Transformer Voltage Drop on Load.

Thread Starter

rlotfalizade

Joined Apr 30, 2020
2
Does the voltage drop depend on the transformer rating, specially when the transformer is overloaded? I am comparing two transformers with the same applied load, but different load rating (different MVA). The load is less than the MVA rating of one of the transformers, but more than the MVA rating of another one. Is the voltage drop the same for these two scenarios?
 

MrChips

Joined Oct 2, 2009
21,150
Yes. Voltage will drop when the transformer is loaded.
Please tell us more about the transformer part number, current and voltage specs, load value and your measurements.
 

MaxHeadRoom

Joined Jul 18, 2013
20,716
An example of this occurrence is in the older linear type wall-warts where the output was a simple rectified output and the rated voltage occurred when the rated load was placed on the unit.
The open circuit voltage, much higher.
Max.
 

crutschow

Joined Mar 14, 2008
24,987
Does the voltage drop depend on the transformer rating
In general, yes.
The drop is caused by the primary and secondary winding resistance and a transformer with a higher VA rating will have a lower winding resistance.
Actually, that's the main reason a higher VA rated transformer is larger than a lower VA rated, to allow room for the larger gauge, lower resistance wire.
If you could wind a transformer with superconducting wire, than the only limit on the transformer VA rating would be the current limit of the wire (the current where it becomes non-superconductive).
 

Tonyr1084

Joined Sep 24, 2015
4,541
Voltage, Current and Load are all related. If the transformer can not carry the load (the amount of current drawn) the voltage will drop. Take for instance some fictitious numbers: Suppose your transformer is 12 volts and 1 amp. That means your load can be as big as 12 ohms. 12V ÷ 1A = 12Ω. But suppose the load is 10Ω. Well, 12V ÷ 10Ω = 1.2 amps. IF the transformer can not push that much amperage then the voltage will drop. So since this fictitious transformer is capable of 1 amp then the voltage will drop to 10V. 1A x 10Ω = 10V. Lets run the numbers even higher. Suppose the load is 8Ω: 1A x 8Ω = 8V. Now, suppose the transformer can push 2A: 2A x 8Ω = 16V. Since the transformer is rated for 12 volts - you won't see any voltage drop off.
 

Thread Starter

rlotfalizade

Joined Apr 30, 2020
2
Yes. Voltage will drop when the transformer is loaded.
Please tell us more about the transformer part number, current and voltage specs, load value and your measurements.
Thanks. The transformers are:
1. 12.5MVA, 132kV/25kV
2. 25MVA, 132kV/25kV
Load current is: 580A.
 

MrChips

Joined Oct 2, 2009
21,150
How big is this transformer anyway if it can handle 25MVA?
We are talking about a different beast here.
What are your voltage readings loaded and unloaded?
Are your transmission feeder lines/connecting cables capable of carrying 100A/580A?
 

Tonyr1084

Joined Sep 24, 2015
4,541
Sounds like a power pole transformer to me. Worrying about a voltage drop? 132,000 volts dropping down to 25,000 volts doesn't sound like a hobby transformer to me. Sounds more like an electric chair type of device (death comes easy with a single mistake).
 

panic mode

Joined Oct 10, 2011
1,831
" The load is less than the MVA rating of one of the transformers, but more than the MVA rating of another one. "

you cannot do that. load MUST be less than transformer rating.
 

ci139

Joined Jul 11, 2016
1,464
(quite frequently) the stacked alloy core tranformers are usually rated for each parameter's maximum = you either get 20V out of it or 1A but not both at the same time . . .
 
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