Question

An electromechanical MHO relay has its operating torque and restraining torque governed by relation $T_o=aVIcos(\varphi -\alpha )$ and $T_r=b{V}^2,\left(Where\alpha isadjustable\right)$ by varying resistance in phase shifting circuit. The condition on ratio a/b and admittance Y when this relay operates is

• A. $\frac{a}{b}>\frac{1}{Ycos(\varphi -\alpha )}$
• B. $\frac{a}{b}<\frac{1}{Ycos(\varphi -\alpha )}$
• C. $\frac{a}{b}>Ycos(\varphi -\alpha )$
• D. $\frac{a}{b}<Ycos(\varphi -\alpha )$

Answer 1

The correct option is A $\frac{a}{b}>\frac{1}{Ycos(\varphi -\alpha )}$

$Given,Operatingtorque{T}_o=aVIcos(\varphi -\alpha )$
$Restrainingtorque{T}_r=b{V}^2$
$Forrelaytooperate,T_o>T_r$
$aVIcos(\varphi -\alpha )>b{V}^2$
$Ycos(\varphi -\alpha )>\frac{b}{a}$
$\frac{a}{b}>\frac{1}{Ycos(\varphi -\alpha )}$