Formula Used
Synchronous speed: Ns = 120 × f ÷ P
Slip speed: Slip RPM = Ns − Nr
Slip percent: s% = ((Ns − Nr) ÷ Ns) × 100
Slip decimal: s = (Ns − Nr) ÷ Ns
Rotor frequency: fr = s × f
Three phase input power: Pin = √3 × V × I × PF ÷ 1000
Torque estimate: T = Pout × 1000 ÷ ω, where ω = 2πN ÷ 60
Air gap power estimate: Pag = Pout ÷ (1 − s)
Rotor copper loss estimate: Pcu = s × Pag
How to Use This Calculator
Enter the supply frequency and motor pole count first. Add the measured rotor speed from a tachometer, drive display, or nameplate. Add power data when you need torque, input power, loss, and load checks.
Select the output power method that matches your available data. Use rated power with load percent for routine estimates. Use electrical input and efficiency when voltage, current, and power factor are measured. Use measured torque when a torque sensor is available.
Press the calculate button. The result appears below the header and above the form. Download the CSV file for spreadsheets. Use the PDF button for a quick service report.
Understanding Induction Motor Slip
Induction motor slip explains the gap between magnetic field speed and rotor speed. The rotating field moves at synchronous speed. The rotor must run slightly slower. That difference lets conductors cut flux. It also creates rotor current and torque. Without slip, torque would fall toward zero.
Why Slip Matters
Slip is a practical health signal. Low slip often shows light loading or oversizing. High slip can show overload, voltage drop, bad bearings, weak bars, or wrong frequency. A motor nameplate gives rated speed. Comparing that speed with calculated synchronous speed reveals normal full load slip.
Key Inputs
Frequency and pole count define synchronous speed. Rotor speed comes from a tachometer, drive display, or nameplate. Line voltage, current, power factor, and efficiency help estimate input and output power. Load percent helps compare actual use with rated capacity. Optional no load speed supports a simple regulation check.
How Results Help
The slip percent shows how far the rotor trails the field. Slip RPM gives the same gap in speed units. Rotor frequency shows the electrical frequency induced in the rotor circuit. Estimated torque links speed with shaft power. Air gap power and rotor copper loss are useful approximations. They assume steady operation and reasonable efficiency data.
Practical Checks
Measure speed after the motor reaches stable temperature. Confirm the supply frequency before judging slip. Use the correct pole count. Check whether a drive is changing frequency. For variable frequency drives, enter the actual output frequency, not only the utility frequency. Compare results against nameplate values and historical records.
Using Slip for Maintenance
Trend slip over time. A rising slip at the same load deserves attention. It may point to mechanical drag, winding problems, or supply weakness. Sudden slip changes can also appear after coupling work or pulley changes. Use the calculator as a screening tool. Final decisions should include temperature, vibration, insulation, and current balance checks.
Limits of the Method
Slip calculations are simple, but real motors are complex. Harmonics, voltage imbalance, saturation, and load pulsation affect readings. Estimated losses are approximate. Still, slip remains a fast and useful diagnostic measure for electrical teams.
Record ambient heat too. Resistance changes can shift calculated values during long production runs often.
FAQs
What is induction motor slip?
Slip is the difference between synchronous speed and rotor speed. It is usually shown as a percentage of synchronous speed. Slip allows rotor current and torque to develop.
Why is rotor speed lower than synchronous speed?
The rotor must lag behind the rotating magnetic field. This lag lets rotor conductors cut magnetic flux. That action induces current and creates torque.
What is normal slip for an induction motor?
Many standard motors run between one and six percent slip at rated load. Exact values depend on motor design, rating, load, voltage, and efficiency class.
Can slip be negative?
Yes. Negative slip occurs when the rotor runs faster than synchronous speed. This often indicates generating action, such as regenerative operation on a driven load.
How do pole count and frequency affect slip?
Pole count and frequency set synchronous speed. Slip is then found by comparing rotor speed with that synchronous value. Wrong pole data gives wrong slip results.
Does a variable frequency drive change the calculation?
The formula stays the same. Enter the drive output frequency, not the fixed utility frequency. This gives the correct synchronous speed for that operating point.
Why does slip increase under load?
More load needs more torque. The rotor slows slightly to induce more rotor current. That extra current helps the motor produce the required torque.
Are loss estimates exact?
No. Air gap power and rotor copper loss are estimates. They assume steady operation and good input data. Use them for screening, not final certification.