Can autotransformer act like line reactor?

[thinker]

The VFD operates at 575V, fed by step-up drive isolation transformer 480/575V. Inside that VFD, there is a switching DC power supply (input power three phase, 400V) fed from a dedicated 400VA control transformer 575/400V. This power supply has failed (as we assume, the failure could be attributed to fast switching transient coming from the grid). We consider that power supply to be not adequately protected from such transients (penetrating via parasitic transformer capacitance), and plan to add a small (3%) line reactor. So, we will have step-down transformer and reactor in series feeding the power supply.

Question to community: can we use the autotransformer instead of combination transformer-reactor to provide both functions (voltage matching and dv/dt attenuation)? In this case, we eliminate capacitive coupling of the transient, but we do not know if the autotransformer can provide transient attenuation properties similar to line reactor.

 

[waross]

You now have the reactance of both the primary and secondary windings helping you.
The auto-transformer may replace capacitive coupling to direct coupling and greatly reduce the amount of reactance in the circuit.
Have you considered adding surge protection directly to the power supply input?
Probably the majority of the reactance in the power supply feed will be the small control power transformer.
The reactance of the step up transformer combined with the reactance of the control power transformer should provide a lot of isolation.
It is possible that the power supply died without outside influence.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Huh, just had a discussion like this elsewhere...

The consensus there was that because the boost transformer is an isolation transformer re-wired as an autotransformer, you are not going to get very much impedance from it so the attenuation effects would no longer be as good as those of a reactor. But I have to still agree with Bill in this case, because you also have the CPT as an impedance source and it's hard to imagine that there is anything getting through ALL of that to cause the SMPS to fail. I'd venture to say it was failing for some other reason until proven otherwise.

One possibility is that the SMPS is of a design that is sensitive to the line frequency and because it was made for 50Hz and you are undoubtedly giving it 60Hz, that is causing premature failure. That is almost unheard of now, but was an issue years ago when some types of SMPS first came out. When I worked for Siemens years ago, you had to make sure you ordered your VFDs as 60Hz here, otherwise you got the 50Hz version, and the stated issue was that control board SMPS. I think they later changed to using a DC-DC converter tapped off of the drive's DC bus as many other VFD mfrs do now, partly because of that issue.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[oldfieldguy]

If you open the neutral connection, the remainder in circuit will act as a reactor. This characteristic is used on some autotransformer reduced voltage start schemes.

old field guy