What the Motor Slip Calculator does
This calculator works out an induction motor's synchronous speed from the pole count and the supply frequency, then the slip in rpm and per cent from the speed you measured, along with the rotor frequency and what the numbers suggest about how hard the motor is working.
Slip is the whole reason an induction motor produces torque. The rotating field has to move faster than the rotor for current to be induced in it, and that relative movement - the slip - is what generates torque. A motor at exactly synchronous speed would produce none at all, which is why a healthy loaded induction motor always runs a few per cent slow.
How to use it
- Pick the number of poles. Poles always come in pairs, so the count is even, and the nameplate speed gives it away if it is not stated: near 2,900 rpm on 50 Hz is 2-pole, near 1,440 rpm is 4-pole, near 960 rpm is 6-pole.
- Enter the supply frequency. Use the actual frequency if the motor is fed from a drive or a generator, because the synchronous speed follows it exactly.
- Enter the shaft speed you measured with a tachometer or a strobe. Measure at the shaft, not at the driven machine, unless you have allowed for the transmission ratio.
- Add the nameplate rated speed if you have it, and the slip becomes a rough indication of load - at a steady supply voltage, slip is close to proportional to torque over the normal running range.
- Compare the reading against the synchronous speed table if you are unsure of the pole count.
Reading the results
Slip in rpm is the difference between synchronous and actual speed. Slip as a percentage is that difference divided by the synchronous speed.
Typical full-load slip runs from under 1% on a large machine to 5% or more on a small one - small motors have proportionally higher rotor resistance and therefore more slip.
Rotor frequency is the slip multiplied by the supply frequency. At 4% slip on a 50 Hz supply, the rotor circuit is carrying 2 Hz - which is what a clamp meter on the rotor circuit of a wound-rotor machine reads, and why rotor-circuit resistance has such a strong effect on the torque-speed curve.
A measured speed above synchronous means the machine is being driven by its load and is generating rather than motoring. That is what happens to a motor on a descending hoist or an over-running conveyor.
Worked example: a four-pole motor on a 50 Hz supply measured at 1,440 rpm
Synchronous speed is 120 x 50 / 4 = 1,500 rpm.
Slip is 1,500 - 1,440 = 60 rpm, which as a fraction is 60 / 1,500 = 0.04, or 4%.
Rotor frequency is 0.04 x 50 = 2 Hz. That is the frequency of the current circulating in the rotor bars - a very different thing from the 50 Hz in the stator, and the reason rotor-bar faults produce sidebands 2 Hz either side of the supply frequency in a current spectrum.
If the nameplate says 1,450 rpm, the rated slip is 50 / 1,500 = 3.33%. Our measured 4% is 1.2 times the rated slip, which suggests the motor is running at roughly 120% of its rated torque - overloaded, and worth investigating.
The caveat matters, though. Slip at a given torque rises when the supply voltage falls, roughly with the inverse square of the voltage. A motor on a 10% low supply produces the same 4% slip at only about 99% of rated torque. So a high slip reading is a prompt to check the voltage before concluding the motor is overloaded.
Formulas and scoring rules
- Synchronous speed
Ns = 120 x f / pf in hertz, p the number of poles (not pole pairs). 120 = 60 seconds x 2 poles per pair.- Slip in rpm
Ns - N- Slip as a fraction
s = (Ns - N) / NsMultiply by 100 for a percentage.- Actual speed from slip
N = Ns x (1 - s)- Rotor frequency
f_rotor = s x fAt standstill the slip is 1 and the rotor sees the full supply frequency.- Load estimate
load ratio is approximately measured slip / rated slipValid only at the rated voltage and over the normal running range.- Pole count from a nameplate speed
p = round(120 x f / rated rpm)Rounded to the nearest even number.
Why an induction motor must slip
The stator's rotating field sweeps past the rotor conductors and induces a voltage in them, which drives current, which interacts with the field to produce torque. If the rotor ever caught up with the field there would be no relative motion, no induced voltage, no current and no torque - so the motor would immediately slow down until slip reappeared.
That self-regulating behaviour is the elegance of the machine. Add load and the rotor slows slightly, slip increases, rotor current increases, torque increases to match the load, and a new equilibrium is reached a fraction of a per cent lower. No control system is involved at all.
Using slip as a measurement
Slip is a genuinely useful diagnostic because it needs only a tachometer, and because it responds to the mechanical load rather than to the electrical supply in the way that current does. A clamp meter on a lightly loaded motor tells you rather little, because the magnetising current dominates; slip at 20% load is clearly different from slip at 80%.
The technique has two conditions. The supply voltage must be at or near nominal, because slip at a given torque rises as the square of the voltage falls. And the motor should be warm, because rotor resistance rises with temperature and a cold motor slips less at the same torque. Both conditions are easy to meet and easy to forget.
Limitations: what the result does not prove
- It applies to induction motors. Synchronous motors run at exactly synchronous speed with zero slip by definition, and their behaviour under load is a load angle rather than a speed change.
- The load estimate from slip assumes the rated voltage and a warm motor. A low supply voltage raises slip at the same torque and would make the motor look more heavily loaded than it is.
- It assumes a balanced supply. Voltage imbalance produces a negative-sequence field that adds heating and a braking torque, which distorts the relationship.
- On a variable-speed drive the supply frequency is whatever the drive is producing at that moment, not the mains frequency, and the synchronous speed follows it.
- Nothing here is a compliance certificate, and a slip reading is not a substitute for a proper condition assessment by a qualified person.
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Standards and sources
- IEC 60034-1 - Rotating electrical machines: rating and performance - checked 19 Sep 2026
- IEC 60034-12 - Starting performance of cage induction motors
- IEC 60034-26 - Effects of unbalanced voltages on the performance of cage induction motors
Frequently asked questions
What is the synchronous speed of a 4-pole motor?
1,500 rpm on a 50 Hz supply and 1,800 rpm on 60 Hz, from Ns = 120 x f / p. The actual running speed is a few per cent below that - typically 1,440 to 1,470 rpm on 50 Hz - and the difference is the slip.
What is a normal slip percentage?
Between about 1% and 5% at full load, with larger motors at the lower end and small ones at the upper end. Above about 6% at nominal voltage suggests an overload or a supply problem; a high-slip design motor, used where a soft torque characteristic is wanted, can be 8 to 13% by design.
How do I find the number of poles?
From the nameplate speed: divide 120 times the frequency by the rated rpm and round to the nearest even number. A 50 Hz motor rated at 1,450 rpm gives 120 x 50 / 1,450 = 4.14, so 4 poles. The synchronous speed table on this page lists the common combinations.
Can I measure motor load from slip?
Roughly, yes - slip is close to proportional to torque over the normal running range, so measured slip divided by rated slip approximates the fraction of rated torque. It only holds at the rated voltage and with a warm motor, and it is much better than a clamp meter for lightly loaded machines where the magnetising current dominates.
What does it mean if the speed is above synchronous?
The machine is being driven by its load rather than driving it, so it is generating and feeding power back into the supply. That happens on a descending hoist, an over-running conveyor and a wind turbine using an induction generator. It is perfectly normal in those applications and a symptom of something wrong in most others.
What is rotor frequency and why does it matter?
It is the slip multiplied by the supply frequency - 2 Hz at 4% slip on a 50 Hz supply. It matters because the rotor's impedance depends on it: at standstill the slip is 1, the rotor sees the full 50 Hz, its reactance is high and the starting torque is limited. As the motor speeds up the rotor frequency falls and the torque characteristic changes accordingly.
Does slip change with supply voltage?
Yes, strongly. Torque follows the square of the voltage, so at a given load a 10% voltage drop needs roughly 23% more slip to produce the same torque. That is why a slip measurement should always be paired with a voltage reading before drawing any conclusion about the load.
Do variable-speed drives change the slip?
The slip in rpm stays roughly constant for a given torque, but as a percentage it grows as the frequency falls, because the synchronous speed is lower. At 10 Hz a 4-pole motor's synchronous speed is only 300 rpm, so the same 60 rpm of slip is 20% rather than 4% - which is one reason motors need derating or forced cooling at low speed on a drive.
Last reviewed by the A2Z.Tools team against the sources listed above.
