Primary transformer resistance

[Jpflex]

You could connect an incandescent light bulb in series with the primary. This would act as a current limiter if the coil is shorted. You can measure the total current and the voltage drop across the light bulb and across the coil and determine the (rough) impedance of the coil from that.

This may work for small transformers but these power plant transformers produce mega watts of power. 34500 volts secondary with dangerous switch gear

 

[jim dungar]

However, it seems i would need to place an ammeter around the primary and calculate the impedance based on actual current reading

with the impedance value i could further calculate the primary henry value based on

For large custom transformers the manufacturer can usually provide this information.

If you need something else the transformer %Z and X/R ratio can be used to calculated the resistance and reactance using some common power systems study formulas.

Why are you doing this? Field verification of transformer values is rarely, if ever, done without using special/dedicated test instruments. Unless you use these test sets regularly, it is better to rent them as needed.

 

[ggunn]

What i mean is that for example a reactor being a coil of wire within preferably an iron core will need less resistance in series with this reactive coil for AC voltage/ current rather than DC applied due to the effect of CEMF which is the same thing you explained in the application of frequency without damaging the inductor coil

I still don't know what you are talking about but I stand by what I said. For a given inductor the AC impedance will be greater than the DC resistance.

 

[wwhitney]

The original calculation gave the 60 Hz fully loaded impedance of the transformer. Not the DC impedance.

Am a bit unclear on the computation in the OP. It takes the transformer rated primary current and divides it into the primary voltage to get an impedance. It seems to me that impedance is (approximately) the equivalent load impedance the primary voltage source sees when the transformer is fully loaded, and that the calculation has nothing to do with any impedance you'd actually measure on the transformer in or out of circuit. Am I missing something?

Cheers, Wayne

 

[CoolWill]

This may work for small transformers but these power plant transformers produce mega watts of power. 34500 volts secondary with dangerous switch gear

Which means they must have a pretty high impedance, otherwise 10s or 100s of thousands of amps would flow when 34.5 kV was connected. I still think a lot could be gleaned from connecting it in series with a load.

 

[LarryFine]

I still think a lot could be gleaned from connecting it in series with a load.

Don't be surprised if the two voltages don't add up to equal the supply voltage.

 

[Jpflex]

Which means they must have a pretty high impedance, otherwise 10s or 100s of thousands of amps would flow when 34.5 kV was connected. I still think a lot could be gleaned from connecting it in series with a load.

I believe i calculated somehere around 3,000 amperes on the primary max but not over that. The secondary would be proportionañly less than that.

This also depends on the loads on secondary as primary feedd secondary and secondary puts limits on primary etc

 

[Jpflex]

Which means they must have a pretty high impedance, otherwise 10s or 100s of thousands of amps would flow when 34.5 kV was connected. I still think a lot could be gleaned from connecting it in series with a load.

Additionally this eould be extremely dangerous to do and could jeopardize ones job. I would not consider this within these grid inverters

 

[CoolWill]

Additionally this eould be extremely dangerous to do and could jeopardize ones job. I would not consider this within these grid inverters

What would be dangerous? We're talking about de-energized transformers. How else did you use an ohm meter on it?

 

[Jpflex]

What would be dangerous? We're talking about de-energized transformers. How else did you use an ohm meter on it?

Placing a load im series with the primary while powered on and modifying manufacture installation can be dangerous for an inverter with this much output.

I didnt say anything about testing dead circuits with an ohm meter

 

[winnie]

Agreed. If the OP has a suitable 480V test supply, measurements with that supply, a resistance, and the transformer would be reasonable.

But a line interactive inverter is not a good test source. The inverter voltage would change dynamically and wouldn't have a proper excitation source

 

[CoolWill]

I guess I missed the part about it being an inverter. I've been thinking transformer this whole time.

 

[LarryFine]

I guess I missed the part about it being an inverter. I've been thinking transformer this whole time.

Ditto.

 

[winnie]

I guess I missed the part about it being an inverter. I've been thinking transformer this whole time.

I think this is a transformer that is part of an inverter installation.

The OP describes the transformer isolated from the inverter by contractors.

Rereading the original post, it isn't actually clear if the transformer primary is correctly identified, but I don't know if this is important. With the transformer not energized and isolated from the inverter electronics, the OP measured low resistance on the inverter side terminals.

Jonathan

 

[CoolWill]

Placing a load im series with the primary while powered on and modifying manufacture installation can be dangerous for an inverter with this much output.

I didnt say anything about testing dead circuits with an ohm meter

Then how did you arrive at the 0.2 ohms of resistance?

 

[Jpflex]

Then how did you arrive at the 0.2 ohms of resistance?

Placing a load im series with the primary while powered on and modifying manufacture installation can be dangerous for an inverter with this much output.

I was refeeing to using an ommeter after placing a load in series with the primary as you mentioned

this would not be as much of an issue as if testing live

 

[Jpflex]

I think this is a transformer that is part of an inverter installation.

The OP describes the transformer isolated from the inverter by contractors.

Rereading the original post, it isn't actually clear if the transformer primary is correctly identified, but I don't know if this is important. With the transformer not energized and isolated from the inverter electronics, the OP measured low resistance on the inverter side terminals.

Jonathan

The transformer is part of the inverter. There are 6 inveeters that output ac current parraleled onto 3 phase buss starting at inverters output contactors and fuse.

From here there is no other sepparation from the Y Delta transformer primary windings. Buss to primary

 

[winnie]

As a very very rough back of the envelope calculation, I'd expect a DC resistance measured L-N on the secondary side to be about 0.22 milliohm, about 1/1000 of what you measured.

I estimate this as follows: based on DOE efficiency standards, I expect the transformer efficiency to be 99.5%, so at full load I expect 18kW of losses. I assume that these losses are 1/3 primary coils, 1/3 secondary coils, and 1/3 core (like I said, very very rough). So about 6kW of losses to resistance in the primary coil. Since this is a 3 phase system I assign 2 kW per coil.

Transformer full load current is about 3000A, so I just use I^2 * R = P with 3000^2 * R = 2000. Since you are measuring the transformer on the wye side this gives the L-N resistance.

Given all of my estimates, I would not be surprised by any value between 0.1 and 2 milliohms. 200 milliohms (the original measurement) is IMHO simply not possible.

IMHO the 0.2 ohm measurement is simply equipment limitations. A typical multimeter is simply not useful below 10 ohms; 0.2 ohms is probably some combination of lead and contact resistance.

To properly measure less than 1 ohm you need an instrument like this:
https://www.tequipment.net/Extech/380560/Milli-Ohmmeter/
or

https://www.tequipment.net/Extech/UM200/Micro-Ohmmeter/DLRO/Bond-Testing/

You will note that these instruments use high current (10A test current) and 'kelvin probe' test leads, where 1 pair of leads injects the current and the other pair of leads measures the voltage across the resistance. This way the test current is not flowing through the measurement leads, and the resistance of the leads gets eliminated from the measurement.