Electrical Engg (EEE) - MCQ Practice Questions

Understanding:

We must identify which quadrant of a torque-speed plane corresponds to reverse rotation with positive (braking) torque.

Step 1: Define the four quadrants

In the torque-speed ($T$–$N$) plane used for electrical drives:

Step 2: Match the condition

The question specifies reverse rotation ($N<0$) and braking torque in the positive direction ($T>0$). This combination lies in Quadrant IV.

Step 3: Physical interpretation

In Quadrant IV the motor is decelerating from reverse rotation. Energy flows back to the supply (regenerative braking in the reverse direction). A dual-converter or four-quadrant chopper is needed to achieve this.

Answer:

Reverse rotation with positive braking torque corresponds to Quadrant IV.

Quick Tip:

A simple mnemonic: Quadrants I and III are motoring (torque and speed have the same sign); Quadrants II and IV are braking/generating (torque and speed have opposite signs).

Understanding:

We must find the stator voltage when a VFD operates at a reduced frequency under constant $V/f$ control.

Formula:

Constant $V/f$ control maintains:

Therefore:

Step 1: Calculate the $V/f$ ratio at base

Step 2: Find voltage at 30 Hz

Answer:

The stator voltage at 30 Hz is 240 V.

Quick Tip:

Constant $V/f$ keeps the air-gap flux approximately constant, which prevents magnetic saturation at low frequencies and avoids torque reduction. Above base speed, voltage is capped at $V_0$ and flux-weakening begins.