Shenzhen Okerda Science & Technology Co., Ltd

Technical Service

Ⅰ.WHAT IS A TRANSFORMER

A transformer is a static piece of apparatus with two or more windings which, by electromagnetic induction, transforms a system of alternating voltage and current into another system of voltage and current of same values or of different values, and at the same frequency for the purpose of transferring electrical power.

Although transformers look simple, its design involves a few very important electromagnetic laws and formulas.

As mentioned above, one of the important building blocks is a two-coil wire - namely primary and secondary windings. These two windings are placed in a very close proximity and are linked by the alternate flux in the core. As the result, the voltage is induced in the winding. The formula below is used to calculate the induced voltage in the secondary winding.

E = kAcNBmf(10-4)
Where,
E – induced voltage
k – constant, 4.44 for sinewave
4.00 for squarewave
Ac – cross section area of core in cm2
N – number of turn which induces the voltage
Bm – maximum flux density of the core in Tasla
f – frequency in Hz

Armed with the above formula, one can increase or decrease the amount of turns of the winding to increase or decrease the induced voltage. Indeed, there are more turns on the secondary winding than in the primary winding for a step-up transformer and vise versa for a step-down transformer.

As many wonder what a transformer may do, if the windings are not correctly designed and manufactured, they may cause harm to the system in which the transformer is used, and put the user in danger because of the risk of electrocution, burns, and fires. We take pride in the design and fabrication of our transformers. We design the transformers to be accurate to the customers’ specifications and make them safe, highly efficient and long-lasting.

Ⅱ. CURRENT TRANSFORMER

1．Types of Current Transformers
Current Transformers are divided into 2 major categories:

MEASUREMENT: Intended for use with indicating or integrating meters
PROTECTION: Intended for use with protection devices

2．Basic Design Concepts
Fundamentally all Current transformers follow the same basic principle:

AT s in primary= ATs in secondary +ATs error
That is: Current x primary turn or turns fundamentally equals:Current x Turns in Secondary
Due to excitation current and internal voltage drop due to internal resistance we do not have a perfect vector transposition between primary and secondary currents. This is due to: Ratio or CURRENT ERROR and Phase angle shift or PHASE ANGLE ERROR
The more accurate the reflection in the secondary of the primary current the smaller the current and phase angle errors.

These errors are dependant on:

Excitation ampere turns
Type of magnetic material used
Configuration of the transformer; in general the smaller the ID and OD the more accurate the transformer for the same excitation ATS
The above basic principles are fundamental to both metering and protection CTs.

3．Construction
Current transformers can be constructed in a number of different ways:

WOUND PRIMARY: with a fixed primary with a number of turns around the core;the winding is fully insulated and form an integral part of the assembly.
BAR PRIMARY: this has a fixed insulated primary conductor passing through the core and essentially the primary conductor is equal to 1 turn.
BUSHING TYPE:this has no primary winding;it is for use with a fully insulated conductor acting as the primary winding.
WINDOW TYPE:this has no primary winding but it normally has insulation in the window so that conductors can be passed through.

Other types are for HV applications.