Previous in Forum: Serious Problem with IGBT Power Ratings / Performance Next in Forum: How to Calculate the Magnetic
Member

Join Date: Mar 2011
Posts: 8

# Converter from CT to Volt

06/13/2011 10:33 PM

Hi All,

I want to make modification on our production system. We have diesel genset and utility source, together the have share load. Utility 2500 amps with CT installed 2800/5, I want output from CT (amperage) is going to convert to volt DC (0-10 VDC) so I can forward this signal to our DG. Why ? because load level handle on DG is depending on how high the volt DC is (range is 0-10VDC to adjust load range (0-550 kVA)).

Interested in this topic? By joining CR4 you can "subscribe" to

Guru

Join Date: Mar 2010
Location: In the pool because it is too hot.
Posts: 3055
#1

### Re: Converter from CT to Volt

06/13/2011 11:35 PM

What is the CT powering now? And how may this be influenced or affected? When you load the CT can with a high burden - resistor that has a value R = U^2/ I you will get 10 volts by 5A current (RMS) . Probably it needs to be rectified and smoothed with a capacitor to get 10 Volts DC. Depending on the rectifier bridge you'll need to adjust the voltage with a factor as like 1.41.

If you are using the CT to measure the actual current, it might be better to install a second one for your purpose.

__________________
Plenty of room here
Member

Join Date: Mar 2011
Posts: 8
#2

### Re: Converter from CT to Volt

06/14/2011 12:05 AM

Hi Guru,

Yes, Im planning use other than CT that use to measure actual load. Apologize me before for not understanding you, for example: I can connect output singnal from CT to variabel resistor eg. 10 K than connect it with rectfier component, so i can get volt DC.

Guru

Join Date: Mar 2010
Location: In the pool because it is too hot.
Posts: 3055
#3

### Re: Converter from CT to Volt

06/14/2011 1:23 AM

variable or fixed resistor, yes. But be safe and do not use the CT WITHOUT any load, because it can generate dangerous and life threatening voltage if the coil ends are free and without any burden.

__________________
Plenty of room here
Guru

Join Date: Dec 2006
Location: Long.92E,Lat.26N
Posts: 1336
#8

### Re: Converter from CT to Volt

06/15/2011 3:27 AM

Remember CT gives you 5Amps for 2800Amp primary.

So to get 10V DCPeak from rms SecondaryAmps5amp--:

Certainly not 10 KiloOhm!!!

Anonymous Poster #1
#4

### Re: Converter from CT to Volt

06/14/2011 3:27 AM

Why don't you go for a simple PT rather than CT?

As yiu are aware (and has been pointed out) the CT basically expects virtual short circuit on the secondary.

To generate a voltage say 10V you need at least 2Ω (Rated 5Ax2Ω=10V) and that is going to dissipate 50W power in the resistor (I2R)

Even worse is at this level of burden, the CT's linearity may be affected.

Power-User

Join Date: Jul 2010
Location: India
Posts: 161
#5

### Re: Converter from CT to Volt

06/14/2011 8:31 AM

A standard product available in market can solve your problem.

It could be current (0-1 Amp) to Voltage (0-10 volts) Converter. Here is link to site giving data sheet of such convertor commonly used in industries.

http://www.prelectronics.com/idd964.asp?pcol=2279

__________________
When was last time you did something for first time.
Guru

Join Date: Oct 2009
Location: sometimes Wales,UK.. was Libya, now Oman!
Posts: 1715
#6

### Re: Converter from CT to Volt

06/14/2011 11:53 PM

A dangerous idea..... get a PT as suggested, as a CT is designed to be a CT. PT's are cheap and easy to obtain, easy to fit and will give you hours/years of trouble free operation.

The problem with trying to use a CT, is getting the ratio right to give you the correct voltage output then convert it, and do whatever else to it so it will work.

A PT is the way to go and whole lot less dangerous.

__________________
The square root of nothing is what you make it!
Power-User

Join Date: Jun 2011
Location: Berkeley, CA USA
Posts: 128
#7

### Re: Converter from CT to Volt

06/15/2011 2:03 AM

I used to design control systems for switch gear and you can NOT replace a CT with a VT. A CT senses current and a VT senses Voltage.

The CT on your system is designed to operate with no more than a 0.5Ω load and possibly only a 0.05Ω load. This is called a burden in this industry. I recommend you take the output of the ct and run a wire of at least 12AWG through a 1000:1 CT like

found through glogal spec.

This will give you an additional level of isolation and you can move the 1K:1 CT close to the genset.

Now with the 100Ω termination you will get a 0 - 0.5VAC signal that will have to go through an amplifier and RMS conversion that you feed to the genset control system.

What is not clear from you question is exactly what the DG function is. You say load share. does that mean that as the signal in approaches 10VDC the generator turns on for peak load reduction? This is important as it affects the circuit between the 1K:1 CT and the DG.

__________________
Working to end the use of carbon for energy
2
Power-User

Join Date: Oct 2010
Location: OZ, otherwise known as Oklahoma were the wind comes sweeping down the plains.
Posts: 159
#9

### Re: Converter from CT to Volt

06/15/2011 9:05 AM

You will find exactly what you need here https://www.ohiosemitronics.com/pdf/catalog/three_phase_rms_current_transducer_model3CTR.pdf, 3 Phase - Input 0-5 Amps, Output 0-10 Volts DC, or here https://www.ohiosemitronics.com/pdf/catalog/single_phase_ac_rms_current_transducer_modelACTR.pdf Single Phase - Input 0-5 Amps, Output 0-10 Volts DC, or here.

Guru

Join Date: Oct 2006
Posts: 1571
#10

### Re: Converter from CT to Volt

06/15/2011 11:25 AM

In DC drives we often use 2 ct's and a six diode bridge to measure the three phase currents. The ct's are often 10,000:1 and the burden resistor is slected to give between 1.5 to 5 volts depending on the manufacturer. The output could be filtered by an appropriate device at that point. As others have mentioned, watch the watts and don't open circuit the ct's. The diodes should be conservatively rated atleast 2x the secondary current, and also rate the resistors at about 50% of their wattage. Gives long life and handles overloads.

Interested in this topic? By joining CR4 you can "subscribe" to