NapkinCalc

AC Motors

Slip — why the rotor lags

continues from lesson 1 — values defined earlier in the course stay live here

ELI5: if the rotor ever caught the field, it would cut no field lines, induce no current, and make no torque — so it can never quite catch up. It settles just below, and that shortfall (as a fraction) is the slip. A loaded motor runs at 1–5% slip; the slip is literally where the work comes from.

nrotor=1740n_{rotor} = 1740 nameplate full-load speed (rpm)
sslip=nsync−nrotornsyncs_{slip} = \frac{n_{sync} - n_{rotor}}{n_{sync}} = 0.03330.0333 slip — typically 0.01 to 0.05
frotor=sslip⋅flinef_{rotor} = s_{slip} \cdot f_{line} = 22 frequency induced in the rotor (Hz)
✓ pass sslip<0.06s_{slip} < 0.06 healthy slip for a standard motor

Real-world hook: rising slip under load is why a drill bogs down when you press hard, and measuring slip is how technicians estimate a motor's actual load without a meter.

Try it yourself: a motor with synchronous speed 1800 rpm runs at 1710 rpm under load. What is its slip? (s = (n_sync − n_rotor)/n_sync.)

slipyou=1800−17101800slip_{you} = \frac{1800 - 1710}{1800} = 0.05000.0500 ✏️ Your turn: compute (1800 − 1710)/1800. (It comes out to 0.05 — a healthy 5%.)
✓ pass abs(slipyou−1800−17101800)<10−9\mathrm{abs}\left(slip_{you} - \frac{1800 - 1710}{1800}\right) < 10^{-9} green when your slip is correct