Transformer Interview Questions Answers

During No load why the power factor of the transformer is very low ?

Current flowing through the transformer consists of two components. Magnetizing current (I_m) which is in quadrature (90⁰)  to the applied voltage and in phase current which is in phase to the applied voltage. During no load condition most of the excitation current drawn by the transformer from the primary winding is to magnetize the path. Hence excitation current drawn by the transformer during no load condition mostly consists of magnetizing component of current which is used to provide magnetic field in transformer circuits (Inductive nature). Therefore as the nature of the load is inductive, hence the power factor of transformer during no load condition will by in the order of 0.1 to 0.2

What happen when Transformer is given DC supply ?

When DC supply is given to the transformer primary winding no back emf is induced in the transformer. Importance of Back emf is that it limits the current drawn by the machine. In the absence of back emf transformer starts drawing huge currents which results in burning down of primary winding.

So when dc supply is given to transformer primary windings will burn

At what power maximum efficiency of power transformer and distribution transformer is designed

Maximum efficiency of the transformer is attained at certain load factor (α) when core losses of transformer is equal to the copper losses.

P_{Core loss} = α² X P_{Copper loss}From the above equation for particular load factor (α) maximum efficiency of a transformer is designed when core loss is equal to copper loss. For a transformer core losses remain same irrespective of the load and copper losses varies based on the loading. For a transformer maximum efficiency is designed for particular load factor when core and copper losses are same. Core loss for a transformer is designed based on the application of transformer so that both core losses and copper losses will be sameFor power transformer delivering bulk power employed in generating stations and other substations power it deliver will not vary round the clock and delivers full load. Hence power transformers are designed to have maximum power at full load. Whereas power deliver capacity of distribution transformers which deliver power vary with time duration of the day. Hence distribution transformers are designed to have maximum efficiency at 50% of the rated full load

What are the conditions for parallel operation of two transformers ?

Transformers which are to be operated parallel should have to satisfy the following conditions:

• Polarities of both the transformers should be same else lead to short circuit
• Voltage rating of the both primary and secondary of the two transformers operating in parallel should be same
• Per unit impedance of the transformers should be same
• Phase displacement of transformers should be same
• Phase sequence of two transformers should be same

Why Silica gel is used in transformers?

Silica Gel is used to remove the moisture in the air entering in to the transformer. During the breathing of the transformer air enters in to the transformer. This air makes contact with the hot transformer oil in the conservator and removes the heat by convection. Air entering in to the transformer should be free from moisture else the properties of transformer oil gets degraded. Therefore silica gel crystals are provided in the breather to remove the moisture content in the air Silica gel crystals will be in blue color initially and after absorbing moisture it turns to pink.