Test: Auto-Transformer - Electrical Engineering (EE) MCQ

Potential divider is resistance division and it does not take part in induction processes.

 Direct electrical contacts is a disadvantage to the auto transformer.
Short circuit current of the auto transformer is higher than the corresponding 2-winding transformer.

 KVA(auto)/KVA(2-W) = V2*I2/V2*(I2-I1)
= 1/(1-I1/I2)
= 1/(1-k)
Losses(auto)= (1-k) *Losses(2-W)
Impedance drop(auto)= (1-k)*Impedance drop(2-W).

The leading p.f. has negative v.r. and lagging p.f. has major portion of positive voltage regulation.

Voltage regulation is independent of the size of the transformer.

Hysteresis losses occur maximum in the ferromagnetic material.

Efficiency = KVA*p.f/(KVA*p.f + Losses); So the efficiency is maximum at unity power factor.

The side which has lower impedance will have lower number of turns and so the low voltage side.

Transformer is nothing but the arranged windings which are magnetically coupled. The windings will inductive predominantly with very low resistance.

Concept:

The percentage change in output voltage from no-load to full-load is termed the voltage regulation of the transformer.

Voltage Regulation of Transformer at Unity Power Factor

The phasor diagram of this case is given

• It shows a phasor diagram for the case of a resistive load (unity power factor) on the transformer 
• Since the current I₂ is in phase with the secondary voltage V_o (_FL), the voltage drop across R_e2 is also in phase with V_o (_FL).
• The drop across the leakage reactance X_e2 leads the secondary voltage V_o (_FL) by 90^o
• The referred value of primary voltage V_o (_NL) is beyond the arc, so it is bigger than the secondary voltage V_o (_FL), which means the voltage regulation calculated is positive.

Voltage Regulation of Transformer at Lagging Power Factor

The phasor diagram for this case is 

• It shows a phasor diagram for the case of an inductive load (lagging power factor) on the transformer (i.e., the load current lags the secondary voltage by 90^o). The referred value of primary voltage V_o (_NL) is beyond the arc,
• So it is bigger than the secondary voltage V_o (_FL), which means the voltage regulation calculated is positive for an inductive case.
• For any current that lags the secondary voltage by 0 to 90^o, the voltage regulation will be positive.

Voltage Regulation of Transformer at Leading Power Factor

The phasor diagram for this case is 

• It shows a phasor diagram for the case of a capacitive load (leading power factor) on the transformer i.e., the load current leads the secondary voltage by 90^o.
• As a result, the referred value of primary voltage Vo (NL) is actually smaller than the secondary voltage Vo (FL), which means the voltage regulation calculated is negative for a capacitive case.