Hardware Description - GE Multilin MM3 Motor Manager 3 Instruction Manual

HARDWARE DESCRIPTION

3.3.1 MM3 Motor
Manager 3
Design
3–6
3.3 Hardware Description
A 16 bit 68HC16 microcontroller IC performs program execution and control logic for the
MM3 Motor Manager 3. It has an 8 or 16 bit bus width which can be selected dynamically
with each external memory fetch allowing a mix of 8 and 16 bit devices. Internal clock rate
is 16 Mhz. Instructions are stored in a 512K × 8 bit flash memory, data is stored in an
32K × 8 RAM while setpoints and accumulated data are stored in a 8K × 8 EEPROM.
An intelligent display module with its own microprocessor, memory and command set is
accessed through communications channel on the main CPU board. The display, a 4 × 4
keypad and the front panel LEDs data are exchanged via modbus data packets.
External switch inputs are driven with the same voltage applied to the control voltage 120
or 240 V AC which triggers an optocoupler for isolation. All control logic based on the state
of these inputs determines operation of up to 4 output relays which are also driven from a
latch under program control. Like the inputs, the relay outputs are driven from an isolated
power supply and optocoupler to prevent switching transient energy from affecting the
CPU.
A 10-bit successive approximation A/D on the 68HC16 CPU with eight channels is used to
measure all analog signals. Separate AC inputs for phase 1, phase 2, phase 3, and ground
fault signals are sampled at a 1.67 ms rate, squared and summed. RMS current is then
determined by deriving the square root of the sampled waveform over several cycles. The
sampling time is set to measure an integral number of cycles to reduce the affects of noise
and harmonics. Thermistor, analog input, control supply voltage and internal reference
voltage are also monitored. An external precision 5 V DC reference is used as the input
reference for the A/D converter.
When power to the unit is removed, a small 8-bit processor (68HC908) will continue to
operate for a period of at least 1 hour. This processor is powered from a large backup
capacitor. The 68HC908 accurately measures the time that the MM3 Motor Manager 3 has
been without control power. When power is re-applied the main processor will read the
time off from the small processor and then very accurately calculate the thermal capacity
value. This time off value is also used for the undervoltage restart feature.
Serial communications at up to 57600 baud is implemented with a UART. All necessary
timing and control is performed inside the chip. An external transceiver chip converts the
digital data to an RS485 interface. Direction, receive data and transmit data are on the
input side with a two wire twisted pair driver on the output.
AC control voltage to power the MM3 Motor Manager 3 can be selected as 120 or 240 V AC
using a switch and dual wound primary transformer. A filter is incorporated between the
incoming supply and transformer primary to prevent transients from affecting the
circuitry.
Separate, isolated secondary supplies are used for CPU power, input/output and
communication drivers. Optocoupling and transformer coupling are used between
isolated circuits to prevent transients from upsetting program execution. The 68HC908 is
used to provide separate watchdog timer and power fail monitoring control to ensure that
the main CPU starts and operates under any input voltage conditions. Should normal
program execution fail, the 68HC908 resets the main CPU.
CHAPTER 3: HARDWARE
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL