I want to implement constant V/f in variable speed Inverter of a 3-phase Induction motor, 1500 rpm, 4-poles. I also know the equation:

V/f = 4.44 × N x Phi Phi is maximum value of flux, f is frequency and N is number of turns.

How can I calculate or measure the constant value of V/f for the above motor using above equation? What will be the value for N and Phi?

Or is there any other way to find out the constant value of V/f of an ACIM?


N is the number of turns in a single coil of the motor windings. The maximum value of flux is the flux that the iron core for the coil can carry without being saturated. N and Phi are selected by the motor designer.

Once the motor has been designed, everything that is of interest to the user of the motor is marked on a rating plate that is permanently attached to the motor. If the motor has been designed for a manufacturer to incorporate in another product, the designer provides documentation that is identified by the motor's identification number and only that ID number is marked on the motor. The design information may be published in catalogs, but it is often provided only to the original purchaser.

If the rating plate is available, the V/f is calculated by dividing the motor's rated operating voltage by its rated operating frequency. If the rating plate is not available, the rated voltage and frequency are often the same as the rated voltage and frequency of the product in which it was used. Sometimes it may be necessary to examine the product to determine if the voltage and frequency applied to the motor were internally derived inside the product.

Note that a constant value of V/f is strictly applicable only at the rated voltage and frequency. It is necessary to determine by observing the motor performance how much V/f adjustment may be necessary as the operating frequency is reduced and increased. Usually constant V/f is sufficient for frequency reductions to 10% of rated frequency or so. If the load torque declines as speed declines as with fan or centrifugal pump, constant V/f is sufficient for any speed reduction.

  • \$\begingroup\$ For a 3-phase motor which voltage value we use for the calculation of V/f ratio.. line voltage or phase voltage? And for the 'f' value what i understand is that its in Hz of the single phase frequency.. for example the frequency of mains single phase is 50 Hz but if we see 3-phase frequency the i think it's 150 Hz (i am not sure). So will i use frequency of single phase or frequency of 3-phase combined? \$\endgroup\$ – alt-rose Oct 12 '19 at 12:35
  • \$\begingroup\$ Three-phase motor voltage ratings are always the line-to-line voltage. The same is true of any load or device that is connected to three-phase power. In three-phase four-wire distribution systems the line and phase voltages are often both given such as 400/230 (line/phase). A motor voltage specified as 400/230 would mean that the motor can be connected in star for 400 V or delta for 230 V. A 690/400 motor could be connected to a 400 V source in star for starting and delta for running to reduce the starting current. Three phase does not multiply frequency 50 Hz supplies are 50 Hz 1 Ph or 3 ph. \$\endgroup\$ – Charles Cowie Oct 12 '19 at 13:43

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