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Starting Torque


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#1 bob

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Posted 18 June 2006 - 08:34 AM

Hi,

My conveyor belt is driven by a 75 kW VSD driven electric motor. When the conveyor is fully loaded , the drive very often tripped on drive overload at start. I have changed the configuration from V/F to sensorless vector and now the drive is tripping on motor overload. It is important to note that the high starting torque is only required to start the conveyor and once it is at speed the current consumption is only around 40-50 amps.I believe I`m left with only two solutions :

1. Change the drive to a larger size to cope with the high starting torque
2. Reduce the load on the conveyor although this is not a practical solution.

The drive used hasn`t got the close vector option. I have increased the acceleration ramp but without any marked improvement.

Any advice ?

Bob

#2 marke

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Posted 18 June 2006 - 07:12 PM

Hello Bob

With a loaded materials conveyor, it is not uncommon for the start torque requirement to be in the order of 160 - 180%. This torque is required with the motor shaft at standstill.

Most drives have problems developing this torque at less than 5Hz and some drives have problems at higher frequencies.
I recently temporarily replaced a drive that had failed in a similar situation. It was a sawdust drying drum which would never start if stopped when loaded. I replaced the drive with a similar rated VFX just to get the customer going while the other drive was repaired. ( the drum had stopped loaded due to a power failure)
We told the drive to start to see what would happen and it started and ramped up with no difficulties at all.
The secret is the internal algorithms and how well the drive can develop torque from zero speed.
If the drive is not able to fully flux the motor at very low frequencies, the motor will be held in the stall condition. The drive frequency increases and soon exceeds the maximum slip of the motor. The torque developed is not sufficient to break the load away, and the motor begins to draw the stall current and the drive then trips on overload or earth fault or something similar. If it is an open loop vector, it may ramp up to a current limit point at a slip frequency beyond the rated slip and hold that frequency until the motor trips on overload. - The VFX was in open loop mode, but seems to give high torque on a stalled shaft.

Best regards,

#3 mariomaggi

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Posted 18 June 2006 - 09:24 PM

Dear bob,
probably there is a third solution, less expensive.

If your motor is a 4-poles motor, you could install a 6-poles motor of the same power.
The inverter drive will remain the same, and you will have about 150% of actual torque.

The 4th possibility is to add a fluid coupling between motor and gearbox.

The 5th possibility is to select a different rotor type, with only one cage, preferably made by copper. Please refer to some motor specialist before to make such modification.

Regards
Mario



Mario Maggi - Italy - http://www.evlist.ithttps://www.axu.it





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