Categories

# Transformer

#### Relationship between induction motor voltage and current

##### Sunday, September 6, 2015

When the induction motor starts the rotor winding is stationary and maximum voltage is induced in the rotor which will produce maximum rotor current. At starting the rotor is stationary and acts in a similar to a transformer with its secondary winding short circuited except that there is an air gap in the motor. At starting the motor current can be like 6 times of the rated current. The voltage is reduced during motor starting to decrease the current but still the current is much higher than ...

#### Current Transformer Secondary Connection

##### Monday, August 31, 2015

In circuit theory, current through two series resistors is the same even if the resistors are of different resistances. It's the voltage across the resistors that is different. Look at reactors/capacitors, and it's the same thing. Now, current through parallel resistors is different through each resistor, as the current divides among the parallel resistors. So, put the meters in series on the secondary of a current transformer.

Now, whether one CAN put two of them in series, versus ...

#### Why isolation transformer is used at the output of online UPS?

##### Saturday, July 25, 2015

An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons.

What this means in reality is that a normal application for an isolation transformer is to either keep the load from generating harmful harmonics back onto the distribution bus, ...

#### Why three phase power system is grounded?

##### Tuesday, July 21, 2015

A three-phase network is, indeed, either grounded or ungrounded. What that means, to my understanding, is that, simply speaking, the neutral point on a wye connection can be either grounded or not. What I really mean when I say "ungrounded" is the situation when there's no closed zero-sequence path for current to flow. A delta-wye grounded transformer is commonly referred to as a grounding bank, because the delta side of the transformer can be left un-terminated--still connected in delta but ...

#### How to select a transformer loaded only by a motor?

##### Sunday, July 12, 2015

Do we have to select a transformer (transformer loaded only by an asynchronous motor) based on the apparent power of the asynchronous motor during startup or based on the apparent power of the asynchronous motor during normal operation?

In other words, does the apparent power of the secondary windings of the transformer need to be larger than or equal to the apparent power of the asynchronous motor during startup or does it need to be larger than or equal to the apparent power of t ...

#### How to protect transformer from damages of surge current?

##### Friday, July 3, 2015

Current variations in the power supply causes severe problems in the electrical devices i.e if it increases or decreases in both cases it causes overheating which melts the conductors. High current is said when the current is above the ratings of any electrical devices, this rises the temperature of the transformer similarly when the current is below the min. rated value of the transformer but the load remains constant because of which it draws heavy amount of current to meet the load current ...

#### Minimizing Transformer Inrush Current

##### Saturday, June 13, 2015

I have used three different Methods depending on the details of a specific project plus I do not use the Method 4.

Method 1. Secondary pre-charge
This is normally the most economical Method. It uses a small pre-charge transformer to apply a voltage to the transformer's secondary and I normally use 90% of the rated transformer's secondary voltage. The required current is approximately the transformer's magnetizing current. A typical value is 0.5% of the rated w ...

#### Tan Delta and Capacitance Test on Transformer

##### Sunday, June 7, 2015

Herewith some information relating to Tan Delta (Power Factor) and Capacitance Test on transformers from our perspective. With regards to your statement relating to whether Tan Delta testing could have any adverse effect on transformer health, the following:

- This test is a diagnostic insulation assessment tool that is a measurement of the fundamental AC electrical characteristics of insulation. These electrical characteristics could provide an indication of moisture, insulation d ...

#### Measure knee point on a transformer

##### Wednesday, May 27, 2015

There was a query in all this about how to measure knee point on a transformer. Knee point relates to the ability of the secondary winding of a current transformer to produce enough voltage to drive the required current around the connected burden and what excitation current is "lost" inside the transformer to do that.

So for a current transformer, leave the primary winding open circuit. Apply a small increasing voltage to the secondary and measure the current that flows into the s ...

#### The effect of harmonics in power system

##### Friday, April 24, 2015

According to standard BS 7821 part 4: A distribution x-former supplying non-sinusoidal load would end up with few percent of overloading with respect to same amount of sinusoidal load which is usually reflected in X-former sizing procedure by a k-factor, I suppose. However, taking up to 20% of spare capacity in distribution x-former sizing, this will hardly result in over temperature trip out unless in case of very marginal sizing design or poorly filtered non-sinusoidal loads e.g. variable f ...

#### Apply DC to a transformer it will burn out

##### Wednesday, March 25, 2015

For a transformer to work, the current in one coil has to somehow make current flow in the other coil and the circuit it's connected to. A DC current in one coil will make a magnetic field on the other coil, but a magnetic field by itself won't drive any electrons around. A CHANGING magnetic field, however, does create an electric force which will accelerate those electrons in the other coil into carrying a current. This process is described by Faraday's law of induction. You get a changing f ...

#### Difference between Power Transformer and Distribution Transformer

##### Sunday, February 22, 2015

We are known that there are two transformer circuits in electrical distribution. One are power circuit & second are control circuit. The transformer who connected HT bus bar and supply feeds to HT motor is all most Power transformer because it is connected with power circuit. Other low voltage transformer is mostly connected in control circuits. It's distributed power (voltage or current) to any machine so it called distribution transfor ...

#### Metering Current Transformer

##### Sunday, February 1, 2015

Instrument security factor is the ratio of instrument limit primary current to the rated primary current. Instrument limit current of a metering current transformer is the maximum value of primary current beyond which current transformer core becomes saturated. Instrument security factor of current transformer is the significant factor for choosing the metering instruments which to be connected to the secondary of the current transformer. Security or Safety of the measuring unit is better, if ...

#### What happens in current transformer core during CT saturation?

##### Wednesday, January 28, 2015

The core saturation is a physical phenomenon. It happens when the coupled magnetic flux is so intense that all magnetic domains on a ferromagnetic material are already aligned and thus does not respond to any further increase in the flux. The implications on the current transformer secondary current may be diverse. Having a saturated core does not mean the current on the current transformer secondary will be high or even constant as the flux increases.

Once saturated, the inductan ...

##### Monday, January 26, 2015

Synchronizing module ensures that voltage, frequency and phase angle difference between the two systems is within acceptable limits relative to one another. Since the upstream source of two identical transformers is a common bus, so synchronizing module is not required (provided tap position of transformer is same). As load transfer is to be done without any interruption, so momentary paralleling is required. When planned outage of one of the transformer is required, the respective incomers s ...

#### What is Flux Walking?

##### Monday, January 19, 2015

Flux walking, also known as flux stair casing, is the successive accumulation of flux in a magnetic core ultimately leading to its saturation. In any circuit involving magnetic elements operating in steady state, the net (volt x time) product applied to the core must be exactly zero. If you have more than one piece of magnetics, the volt-second balance must hold for every one of them individually. Voltage polarity should be accounted for and summed over one switching cycle. Even the slightest ...

#### Why Neutral Grounding Transformer is used for generator?

##### Sunday, January 18, 2015

A Neutral Grounding Transformer is NOT a three phase transformer, but a single phase transformer, with the primary (HV) rated voltage equal to the system phase-to-neutral voltage and the secondary (LV) rated voltage either 110V or 240V.

Why is it required? For economic reasons. Let us see how. Supposing you have an 11kV System, whose neutral you want to ground through a resistance. The desired ground fault current, let us say, is 10A. Now, if you want to connect a resistor directl ...

#### Short Circuit Calculation for Transformer

##### Monday, January 12, 2015

Too hard to tell you what to do without knowing more about the power system both on the primary and secondary side of the transformer. Find a good reference on doing this using per unit quantities. A good method recommended by ANSI involves creating separate R and X networks to simplify the complex impedance calculation. You should also take into account motor contribution if appropriate.

The secondary side short circuit current for a 3 phase transformer = Volt amp transformer rat ...

#### Transformer Sizing

##### Tuesday, January 6, 2015

Transformer sizing is a common detailed subject and it's having different conditions / philosophy based the different type of the transformer to be sized. Generator step up transformer of an ac power plant is having different concept from that is applicable for distribution transformer. Station or startup transformer is having it's own philosophy to size the optimum rating. In general, the distribution transformers are widely sized and the parameters to be considered as following points:

...

#### Different applications of Dy transformer and Yd transformer

##### Thursday, January 1, 2015

The Simple rule which normally dominates the selection of vector group of a transformer is the network where it is to be installed. Normally there are three networks: Generation, Transmission and Distribution. The inherent advantage of the D connection is prevention of zero sequence and harmonics to enter in the network.

Now starting from the Generation. The generators are connected in the Y. Now from the Generators the Unit Transformers are used to transmit the power. The prefera ...