1. INTRODUCTION
When applying protective relays to motors or
any other equipment, we always ask how much protection is enough. The answer depends on rewind cost, loss of production, effect on downtime, new versus old installation, need for communication, metering, control and the consequences of a motor failure on the electrical system and process.
This publication presents an overview of motor hazards and a discussion of detection and protection options. Basler relay models are offered with typical setting value ranges and considerations to help designers and users select Basler relays for motor protection. Most of the protection functions apply to squirrelcage, wound induction motors and synchronous motors. Additional protection is usually provided for synchronous motors and will be mentioned in this document.
2. OVERVIEW OF MOTOR HAZARDS
Motor protection is a challenge because there
are so many different things that can go wrong
with a motor and its associated load:
Motor induced
• Insulation failure (within the motor)
• Bearing failure
• Mechanical failure
• Synchronous motors-loss of field
Load induced
• Overload and underload
• Jamming
• High inertia
Environment induced
• High ambient temperature
• High contaminant level or blocked
ventilation
• Cold or wet ambient conditions
Source induced
• Loss of phase or phases
• Voltage unbalance
• Overvoltage
• Undervoltage
• Phase reversal
• Out of step condition resulting from system
disturbance
Operation induced
• Synchronizing or closing out of phase
• High duty cycle
• Jogging
• Rapid reversing
3. PROTECTION
3.1 Stator Faults
3.1.1 Phase Fault Overcurrent Protection
Phase to phase and three phase faults are
usually detected with nondirectional
instantaneous or definite time overcurrent
relays. If the available 3-phase fault current is a low multiple of the relay setting (weak system), quick pickup is not assured. Differential relaying should then be considered. Instantaneous relays are typically applicable when the motor rating is less than one-half of the supply transformer KVA rating.
The instantaneous phase relay should be set at no less than 1.6 times the locked rotor current using the value of locked rotor current at maximum starting voltage. This setting also
assumes the relay is sensitive to the transient
overreach (DC offset) of an asymmetrical fault.
Lower settings are possible if the relay disregards the transient component or if a time delay longer than the transient time (6-15cy) is added.
Verify that the minimum 3-phase fault current at the motor terminals is at least 3 times the relay setting. Fig. 1 illustrates the relay settings in relation to the starting current and the minimum short circuit current.
Download Timer
