available as an option. Regulated ±5 V sup-
plies are created for use by logic and analog Is. A regulated +10 V supply is used to drive the RTD selection
relays and LED indicators, and an isolated +10 V supply is used to drive the output relays and read the contact
inputs. +2.5 V reference voltages are derived from temperature compensated precision voltage reference
diodes to provide stable, drift-free references for the analog circuitry. A power fail detector circuit is used to
reset the relay whenever the supply voltage goes out of the proper operational range. This hardware watchdog
circuit must be signalled regularly by a firmware generated voltage or else the microcomputer will be reset.
Three-phase CTs are used to scale the incoming current signals to the 269Plus. The current waveforms are
then rectified and fed through fixed burdens to produce a voltage signal of 430 mV peak / FLC. This signal is
then multiplexed. The multiplexed signal is buffered and fed to an A/D converter. The digital signal is then fed
to the microcomputer for analysis. A 269Plus provides a separate ground fault CT to scale the input ground
fault current. This current signal is rectified and fed through a resistive burden to convert it to 1.25 V peak/sec-
ondary amps rating. This is then fed to the same multiplexer as the phase input signals.
The 269Plus temperature monitoring circuitry consists of 10 RTD connections multiplexed by miniature relays
and a 4-to-10 decoder. Mechanical relays are used because of their excellent isolation, transient immunity, and
almost zero on-resistance. A stable current source feeds each of the RTDs in turn, and 128 readings are taken
over a period of one second for each RTD. This provides for stable averaging and good 50/60 Hz noise rejec-
tion. An RTD lead compensation circuit subtracts the RTD lead resistance and then the analog RTD voltage is
multiplexed along with the phase and ground fault signals. A NO Sensor Detector circuit indicates when no cur-
rent flows in an RTD in order to distinguish a faulty sensor from a high temperature reading.
6
The 80C32 microcomputer interfaces with an 8155H I/O port and static RAM to drive an intelligent display
module and provide a digital output signal for a D/A convertor. The analog output signal from the DAC is then
converted to a current and scaled to be 4 to 20 mA. The microcomputer also drives an 8255A I/O port which
handles keypad inputs, LED drivers, and external switch inputs. The data lines from the 80C32 are latched
before being passed to the address lines of the EPROM and NOVRAM. NOVRAM store cycles are initiated
every time control power goes out of the recommended operating range. The output relays are controlled by
the microcomputer through opto-isolators and are powered by a separate, isolated +10 V supply. An SN75176
transceiver provides an RS485 communications interface for programmable controllers and computers con-
nected to the 269Plus.
All connections to the 269Plus are made on the I/O circuit board; transient protection and filtering are provided
on all inputs.
GE Power Management 269Plus Motor Management Relay 6-
1
Courtesy of NationalSwitchgear.com