Autotransformer configuration
Autotransformer configuration
(OP)
I have an 5kV autotransformer used to start a 5kV motor. The autotransformer arrangment is such that there is a main contactor, a wye-contactor to connect the transformer coils, and a 3rd by-pass contactor.
When starting the main contactor closes along with the wye contactor. During transition the wye contact opens and then the by-pass contacts close to go across the line.
I wanted to learn in theory how this arrangment works? What is the purpose of closing the wye contactor to put the coils in a wye configuration? I'm assuming its used to help create/control the voltage for the various taps?
When the wye contactor is removed before the by-pass contactor is engaged I am guessing that the coils are then being used similar to a reactor starter? How does this reactor starter work in theory? Does it limit just the current, or the current as well as voltage?





RE: Autotransformer configuration
A reactor starter is simpler. And is basically the same sans the wye connection. Start with the reactors in then totally bypass them with the bypass contactor.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Autotransformer configuration
Thanks itsmoked
Can you explain how the voltage division works with this wye arrangment, I'm having a hard time visualizing it?
How does a reactor start work? Does it simply limit the current or is there a voltage drop across the impedence that you mentioned thus reducing the voltage to the motor terminals?
RE: Autotransformer configuration
Here's one I just drew to refer to.
During start you have:
(A) hooked to the line
(B) hooked to a motor lead
(C) hooked to 'the other side of the line' or a neutral point. (wye in your case)
As an auto transformer the voltage will be reduced in the ratio of turns spread over the two parts of the transformer.
If (C) is disconnected there is NO ratio. Transformer action ends. The full voltage is conducted from (A) to (B). No more ratio..
There will be some impedance due the magnetic nature of the transformer but very little as that is not what it's optimized for.
BTW: This disconnection of (C) is why you should not use an autotransformer for large voltage step downs. If you supplied, say, 4160V to (A) and neutral tied (C) to get 460V out of (B) what would happen if the transformer failed open between (B) and (C)? (B) would go to 4160V!! This would be, um, not good.
To answer your second question. There is a restriction of the current by the reactor. That is the mechanism that reactors work with. Of course limiting current results in a voltage reduction. A reactor starter is still referred to as a reduced voltage starting system.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Autotransformer configuration
Ah I see it now! I see the voltage divider you were refering to. So if I understand this correctly then this configuration can be written as follows from the standard voltage divider equation:
Vout=Vin(R2/R1+R2)
so...
Vb = Vac(Xbc/Xab+Xbc)
Does C have to be connected to the other two phases as in this wye configuration or can it be connected to ground?
As I just typed that last question I think I answered it. By connecting C in a wye configuration with the other two phases it establishes an input voltage Vin of 4160 an therefore the Vb is a ratio function of 4160. If C were to be connected to ground then the input voltage Vin would have been Van which in this case would be 2400V. Therefore the Vb output would be a ratio based off of 2400V instead of 4160. Does this sound right?
RE: Autotransformer configuration
Vout = Vin[(B)(C)turns /(A)(C)turns]
No R or X involved. (first order)
Keith Cress
kcress - http://www.flaminsystems.com
RE: Autotransformer configuration
this may not be the best question, but can resistors be used instead of coils to create the same voltage divider in this Autotransformer above?
In thinking about this, the only thing I can come up with is that there will be alot of wasted energy in the form of heat loss.
Was my example right about tying each of the legs to ground to create an Vin of 2400V?
RE: Autotransformer configuration
Reduced voltage starting methods for large asynchronous AC motors:
Autotransformer, open or closed transition (MV or LV)
Primary Reactor (MV or LV)
Primary Resistor (mostly LV only)
Wye-Delta (a.k.a Star-Delta), open or closed transition (mostly LV only)
Solid State; using Voltage Ramp, with or without Current Limit, Current Ramp, Current Step (a.k.a Pedestal) or Torque Ramp. Some manufacturers offer several or even all of these methods. (MV or LV)
In addition, especially in large MV motor applications, you also sometimes have Drives used to start motors. LCI (Line Commutated Inverter) are very commonly used where the line supply is too weak to accommodate other starting methods.
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RE: Autotransformer configuration
Jaref
In theory though would the Resistors work the same way?
RE: Autotransformer configuration