What is inrush current in transformer?

What is inrush current in transformer?

Inrush current, input surge current, or switch-on surge is the maximal instantaneous input current drawn by an electrical device when first turned on. Alternating-current electric motors and transformers may draw several times their normal full-load current when first energized, for a few cycles of the input waveform.

How is transformer inrush current calculated?

Inrush current calculations: Approximately inrush current I(peak) is equal to the ratio between the root 2 times of maximum applied voltage to DC winding resistance. This calculation gives you the approximate value of the inrush current to understand.

What is the problem of current inrush in a transformer Why does it happen?

Inrush current may cause fuses, reclosers, or relays to falsely operate. It may also falsely operate faulted-circuit indicators or cause sectionalizers to misoperate. When the transformer is switched in, if the system voltage and the transformer core magnetization are not in sync, a magnetic transient occurs.

Does inrush current damage transformer?

As previously mentioned, inrush current is the maximum peak current, experienced in the system and it can be twice or ten times of the normal rated current. This unwanted current spike can damage the device like in transformer, inrush current can cause tripping of the circuit breaker, every time it turns ON.

What causes high inrush current?

High inrush current is the result of the maximum instantaneous input current drawn by an electrical device during the initial power up. As technology continues to advance, most systems today run efficiently and maintain a low impedance which in turn contributes to high inrush current.

How can we reduce the inrush current of a transformer?


  1. Hysteresis flux of the transformer can be reduced.
  2. Switching on the transformer at the voltage peak.
  3. Increasing the frequency at the switching time.
  4. Reducing the voltage at the switching time.

How is inrush current measured?

To measure the inrush current, technicians can use either a hard-jawed clamp meter or a flexible current probe. Only meters that offer an inrush button can measure inrush current.

How do you prevent an inrush current in a transformer?

How do you control inrush current?

Inrush current can be reduced by increasing the voltage rise time on the load capacitance and slowing down the rate at which the capacitors charge. Three different solutions to reduce inrush current are shown below: voltage regulators, discrete components, and integrated load switches.

What are the effects of inrush current?

High inrush current can affect electrical systems by tripping fuses and circuit breakers unnecessarily. If inrush current protection is not in place, relays and fuses must be used that are rated higher than any possible inrush current.

How long does transformer inrush current last?

Current could be anywhere from 3-14 times FLA of transformer and typically last up to 0.1s and in some special cases up to 1s. Some factors that determine magnitude and duration of inrush current are: Point of voltage waveform switching. Magnitude and polarity of residual magnetism in core.

Why is inrush current important?

The terms inrush current and starting current are often used to mean locked rotor current.” High inrush current can cause nuisance tripping of protective devices or motor damage. It can also cause voltage dips in the supply line (which can affect other equipment), or even prevent the motor from starting properly.

How do I stop inrush current?

Inrush current can be reduced by increasing the voltage rise time on the load capacitance and slowing down the rate at which the capacitors charge.

What is inruch current in transformer?

Transformer inrush current is defined as the maximum instantaneous current drawn by the transformer when the secondary side is unloaded or in open circuit condition. This inrush current harms the magnetic property of the core and cause an unwanted switching of transformer’s circuit breaker.

How can we reduce an inrush current in a transformer?

– Lower operating flux density reduces inrush. Typically larger core and/or more turns. – Leakage Impedance. Leakage of primary winding to core not just impedance defined by the primary/secondary leakage. – Large primary winding area. – Relative number of conductor turns and relative core size – Core magnetic properties and its geometry.”

What is exciting current in transformer?

Answer: Exciting current, when used in connection with transformers, is the current or amperes required getting the transformer to the point where it will operate. A certain amount of energy is required to overcome the internal resistance of the steel core.

What type of current must be used with a transformer?

The transmission current is AC. It is commonly used to increase or decrease the supply voltage without a change in the frequency of AC between circuits. The transformer works on basic principles of electromagnetic induction and mutual induction.

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