Question

Two coupled DC series motor with constant load torque are changed over from series to parallel connection across a fixed voltage supply. How does the set speed changes compared to the original speed?

• A. Half
• B. None
• C. Double
• D. Equal

Answer 1

The correct option is C Double

$\text{For series DC motor,}$
$T\propto I^2$
$\text{As torque is constant means current also remains constant}$
$T=\frac{E_b{I}_a}{\omega }$
$\text{As both T and}{I}_a\text{are constant}$
$E_b\propto \omega$
$\text{In case of series connection,}{E}_b=\frac{V}{2}$
$\text{For parallel connection,}{E}_b=V$
$\text{So speed becomes double}$