Large grinding mills, typically 5 MW or bigger, use wound rotor induction motor drive-trains as high starting torque is required.
The first 20 seconds during the start-up of a Wound Rotor Induction Motor is the most stressful time for the rotor circuits and it is during this time that things often go wrong.
Standard motor protection relays monitor and protect the motor on the stator side. This protection is not able to detect faults on the rotor side.
The SafeStart provides rotor side protection. In an event of a fault, a high-speed trip signal is issued directly to the MV breaker within milliseconds from the fault occurring.
Protection elements include:
Over-current protection
Instantaneous over-current protection is specifically tailored to be sensitive during most of the start-up but to exclude nuisance tripping when the shorting contactor closes.
No Run protection
This protection activates when the MV breaker is closed but the control signal from the PLC is not getting through to the LRS. This fault will cause the LRS to heat up. A large amount of energy will be dissipated in the LRS and may cause a starting torque spike if the motor is started with an overheated LRS.
Rotor earth fault protection
When an earth fault on the rotor circuit results in a flash to earth, the SafeStart detects it and a trip signal issued. Refer to the graph above. A trip was issued after 6 ms for this line to neutral flash on the blue phase.
A rotor earth fault on the rotor side needs to be detected before it escalates into a more serious and damaging line-to-line fault. This fault is normally invisible to the Stater motor protection relay.
Open Phase protection
Open phase protection activates when the brushing fails or the protection module detects an open circuit in the rotor circuits. This issues a trip to the MV breaker.
Jamming protection
This protection module activates a jamming trip signal when the mill does not accelerate as expected.
Jamming protection is important in the case where the barring gear did not disengage and the interlock proximity switches failed. This is protection prevents the barring gear motor from over-spinning and even exploding.
Locked Charge Protection
The protection module calculates the ore angle during start-up and issue a trip signal if the ore angle exceeds a preset value to prevent drop charges from occurring.
