OSÂ Overspeed fault in Toshiba drive
OS Overspeed fault in Toshiba drive
Description The OS fault occurs when the motor's actual rotational speed exceeds the "Maximum Frequency" or "Overspeed Threshold" setting in the drive parameters. This usually happens in applications where the load can "drive" the motor (like a downhill conveyor or a hoist lowering a heavy weight), or in closed-loop vector control where the encoder feedback indicates the motor is spinning uncontrollably fast. It is a critical safety trip to prevent the motor rotor from exploding due to centrifugal force or the machine from destroying itself. Causes
1. Overhauling Load: Gravity or wind is spinning the motor faster than the VFD is commanding it to go.
2. Improper Tuning: In Vector Control mode, if the PID gains (Proportional/Integral) are set too aggressively, the drive may oscillate and surge speed, triggering OS.
3. Encoder Noise: Electrical noise on the encoder cable can be interpreted as high-frequency pulses, tricking the drive into thinking the motor is doing 5000 RPM when it is actually stopped.
4. Parameter Error: The "Maximum Frequency" (F011) is set lower than the "Upper Limit Frequency" (UL), creating a logic conflict where normal operation is considered "overspeed." Solution Check the application mechanics. If this is a hoist or lift, does the brake hold? If the brake is slipping, the load will accelerate due to gravity, causing an OS trip. Ensure the mechanical brake is engaging properly.
If the mechanics are fine, check the "Overspeed Detection Level" parameter (typically F626). It is usually set to 120% of the maximum frequency. If your process requires high speeds, you may need to increase this limit slightly.
For closed-loop systems (using an encoder), inspect the shielding of the encoder cable. The shield must be grounded at the drive end only. If the cable is run next to the high-voltage motor leads, induction will create phantom pulses. Reroute the encoder cable in a separate metal conduit. If the motor surges or oscillates before tripping OS, you need to perform an Auto-Tune again or lower the "Speed Loop Proportional Gain" to stabilize the control loop.
Description The OS fault occurs when the motor's actual rotational speed exceeds the "Maximum Frequency" or "Overspeed Threshold" setting in the drive parameters. This usually happens in applications where the load can "drive" the motor (like a downhill conveyor or a hoist lowering a heavy weight), or in closed-loop vector control where the encoder feedback indicates the motor is spinning uncontrollably fast. It is a critical safety trip to prevent the motor rotor from exploding due to centrifugal force or the machine from destroying itself. Causes
1. Overhauling Load: Gravity or wind is spinning the motor faster than the VFD is commanding it to go.
2. Improper Tuning: In Vector Control mode, if the PID gains (Proportional/Integral) are set too aggressively, the drive may oscillate and surge speed, triggering OS.
3. Encoder Noise: Electrical noise on the encoder cable can be interpreted as high-frequency pulses, tricking the drive into thinking the motor is doing 5000 RPM when it is actually stopped.
4. Parameter Error: The "Maximum Frequency" (F011) is set lower than the "Upper Limit Frequency" (UL), creating a logic conflict where normal operation is considered "overspeed." Solution Check the application mechanics. If this is a hoist or lift, does the brake hold? If the brake is slipping, the load will accelerate due to gravity, causing an OS trip. Ensure the mechanical brake is engaging properly.
If the mechanics are fine, check the "Overspeed Detection Level" parameter (typically F626). It is usually set to 120% of the maximum frequency. If your process requires high speeds, you may need to increase this limit slightly.
For closed-loop systems (using an encoder), inspect the shielding of the encoder cable. The shield must be grounded at the drive end only. If the cable is run next to the high-voltage motor leads, induction will create phantom pulses. Reroute the encoder cable in a separate metal conduit. If the motor surges or oscillates before tripping OS, you need to perform an Auto-Tune again or lower the "Speed Loop Proportional Gain" to stabilize the control loop.
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