Electrical Machines: Transformer Losses, Induction Motor V/f Control & Synchronous V-Curves
High-yield electrical machines concepts covering copper losses, eddy currents, hysteresis, V/f drives, and power factor correction.
In-Depth Interview Questions & Model Solutions
Q1Why is the Open Circuit (OC) test performed on the LV side and Short Circuit (SC) test on the HV side of a transformer?
OC Test: Performed at rated voltage to determine core losses (iron/hysteresis/eddy) and shunt parameters (Rc, Xm). Applying rated voltage on the Low Voltage (LV) side is safer and requires readily available laboratory supplies, while no-load current (Io ~ 2-5% of full load) is easily measurable. SC Test: Performed at rated current to measure full-load copper losses and series parameters (Req, Xeq). Performing it on the High Voltage (HV) side requires only 5-10% of rated voltage to circulate rated full-load current, which is easily controlled, and the lower rated current is accurately read on standard meters.
- OC Test yields core loss (assumed constant since voltage is rated).
- SC Test yields I²R copper loss at rated current.
- HV side SC test keeps testing current lower and safer.
Q2Explain V-Curves and Inverted V-Curves of a Synchronous Motor and their application in Power Factor Correction.
A V-Curve plots Armature Current (Ia) vs Field Current (If) at constant mechanical load. At normal excitation, Ia is at its minimum and power factor is unity (cos φ = 1). Under-excitation causes the motor to draw lagging current (operating like an inductor). Over-excitation causes the motor to draw leading current (operating like a capacitor). An over-excited synchronous motor running without mechanical load is called a Synchronous Condenser, widely used in substations to supply reactive power and improve grid power factor.
- Over-excited synchronous motor draws leading reactive power.
- Synchronous condenser eliminates the need for bulky capacitor banks in large plants.
Technical Panel Interview Strategy Tips
- Draw the V-curves with unity pf line, 0.8 leading, and 0.8 lagging compounding curves.