Rockwell Automation MV SMC Flex Motor Controller (for Series K or later) - Manual (Page 23)

Rockwell Automation Publication 1560E-UM051F-EN-P - June 2013 13
Product Overview Chapter 1
Ground Fault Alarm
The MV SMC Flex will indicate a Ground Fault Alarm if:
• No warning currently exists
• Ground fault alarm is enabled
• GF Inhibit Time has expired
• GF Current is equal to or greater than the Gnd Flt A Lvl
Parameter 77, Gnd Flt A Lvl, allows the installer to define the ground fault
current at which an alarm will be indicated. It is adjustable from 1.0 to 5.0 A.
Parameter 78, Gnd Flt A Dly, allows the installer to define the time period a
ground fault alarm condition must be present before a trip occurs. It is adjustable
from 0.1 to 25 seconds.
Thermistor/PTC Protection
The MV SMC Flex provides terminals 23 and 24 for the connection of positive
temperature coefficient (PTC) thermistor sensors. PTC sensors are commonly
embedded in motor stator windings to monitor the motor winding temperature.
When the motor winding temperature reaches the PTC sensor’s temperature
rating, the PTC sensor’s resistance transitions from a low to high value. Since
PTC sensors react to actual temperature, enhanced motor protection can be
provided to address such conditions as obstructed cooling and high ambient
temperatures.
Table 1 defines the MV SMC Flex PTC thermistor input and response ratings:
Table 1 - PTC Input Ratings
IMPORTANT The ground fault inhibit timer starts after the maximum phase of load current
transitions from 0A to 30% of the device’s minimum FLA Setting or the GF
Current is greater than or equal to 0.5 A. The MV SMC Flex does not begin
monitoring for a ground fault condition until the Gnd Flt Inh Time expires.
Response Resistance 3400Ω ± 150Ω
Reset Resistance 1600Ω ± 100Ω
Short-circuit Trip Resistance 25Ω ± 10Ω
Maximum Voltage at PTC Terminals (R
PTC
- 4kΩ) < 7.5V
Maximum Voltage at PTC Terminals (R
PTC
= open) 30V
Maximum Number of Sensors 6
Maximum Cold Resistance of PTC Sensor Chain 1500Ω
Response Time 800 ms