2.3 INPUTS AND OUTPUTS
2.3 INPUTS AND OUTPUTS
a) PHASE CT INPUTS
For motor full-load currents up to 250 A, the phase conductors can be directly connected to the MM3 without
phase CTs. If external CTs are required, the secondary winding should be looped though the MM3 doughnut
Selected CTs should be capable of supplying the required current to the total secondary load which consists of
the connection wiring burden. The CT must not saturate under maximum current conditions which can be up to
8 times motor full load during starting.
b) GROUND FAULT CT INPUT (19/20)
The ground fault detection consists of a 50:0.025 input (Terminal 19) and a common input (Terminal 20), or
residual connection of phase CTs ( only the common input should be externally grounded ). The 5 A input on the
ground CT is used for 5 A secondary CTs or for residual connection of phase CTs. Residual ground fault pro-
tection provides a sensitivity of 10% of motor full load current. The 50:0.025 core balance (zero-sequence) CT
input can be used for improved sensitivity when measuring the ground fault current.
Care must be taken when turning ON the Ground Fault Trip feature. If the interrupting device
(contactor or circuit breaker) is not rated to break ground fault current (low resistance or sol-
idly grounded systems), the feature should be disabled. The 50:0.025 input is only recom-
mended to be used on resistance grounded systems. Where the system is solidly grounded
or high levels of current are to be detected use the 5 A ground input.
c) SUPPLY VOLTAGE (22/23/24)
A supply voltage of 120/240 V AC at 50/60 Hz is required to power the MM3. The label on the back of the unit
specifies the voltage that has been internally set. To change the voltage setting, open the sliding door on the
back of the MM3 and locate the supply voltage selector slide switch. The selector slide switch has a label
affixed to show the 120/240 V AC positions. Set the slide switch to the desired voltage.
d) GROUND (21/48) SURGE
This is an additional ground terminal provided for dissipating transient signals and surges. This must be con-
nected by a thick wire or braid to the system ground for reliable operation.
e) EXTERNAL CONNECTIONS
Signal wiring is to box terminals that can accommodate wire as large as 12 gauge. CT connections are made
using #8 screw ring terminals that can accept wire as large as 8 gauge. Consult Figure 2–3: TYPICAL WIRING
DIAGRAM on page 2–3. Other features can be wired as required.
f) THERMISTOR INPUT (17/18)
Either a Positive Temperature Coefficient (PTC) or Negative Temperature Coefficient (NTC) thermistor may be
directly connected to the MM3. By specifying the hot and cold thermistor resistance, the MM3 automatically
determines the thermistor type as NTC or PTC. Use thermistors with hot and cold resistance values in the
range 100 to 30000 Ω. If no thermistor is connected, the S1: CONFIGURATION \ THERMISTOR \ THERMISTOR TRIP
and S1: CONFIGURATION \ THERMISTOR \ THERMISTOR ALARM setpoints must be set to DISABLE.
g) ANALOG INPUT (28/29)
The MM3 accepts an analog input from a standard 4 to 20 mA source. This input can be used for process con-
trol monitoring to provide status and/or alarm and tripping signals related to the input signal level. The analog
input messages (S3: PROCESS \ ANALOG INPUT setpoints page) can be programmed to show user-defined
names and units.
MM3 Motor Manager 3
GE Power Management