I am implementing a Variable frequency Drive control of single phase induction motor. I am little bit confused about the V/f ratio and voltage level. i boosted voltage upto 380V for Inverter . if i operate the motor at 220 volt with 50 HZ. so the 220/50 = 4.4 . am i right . otherwise i have to use (1.414*220/50)=6.2

how can i calculate the modulation index. please help me i am confused about the calculation of V/f constant Control techniques .

  • \$\begingroup\$ Please capitalise your posts properly for legibility. i.e., First letter of each sentence, "I" and correct units (Hz) and remove the random capitalisation you did add. It will give a better impression of the author and may gain you better answers. \$\endgroup\$
    – Transistor
    Aug 18 '16 at 17:54

I am little bit confused about the V/f ratio and voltage level

Any power magnetic component running from AC will magnetically saturate the core if the volt.seconds starts to be exceeded i.e. too many volts of the same polarity for too long and you get trouble.

Volts per hertz is exactly the same measure because hertz are cycles per second hence V/f are volt.seconds per cycle.

So if in one cycle of AC the volts.seconds gets too big you get magnetic saturation problems.

For variable frequency drives and induction motors, as speed is ramped down by lowering frequency so volts must also fall to prevent saturation.

For any particular motor it's volt.seconds will be determined by the motor manufacturer or specifier so I can't help you in that respect other than to say that if a motor is OK for 220V AC RMS at 60 Hz then at 30 HZ (half speed) you should have a pretty good reason for running at a voltage level higher than 110 V AC RMS.

  • \$\begingroup\$ Andy - I suspect that the phrase "you should have a pretty good reason for running at a voltage level higher than 110 V AC RMS." MAY be less obvious in meaning to some than may be obvious. eg it COULD be construed to mean that it is likely to be OK to run at > 110 VAC RMS. As OP is in Pakistan his Nglish comprehension may be anything from supeb to marginal. \$\endgroup\$
    – Russell McMahon
    Aug 18 '16 at 17:58
  • \$\begingroup\$ Ok who's the downvoting but silent moron? \$\endgroup\$
    – Andy aka
    Aug 18 '16 at 22:42
  • \$\begingroup\$ THank you ..@ can you suggest me the book or article where i can understand about V/F constant ratio calculation \$\endgroup\$
    – Misal313
    Aug 18 '16 at 23:11
  • \$\begingroup\$ I certainly didn't down vote and I will counteract it. \$\endgroup\$ Aug 19 '16 at 2:12
  • \$\begingroup\$ @Andy Not me. Few and far between indeed are my downvotes. I'll see Charles upvote and raise him an upvote. BUT, it would be a good idea to change the wording (to clarify YOUR meaning) just slightly, as noted above. eg " ... then at 30 Hz it would usually be unwise to use more than 110 VAC RMS unless you had a very good reason to do so, and knew what you were doing." \$\endgroup\$
    – Russell McMahon
    Aug 19 '16 at 2:36

The RMS value of the fundamental should be 220 volts at 50 Hz and 4.4 V/Hz for operation below 50 Hz. With V/f control of 3-phase motors, the V/f is "boosted" above the constant value at frequencies below 10 Hz or so to prevent reduced torque capability at low frequencies. The boost is needed because the voltage drop in the winding becomes more significant as the frequency is reduced. With a single-phase motor, there is not much that can be done about reduced torque capability at low frequencies because it is mostly due to reduced current and/or reduced phase shift in the auxiliary winding current. The motor will only be suitable to drive a fan or centrifugal pump. It will not have enough torque to drive a load that requires very much torque to start or operate at a low speed.

The motor needs to be a shaded-pole or permanent-split-capacitor (PSC) type motor. A capacitor-start motor will not tolerate continuous current in the auxiliary winding circuit.

The PWM scheme should be designed to minimize the lower order harmonics. The modulation needs to provide constant V/Hz of the fundamental component. Perhaps another answer will provide more detail on that or you can find detailed information at a more tutorial type of web site.

i boosted voltage upto 380V for Inverter.

If that is the DC input voltage to the inverter section, that seems adequate.

otherwise i have to use (1.414*220/50)=6.2

No. The V/Hz is based on RMS voltage, not peak.


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