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# VFD frequency/voltage pattern

A standard VFD starts with reduced voltage and frequency and operates with a constant ratio of voltage to frequency as the frequency increases. This maintains the motor at its full load current rating for a constant torque load. With variable torque loads - fans/pumps - the current is low at low frequency rising to the motor's full load current rating at rated frequency.

The V/F pattern in VFD can be
1. Linear
2. Ratiometric
3. User Defined

Depending upon your application, the V/F pattern can be set
Suppose you set V/F pattern as Linear and Starting voltage is 30, then
At 1 Hz, voltage is 37
At 2 Hz, voltage is 44
At 3 Hz, voltage is 51
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At 10 Hz, voltage is 100
At 20 Hz, voltage is 170
At 30 Hz, voltage is 240
At 40 Hz, voltage is 310
At 50 Hz, voltage is 380

Now, to know about how much current the motor will take.

You need to know that
1. Induction Motors behave as a 3 phase short circuit transformer (Secondary terminals shorted out) if its rotor is jam fully.
2. If its rotor starts rotating at speed of stator magnetic field than motor take 0 Ampere current (Slip = 0). This is practically impossible, as emf need to induced.

Now from the above two points you can easily understand that as VFD starts increasing frequency causes stator magnetic field to rotate while rotor unable to catch the speed of rotating magnetic fields of stator, than current will start to increase.
The current will start to increase till rotor approx catch the speed of stator magnetic field.

If current is flowing above the maximum limit of motor winding, than you need to start motor with further reduced frequency. Usually VFDs available with fractions of frequency that is 0.01Hz. Torque can be increased at constant power if frequency reduces.

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AC Motor Control

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