Power Factor - explanations of? 1

[nigeld]

I am currently trying to understand how Power Factor effects the output of a welding transformer. I understand how it is calculated, in terms of difference of phase angle between volts and amps, but how does the voltage affect the output? A transformer relies upon a changing magnetic field on the primary (caused by moving electrons-amps) to create the current movement in the secondary. Can someone explain how the voltage effects the maximum power available?

Regards, Nigel.

 

[moturcu]

Hi Nigeld,

A welding transformer's secondary winding is subjected short circuit during the welding process. In order to minimize this current surge on the primary side, transformer's magnetizing inductance is made smaller on purpose. This essentially means lower short circuit reactance and higher short circuit ratio which is an indicator of short circuit handling capability and/or short circuit power.

On the other hand short circuit reactance is inversely proportional with the voltage. So as you increase the voltage yo will get more short circuit power from welding machine so long as you do not exceed thermal limits.

I hope this answers your question.

 

[nigeld]

Thankyou moturcu for your response,

to clear up my original question....

Why is it that when there exists a phase angle between the volts and amps that the 'useful' power is reduced? What is happening in the part of the cycle when volts and amps are opposing each other, or even when they are both +ve or -ve, just that dV/dt is opposite in sign to dI/dt to cause 'reactive current'.

In a transformer, if you have electrons moving in the primary, will this still not cause electrons to move in the secondary regardless of relative phase of the voltage?

What is 'Reactive Current', what causes it, and why can it do no useful work? What elements can reactive current do work in (ie resistive, capacitive or inductive?)

What does a reactive electron do compared to a non-reactive one?

 

[moturcu]

Let's start with an analogy to expalain the meaning of active and reactive power(energy).

Suppose there are 20 boxes each of which is 50kg such that total is 1000kg. One person wants to cary this boxes from point A to point B with a carrying tool whose weight is 5kg.
and also suppose that the distance between point A and B is 10m. So the person carries all boxes by making 20 trips between point A and B. The total energy he consumed during this work is
Wtotal=20*50*10+20*5*10
But only 20*50*10=10000joule is spent to cary boxes and can be considered as active energy. On the other hand he consumed 2*20*5*10=2000joule for carrying tools so this could be considered as reactive energy.

In an electrical system, power relations can be more easily expalined by looking at average value of voltage and current multiplication instead of their instaneous interrelation. Their instantenous(phase) relation ultimately effects the average of their multiplication. As a result, phase relation effects how much of the energy circulate in the system as a reactive energy and how much of it is delivered from pne point to another.

 

[busbar]

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[Aranrhod]

Hi Nigel,

Here's an alternative way of looking at it. Instantaneous power in a circuit is Volts x Amps. WHen the circuit is resistive Amps always takes the same sign as volts. So plus x plus = plus, and minus x minus = plus. So there is always power being transferred from source to load.
When you have a reactive circuit, let's assume 100% reactive with no resistance at all, then the signs of Volts and Amps are not always the same. In fact 50% of the time they will be the same (so real instantaneous power is deliverd to the load) but 50% of the time they will be different (so real instantaneous power is delivered from the LOAD to the SOURCE). The net transfer of power is zero.
In most real cases you have a mixture of the above, so the MAJORITY of the time the signs are the same and a MINORITY of the time they will be different. So there is a net transfer of power from the source to the load. The ratio of how much is actually transferred to how much could be transferred for a fully resistive load is the Power Factor.

Hope this helps. . .

 

[nigeld]

Thankyou so very much for your help in this!!!

I am most grateful

Regards,
Nigel.