Complete Guide to Motor Slip: Calculation, Synchronous Speed, Rotor Frequency
The Slip Calculator determines slip %, rotor speed, and rotor frequency in induction motors. Essential for performance analysis.
What is Slip?
Slip is the difference between synchronous speed (N_s) and rotor speed (N_r), expressed as %.
s = (N_s - N_r) / N_s × 100%
Synchronous Speed
N_s = (120 × f) / P
f = frequency (Hz), P = poles
Rotor Frequency
f_r = s × f (s in decimal)
Induces current in rotor bars.
Why Slip Matters
- Torque Production: No slip → no torque
- Efficiency: Lower slip → higher efficiency
- Speed Control: VFD adjusts slip
Use with Synchronous Speed and Torque calculators.
Formulas Summary
| Parameter | Formula |
|---|---|
| Slip % | s = (N_s - N_r)/N_s × 100 |
| Rotor Speed | N_r = N_s × (1 - s) |
| Rotor Frequency | f_r = s × f |
| Synchronous Speed | N_s = 120f / P |
Typical Slip Values
| Motor Type | Full Load Slip |
|---|---|
| NEMA Design B | 3–5% |
| Design C (High Torque) | 5–13% |
| Design D | 8–13% |
| Synchronous | 0% |
Calculation Steps
- Find N_s: 120f/P
- Measure N_r: Tachometer
- Compute Slip
- Check Efficiency
Example: 4-pole, 60 Hz, 1720 RPM
- N_s = 120×60/4 = 1800 RPM
- s = (1800-1720)/1800 × 100 = 4.44%
- f_r = 0.0444 × 60 ≈ 2.67 Hz
Slip vs Load
Slip increases with load. At no load: ~0.1%
High Slip Effects
- Lower Efficiency: I²R losses
- Higher Starting Torque: Design D
- Overheating: If > rated
Testing Slip
- Stroboscope: Freeze rotor bars
- Nameplate: Full load RPM
- Dynamometer: Load test
Common Mistakes
- Wrong Poles: 2-pole ≠ 3600 RPM at 60 Hz
- Slip = 0: Only synchronous motors
- RPM vs RPS: Use consistent units
Advanced Topics
- Slip Power Recovery: Wound rotor
- VFD Control: Adjusts effective slip
- Pull-out Torque: At ~20% slip
FAQs
Can slip be negative? Yes — regenerative braking.
Why not 100% efficiency? Slip needed for torque.
How to reduce slip? Use more poles, lower load.
Conclusion
Slip defines induction motor behavior. Our Slip Calculator delivers instant results for design and troubleshooting. Optimize with Synchronous Speed, Torque, and Construction tools. Power ahead with 1000 Calculators.