voltage regulation with significance

voltage regulation with significance

1. What Voltage Regulation Is

Voltage regulation = measure of how much secondary voltage $V_2$ drops from no-load ($E_2$) to full-load due to winding $R$ and $X$ drops, expressed as percentage of full-load voltage:

where $E_2$ = no-load secondary emf (equal to $V_1·N_2/N_1$, i.e., ideal), $V_2^{FL}$ = full-load terminal voltage (with $I_2$).

For ideal ($R=0,X=0$), $E_2=V_2$ ⇒ regulation $0\%$.

Alternatively referenced to primary:

but secondary form is standard.

2. Expression in Terms of $R, X, pf$

Approximate (for small drops):

where

referred to secondary (or primary, be consistent),

= load power factor angle (+ for lagging, - for leading).

So

+ for lagging, - for leading (leading can give negative regulation — voltage rises on load due to capacitive $X$ cancelling inductive drop).

3. Why Significance — Why Keep It Low

• Good regulation (low %, e.g., $2$–$5$% for distribution transformer) ⇒ $V_2$ nearly constant despite load variation ⇒ equipment operates at rated voltage (motors, electronics).
• Poor regulation (high %) ⇒ large voltage swing ⇒ flicker, under-performance, over-current at low $pf$.

Design goal: minimize $R_{eq}, X_{eq}$ (low leakage) and operate near unity $pf$ (or leading to cancel).

4. Worked Examples

(a) E2=240V (no-load), full-load V2=230V ⇒ Reg=(240-230)/230=4.35% ✓

(b) Transformer R_eq=1Ω, X_eq=2Ω, I2=10A, pf=0.8 lagging (φ=36.9°): V_drop≈10(1·0.8+2·0.6)=10(0.8+1.2)=20V. If V2=230V, Reg≈8.7% (a bit high) ✓

(c) Same leading pf 0.8 (capacitive): V_drop=10(0.8 -1.2)= -4V ⇒ V2=244V ⇒ Reg=-1.7% (rise) ✓

(d) How to improve? Reduce R (thicker wire), reduce X (tighter coupling, interleaved windings), keep pf near 1 via correction.

(e) Distribution transformer spec: regulation <5% typical.

5. Exam Points

• Define regulation $(E_2-V_2)/V_2×100\%$ — 5 marks, very frequent (almost certain to ask definition).
• Give approximate $I(R cosφ ± X sinφ)/V$ — 5-7 marks, very frequent (common numerical: find regulation for lagging/leading).
• Explain significance (low regulation ⇒ constant voltage for loads) — 5 marks, very frequent.
• Mention negative regulation for leading pf — 3-5 marks, frequent.