Measurement of Open Circuit Voltage Across a Rectifier

What are the AC 3 phase supply Voltage? What are the three phase rectifier ratings? While it may be normal to measure open circuit DC voltage (except dangerous levels) out put, It will be dangerous & risky to attempt measurement of short circuit current. What do you expect to prove? With my limited knowledge I can not imagine the rectifier surviving such abuse.

The test i am doing is on axial flux permanent magnet generator. At 300 RPM about 25V DC is seen across the rectifier. And the bridge rectifier i am using is 50A which is more than enough for the short circuit current at that speed. I am just curious whether the voltage seen across the rectifier is the actual value or the value that might be leveled up due to junction capacitance.

Yes, the junction capacitance will have an effect in your circuit performance. The magnitude of this effect will be determined by this capacitance value, the parasitic capacitance of your test circuitry and the frequency of your AC signal. If the frequency is any nominal power distribution frequency then I greatly doubt that your instrumentation will be sensitive enough to read the effect of junction capacitance.

What is actually happening in the rectifier itself requires a reasonable grasp of quantum mechanics, the energy states of an atom and a little solid state crystal theory to explain. Each of these topics require much more time and discipline than a blog entry can tolerate.

I am very worried for your safety. Measuring a short circuit current flow of three phase power can very easily be an explosive test with molten metal spraying around. This is what actually happens inside a fuse. Your description of your test and understanding of semiconductors tells me that you are a novice to electricity and the safety protocols of using three phase power.

Be very careful! Listen carefully to whoever is supervising you. If CR4 is the only one supervising you then STOP!

I sliped to mention that i was testing a variable frequency Permanent magnet generator which is coupled to DC motor drive to measure voltage and current at different speeds. I am sorry that my context was not clear. But i just wanted to know what actually happens when testing open circuit voltage at rectifier terminals. Thanks for your comment, it was helpful.

"The test i am doing is on axial flux permanent magnet generator."

Is this for a DIY wind generator? Do you really want/need a short circuit measurement?

If you are trying to measure/characterize the performance and losses, I suggest you measure the voltages open circuit and with "reasonable" current loads. I would use properly rated resistive loads where the current is close to normal operating conditions.

Trying to "crowbar" (short circuit) the output of a wind generator to prevent over-speed (in high winds) is bad idea because the excessive current/heat generated could easily destroy the generator windings and rectifiers. I suggest a bank of resistive water heater elements in a water storage tank (safely pressure vented). When the system is properly designed, the excess power can be used to heat water for climate control and bathing. If this is going to be a wind generator application, I'd further suggest some type of mechanical over-speed protection like friction braking or blade furling.

Be safe & good luck!

Measuring open circuit voltage and short circuit current by a person with one post.

Possibly a Thevenin and Norton equivalent assignment is due tomorrow?

I remember a PHD candidate grad instructor that came from a land where they don't seem to teach common sense. How he got past BS and MS I don't know (BS I guess). Anyway, he demonstrated Thevenin and Norton of a transformer to a lab class. He somehow bypassed the detail of using an audio generator for the test. The lab must have had big breakers because he got enough smoke out of the circuit to set off the fire alarms and clear the building.

An important note to all students out there. Electronic circuits work because the semiconductor house has squeezed quite a bit of black smoke into the little plastic packages. Let the black smoke out and the circuit won't work any more.

At no load there may be a high peak voltage. This voltage seems to depend on voltage drop across rectifiers, transients on the line, stray capacitance and probably other things. A very light load usually got rid of the peak voltage.

When testing welders, we used to use ordinary iron pipe with no plating. Ran tap water through the pipe and used a moveable clamp to adjust the current.
Just guessing, but 1/4 inch pipe maybe 8 feet long should be about right for this current and voltage.