Motor Protection
ATL Motor Starter with Customer Specified Protective Relay
If a motor protective relay is ordered at the time the motor starter order was placed, the relay is installed at the factory along with other components (including potential transformers and current transformers) needed for the relay to operate correctly. The relay will be wired to de-energize and open the contactor if an event is detected.
The customer is responsible for setting the parameters in the motor protective relay and for testing and ensuring the operation of the protective relay.
ATL Motor Starter with Basic Electronic Protective Relay
If a motor protective relay is not specified when the across-the-line (ATL) motor starter is ordered, the ATL motor starter will come with a basic electronic protective relay. Refer to Basic Electronic Protective Relay for instructions and an example of determining the correct relay setting for the electronic protective relay.
Basic Electronic Protective Relay
The electronic protective relay must be configured for full load current as indicated on the motor controller nameplate. To determine the correct overload setting, you need to know the current transformer ratio of the system. This ratio is on the current transformers label and also indicated on the system schematic.
To determine the correct relay setting, divide the motor controller full load amps (FLA) by the current transformer (CT) ratio.
Electronic protective relay example:
Motor FLA = 66 A, CT ratio = 300:5
The corresponding overload (O/L) setting would be calculated as follows:
In this example, the dial on the motor overload should be set as close to 1.1 A as possible. Refer to the electronic protective relay catalog information shipped with the motor starter for more details about settings for overload, power factor, and time.
ATL Motor Starter with MotorSeT Advance Motor Control System
Motor Overload Protection
The MotorSeT Advanced ATL soft starter offers motor overload protection based on an advanced motor thermal model. The motor thermal model monitors the performance of the motor at all stages of operation and constantly calculates its temperature.
The thermal model offers a choice of 80 NEMA-based overload curves and includes current imbalance overload compensation, adjustable hot and cold load compensation, and adjustable exponential load cooling. Fro programming and configuration settings, refer to BRU3897001 MotorSeT Advanced Motor Control System instruction bulletin.
Feeder Configuration for Variable Frequency Drives
Feeder configurations used as upstream protection for variable frequency drives, have built-in protection and will not include any additional protective relays. The drive event permissive signal should be wired to open the contactor and turn off power to the drive when there is an event. The MotorSeT advanced motor control system cannot be used in a feeder configuration.
Feeder Configuration for Transformers
Feeder configurations used as upstream protection for transformers, require a feeder protective relay. If a feeder protective relay is ordered at the time the feeder configuration order was placed, the relay is installed at the factory along with other components (including potential transformers and current transformers) needed for the relay to operate correctly. The relay will be wired to de-energize and open the contactor if an event is detected.
The customer is responsible for setting the parameters in the feeder protective relay and for testing to ensure the operation of the protective relay. The MotorSeT advanced motor control system cannot be used in a feeder configuration.
Fuses
Fuses provide short-circuit protection and are selected based on the application.
R-rated Fuses
R-rated fuses are current-limiting, high interrupting rating fuses that provide short-circuit protection for medium voltage motor starters (loads) and controllers. R-rated fuses are backup fuses that have a minimum interrupting rating, and must be used in coordination with overload relays and/or overcurrent protection in combination motor starters.
E-rated Fuses
E-rated medium voltage fuses are general purpose fuses primarily used to help protect transformers. E-rated medium voltage fuses provide both current overload and short-circuit protection.