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#1 anu_rags

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Posted 22 February 2008 - 10:38 AM

Dear All,

I have been trying to start a Horizontal Centrifuge for last 2-3 days without any success. Traditionally the Centrifuge has been started either with VFD or with Slip Ring motor and starter. We tried to install a Softstarter on a 37kw 415V 1485rpm motor driving a Horizontal Centrifuge through a V Belt in between. The centrifuge was been started in unloaded condition ie the basket was empty. I tried possibly all the settings possible by Softstarter in both Voltage Ramp and Torque Ramp, but faced majorly one issue which was not getting solved.

Both in Voltage and Torque ramp, just when the motor was getting to reach full speed, high amount of slip started at motor shaft between shaft and belt. The sound suddenly increased to high levels, with high amount of friction between motor and belt, smoke starts coming out and current suddenly jumps for 2-3 seconds and then come back to low but since the friction is so high, I have to stop the SS.

In torque ramp, I started with 60 sec ramp, start torque of 15% and end torque of 100%. Increased it upto 90sec ramp, end torque 70% and then increased upto 120sec ramp, end torque 60%. But just when the motor seems to be coming up at full speed at the end of ramp up time, slip starts happening.

In voltage ramp, I started with 30 sec ramp, start voltage 30%. Then increased it to 60 sec ramp, start voltage 45%. But same slip at around full speed.

We started the same motor with Star Delta starter, and it went to full speed in 120 sec without any slip problems.

Any suggestions why this is happening. If Im unable to start an empty basket, how will I start fully loaded centrifuge.

Regards,
Anurag

#2 marke

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Posted 22 February 2008 - 07:14 PM

Hello anu_rags

The problem that you are seeing is that your starting torque is too high for the number of belts fitted. You need to either increase the number of belts, or reduce the starting torque.

If you use a voltage ramp, or a controlled current soft start, you will get a high peak torque as the motor reaches full speed.
I would suggest a torque ramp with a lower start torque and a much longer ramp time. i.e. the torque does not ramp up much at all.
Ideally, you want to have a constant torque start with the torque below 100 % for all of the start period. If the belts still slip when the torque is less than 100%, there is a problem with the belts, either insufficient number of incorrect tension.
I would suggest an initial torque at start of around 40 % and an end torque at start of around 60% and a start time of around 300 seconds and see what happens.

The load is purely inertial so it will start, it is just a matter of how long it takes. This should keep the torque well below the belt slipping torque.

Best regards,

#3 dennis3834

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Posted 22 February 2008 - 07:46 PM

To reduce slip, try to use Poly Vee belt wherein there is a groved alongthe inside. Just a thoguht..

#4 anu_rags

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Posted 22 February 2008 - 08:07 PM

Hello Mark,

Thanks for your reply. Closest setting that I had as per your suggestion was 15% start torque, 60% end torque and ramp of 120 sec. I understood that in order to reduce the slip, the end torque should be low. Current used to start from around 200% and used to reach around 300%, when slip started happening at end of 120sec. Since the current was too high for such a long period, SS tripped once on internal motor thermal protection. Just to get around for start purpose, I decreased the thermal protection setting.

As per your suggestion if I keep start torque of 40%, will not the current be higher and will remain around same level for full start time. It may heat up the motor and strain the supply. Maximum start time I have with SS is 250sec. I also dont understand when I start the motor with Star Delta Starter, no slip occurs at full speed and motor runs without any problem.

Dennis-Thanks for your advice too.

Regards,
Anurag

#5 marke

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Posted 22 February 2008 - 08:56 PM

Hello Anurag

The heating in the motor is a function of the power loss in the motor during start.
Provided that the start torque is high enough to be well above the residual load torque at all speeds, the power dissipated in the motor is essentially independent of start time. In the case of a purely inertial load, this is true. In the case of loads that are doing work during start, i.e. pump and fan, you need to keep the torque significantly higher than the load torque at all speeds. - extending the start time should not be an issue in this case provided that it is still reasonable.

The star/delta starter supplies one third of full voltage start torque to the motor. This would typically be in the order of 40% torque to 60% torque.
The start current with a soft starter will be higher than the motor in star due to the effective transformer action when in star.
I would expect that start current with the soft starter to be in the order of 300% to 400% whereas while the motor is in star, it will be in the order of 200% to 250%.

Best regards,




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