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Load damage


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#1 bob

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Posted 05 November 2005 - 05:41 AM

Hi,

We have one 1.5 k W 4 pole 400 V motor driving a load through a reduction gear box which is rated at 73 kg m outputtorque. There is only a simple thermal relay to protect the motor against overload. We have experienced so far many load damages without the motor tripping at the motor rated current. I believe that the thermal relay function is to protect the motor against overload and not to protect the load.
What are the basic types of load protection that exist on the market ?

Bob

#2 marke

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Posted 05 November 2005 - 06:07 AM

Hello Bob

Yes, you are correct, the thremal overload is designed to protect the motor only. Load damage is often caused by poor starting techniques with the star delta starter being one of the worst culprits.
In order to protect the load, we need to understand what the load is, what the damage is and when the damage occurs.

One way of protecting some loads, is to use a shaft torque monitor, sometimes a shear pin and sometimes just an overcurrent/undercurrent realy is sufficient.

If the damage is occuring at start, use a soft starter to overcome the problem.

Best regards,

#3 bob

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Posted 05 November 2005 - 06:20 AM

Thanks Marke for your very prompt reply. It appears that the damage happened during the starting of the agitator particularly when it is jammed with mud. If this holds true, I do not believe that a soft start will solve the problem. Basically they should clear the mud before starting the load but this is quite difficult to achieve. I do not understand how a soft start will solve the problem.

Bob

#4 marke

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Posted 05 November 2005 - 06:26 AM

Hi Bob

What is the current starter used?
What was the damage?

Best regards,

#5 bob

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Posted 05 November 2005 - 06:43 AM

Hi Marke,

The starter is a current D.O.L one and the driven load is an agitator with multiple screws attached to it. The screws were found torn which basically indicates a jammed load.

Bob

#6 marke

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Posted 05 November 2005 - 06:53 AM

Hi Bob

There are two mechanical considerations.
1. the DOL starter provides maximum start torque to the driven load
2. the DOL starter steps from zero torque to LRT in an instant resulting in a high torque transient.

If you use a voltage ramp soft starter, the torque transient will be eliminated and although maximum torque will be slowly applied to the load, there will be no "hammering" type damage from the sudden transient.
We commonly use small voltage ramp soft starters to eliminate mechanical damage to chains etc.

If you use a current limited soft starter, you can eliminate the transient and also limit the start torque. Set up the start current for a normal start and set up a maximum start time so that if the motor does not reach full speed in a period of time it tirps. This would indicate that the agitator is overloaded.

The current limited soft starter is not the cheapest option, but if damage is eliminated, you could get a reasonable payback!!

Best regards,

#7 GGOSS

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Posted 06 November 2005 - 10:48 PM

Hello Bob & Marke,

I would be looking to start/control this agitator via a good quality constant torque rated variable speed drive.

At 1.5kW, the cost difference between a soft starter and a drive is very small. To my way of thinking the VSD will outperform a soft starter in this application and provide other advantages such as reduced motor heating, reduced starting currents etc etc.

Regards,
GGOSS




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