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Auto Transformer Motor Starting


noble

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I am carrying out motor starting study for one of the existing plant. The existing motors rated 3.3kV 770kW are started with the auto transformer at 67% tap. Auto Transformer is in ckt for 2.5 sec and then bypassed.

 

According to my calculations the voltage drop at motor terminals is about 87% during motor starting if started direct on line with worst system condition. And the simulation shows motor can be succesfully accelarted.

 

Hence I am unable to understand why they would have gone for Auto Transformer Starters.

 

My simulation with Auto transformer starting shows that the drop during first 2.5 sec is reduced compared to direct on line, however after bypassing, motor again draws the same current as Direct on line for some time leading to same voltage drop. Also the accelation time is increased. Then what is the gain in having auto transformer start.

 

Please shed some light!

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Hello noble

 

Welcome to the forum.

Reduced voltage starting is usually employed to either reduce the starting current, or reduce the starting torque, or both of the above. The auto transformer starter, correctly used, is effective in both reducing the starting current and the starting torque, however in many cases, the advantage gained is eliminated because the step through to full voltage occurs when the motor has not reached full speed.

 

The desire to reduce the start current, is generally in effort to reduce the voltage deflection at the point of common coupling. If you direct on line start the machine and cause the voltage to drop to 87% of rated voltage, then you could cause interference with other consumers or connected load. In many regions, the voltage disturbance must be limited to no more than 5%.

 

Torque reduction is generally required to protect the drven load and minimise mechanical breakdowns.

 

Sometimes, reduced voltage starting is employed because the local supply authority requests it.

 

I would say that yes, you could start the machine DOL, but there may be some problems resulting from it. I recommend that you try to find out why the reduced voltage starter was employed in the first place before eliminating it.

 

Best regards,

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Thanks Mark,

 

The 13% voltage at common coupling is acceptable in this case. Since the clients standards states that the voltage drop at the end user (say other motor terminals) can be allowed upto maximum of 20% during particular motor starting and under normal operation max upto 5%.

 

You have stated that higher starting torque can cause mechanical breakdowns. Is there way to check that it will not happen?

 

Also one more doubt I have, about the inrush current of the transformer. At the instant of starting when transformer and motor is energised from the supply, the inrush current of transformer and the starting current of the transformer, added together will it have any harmful effect

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Hello noble

 

The mechanical damage thing is something I suspect that only a mechanical engineer can comment on. Certain types of loads are more susceptible than others. What sort of load is it?? How often is it started?

 

Yes, with an auto transformer, as with a motor, there is a high inrush current for cycles rather than seconds, that is required to establish the flux in the iron. This is also true for the DOL started motor, so will probably not be too different. It will appear as a transient current, and any transient will have an effect on the supply, especially if there is a lot of power factor correction and the ability to resonate.

 

Best regards,

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Hello noble

 

Well, I would say that there would not be a significant disadvantage in running these DOL because I would expect that on the 67% tap, there would not be enough torque to get the pumps to full speed. I would expect that to get a significant improvement, you would need to use the 80% tap, or multistage. i.e. 67% - 80% - 100%

 

The reduced voltage starter on Pumps can be useful to reduce the effect of the pressure surge on startup. This can cause pipes to fracture if the pump is started with essentially an empty pipe.

 

Hope this helps,

Best regards,

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