Why is slip needed for torque?

If rotor speed equaled synchronous speed exactly, there would be no relative motion between the rotor bars and rotating field, so no rotor EMF and no torque. A small positive slip induces rotor current and torque.

What is breakdown torque?

Breakdown torque is the maximum steady torque on the curve. If the required load torque exceeds it, the motor cannot maintain speed and stalls or accelerates only after conditions change.

Induction Motor Torque-Speed Curve

An induction motor produces torque because rotor currents are induced by slip between the rotating stator field and the rotor. This simulator uses a simplified per-phase equivalent circuit to draw the torque-speed curve: synchronous speed is ns = 120f/p, slip is s = (ns - n)/ns, and torque is proportional to Vph²(R2/s)/[(R1 + R2/s)² + Xeq²]. It marks starting torque/current, breakdown torque and slip, and the stable operating point for a selected load torque. The V/f option sketches variable-frequency-drive behavior: voltage is reduced with frequency below base speed to keep air-gap flux roughly constant.

Who it's for: Electrical machines, motor drives, industrial automation, power electronics, and undergraduate AC machinery courses.

Key terms

Induction motor
Torque-speed curve
Slip
Breakdown torque
Starting current
V/f control

This is a steady-state approximate per-phase model. It omits magnetizing branch details, saturation, deep-bar rotor effects, thermal limits, inverter harmonics, and transient acceleration.

Live graphs

How it works

Induction motor torque-speed simulator with slip, breakdown torque, starting current, V/f control, synchronous speed, and operating point.

Key equations

n_s = 120f/p, s = (n_s − n)/n_s

T ≈ 3Vph²(R2′/s) / [ω_s((R1+R2′/s)² + Xeq²)]

Frequently asked questions

Why is slip needed for torque?: If rotor speed equaled synchronous speed exactly, there would be no relative motion between the rotor bars and rotating field, so no rotor EMF and no torque. A small positive slip induces rotor current and torque.
What is breakdown torque?: Breakdown torque is the maximum steady torque on the curve. If the required load torque exceeds it, the motor cannot maintain speed and stalls or accelerates only after conditions change.

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