** How do I modify the ratio of my current transformer?**

### RC, SC & CT Current Transformer Ratio Modifications

The current ratio between the primary and secondary winding is defined by the following formula:

Ns x Is = Np x Ip

Where:

Ip = Primary Current

Is = Secondary Current

Np = No. of turns on the Primary Winding

Ns = No. of turns on the Secondary Winding

Example:

On a 300:5 current transformer,

Is = 5 Amps when Ip = 300 Amps,

the number of primary turns is 1.

Ns x 5 = 300 x 1

Ns = 60

The ratio of the current transformer can be modified by altering the number of secondary turns. Forward or backwinding the secondary leads through the window of the current transformer will add or subtract secondary turns respectively.

By adding secondary turns the same primary current will result in a decrease in secondary current output. By subtracting secondary turns the same primary current will result in greater secondary output.

Example:

On a 300:5 current transformer, if a 325:5 current ratio is desired it is necessary to add five turns to the secondary winding.

Ns x 5 = 325 x 1

Ns = 65

Deducting 5 secondary turns will create a transformer with a current ratio of 275:5.

Ns x 5 = 275 x 1

Ns = 55

### SC Series Ratio Modifications

**Primary Turn Ratio Modifications**

Formula:Ka = Kn x Nn /Na

Where:Ka = Actual Transformer RatioThe ratio of the current transformer can be modified by adding more primary turns to the transformer. By adding primary turns, the current required to maintain five amps on the secondary is reduced. (Example: A 100:5 current transformer designed for one primary turn.)

Kn = Nameplate Transformer Ratio

Na = Actual Number of Primary Turns

Nn = Nameplate Number of Primary Turns

Square Case Transformer Polarity

**Secondary Turn Ratio Modification**

The current ratio between the primary and secondary winding is defined by the following formula:Where:Ns x Is = N<p x IpIp = Primary CurrentThe ratio of the current transformer can be modified by altering the number of secondary turns. Forward or backwinding the secondary leads through the window of the current transformer will add or subtract secondary turns respectively.

Is = Secondary Current

Np = No. of turns on the Primary Winding

Ns = No. of turns on the Secondary Winding

By adding secondary turns the same primary current will result in a decrease in secondary current output. By subtracting secondary turns the same primary current will result in greater secondary output.

**VA is the Volt-Amp Rating of the current transformer.**

The VA rating is an indication of the maximum burden (resistive and/or inductive load) that can be placed across the secondary connections of a current transformer operating at rated current. A burden above the VA rating may result in core saturation and measurement inaccuracy.

** How Do I calculate my VA rating?**

Power = Current x Voltage e.g.

**VA = Is • V**

Voltage = Current x Resistance e.g.

**V= Is • R**

Where:

**VA**= Volt-Amp Rating (VA)

**Is**= Secondary Current (Amps)

**R**= Burden (ohms)

Combining these equations results in:

**VA = (Is)2 • R**

In some cases, required VA ratings are listed on the device the current transformer will connect with, ie. a meter, relay, etc. Typical internal meter burdens are in the milliohm range.

The most common source of excess burden in a current measurement circuit is the conductor between the meter and the CT. Often, substation meters are located significant distances from the meter cabinets and the excessive length of small gauge conductor creates a large resistance. This problem can be solved by using a larger gauge secondary wire, or a CT with 1 ampere secondary which will produce less voltage drop between a CT and its metering devices (used for remote measurement).

(Accuracy)-B(Burden) e.g.

**1.2-B0.5**

The above example has a 1.2% accuracy at 0.5 ohm burden.

The relay class transformers are classified as follows:

**20 • (Rated Secondary Current) • (Burden) = Relay Class Rating**

Example:

**20 • (5 amps) • (1.0 Ohm Burden) = C100**

MCT has developed many different types of relay class transformers in the past for a variety applications.

** How is the CT core constructed?**

CRGO electrical steel is an iron-silicon alloy that was developed to provide the low core loss and high permeability required for more efficient and economical electrical transformers. CRGO electrical steel is usually less than 2mm thick, has a silicon level of 3%, and is coated to increase electrical resistance between laminations, and to provide resistance to corrosion and rust.

Additionally, the steel is processed in such a way that the optimum properties are developed in the rolling direction, due to a tight control of the crystal orientation relative to the sheet. The special orientation of the crystals within the steel increase the magnetic flux density by approximately 30% in the coil rolling direction. This allows cores to have a higher efficiency and smaller size.

** What methods of payment do you accept?**

** Do I have to go through a distributor?**