measuring current waveform

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abhishek3330

Joined Mar 19, 2013
3
I have to differentiate between inrush current and fault current of power transformer, for that i have to check the current waveform of both.
The only measuring instrument i have is CRO so please tell me how to observe current waveform on CRO, starting from how to connect probe.
 

ramancini8

Joined Jul 18, 2012
473
Inrush current will always be accompanied by rising input current. Fault current will usually not be accompanied by rising input current. Use a dpdt switch for power on; half the switch for tx power, and half the switch to trigger the scope. When the input power is on detect the rise in current to see the fault current. This is easily accomplished with a recorder.
 

crutschow

Joined Mar 14, 2008
34,470
If the transformer is line powered, you need an isolated current sensor such as a Hall effect device or a current transformer to safely observe the current. Current transformers are the most commonly used device for this. Never try to measure line current directly with an oscilloscope.

Alternately you could use a power line isolation transformer to power the transformer under test. That will reduce the hazard of directly measuring the current with a shunt resistor. But you still have to be careful of the high voltages present.
 

ErnieM

Joined Apr 24, 2011
8,377
Since the scope return is (typically, do check first!) the AC neutral you could put a burden resistor in line with the scope and see the current.

If you need any more detail then that you don't have the experience to accomplish a dangerous measurement, so that is all I will say. Sorry.
 

crutschow

Joined Mar 14, 2008
34,470
Since the scope return is (typically, do check first!) the AC neutral you could put a burden resistor in line with the scope and see the current.

If you need any more detail then that you don't have the experienced to accomplish a dangerous measurement, so that is all I will say. Sorry.
Sorry but that is a dangerous thing to do and I most emphatically state that it should not be done, even by an experience person (I'm surprised you recommended that :eek:) The ground and neutral can be inadvertently switched and you can get much spritzen sparks with dangerous consequences. Don"t do it.
 

thatoneguy

Joined Feb 19, 2009
6,359
See if you can borrow/buy/"demo" a current probe for your scope.

They are contact free and simply clamp around the insulated wire. Price isn't cheap, however. If you have a "big name" scope, such as Tek, Agilient, LeCroy, contact the dealer to see if you can "demo" a current probe if you only need it for one use. This may work out well if you have an off brand scope as well. You then get one for 1 week to a month or more in the hopes of you liking it a lot and buying one. :)

You can connect to the secondary without a problem, it is isolated from the primary, and hence, ground/neutral of your power system. Inrush is typically on the primary side, though. A 500W 1:1 power isolation transformer can be had for about $125 (far cheaper than a current probe). If you work with line powered devices and a scope often, the usefulness and safety of an isolation transformer can't be understated. A Variac is also nice, but they often aren't isolating, so you'll need both if wanting to tweak voltages to simulate droops and peaks.
 

t_n_k

Joined Mar 6, 2009
5,455
If one intends to use a standard toroidal core current transformer to measure line currents [such as one would use with a current indicating panel meter] there is usually a caveat that strongly recommends that the user terminate the CT secondary in a burden resistor of suitable value. This reduces the likelihood of the user receiving a shock across the unterminated CT secondary terminals due to the high induced voltage likely to occur with primary current flowing.
 
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