Brochure

PowerPact H-, J-, and L-Frame Circuit Breakers
Motor Circuit Protection
50
02/2016 © 2011–2016 Schneider Electric
All Rights Reserved
Additional Electronic Protection
• Locked rotor
• Under-load
• Long starts and stalled rotor
• Insulation faults
Trip Class of a Overload Relay Device
The motor branch circuit includes thermal protection that may be built into the circuit breaker. The
protection must have a trip class suited to motor starting. Depending on the application, the motor
starting time varies from a few seconds (no-load start) to a few dozen seconds (high-inertia load).
Example: In class 20, the motor must have finished starting within 20 seconds (6 to 20 s) for a starting
current of 6 x FLA.
Asynchronous-Motor Starting Parameters
The main parameters of direct on-line starting of three-phase asynchronous motors (90% of all
applications) are listed below.
• FLA: full load amperes
This is the current drawn by the motor at full rated load.
• Id: locked rotor current
This is the current drawn by the motor during starting, on average 6.0 x I
n
for a duration of 5 to 30
seconds depending on the application. These values determine the trip class and any additional
“long-start” protection devices that may be needed.
• Id: peak starting current
This is the subtransient current during the first two half-waves when the system is energized, on the
average 14 I
n
for 10 to 15 ms (e.g. 1840 A peak).
The protection settings must effectively protect the motor, notably through a suitable overload relay trip
class, but let the peak starting current through.
Motor-Feeder Solutions
PowerPact™ H-, J-, and L-frame circuit breakers motor circuit breakers are designed for motor-feeder
solutions using:
• three devices, including an electronic MCP or 1.3 M instantaneous-only trip unit
• two devices including a 2 M electronic trip unit.
Table 48: Trip Class of Overload Relays as a Function of Their FLA Setting
Class 1.05 FLA
1
1
Time for a cold start (motor off and cold).
1.2 FLA
1
1.5 FLA
2
2
Time for warm start (motor running under normal conditions).
6.0 FLA
1
5 t > 2 h t < 2h t < 2 mn 2 s < t  5 s
10 t > 2 h t < 2h t < 4 mn 4 s < t  10 s
20 t > 2 h t < 2h t < 8 mn 6 s < t  20 s
td
t”d
I*d
Id
FLA
Starting
time
Typical motor-starting curve
t
I