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AmpSize

Motor calculator

Find motor full load current, starting current and torque.

Try a job 6

Each job fills in example values. Replace them with your values.

Inputs

Worked example with the default inputs. The live calculator replaces it when the page loads.

Shaft power
15 kW
Phases
Three phase
Voltage
400 V
Power factor
0.85
Efficiency
0.9
Poles
4
Frequency
50 Hz
Slip
3 %

Full load current

28.3A

Starting current, DOL
169.81A
Synchronous speed
1,500rpm
Shaft speed
1,455rpm
Full load torque
98.45Nm

This result is a design aid. Verify it against the current standard.

Calculation steps

  1. Full load currentResult28.3A
  2. Synchronous speed1,500rpm
  3. Shaft speed1,455rpm
  4. Full load torque98.45Nm
  5. Starting current169.81A

About this calculator

How this is calculated

The tool finds the running and starting current of a motor, with its speed and torque. It works from the shaft rating, the voltage, the power factor, the efficiency and the number of poles. It reads no table, so it cites no clause.

  1. Full load current. The tool divides the shaft power by the efficiency. That gives the input power. It divides the input power by the power factor. It then divides by the voltage, and on three phases by the square root of three.
  2. Synchronous speed. It multiplies the supply frequency by 120 and divides by the number of poles.
  3. Shaft speed. It reduces the synchronous speed by the slip you enter, as a percentage.
  4. Full load torque. It divides the shaft power by the shaft speed, with the usual constant for rpm.
  5. Starting current. It multiplies the full load current by 6. This assumes a direct-on-line start.

Worked example

  • 15 kW four-pole motor at 400 V. The power factor is 0.85, the efficiency is 0.9 and the slip is 3 %. The full load current is 28.3 A. The synchronous speed is 1,500 rpm. The shaft speed is 1,455 rpm. The full load torque is 98.45 N·m. The direct-on-line starting current is 169.81 A.

Limits

The factor of 6 is a general assumption. A real motor can draw more or less. Read the nameplate or the data from the maker when you have it. A soft starter or a drive gives a lower starting current, and the tool does not model them.

The tool does not size the cable, the breaker or the overload relay. It does not check the voltage drop at start. The slip is an input. It is not calculated from the load.

See also: Motor full load current, 11 kW at 400 V, Motor starting current, DOL, 55 kW and Motor circuits and starting.

Questions

Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.

What does the motor calculator do?

The tool reports the full load current, the synchronous speed, the shaft speed, the full load torque and the starting current of an induction motor. It works from the shaft rating in kilowatts, the voltage, the phase count, the pole count and the slip.

Which inputs matter most?

The shaft rating and the voltage set the current. Power factor and efficiency both raise the current for the same shaft rating. The pole count and the supply frequency set the synchronous speed. The slip percent sets how far the shaft speed falls below it.

How is the starting current worked out?

The tool assumes a direct on line start at six times the motor's full load current. It says so in a note with every result. A soft starter, a star delta starter or a variable speed drive gives a different starting current. Use the nameplate figure when you have one.

Can I use the result to size a cable?

Use the full load current as the design current in the cable size tool. Motor starting also affects the voltage drop and the protective device choice, and this tool does not check those. The inputs sit in the page address, so a link reproduces the result.

Worked examples

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