I am trying to get a better understanding of magnetic induction. Lets say you have an electromagnet wrapped around an iron rod (primary coil). On the other end of the rod you have another coil(secondary) hooked up to a meter. If you simply pulsed the primary coil with DC would this produce a lower voltage in the secondary coil if instead you instead ran AC through the primary coil?
Hi,
You may want to reword your forth (last) sentence there because it is not written correctly. It is written like a statement but sounds like you really wanted to make a comparison and ask a question about that comparison. In other words one interpretation would be:
"If you pulsed the primary coil with DC would this produce a lower voltage than if you instead applied regular AC to the primary?"
The short answer is that you can get a higher voltage from using pulsed DC than you can with regular sinusoidal AC because if the driver is designed for flyback operation then you can get a high pulse output. But it's worth looking into it a little more because there are constrains that apply for both.
The answer for AC alone is fairly simple because we assume the turns ratio governs the output voltage, so with a turns ratio of 10:1 and 100 volts AC on the primary we'd see 10 volts AC on the secondary. With a turns ratio of 1:10 however with 100 volts AC on the primary we'd see 1000 volts AC on the secondary. So that depends mostly on the turns ratio.
The answer for DC pulsed is more complex, because it involves having knowledge of the driver circuit. We have to know what kind of driver it is and what mode the designer intended.
Mode 1 might be called the "standard" or "non flyback" mode and Mode 2 would be called the "flyback" mode.
With Mode 1 we would apply a symmetrical DC pulse (positive and negative half cycles have the same amplitude and pulse duration) and that would be almost the same as the AC mode above, where we use an AC signal. The output then matches the turns ratio in the same way.
This also requires knowledge of the transformer inductance (or use a ready made formula) so that we dont saturation the core. But assuming all that has been worked out, we get basically the same as we get as with the AC wave except the waves are rectangular not sinusoidal.
As i said though, there are conditions that must be met to get this kind of operation.
With Mode 2 we would apply a non symmetrical DC pulse and when we turn it off we would expect a large inductive kick back in the primary. This means that the inductance causes a very high voltage pulse for a shorter time than the applied pulse. Although the voltage will be much higher than the applied rectangular signal the volt seconds of both pulses will be equal. This higher voltage pulse then appears on the secondary according to the turns ratio. So now we might apply a 100 volt pulse, turn it off, then see a 500 volt pulse, and that 500 volt pulse gets stepped up by the secondary according to the turns ratio. This kind of design allows for a very very high output voltage, but higher than with an ordinary AC signal on the primary. The driver must be designed with the knowledge that the primary voltage itself will shoot up to a much higher value once the pulse is turned off, The turns ratio then boosts this voltage to an even higher level.
The two reasons this happens are:
1. The inductive kick back on the primary generates a voltage amplitude that is higher than the applied pulse amplitude.
2. The turns ratio is a step up, which raises that high kick back pulse even higher.
We dont get something for nothing however, as the volt seconds for both half pulse cycles have to be equal on the primary side. This means that for a high voltage output pulse of a given duration we have to have an applied pulse on the primary of longer duration, and it may have to be much longer if the inductive kickback is very high. The kickback however allows us to get a very very high output voltage and sometimes that's the most important thing for the application.
Square wave inverters use the standard method where a DC symmetrical pulse is used to generate a secondary voltage. Automobile ignition coils use the flyback method to get a very high output voltage pulse.
There are other modes too that are a bit more specialized such as a "resonant" mode where the whole thing resonates and produces an AC output from a DC pulsed input. In this case the whole construction acts like a transformer plus a filter.
As long as we are talking about flybacks and associated voltage boosting, we might as well mention that we can get a high voltage without using a secondary coil in this same manner. By pulsing an inductor we can get a very high output voltage because of that inductive kick back. This is how many boost circuits are made.
The driver circuits for the regular mode and flyback mode are different because for regular mode we want to force the primary to be a certain voltage for both half cycles. For the flyback mode we only force the voltage to be a certain level during the driver 'on' time, and when the driver turns 'off' it really turns off and so it relinquishes control solely to the inductance of the primary.