can a transformer replace a reactor ?

Kataeb

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Can we use a line transformer with ratio 1/1 , in place of a line reactor choke , for VFD drives application? Will the transformer do the job of the reactor? I checked the net and found some contradictory answers... Thank you
 
It seems like it "might" help vs. having nothing, but what are the chances some random 1:1 transformer will have the desired impedances? Plus you may get stuck in a strange situation where you need to size the transformer fuses conservatively to satisfy your electric code, but may not be liberal enough to keep the VFD from tripping those small fuses. Or if you go with a huge transformer to solve that, the fuses may be far too big for the VFD and motor.
 
No, the phase angle is changed through a reactor, so that harmonics can't pass through. A normal transformer will generally not do this.
 
A line reactor is the obvious best choice. Though I have seen two applications where an isolation transformer was considered the better choice (Both times were early in my career and I was just the grunt).

The first one involved a very large drive powered by 4160 VAC, the second was a large panel with 15 small drives. The design engineer decided that one isolation transformer was more cost effective than 15 reactors.
 
An isolation transformer is a rector...

A standard 1:1 transformer is not an isolation transformer.
 
Any transformer is an isolaion transformer between its non-connected coils.
It is autotransformer that is not an isolation transformer.
In both cases this has nothing with the ratio.
 
Any transformer is an isolaion transformer between its non-connected coils.
It is autotransformer that is not an isolation transformer.
In both cases this has nothing with the ratio.

Maybe just a terminology difference between countries, but an isolating transformer needs to alter the phase angle. Otherwise it's just a transformer.
 
A proper 'Drive Isolation Transformer' alters the phase.

The primary is connected in Delta and the secondary is connected in Wye, with the common grounded.

Typically, a properly sized transformer is BETTER than a simple Line Reactor.
 
Drive isolation transformers were around long before the widespread use of reactors. It is an isolation transformer because it has two separate sets of windings on a common core, so there is no direct electrical connection from one side to the other; they are only magnetically coupled while being electrically isolated. Drive isolation transformers ARE also typically delta-wye so that there IS a phase shift from primary to secondary, even if the ratio is 1:1. This is because drives, both AC and DC, are typically designed for solidly grounded wye power systems. So you need that drive isolation transformer if your source is NOT a solidly grounded wye. Reactors on the other hand are a direct connection from line to load, there is no separation.


From the standpoint of mitigating harmonics, the transformer and reactor will have about the same effect, but reactors are typically available in different impedance values, where as drive isolation transformers are not, they are usually about 3-6% impedance, but not really selectable in that way, you usually get what you get when you buy them based on the motor size.


But electrically, an isolation transformer is basically equivalent to two back-to-back reactors, or look at the other way, a reactor is half of an isolation transformer. From a standpoint of their main benefit, that of slowing down the rise time of transients, they are basically the same. Adding either one ahead of a drive adds an inductive time constant that will not allow a rapid change in current or voltage through the coils. That's what you want. Shielded isolation transformers, as most drive isolation transformers are, also add the the ability to isolate common mode noise created by the drive from getting back onto the incoming line and affecting other equipment. Reactors can't do that.
 
Last edited:
jraef what will the phase shift exactly do compared to the inductive impedance against harmonics?

I'm curious because sometimes there is single phase application with a transformer (I mean a vfd that take single phase input to 3 ph output)
A reactor for this is usually build to order and long lead time when a transformer can be stock.....
Or if you want to run industrial 480/600v special equipment on domestic single phase, you can step up voltage and use a larger drive to run the load. i'm i right to expect the tfo will stop some of the harmonic reflexion ?
 

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