Go Down

Topic: TT 100-SD  (Read 799 times) previous topic - next topic


Apr 28, 2020, 02:46 am Last Edit: Apr 28, 2020, 02:31 pm by raschemmel

If the datasheet for the CT says that Ip /Is = 100/0.033
0.033A /per 100 , doesn't that mean the turns ratio is 3030 ? (100/0.033)

I think you are over complicating things.
I made a device to disconnect a computer from the internet using a 8PDT relay by sensing the
computer monitor voltage before and after the power saver turned off the monitor.
When the power saver turned off the monitor, the monitor current dropped significantly.
I used an op amp comparator voltage threshold detector to detect the burden resistor voltage and
and energized the relay when it dropped below the threshold.
I didn't need to know anything about anything.  All I had to do was measure the current with the CT
when the monitor was on and then wait 2 minutes for the computer power utility to turn off the monitor
and then measure it again. I used a DMM on mA scale to read the CT output, then added a burden resistor and used the DMM to measure the voltage across the resistor to obtain the On current, Off current, On voltage (across the burden resistor) and the Off voltage.
The rest was just building the op amp comparator circuit and adding a transistor driven relay.
I don't know why you couldn't do something similar.

I don't see any information about the CNC machines so what exactly do you know ?
Where are you measuring voltage ?
I don't know anything about CNC machines I don't know why I would need to. I would just make an
extension cord for the CNC power to use to measure the current, or put the CT on one of the 3-phase
lines. The current on any one phase is approximately 1/3 the total current but that isn't even needed.
just program the CNC to drill a hole through an aluminum block and then stop. That should be enough
time to measure the current during the drilling and after the machine stops and goes into idle mode.

On a separate note,

Unlike a voltage transformer, the primary current of a current transformer is not dependent of the secondary load current but instead is controlled by an external load.

 The secondary current is usually rated at a standard 1 Ampere or 5 Amperes for larger primary current ratings.
Since without the burden resistor, the CT is an UNLOADED OPEN CIRCUIT.
Clearly the only "external load" here is the burden resistor.

= (primary current ÷ secondary current) ÷ burden = x
x = (100A / 0.033 A)/ 51
             = 3030/51
             = 59.41

For an ideal transformer:

Ip Tp = Is Ts

Ip / Is = Ts / Tp
100/0.033 = Ts / Tp  (NOTE: Primary current is in the nominator on the LEFT , WHEREAS Secondary
                            turns ration in the NOMINATOR on the RIGHT)
                           (Likewise Secondary current and Primary Turns ration are the DENOMINATORS)
                            (Which means there are 3030 more Secondary turns than Primary Turns)
                            (This makes perfect sense when you consider the massive field generated by
                             100A when compared to the relatively very small field generated by 33mA)

Ip * Tp = Is * Ts
Let Ip = 100A
Tp = 1
Is = 0.033A
Ts = 3030

(100)(1) = (0.033)(3030)
100 = (99.9999) ~ 100

It gets interesting when you consider the INDUCTANCE ration is the SQUARE of the TURNS ration,
if the turns ration is 3030:1(s/p)
then the inductance ratio is 9,180,900‬:1 (p/s) (AFAIK)


I have never used ct sensor, so it's all new, sorry if I'm off sometimes!

It needs to be non intrusive so it didn't change the actual wiring ( insurance would claim our fault if anything's wrong of the wiring is changed)

I just want to know when our machines are up and running and transmit it over web :)


You should be able to use the same approach
I did.

Go Up