List of top Electrical and Electronic Measurements Questions on Instrument transformers

A DC series motor with negligible series resistance is running at a certain speed driving a load, where the load torque varies as the cube of the speed. The motor is fed from a 400 V DC source and draws 40 A armature current. Assume a linear magnetic circuit. The external resistance in ohms that must be connected in series with the armature to reduce the speed of the motor by half is closest to:
The table lists two instrument transformers and their features:
\begin{tabular}{|l|l|} \hline Instrument Transformers & Features
\hline X) Current Transformer (CT) & P) Primary is connected in parallel to the grid
& Q) Open circuited secondary is not desirable
& R) Primary current is the line current
\hline Y) Potential Transformer (PT) & S) Secondary burden affects the primary current
\hline \end{tabular}
Which matches are correct?
A single-phase triac based AC voltage controller feeds a series RL load. The input AC supply is 230 V, 50 Hz. The values of R and L are 10 \(\Omega\) and 18.37 mH, respectively. The minimum triggering angle of the triac to obtain controllable output voltage is