Motor RPM Calculator

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Find an AC motor's speed from its number of poles and supply frequency, including slip.

Inputs

Supply frequency (Hz)

Number of poles

Slip (%)

Motor rating (kW)

How to use this calculator: Motor RPM Calculator

Find an AC motor's speed from its number of poles and supply frequency, including slip. The example below is calculated by this page's real engine from the displayed inputs.

1Confirm that the Motor RPM Calculator matches the quantity or design check you need.
2Enter Supply frequency, Slip, and Motor rating using the units printed beside each field.
3Select the applicable Number of poles options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.

Input guide and example values

Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.

Input	Example value	Why it matters
Supply frequency	50 Hz	Measured or known supply frequency used by the calculation engine.
Number of poles	4 poles (≈1500 RPM)	Select the option that matches the real installation or scenario.
Slip	4 %	Measured or known slip used by the calculation engine.
Motor rating	1.5 kW	Optional input; leave the supplied default only when it matches your case.

Formula inputs & variables for Motor RPM Calculator

These are the named quantities used by this calculator. When the source formula does not define a mathematical symbol, OneCalcApp keeps the real input label instead of inventing one.

Variable / input	Unit	Meaning in this calculation
Supply frequency	Hz	Measured or known supply frequency used by the calculation engine.
Number of poles	—	Select the option that matches the real installation or scenario.
Slip	%	Measured or known slip used by the calculation engine.
Motor rating	kW	Optional input; leave the supplied default only when it matches your case.

How the Motor RPM Calculator works

The Motor RPM Calculator uses Supply frequency, Number of poles, Slip, and Motor rating to calculate Actual motor speed, Synchronous speed, Slip speed, Full-load torque, and Speed in rad/s. Its engine applies Synchronous speed Ns = 120 × f ÷ poles; the worked values below come from that same live calculation rather than a separately typed example.

With Supply frequency 50 Hz, Number of poles 4 poles (≈1500 RPM), Slip 4 %, and Motor rating 1.5 kW, the main worked-example result is Actual motor speed = 1,440 RPM.

How each Motor RPM Calculator input is used

Supply frequency

The Motor RPM Calculator worked example uses Supply frequency = 50 Hz. This value is passed directly into the calculation, with an allowed minimum 1.

Number of poles

The Motor RPM Calculator worked example selects “4 poles (≈1500 RPM)”. Available choices include 2 poles (≈3000 RPM), 4 poles (≈1500 RPM), 6 poles (≈1000 RPM), and 8 poles (≈750 RPM). This selection may change the method or factor used by the engine, so choose the option that matches the real case.

Slip

The Motor RPM Calculator worked example uses Slip = 4 %. This value is passed directly into the calculation, with an allowed minimum 0 and maximum 20.

Motor rating

The Motor RPM Calculator worked example uses Motor rating = 1.5 kW. This value is passed directly into the calculation, with an allowed minimum 0.

Motor RPM Calculator formulas and result interpretation

Formula 1: relationship used

In the Motor RPM Calculator, Synchronous speed Ns = 120 × f ÷ poles. The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 2: relationship used

In the Motor RPM Calculator, Actual speed N = Ns × (1 − slip). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Formula 3: relationship used

In the Motor RPM Calculator, Torque = 9550 × kW ÷ RPM (Nm). The quantities in this relationship come from the named inputs or from an earlier calculation step shown in the worked example.

Actual motor speed

For the displayed Motor RPM Calculator worked example, Actual motor speed is 1,440 RPM. synchronous 1,500 RPM Verify Supply frequency, Number of poles, and Slip and their units before relying on this output.

Synchronous speed

For the displayed Motor RPM Calculator worked example, Synchronous speed is 1,500 RPM. Verify Supply frequency, Number of poles, and Slip and their units before relying on this output.

Slip speed

For the displayed Motor RPM Calculator worked example, Slip speed is 60 RPM. Verify Supply frequency, Number of poles, and Slip and their units before relying on this output.

Full-load torque

For the displayed Motor RPM Calculator worked example, Full-load torque is 9.95 Nm. Verify Supply frequency, Number of poles, and Slip and their units before relying on this output.

Speed in rad/s

For the displayed Motor RPM Calculator worked example, Speed in rad/s is 150.8 rad/s. Verify Supply frequency, Number of poles, and Slip and their units before relying on this output.

Motor RPM Calculator accuracy, checks and limitations

Motor RPM Calculator units check: confirm Supply frequency (Hz), Number of poles, Slip (%), and Motor rating (kW) before calculating.
Motor RPM Calculator result check: compare Actual motor speed, Synchronous speed, Slip speed, Full-load torque, and Speed in rad/s with the substituted formula steps and the displayed rounding precision.
Motor RPM Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

Formula, derivation and worked example

An induction motor's magnetic field rotates at the synchronous speed set by frequency and pole count. The rotor always lags slightly behind — that lag is the slip, typically 3–5% at full load, which is why a 4-pole motor runs at 1,440 RPM rather than 1,500.

Synchronous speed Ns = 120 × f ÷ poles

Actual speed N = Ns × (1 − slip)

Torque = 9550 × kW ÷ RPM (Nm)

Substitution steps

1. Synchronous speed
120 f ÷ poles
= 1,500 RPM
2. With slip
× (1 − 4%)
= 1,440 RPM

Computed example results

Actual motor speed: 1,440 RPM; synchronous 1,500 RPM
Synchronous speed: 1,500 RPM
Slip speed: 60 RPM
Full-load torque: 9.95 Nm
Speed in rad/s: 150.8 rad/s

Understanding the result

Read the main result together with supporting checks, assumptions, limits and intermediate values.

For a manual check, repeat the first equation, confirm the units and change one input at a time.

Assumptions and calculation scope

Supply frequency is assumed to be entered in Hz; converting from another unit before entry avoids changing the numerical meaning of the calculation.
Number of poles is assumed to match the real case. Choosing a different available option can change the method, factor or interpretation used by this calculator.
Motor RPM Calculator applies the displayed Synchronous speed Ns = 120 × f ÷ poles relationship to the entered engineering inputs. Factors that are not exposed as inputs or stated assumptions are outside this calculator's calculation scope.

Common mistakes when using Motor RPM Calculator

Do not mix units for Supply frequency (Hz), Slip (%), Motor rating (kW). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave Number of poles on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed Synchronous speed Ns = 120 × f ÷ poles relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Actual motor speed = 1,440 RPM from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.

When the Motor RPM Calculator is useful

Motor RPM Calculator is designed for cases where Supply frequency, Number of poles, Slip, Motor rating are known and you need Actual motor speed, Synchronous speed, Slip speed. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.

Use the calculator for the scope described by its inputs and notes. The displayed method is Synchronous speed Ns = 120 × f ÷ poles. If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.

Supply frequency and Number of poles: what changes the answer

The worked example uses Supply frequency = 50 Hz, Number of poles = 4 poles (≈1500 RPM), Slip = 4 %, Motor rating = 1.5 kW. With those values, Actual motor speed is 1,440 RPM. Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.

For this calculator, the main input roles are: Supply frequency (Hz): Measured or known supply frequency used by the calculation engine. Number of poles: Select the option that matches the real installation or scenario. Available choices include 2 poles (≈3000 RPM), 4 poles (≈1500 RPM), 6 poles (≈1000 RPM), 8 poles (≈750 RPM). Slip (%): Measured or known slip used by the calculation engine. Motor rating (kW): Optional input; leave the supplied default only when it matches your case.

How to sanity-check a Motor RPM Calculator result

Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Actual motor speed, because it is the first worked-example output shown by the live engine.

Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.

Next logical calculator

Continue with Pulley Size & RPM Calculator

Pulley Size & RPM Calculator is directly connected from Motor RPM Calculator as a source-defined continuation or comparison.

Open Pulley Size & RPM Calculator

Formula

Synchronous speed Ns = 120 × f ÷ poles
Actual speed N = Ns × (1 − slip)
Torque = 9550 × kW ÷ RPM (Nm)

Worked example

14 poles on 50 Hz → Ns = 120 × 50 ÷ 4 = 1,500 RPM.
2With 4% slip the shaft runs at 1,440 RPM.
3A 1.5 kW motor at 1,440 RPM produces 9.95 Nm of torque.

Tips

A VFD changes the frequency, so the speed changes in direct proportion.
Slip rises with load — a motor running very slowly is usually overloaded.

Frequently asked questions

Why is my 1500 RPM motor showing 1440?

1,500 is the synchronous speed; 1,440 is the loaded speed after about 4% slip.

How do I find the poles?

Divide 120 × frequency by the nameplate RPM and round to the nearest even number.

Can I run a 50 Hz motor on 60 Hz?

It will run 20% faster with reduced torque, and the fan load rises sharply. Check with the manufacturer.

What inputs does the Motor RPM Calculator use?

It uses Supply frequency, Number of poles, Slip, and Motor rating. Follow the unit printed for each field and choose any selectable option to match the real scenario.

What does the Motor RPM Calculator calculate?

It calculates Actual motor speed, Synchronous speed, Slip speed, Full-load torque, and Speed in rad/s. With the displayed default inputs, Actual motor speed is 1,440 RPM.

Which formula does the Motor RPM Calculator use?

The primary relationship is Synchronous speed Ns = 120 × f ÷ poles. The page also shows substituted values and calculation steps so the result can be checked independently.

How can I verify a Motor RPM Calculator result?

First verify the units for Supply frequency, Number of poles, and Slip. Then compare the substituted formula steps with Actual motor speed and its displayed precision.

What are the limitations of the Motor RPM Calculator?

Motor RPM Calculator: Use this result for preliminary design and cross-checking. Confirm the applicable code edition, manufacturer data, site conditions and qualified-engineer approval before final design or installation.

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