How to determine the reference voltage for three-phase transformer?

Roland chung

Trusted Information Resource
#1
Dear all,

This is my first post here. I found some experts herein and hope to get some useful information from this forum.

We manufacture X-ray equipment for diagnostic purpose. As you can see from the caption, my question is related to reference voltage of three-phase transformer (reference voltage used for determination of clearance & creepage, etc). In our X-ray equipment, the primary part (line to line voltage is 400V) of three-phase transformer is delta connection (without neutral) and the secondary part (line to line voltage is 208V) is star connection (the neutral combines the earth, as referred to PEN).

Wondering how to consider the reference voltage between input line (primary) and earth (barrier A-a1 in iec 60601-1), between input line and output (barrier 17.g.4 in iec 60601-1).

Thanks in advance.
 
Last edited:
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Stijloor

Staff member
Super Moderator
#2
Dear all,

This is my first post here. I found some experts herein and hope to get some useful information from this forum.

We manufacture X-ray equipment for diagnostic purpose. As you can see from the caption, my question is related to reference voltage of three-phase transformer (reference voltage used for determination of clearance & creepage, etc). In our X-ray equipment, the primary part (line to line voltage is 400V) of three-phase transformer is delta connection (without neutral) and the secondary part (line to line voltage is 208V) is star connection (the neutral combines the earth, as referred to PEN).

Wondering how to consider the reference voltage between input line (primary) and earth (barrier A-a1 in iec 60601-1), between input line and output (barrier 17.g.4 in iec 60601-1).

Thanks in advance.
Any X-Ray equipment experts that can help here?

Thank you!!

Stijloor.
 

Tim Folkerts

Trusted Information Resource
#4
This seems like more of an electrical engineering question than a question about either quality or x-rays.

There are a wide variety of experts here, but I don't recall many electrical engineers answering questions, so there may not be many people here with the expertise to address your question. I have a vague understanding of three phase wiring and delta connections on transformers, but I don't know enough to answer your specific question.
 

BradM

Staff member
Admin
#5
Any voltage reading off of a transformer is going to depend on the incoming voltage supplied to that transformer. What is specified by you as the mfg.?

When you state "primary part" and "secondary part", is that referring to the primary voltage (feeding a transformer) and then secondary voltage (coming off the transformer)?

Also, if you don't mind, can you explain to me what PEN is? I am not familar with that term.

As far as considering any of the voltage, again I would think that is highly dependent on the nominal voltage (and the deviation of actual voltage from nominal voltage) supplying the transformer.

Are you asking what the acceptable leak rate (in volts) should be from the supply voltage to ground?

Sorry for so many questions. Just trying to figure out a little better exactly what you're looking for.:D
 

BradM

Staff member
Admin
#6
No one wants to share his/her experience, a little bit of disappointment. :(
Roland, I don't think that is the case. That's a pretty specialized question you got there; that requires asking an electrical contractor (and/or an electrical engineer) that does work in the medical industry. Most of us are a bunch of quality nuts!!:lol::lmao:

However, I'll certainly try to answer any question for you.:agree1:
 

Al Rosen

Staff member
Super Moderator
#8
To answer the original question using my limited knowledge of the system, I believe that you can ignore the primary to secondary since they are isolated from each other. Use the 120v line to neutral for the spacing of the secondary & neutral/ground. Use 208 volts for calculating the spacing requirements between the lines. Use 400v for spacing requirements on the primary. Just my opinion.
 

Peter Selvey

Staff member
Super Moderator
#9
This can be worked out theoretically by knowing the peak voltage (with respect to earth) and phase relationship between each of the primary and secondary windings. Alternately, you may find it easier to simply to measure the working voltage between each points in turn, e.g.

P1 - S1
P1 - S2
P1 - S3
P2 - S1
etc until
P3 - S3
(total 9 measurements)

It is possible depending on the construction that at one or more of these nine combinations will have a secondary voltage that is 180 degrees out of phase with the primary, so that the working voltage at that point will be simply Vw = Vp + Vs, where Vp and Vs are the primary and secondary rms voltages with respect to earth (not line to line). So, in your example, Vp = 230Vrms, Vs = 208Vrms, so Vw = 438Vrms.

It is normal in safety testing that engineers ignore the secondary voltage, which is reasonable when the voltage is relatively low (e.g. 24Vrms). They do this because with the second edition, the real working voltage can often be slightly above 250Vrms (e.g. 230Vrms + 24Vrms = 254Vrms). This puts the cr/cl in the next column which is a massive overkill. So everyone has quietly agreed to ignore the secondary voltage.

However, this should not be done if the secondary voltage is large, such as your case. In this case there may be real working stress of 438Vrms, which should have appropriate cr/cl for that voltage. That said, the 3rd edition has some more realistic values than the 2nd edition.
 

Roland chung

Trusted Information Resource
#10
Dear all,

Many thanks for your input.

Peter, your answer makes more sense. In fact, I have measured the working voltages between primary and earth, between primary and secondary.

As you said, the maximum working voltage between primary and secondary is equal to primary rms voltage plus secondary rms voltage with respect to earth, i.e. 230V+ 120V= 350V. (I think you made a little mistake, the 208V is line to line voltage)

This raised another question. That is the measured vlotage of 350V less than the input voltage of 400V. I would think the reference voltage shall be 400V at least.

Furthermore, the primary part of three-phase transformer is delta connection without neutral. Why do not you consider the primary line to line voltage plus secondary line to line voltage (i.e. 400V+ 208V= 608V) when determined the voltage between primary and secondary?

Best regards.
Roland
 
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