Autotransformer configuration

You have it! If the wye isn't closed then there exists no transformer. You get no voltage division. Only the impedance of the first part of the coils.

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 -

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?

Forget that it is a 'wye' connection. Just consider that it is an autotransformer.

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 -

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?

Well I'd rather say the result is:

Vout = Vin[(B)(C)turns /(A)(C)turns]

No R or X involved. (first order)


Keith Cress
kcress -

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?

There are what are called Primary Resistance reduced voltage starters. They are rarely (if ever) used on Medium Voltage however, they would be entirely too dangerous. The extreme heat given off by the resistors means they must be open to the air, not something you want to do with MV.

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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Jaref

In theory though would the Resistors work the same way?

Yes, very similar. However as the resistors heat up the resistance changes, which makes the motor response slightly more dynamic compared to an RVAT, and not always in a good way. Primary Resistor starting usually is limited to applications that can accelerate very very quickly, i.e. a few seconds, because of this. Overhead crane manufacturers used them on travel drive motors for years before the advent of inexpensive and reliable solid state starters. Now they are extremely rare.