Inrush question

[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?

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[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

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[S'mise]

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.

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[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]

Originally Posted by 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.

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]

Originally Posted by 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.

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.

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