# Transformer inrush current calculation xls

• Calculate Inrush Current in Three Steps
• SIZING CALCULATION
• Voltage Disturbance
• Generator Sizing kVa Calculator
• Transformer Inrush Current Calculator With Formula
• ## Calculate Inrush Current in Three Steps

Please contact Atlas CEA for further information Suitable Generators How to Convert kVa to KW for Generators The most important thing to consider when sizing a generator is the high inrush currents associated with starting electric motors and transformers, which are typically six times the full load current.

However, inrush currents for the type of high efficiency motors being specified today can be almost double that amount. As a result, it has been common practice to take motor and transformer starting kVA requirements as a yardstick to determine the size of a generator.

This approach often results in generators being oversized for the motor running load and not based on the actual needs of the application. Moreover, it disregards other key factors that play a key role in sizing generators. For instance, harmonics caused by variable frequency drives and sequential starting of motors.

When starting motors or transformers, large voltage and frequency dips can also occur if the generator set is not sized properly. Furthermore, other loads connected to the generator output may be more sensitive to voltage and frequency dips than the motor or motor starter, which can cause problems.

Thankfully help is at hand. Many generators can now be equipped with solutions to overcome the extra excitation systems required in the alternator. Typically, two options are offered: permanent magnet or auxiliary winding. Both provide the generator with three times their nominal current to cover inrush peaks from the electrical motor, for a minimum duration of ten seconds, via a residuary excitation current.

In certain cases even more advanced options are available. For instance, some generators feature a digital automatic voltage regulator D-AVR that is specifically designed to handle the high inrush currents associated with starting motors and transformers. In specific applications, this type of voltage controller allows operators to downsize the generator requirement because the transient behaviour of the power is better managed.

This enables the excitation to increase gradually as the speed of the engine does, allowing for very soft start of loads connected to the generator. This is especially useful for magnetising step up transformers in installations where medium voltage is required. As a result, it is no longer necessary to buy larger generators than needed just to cope with the initial electrical surge upon starting.

An Inrush Current Limiter ICL can protect electrical equipment from overheating when switched on because of inrush current.

And, because inrush current equals the maximum instantaneous surge of incoming current from a power source, it can be as much as 2 to 3 times the steady state current of the attached device.

Inrush Current Calculators are the best way to measure the resistance versus temperature curve to get the right Inrush Current Limiter. Switching power supplies, DC motors , and lighting ballasts can develop extremely high peak inrush current at turn-on unless you implement inrush current protection.

Additionally, magnetic devices that produce alternating current such as electric motors or transformers can draw as much as 30 times their steady state current at switch-on. Without protection from inrush current, the only limits to the amount of inrush current drawn are the line impedance and the equivalent series resistance of a capacitor.

Unfortunately, that is not the case. In fact, the overall efficiency of low impedance also contributes to high inrush current, increasing the possibility of equipment overheating during start-up. Using inrush current limiters will help ensure that systems do not overheat due to inrush current.

On our website, you can view our full line of inrush current limiters and learn how to reduce inrush current. Inrush current limiters are designed with different characteristics like resistance versus temperature curve to accommodate numerous applications.

Because of this, it is necessary to make some calculations based on your system requirements to select the best inrush current limiter for your needs. At a minimum, you will need to know: Maximum Output Power: Based on specific system or design requirements, this is a variable and is relevant to the steady state current of the application Input Voltage: This is the available incoming line voltage to the application in question.

Quantifies the magnitude of capacitance Scope Trace of Inrush Current: A snapshot of inrush current at a moment in time Fuse or Circuit Breaker Rating if applicable Inrush Current Rating of Diode Bridge if applicable Example Problem Statement Current Situation: A watt switch mode power supply trips a amp breaker at turn-on due to high inrush current. How do you stop the breaker from tripping?

But as can be seen from B-H curve of figure 2 even for a flux lower than twice maximum significant inrush current can flow. Switching at zero voltage crossing-maximum residual magnetism in reverse polarity This scenario will have transformer switched on during zero voltage crossing with a residual magnetism in the opposite polarity to that which varying flux would normally have.

Inrush current will also take longer time to reach steady state conditions. Practically, transient flux does not reach three times the normal flux density due to large current inrush through the primary circuit and resulting voltage drop. Switching at zero voltage crossing-maximum residual magnetism in same polarity This scenario will have transformer switched on during zero voltage crossing with a residual magnetism in the same polarity to that which the varying flux would normally have.

## SIZING CALCULATION

This case is an ideal situation and there will not be any inrush current theoretically. Switching at maximum voltage crossing- no residual magnetism This scenario will have transformer switched on during maximum voltage with no residual magnetism.

Since the switching occurs at peak voltage, flux can easily start from zero and attain its nominal value. There will not be any inrush current in this scenario.

Switching at maximum voltage crossing- maximum residual magnetism in opposite polarity This scenario will have transformer switched on at peak voltage with a residual magnetism in the opposite polarity to that which the varying flux would normally have.

Switching at maximum voltage crossing- maximum residual magnetism in same polarity This scenario will have transformer switched on at peak voltage with a residual magnetism in the same polarity to that which the varying flux would normally have. Summary of all possible scenarios for point of wave transformer switching inrush is given below: 2. Magnitude and polarity of residual magnetism in core Residual flux magnitude will depend on quality of steel and point of wave at which transformer is deenergized.

Remember that flux lags voltage by 90 degree. This means that when transformer is de-energized at peak voltage, residual flux will be zero and when de-energized at zero voltage, residual flux will be at its maximum.

Type and quality of core steel determines how much residual flux remnant flux is stored.

### Voltage Disturbance

In general, higher the quality of steel, larger the remnant flux and greater the inrush current. System Damping Source impedance has some influence in the magnitude and duration of transformer inrush. Large DC offset creates large DC flux in transformer core. DC flux decays slowly prolonging the inrush.

Figure below shows a 2. Note that the inrush persists for a long duration. For this specific example inrush has not completely died down even after 1. Figure 9: Transformer energized from MV Generator Winding resistance provide additional damping to transient switching oscillations. Transformers with low winding resistances example would be low temperature rise transformers tend to have inrush current persist for longer duration.

This means 80deg C transformer could have a higher inrush current at 0. Winding Energized-Primary or Secondary Among other things inrush current depends on the length and diameter of coils and the number turns in the energized winding.

Smaller the diameter or length of turnhigher the inrush.

### Generator Sizing kVa Calculator

For step down applications, secondary low voltage winding is wound close to core and primary HV winding is on top of secondary winding. So, if this stepdown transformer is back fed and used as a step-up unit then higher inrush up to times the normal inrush current can be expected.

Transformer Size Larger the kVA larger the inrush current. Thankfully help is at hand. Many generators can now be equipped with solutions to overcome the extra excitation systems required in the alternator.

## Transformer Inrush Current Calculator With Formula

Typically, two options are offered: permanent magnet or auxiliary winding. Both provide the generator with three times their nominal current to cover inrush peaks from the electrical motor, for a minimum duration of ten seconds, via a residuary excitation current. In certain cases even more advanced options are available. For instance, some generators feature a digital automatic voltage regulator D-AVR that is specifically designed to handle the high inrush currents associated with starting motors and transformers.

In specific applications, this type of voltage controller allows operators to downsize the generator requirement because the transient behaviour of the power is better managed. This enables the excitation to increase gradually as the speed of the engine does, allowing for very soft start of loads connected to the generator.

This is especially useful for magnetising step up transformers in installations where medium voltage is required.

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