Inrush question

[sparrott4]

I understand that when an outdoor landscape lighting transformer with a toroidal core is powered up, an inrush current is produced that can generate up to about 25 times the nominal operating amperage.

If the transfomer is mounted very near the breaker panel then the inrush is much higher than if the transformer is mounted far from the panel. I understand that the length of wire between the two somehow absorbs the inrush (let's call it buffering).

Does anyone know if it is possible to calculate the buffering extent of the wire based on gauge and length? For example, if the inrush can reach 25X nominal amps when transformer is mounted 5 feet from panel, then at what length/guage of wire will the inrush be limited to 10X nominal amps?

This may seem to be a somewhat esoteric question, but it's often helpful to know when a high magnetic breaker may be needed or not. If there's a simple chart or equation to predict at what mounting distance the HM breaker is not needed, then that would be helpful.

Thanks.

 

[S'mise]

This "buffering" as you call it, is really capacitance caused by the long straight run of wire. Why not look for a capacitor to counter act this in rush (inductance)? Put it at the transformer connections.

 

[Besoeker]

This "buffering" as you call it, is really capacitance caused by the long straight run of wire. Why not look for a capacitor to counter act this in rush (inductance)? Put it at the transformer connections.

For standard wiring, is is much more likely to be the combined inductive and resistive impedance.
Capacitance would make the situation worse, possibly much if you get resonance.

 

[Besoeker]

If the transfomer is mounted very near the breaker panel then the inrush is much higher than if the transformer is mounted far from the panel. I understand that the length of wire between the two somehow absorbs the inrush (let's call it buffering).

The additional impedance introduced by the length of the wire results in a voltage drop so the transformer sees a lower voltage.

I think you'd need some kind of circuit model with cable and transformer parameters to predict how much the reduction in inrush current would be.
Maybe someone has already done that.
I have never used toroidals large enough for that to be a concern.

There are a couple of links that might be of interest:
http://www.amveco.com/Technical_Notes_7.htm
http://www.emeko.de/uploads/media/04-inrush-currents-can-be-avoided-E_01.PDF
[/QUOTE]

 

[S'mise]

For standard wiring, is is much more likely to be the combined inductive and resistive impedance.
Capacitance would make the situation worse, possibly much if you get resonance.

Care to explain a little more? Capacitance reaction is the oposite of inductive reactance. Wouldn't resonance only be a problem if both were about equal? Capacitors are used to minimize the inrush on devices like motors and lighting transformers. Why would this be any different?

 

[Mayimbe]

For standard wiring, is is much more likely to be the combined inductive and resistive impedance.
Capacitance would make the situation worse, possibly much if you get resonance.

Care to explain a little more? Capacitance reaction is the oposite of inductive reactance. Wouldn't resonance only be a problem if both were about equal? Capacitors are used to minimize the inrush on devices like motors and lighting transformers. Why would this be any different?

I believe hes right.

Higher source impedance relative to the transformer
size limits the current that the transformer can pull from the
system. The peak inrush with significant source impedance (Westinghouse
Electric Corporation, 1950) is

Ipeak = Io/(1+Io*X)

where
Io = peak inrush without source impedance in per unit of the transformer
rated current
X = source impedance in per unit on the transformer kVA base

If you add a Capacitance, the X value will be reducted. Thats leads to an X value reduction and ultimately to the increase of Ipeak.

 

[Besoeker]

Care to explain a little more? Capacitance reaction is the oposite of inductive reactance. Wouldn't resonance only be a problem if both were about equal? Capacitors are used to minimize the inrush on devices like motors and lighting transformers. Why would this be any different?

Do you want the capacitor in series or in parallel with the transformer?

 

[gar]

091104-1623 EST

Make some assumptions.

Assume the transformer's rated primary current is 10 A, and the peak current at that rating is 14.14 A. This consisting of both steady-state magnetizing current and the load current. The steady-state magnetizing current is small compared to full load rated current, maybe a few percent.

Using the 25 times criteria the peak inrush is 354 A. Assuming a 120 V source and its peak voltage of 170 V and no source impedance, then we can estimate the load transformer impedance at the inrush instant as 170/354 = about 0.5 ohm. Assume this is resistive and connect the transformer at the end of a 50 ft run of #12 wire (100 ft of wire) 0.1588 ohms, and still assume 0 source impedance. Now peak inrush will be about 170/(0.5+0.16) = 258 A. The voltage drop from the steady-state 10 A load is 6.6 V .

See a scope picture of transformer inrush at my web site
http://beta-a2.com/EE-photos.html
Photo P6. The transformer is not a toroid, but a simple EI core. The rated input current at 120 V is 175/120 = 1.45 A RMS. Peak inrush to rating is 40/1.45 = about 26 times.

In the real world you can expect circuit inductance to do some of the limiting, but to illustrate the principle it is simpler to just consider resistive components.

An effective way to reduce peak inrush current is to insert a current limiting resistor in series with the primary. Then add a time delay relay that closes maybe 16 MS after power is applied to short this series resistor. Another is to use a negative temperature coefficient thermistor in series with the primary. This has the disadvantage of always dissipating power, and requires a cooling period before reapplying power.

.

 

[S'mise]

Do you want the capacitor in series or in parallel with the transformer?

Parallel like you see them used on motors to limit in-rush.

I don't know how a torridal transformers differ from a traditional wound transformer. I havent had time to look at the links, yet (thank you for posting) But, Why aren't Soft start circuits (resistance added) used exclusively instead of capacitors on inductive loads? Yes, I have seen them used on large motors, but thats kind of rare. Time to go back to the books.

 

[gar]

091105-1758 EST

A capacitor either in series or in parallel with anything will not limit inrush current. In parallel, if the initial charge on the capacitor is zero, as it most likely is, then the inrush current is worsened. In series, no initial effect.

An inductor in series with anything will limit inrush current.

On a toroid vs an EI or similar core. A tape wound toroid has no air gap and for cores with similar ferromagnetic material a higher inductance can be obtain with the toroid with fewer turns.

When a core is saturated it is nearly equivalent to being an air core coil. Fewer turns means lower inductance and lower resistance. Thus, higher inrush current under saturation conditions for a toroid transformer compared to a standard transformer of the same VA rating.

.

 

[S'mise]

The links were very helpful in explaining. So... if a cap is not used to limit inrush (As I was always told) why do they use them on lighting fixtures and similar things? Yes, I realize they are used to help start motors (phase shift)

 

[gar]

091106-1351 EST

S'mise:

First, about capacitors. If a capacitor has zero initial charge, then its initial voltage is zero. At the instant of connecting a circuit to this capacitor the initial current will be the same as if the capacitor had a short across it. If the source was a 100 V battery and the circuit was a resistor of 1 ohm, then the initial current would be 100 A. Some time in the future that current will drop to zero. In between it follows an exponential curve for the simple RC circuit with a fixed DC source voltage.

Second, what are lighting fixtures and similar things?

.

 

[gar]

091107-0823 EST

S'mise:

Following is a site that you might find informative on a simple RC circuit. There are three pages.

http://www.electronics-tutorials.ws/rc/rc_1.html

.

 

[S'mise]

Ahh ha!

Ahh ha!

Thanks Gar, I now see where my logic was flawed. I did remember correctly that Inductive reactance and capacative reactance counteract each other. But, I was confusing inrush and inductive reactance as the same thing. The instances where I have seen caps used with inductive devices were not "used to lessen inrush" (as I was mistakenly told some 20 years ago) but were actually used as a snubber to filter out voltage spikes (noise if you will) after turning off power to these inductive devices. So you were right that a cap would not help inrush current, but they can be used control spikes(outrush current) Thanks for setting me straight.
I love those ah-ha moments where I learn something.

 

[gar]

091107-1554 EST

S'mise:

Yes the capacitors can help reduce transient spikes on turn-off of an inductive load.

Sometimes these include a series resistor with the capacitor to reduce the time of the transient decay. This is especially useful in a DC circuit, such as an electromechanical clutch, where the desire is to minimize the drop-out time without an excessive peak transient voltage.

.

 

[S'mise]

yes, I remember rc circuits from electronics.

As to the op, Sounds like a resistor would be a practical solution to limit inrush current provided that vd would not be a problem.

 

[Besoeker]

yes, I remember rc circuits from electronics.

As to the op, Sounds like a resistor would be a practical solution to limit inrush current provided that vd would not be a problem.

One technique to reduce inrush is to fit the resistor then short it out after a small delay.
We sometimes do that in our works when we are testing high current low voltage rectifier units where the transformer inrush would exceed the rating of the test outlet being used.

 

[S'mise]

Sounds like a standard soft-start ckt.
I have heard of tsr relays that greatly reduce inrush by incorporating thyristors that fire correct polarity, followed by relay shunting the ckt closed.
Very clever stuff but they sound expensive.

 

[Besoeker]

Sounds like a standard soft-start ckt.
I have heard of tsr relays that greatly reduce inrush by incorporating thyristors that fire correct polarity, followed by relay shunting the ckt closed.
Very clever stuff but they sound expensive.

Using a resistor to limit inrush current isn't like a soft start circuit.
The resistor is just two discrete stages. It's in circuit or shorted.
By contrast, the thyristor soft start provides a continuously variable voltage.
Clever and expensive? I wouldn't have thought so.
Simple light dimmers can use either triacs at the low power end or thyristors at higher powers. It's fairly well-established technology. Light dimmers are not generally very expensive.

 

[gar]

091109-0915 EST

To limit transformer inrush current that results from core saturation there is no valid reason to use a phase shift controlled device. A resistor will do the job. This inrush is largely a 1/2 cycle effect as shown in my previously mentioned photos. Keeping a suitable resistor in series with the transformer primary for several cycles is more than adequate to reduce the inrush problem.

.