Question

Compared with an induction motor, AC servo motor speed-torque characteristics and X/R ratio respectively are

• A. nearly linear, large
• B. more non-linear, large
• C. nearly linear, small
• D. more non-linear, small

Hint:

A high rotor resistance eliminates the "positive slope" region seen in normal induction motor curves, preventing unstable operation at lower speeds.

Answer 1

The Correct Option is C

Concept: An AC servomotor is essentially a two-phase induction motor optimized specifically for high-precision position and speed control applications. Standard industrial induction motors prioritize high operating efficiency and low running slip, whereas servomotors prioritize rapid dynamic response, zero-backlash stability, and a highly predictable control loop.

Step 1: Analyze the Speed-Torque Characteristics.
A standard induction motor has a high rotor reactance-to-resistance ratio, producing a highly non-linear speed-torque curve with a distinct peak torque point (breakdown torque). An AC servomotor is modified with a very high rotor resistance ($R$). This changes the shape of the speed-torque curve, flattening the peak and providing a nearly linear negative slope. This negative slope provides inherent aerodynamic-like damping, preventing single-phasing or hunting oscillations when the control voltage drops to zero.

Step 2: Evaluate the physical significance of the X/R ratio.
To achieve this high rotor resistance ($R$), the rotor conductors are made of high-resistivity materials (or thin drag-cup configurations). This keeps the rotor leakage reactance ($X$) low relative to the resistance: \[ \text{X/R Ratio} \approx \text{Small} \] A small $X/R$ ratio minimizes the mechanical and electrical time constants, enabling rapid acceleration, deceleration, and direction changes without high inductive lag. Therefore, an AC servomotor features a nearly linear characteristic combined with a small $X/R$ ratio. This matches Option (C).