Motor Slip & Speed Calculator
Enter frequency, poles and rotor speed to get synchronous speed, slip percentage and rotor frequency.
Step-by-step
- Enter valid values to begin.
Assumes a three-phase squirrel-cage induction motor on a sinusoidal supply. Rotor speed above synchronous is rejected because that is generator action, not motoring. Typical full-load slip: about 3–5% for NEMA design B, 5–13% for design D.
Motor Slip and Speed: Why the Shaft Runs Behind the Field
Core Engineering Principles
Three-phase current in the stator makes a magnetic field that rotates at synchronous speed, Ns = 120 × f / P. The rotor can’t run at exactly that speed, because at synchronous speed the rotor conductors would see no relative motion, no induced voltage, no current and no torque. The rotor falls a little behind, and that shortfall is slip. It’s the price of making torque. The harder the load pulls, the more slip, the more rotor current, the more torque, until the machine reaches breakdown.
Slip also decides the rotor frequency, fr = s × f. At full load on a 60 Hz motor that’s only 1 to 3 Hz, which is why the rotor iron can be solid laminations without overheating, and why a tachometer on a lightly loaded motor reads just under synchronous. A quick field trick follows from this: read the nameplate speed, round up to the nearest synchronous speed, and you know the pole count. 1750 rpm is a four-pole machine at 60 Hz; 1450 is four-pole at 50 Hz.
Nr = Ns × (1 − S)
NEC & Standard References
NEMA MG-1 Part 1 and Part 12 cover speed and slip characteristics of the standard designs, with design B running about 3–5% slip at full load and design D (high-slip) 5–13%. IEC 60034-1 requires rated speed on the nameplate, and IEC 60034-2-1 describes how efficiency is determined from measured slip and losses. NEC 430.7(A) requires the nameplate to show rated full-load speed, which is the number you use here.1. Ns = 120 × 60 / 4 = 1800 rpm.
2. Slip = (1800 − 1750) / 1800 = 2.78%, or 50 rpm.
3. Rotor frequency = 0.0278 × 60 = 1.67 Hz.
4. If the tach later reads 1710, slip is 5.0%. The rotor is carrying a much heavier load, or the supply voltage has sagged, and the winding is running hotter.
- Rising slip is a warning light. A motor that used to run at 1760 and now runs at 1730 is overloaded, on low voltage, or has a failing rotor bar.
- Slip near zero means no load. If a loaded pump motor reads synchronous speed, suspect the tachometer or that the load is gone.
- Don’t add a frequency and a pole count blindly. A two-speed motor changes poles, so check which winding is energised before you read the dial.
- Speed above synchronous is not a motor. That is a regenerating induction generator, and it will push voltage back onto the bus.