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---------auto Ttransformer Starter---------


chinni

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Are you asking about something that has already happened and you want to know what kind of damage was done, or are you wondering if you can do this? Hopefully you are asking in advance because you are wondering what if, not what happened. So I'll address the what if.

 

Assuming you could do this (see below), applying low voltage to the motor will decrease the motor output torque by the square of the voltage applied. So if you apply 230V to a motor wound for 415V, the voltage is only 55% of rated, so the motor torque will be .55 x .55 = 30% of rated torque, and consequently the motor output power will be reduced to 30% of rated as well. So if it is unloaded, it will probably spin normally, but any load attached to it that is over 30% of what it was rated for will cause it to slow down, probably stall. If by chance it doesn't stall, the slowdown will drastically increase slip and the motor will keep drawing more and more current in a vain attempt to pull the load, which will result in an overload.

 

In reality, most autotransformer starters have a severely limited duty cycle in order to keep their size and cost down, so they typically are capable of being on-line for only 15 seconds or so and only with 15 - 20 minute cooling periods between starts. To prevent you from burning up the transformer windings, they install thermal cutout switches tied to the control circuit which will open up if the transformer over heats. So in real life, you probably will not be able to run through the transformer for 10 minutes anyway.

"He's not dead, he's just pinin' for the fjords!"
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