Transformer Interview Questions & Answers Part-4

Transformer Interview Questions & Answers Part-4: In this section we shall discuss more questions on transformer.

Why is flux constant in transformer?

The net MMF in the primary remains constant. When load increases, the secondary MMF(N2I2) reflects in the primary side, which is just opposite to the primary flux. Thus net flux in the primary increases monetarily.

The decreased flux cause less primary emf and primary draw more current to compensate the decreased flux. This way again the flux in the core reached at its previous flux level.

Thus, the MMF adjusts according to the  increase/decrease in the secondary load and flux in the core remains constant.

Can we use transformer with DC supply?

No, transformer can not be used with SC supply.

Why an induction motor is called a rotating transformer?

Transformer and induction motor both function on the principle of the Faraday’s law of electromagnetic induction. In induction motor, the rotor winding is short circuit, whereas, in transformer, secondary is open circuit. Th short circuit rotor winding cause current to flow through it. The current flowing in the rotor and the flux in the air gap cause production of torque. Therefore, induction motor is called a rotating transformer.

What are the cooling methods to cool a transformer?

The cooling methods are:

  • Oil Natural Air Natural (ONAN)
  • Oil Natural Air Forced (ONAF)
  • Oil Forced Air Forced (OFAF)
  • Oil Forced Water Forced (OFWF)

What is Scott- connected transformer?

A Scott-T transformer (also called a Scott connection) is a type of circuit used to derive two-phase power from a three-phase source or vice-versa. The Scott connection evenly distributes a balanced load between the phases of the source.

Scott T Transformers require a three phase power input and provide two equal single phase outputs called Main and Teaser. The Main and Teaser outputs are 90 degrees out of phase. The MAIN and the Teaser outputs must not be connected in parallel or in series as it creates a vector current imbalance on the primary side.

Does the transformer draw any current when its secondary is open?

Transformer draw no-load primary current.

Does copper loss depends on load power factor?

The transformer draw a large current when load power factor is low. The increased current leads to more copper loss ( I2 R).

What are Instrument Transformers?

Current Transformer and Potential Transformer are Instrument transformers. The instrument transformers are used for metering and protection applications.

current transformer is a device which produces an alternating current in its secondary which is proportional to the alternating current in its primary. A current transformer (C.T.) is used where the current is too high to measure directly, or the voltage in the primary circuit is too high, so the galvanic isolation between primary and secondary is important for safety or technical reasons.

Potential Transformer (PT) is a step down transformer and steps down the voltage to a known ratio. ts just a step down transformer which is used for metering the voltage and current flowing through high voltage lines.It consists of two sets of coils primary and secondary.Primary is used to connect to the HT line via series connection and metering devices like kWh/VAF (specially designed for potential transformer) meters are connected in parallel through secondary coil of transformer.

What is the difference between a voltage transformer and a potential transformer?

Both are essentially same, different names for one thing. Potential transformers (PT) or voltage transformers (PT) are step down instrument transformers. They are used to measure high voltages. They offer negligible load to supply being measured,and have highly accurate voltage ratio and phase relation to voltage on primary side.

Capacitive voltage transformers (CVT) are different things , where capacitor voltage divider is used to tap a small part of voltage across a capacitor in a series of capacitors across high voltage.

What is the basic difference between a potential transformer and a step down transformer?

A PT is a special case of the wider set of things that are step-down transformers.

A PT is designed for measuring, not supplying load. It has precise turns ratio, whereas general service transformers might be over-wound to emulate load regulation.

The power rating of a PT is usually negligible but the voltage rating will be very high so it can be relied upon in fault conditions.

What conditions can cause a Potential Transformer to be damaged?

Transformer can damage Due to the following reasons.

  • Continuous overload.
  • Over voltage to its primary Winding
  • Weak Insulation due to aging or any other reason,
  • An air cooled Transformer is apt to fail due to too much moisture in the surrounding.
  • An oil cooled Transformer ids prone to fail due to impurities or moisture in the Cooling oil.
  • It can be partially or fully damaged due to short circuit between inter turn or inter layer winding
  • It can also damage due to damage to its lamination/Core.
  • Short circuit