Dashboard › Deep Learning › Electrical Machines › Induction machines › No-load + blocked-rotor tests

No-load + blocked-rotor tests

Two bench tests — the direct parallel of transformer OC/SC — that pin down every equivalent-circuit parameter.

Freshman ~9 min

Step 1 — Two bench tests pin down every equivalent-circuit parameter

Animation speed 0.55×

V — A — W —

Reference notes

Use Next → on the narrator above to step through both bench tests and watch the parameter table fill in.

Two tests, full equivalent circuit — the direct parallel of transformer OC/SC

An induction motor's per-phase equivalent circuit has the same shape as a transformer's. The same two-test trick pins down every parameter:

No-load (NL) test — apply rated voltage, no shaft load. Measures the shunt branch: core loss, magnetising reactance.
Blocked-rotor (BR) test — mechanically lock the rotor, apply a reduced voltage to drive rated current. Measures the series branch: stator + referred rotor resistance and leakage reactance.

No-load test — measures the shunt branch

Setup: rated three-phase voltage applied to the stator; rotor uncoupled from any load. The rotor runs at near-synchronous speed (slip ≈ 0).
Measure: V_NL, I_NL, P_NL.
What we learn: at very small slip, R₂/s is enormous → no current flows in the rotor branch → essentially all the current flows in the shunt magnetising branch. P_NL equals (essentially) the core loss plus a small amount of friction & windage.

From the readings (per-phase):

cos φ_NL = P_NL / (√3 · V_NL·I_NL)
R_c = V_NL² / P_core X_m = V_NL² / Q_NL

(after subtracting stator copper loss I_NL²·r₁ and the friction-windage component from P_NL, which is usually obtained from a separate spin-down test).

Blocked-rotor test — measures the series branch

Setup: mechanically lock the rotor (s = 1, like a stalled motor). Apply a reduced voltage and crank it up until rated current flows.
Measure: V_BR, I_BR, P_BR.
Why a reduced voltage? Because at standstill R₂/s = R₂ is small — full rated voltage would drive 5–7 × rated current. We need only enough V to push rated current through the small series impedance.
What we learn: at low V_BR, the magnetising current is negligible (the shunt branch is "starved"). All the current flows through the series branch. P_BR equals (essentially) the rotor + stator copper losses at rated current — i.e. the full-load copper loss.

From the readings:

Z_eq = V_BR / I_BR R_eq = P_BR / (3·I_BR²) X_eq = √(Z_eq² − R_eq²)

The stator r₁ is usually measured separately by DC-resistance test, and r₂′ = R_eq − r₁. The leakage reactance splits roughly equally between stator and rotor (a common approximation in design problems).

Why these two tests work in tandem

The NL test isolates the shunt branch by making the series branch (rotor) carry essentially no current. The BR test isolates the series branch by starving the shunt branch with a low voltage. Each test eliminates the other's effect, and between them they pin down every parameter the equivalent circuit needs. Exactly the same logic as the transformer's OC/SC tests.

What you can compute once you have all the parameters

Steady-state operating point at any slip: stator current, power factor, efficiency.
The full torque-slip curve (previous lesson).
The famous circle diagram — locus of stator current as load varies, on the complex plane. A two-test graphical construction that reads off all operating-point information at once.
Starting current and starting torque for protection and starter sizing.

Take-away. Two simple bench tests — about an hour's work on a small motor — give you a complete electrical model. From there, you predict every behaviour the motor will ever exhibit. This is the engineer's leverage.

Keyboard shortcuts

→ next step · ← previous step
R replay narration · M mute / unmute · F fullscreen