Transformer Impedance Computations

[tersh]

I forgot about my lessons 20 years ago.

How do you compute the impedance of this transformer? It's 500va 230vac to 115vac isolation transformer with this specs:

https://www.hammfg.com/electronics/transformers/line/172.pdf

The 500VA step down isolation transformer would be put before the type 3 SPD in the following. I'd like to know the impedance of the transformer and how many length of say AWG 10 it is equivalent to so I can decrease the length of the wire which can be done.

Thank you.

 

[winnie]

You can't really calculate the impedance of the transformer with the information given. The impedance should be in the datasheet, and then you might have to do calculations to convert the given value into useable units.

You can _measure_ the impedance with a simple test. Use a variac to supply power to the transformer primary. Short circuit the secondary. Measure the secondary current with a clamp meter. Adjust the supply voltage (starting at 0) going up until rated secondary current flows. The % impedance is the ratio of test primary voltage to rated primary voltage.

So if the rated input is 240V, and 24V makes full current flow into a short, then the impedance is 10%.

-Jon

 

[tersh]

You can't really calculate the impedance of the transformer with the information given. The impedance should be in the datasheet, and then you might have to do calculations to convert the given value into useable units.

You can _measure_ the impedance with a simple test. Use a variac to supply power to the transformer primary. Short circuit the secondary. Measure the secondary current with a clamp meter. Adjust the supply voltage (starting at 0) going up until rated secondary current flows. The % impedance is the ratio of test primary voltage to rated primary voltage.

So if the rated input is 240V, and 24V makes full current flow into a short, then the impedance is 10%.

-Jon

Based on experience with that kind of transformer. What is the impedance of typical 500va transformer? Does it equal 10 meters or 50 meters or 100 meters of wire? just estimate to have idea. Thanks.

 

[tersh]

Based on experience with that kind of transformer. What is the impedance of typical 500va transformer? Does it equal 10 meters or 50 meters or 100 meters of wire? just estimate to have idea. Thanks.

Btw.. the orange loop of wire you saw is to satisfy UL 10 meters requirement that type 3 SPD must at least have 10 meters of wire from it to the main panel for impedance. So if the transformer impedance is at least 5 meters AWG 10, then I can remove all the loop. What do you think? Based on experience and estimate, can a transformer impedance equal 5 meters of AWG 10 wire?

 

[winnie]

There is not a simple equivalence between the transformer impedance and a length of wire. The transformer impedance will have a large inductive component; the wire is mostly resistance.

You can do a rough calculation to get a feel for the magnitude of things, however.

1) consider a 20A circuit. In the us this would be made with 12ga wire. A 30m circuit with 12ga wire has a resistance of 6.34 (ohms/km) * 2 (there and back again) * 0.030 (km) = 0.38 ohms.

2) consider a 500VA transformer with 5% impedance (a pretty typical value). Rated current is 4.17A, and the equivalent of 6V at the secondary is what makes this current flow. This is an _impedance_ (which has resistive and reactive components) of 1.44 ohms

My gut feeling is that the transformer provides all the impedance that the SPD 'needs' and more, however because this is not a pure resistance this is not actually shown.

-Jon

 

[topgone]

I forgot about my lessons 20 years ago.

How do you compute the impedance of this transformer? It's 500va 230vac to 115vac isolation transformer with this specs:

https://www.hammfg.com/electronics/transformers/line/172.pdf

The 500VA step down isolation transformer would be put before the type 3 SPD in the following. I'd like to know the impedance of the transformer and how many length of say AWG 10 it is equivalent to so I can decrease the length of the wire which can be done.

Thank you.

You could specify what value of impedance you want to the manufactures to suit your needs. These are not something cast in stone, you know?

 

[winnie]

You could specify what value of impedance you want to the manufactures to suit your needs. These are not something cast in stone, you know?

No, but a custom wound 500VA isolation transformer is probably not cheap

-Jon

 

[topgone]

No, but a custom wound 500VA isolation transformer is probably not cheap

-Jon

Okay, you wanted estimates because they're expensive? Here is an estimate:

Transformer Rating*	100VA	250VA	500VA	1kVA	10kVA
Typical Impedance	10%	8%	5%	4%	1.5-2%
Typical Fault Current	10x	12x	20x	25x	50-65x

LINK

 

[tersh]

Okay, you wanted estimates because they're expensive? Here is an estimate:

Transformer Rating*    100VA    250VA    500VA    1kVA    10kVA
Typical Impedance    10%    8%    5%    4%    1.5-2%
Typical Fault Current    10x    12x    20x    25x    50-65x

LINK

Are these values also true for Toroid transformers only are they only for shell or core type transformers? Here's the toroid transformer:

http://hammondmfg.com/1182.htm

 

[victor.cherkashi]

impedance of transformer given on specification is for 60hz. impedance of transformer for surge voltage is almost infinity. what are you trying to do here?

Sent from my ONEPLUS A6013 using Tapatalk

 

[kingpb]

Impedance is given on nameplate; as required by IEEE C57

 

[tersh]

winnie and others,

I was asking the transformer impedance because an EE told me to use it months ago to take advantage of lower VPR at the equipment.

Let's first review about basic concept of SPD (Surge Protection Devices) and Impedances, I googled about the topic a lot yesterday but some things still escaped me.

This is the normal SPD cascading of Type 2 and Type 2 Spds.

I added an isolation transformer before the type 3 SPD so that the 240v AC would be step down 240v-120v to make use of 120v SPD for lower VPR value (for better protectdion) (as advised to me by a 65 year old EE).

Let's first review about basic operation of SPDs or MOV (Metal Oxide Varistors).

The purpose of having high source impedance requirement (between the MOV and the AC source) is so that it can initiate voltage divider action so the varistor at center can have lower current. But this is what I want to understand. Wouldn't the current go to the load directly since it has resistance too?
The other requirement is there must be very low impedance between the varistor and the load.

In the following case a transformer is added before the type 3 SPD

So there is now high impedance to the load. What would happen to the first varistor. Would it have less current since the current is now divided between the first source impedance and middle impedance (caused by the transformer)? Thanks a lot!

 

[victor.cherkashi]

Tersh
Your drawing is incomplete, when it comes to a pulse of energy (high frequency) it different theory and different solution, simple 60hz formula doesn’t work. It is not easy to solve and get exact numbers.
Where is an impedance of your wiring?
What is a frequency of voltage surge?
It easy to understand what is happening here by thinking in energy dimensions and wave theory, see below picture.

Do you know why it's recommend to have additional SPD’s close to equipment?

 

[tersh]

Tersh
Your drawing is incomplete, when it comes to a pulse of energy (high frequency) it different theory and different solution, simple 60hz formula doesn’t work. It is not easy to solve and get exact numbers.
Where is an impedance of your wiring?
What is a frequency of voltage surge?
It easy to understand what is happening here by thinking in energy dimensions and wave theory, see below picture.

Do you know why it's recommend to have additional SPD’s close to equipment?

So reason there must be minimum impedance between SPD and equipment is to limit the voltage rise? But when you add the transformer before the equipment, you increase the impedance and hence increase the voltage rise at particular frequency?

 

[victor.cherkashi]

So reason there must be minimum impedance between SPD and equipment is to limit the voltage rise? But when you add the transformer before the equipment, you increase the impedance and hence increase the voltage rise at particular frequency?

Where did you read this requirement? "minimum impedance between SPD and equipment" What is minimum impedance in numbers?
I found only "[FONT=Arial, Helvetica Neue, Helvetica, sans-serif]Type 3 – Point of utilization SPDs, installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilization"[/FONT]

Bottom line, 30 feet of conductor is not equal to any transformer when it comes to Transient Voltage (high frequency)

 

[tersh]

Where did you read this requirement? "minimum impedance between SPD and equipment" What is minimum impedance in numbers?
I found only "Type 3 – Point of utilization SPDs, installed at a minimum conductor length of 10 meters (30 feet) from the electrical service panel to the point of utilization"

Bottom line, 30 feet of conductor is not equal to any transformer when it comes to Transient Voltage (high frequency)

I read this paragraph by Golddigger in another thread (kindly comment on it): msg 29 of http://forums.mikeholt.com/showthread.php?t=195234&page=3

"

The operation of a surge protective device depends on its being able to shunt away any voltage spikes above a certain level with a very low resistance, making a voltage divider with the source impedance of the fault that is causing the voltage excursion. That means that the SPD needs to have the absolute minimum impedance between it and the protected circuit conductors.

A transformer, even an autotransformer, will represent a relatively large impedance on the scale of an operating SPD. With only one nominal 120V SPD to work with you will not be able to protect your 240V wiring and its connected loads.
If you had a way to run a solid metal path between the service transformer secondary at the pole and the location of your SPD(s) you would be able to use 120V SPDs in a symmetric circuit on either side to the line to that neutral/ground wire.
Just a ground electrode will give you little or no effective SPD operation unless you run two 120V SPDs symmetrically to that ground electrode or a single 240V SPD between the hot wires with no neutral/ground connection."


So why must the "SPD needs to have the absolute minimum impedance between it and the protected circuit conductors"? Is it to avoid the source impedance from dividing itself towards the equipment?​

 

[Russs57]

First off, transformer impedance equals the square of the turns ratio. However that isn’t commonly used as each transformer tap connection would have a different impedance.

In any case I don’t think this is important for what you want to accomplish. IMHO you want a high quality SPD at service entrance where neutral and ground are bonded. Wiring from source to SPD, and then to ground, needs to be very short and low impedance. No more than a meter of wire total. I think an air gap type should be used. MOV types can be used downstream but care should be used in coordinating.

SPD’s with LC filters might accomplish what you are trying to do with a transformer.

 

[Russs57]

Because you will have a very rapid rise in current. Therefore any impedance between source, SPD, and ground will have a high voltage developed across it. This voltage will be presented to the equipment you wish to protect.

 

[tersh]

First of all. I didn't add transformer near the equipment to improve the impedance or as filter. I used it because my country power ac system is 240v. But the VPR of 240v SPD are higher. Lower VPR is more protective. Therefore I used an 240v-120v isolation transformer to drive or use 120v SPD.

In my main panel. I have the Siemens First Surge 140,000A SPD type 2. Then 10 meters from it. I have a step 240-120v step down transformer to drive the 120v Leviton Strip surge protection and a 120v Honeywell fire alarm system (It can take 88v to 256v).

I was asking whether the transformer big impedance can affect the performance of the Type 2 SPD at the main panel. Again depicted here. The actual meters is 10 meters.

I used the following illustration to ask whether having high impedance between the type 2 and ultimate load can affect the performance of the type 2 SPD at main panel, and to what extend?

 

[topgone]

First of all. I didn't add transformer near the equipment to improve the impedance or as filter. I used it because my country power ac system is 240v. But the VPR of 240v SPD are higher. Lower VPR is more protective. Therefore I used an 240v-120v isolation transformer to drive or use 120v SPD.

In my main panel. I have the Siemens First Surge 140,000A SPD type 2. Then 10 meters from it. I have a step 240-120v step down transformer to drive the 120v Leviton Strip surge protection and a 120v Honeywell fire alarm system (It can take 88v to 256v).

I was asking whether the transformer big impedance can affect the performance of the Type 2 SPD at the main panel. Again depicted here. The actual meters is 10 meters.

I used the following illustration to ask whether having high impedance between the type 2 and ultimate load can affect the performance of the type 2 SPD at main panel, and to what extend?

When you inserted an isolating transformer, you actually inserted a device which will block the DC component of the surge! Your SPD will not detect the whole surge but an attenuated surge! If that the efficacy of your SPD depends on the thing detecting the surge and clamp the same, I doubt your idea of an SPD being more effective at lower voltage floats!
If you wanted to block the common mode noise from the source, perhaps an isolating transformer can be enough. If you wanted to block differential mode noise, then go for the SPD and do away with the isolating transformer, IMO.