Dual-purpose 3-phase motor?

[Usual Suspect]

In my business I come across old German equipment with 3-phase induction
motors that are connected in Y-configuration to start and within a few
seconds switched to delta for running. This is done, I'm told, to lessen the
inrush current at switch-on.

Typically the motors have 3 independently-wound windings with 6 connections,
both ends of each winding are brought out to the terminal block.

I'd like to understand a little more about why this works. Is it as simple as
the fact that between 2 supply phase connections in the Y configuration there
are 2 windings and in delta there's only 1? (The Y configuration's common
point is not connected to ground or neutral.)

Thanks,

 

[John Popelish]

In my business I come across old German equipment with 3-phase induction
motors that are connected in Y-configuration to start and within a few
seconds switched to delta for running. This is done, I'm told, to lessen the
inrush current at switch-on.

Typically the motors have 3 independently-wound windings with 6 connections,
both ends of each winding are brought out to the terminal block.

I'd like to understand a little more about why this works. Is it as simple as
the fact that between 2 supply phase connections in the Y configuration there
are 2 windings and in delta there's only 1? (The Y configuration's common
point is not connected to ground or neutral.)

You have it, except that there is a phase angle between the
windings, so they do not share the total voltage as if they
were two independent inductors. Instead of the volts per
turn dropping by half, it drops by a factor of 1/square root
of 3.
http://www.usmotors.com/Service/Bulletins/Issue7-May03.pdf

 

[Usual Suspect]

Thanks, John. Much clearer now. Good reference.

 

[neon]

maye if you consider that Y transformers can generate 1:1 the voltage inpress on to them. while a D transformers will genrate half of the voltage. in your case the Y has higher voltage less current same power then switch to less voltage same power. maybe i did not eplained it correctly.