I am designing a gear+motor arrangement that will start the driven equipment shaft train from rest ("Breakaway"). The motor is sized such that its 300% starting torque (locked rotor torque) is required to achieve breakaway. There is considerable backlash in the gearbox and inherent to its design I cannot reduce the backlash. This results in a high impact load when the motor is started DOL. I need to find a way to engage the gearbox slowly and/or softly to minimize or eliminate the impact load at start-up.
Here are the options I have considered so far:
1) Mechanical soft-start, i.e. fluid coupling- The fluid coupling rating would be equal to the full load rating of the motor, but we need to take advantage of the 300% starting torque of the motor to achieve breakaway, which the fluid coupling will not transmit.
2) VFD- A variable frequency drive would allow me to creep the gears into engagement, but I believe most VFDs will only allow ~150% rated motor torque which only gets me half way to where I need to be
3) Electrical soft-starter- We have tried these before and have found that as soon as the soft start module detects no load, it bypasses the soft-start function and allows full power to the motor. In our case, this is immediately upon start-up due to the backlash in the gears.
4) Pre-engagement motor- I have considered using a fractional-power motor mounted to a free shaft, or piggybacked onto my main motor, whose only function is to take up the backlash in the gears, but is not large enough to cause any damage or start the driven equipment turning. From my perspective this is the easiest solution, however it is not the most 'elegant' solution in that it causes the need for an additional motor starter and control logic to properly sequence the operation of the small pre-engagement motor and larger main drive motor.
5) Manual pre-engagement- Another simple solution, but the intent of the system is to be fully automatic and therefore not requiring manual operation. Plus the lock-out / tag-out procedures required to prevent accidental energizing of the motor while manually turning are cumbersome.
I am open to any suggestions you all might have! Thanks in advance for any help.
Here are the options I have considered so far:
1) Mechanical soft-start, i.e. fluid coupling- The fluid coupling rating would be equal to the full load rating of the motor, but we need to take advantage of the 300% starting torque of the motor to achieve breakaway, which the fluid coupling will not transmit.
2) VFD- A variable frequency drive would allow me to creep the gears into engagement, but I believe most VFDs will only allow ~150% rated motor torque which only gets me half way to where I need to be
3) Electrical soft-starter- We have tried these before and have found that as soon as the soft start module detects no load, it bypasses the soft-start function and allows full power to the motor. In our case, this is immediately upon start-up due to the backlash in the gears.
4) Pre-engagement motor- I have considered using a fractional-power motor mounted to a free shaft, or piggybacked onto my main motor, whose only function is to take up the backlash in the gears, but is not large enough to cause any damage or start the driven equipment turning. From my perspective this is the easiest solution, however it is not the most 'elegant' solution in that it causes the need for an additional motor starter and control logic to properly sequence the operation of the small pre-engagement motor and larger main drive motor.
5) Manual pre-engagement- Another simple solution, but the intent of the system is to be fully automatic and therefore not requiring manual operation. Plus the lock-out / tag-out procedures required to prevent accidental energizing of the motor while manually turning are cumbersome.
I am open to any suggestions you all might have! Thanks in advance for any help.
