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Vfd Driven Motor Problem

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#1 AB2005


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Posted 24 October 2006 - 04:00 AM

Dear All,

I am confused about motor heat up problem when it powers through VFD. Why it heats up? As we set the frequency 50HZ same the input frequency of VFD.
"Don't assume any thing, always check/ask and clear yourself".

#2 mariomaggi


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Posted 24 October 2006 - 05:38 AM

Dear AB2005,
the current coming from inverter to the motor is not perfectly sinus-shaped. Usually there is a certain distortion, specially at high frequency. These harmonics or interharmonics generates heat.

Another possibility: the output voltage of this inverter is not exactly the same voltage in the main line, this means that the motor under load could have an higher slip and will generate more heat.


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#3 jOmega


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Posted 27 October 2006 - 05:27 AM

Harmonic content in output waveform of most VFDs today is minimal. Tests using a Spectrum analyzer and/or Harmonic analyzer confirm this to be so.

So if the heat isn't due to harmonics, then one must look elsewhere.

With the VFD operating at 50 Hz output, check the RPM of the motor. If it is less than the RPM on the nameplate, one of two things is most likely to be true.

1. you have too much mechanical load causing the motor to slip more than rated and draw more current...


2. the VFd is not sourcing sufficient RMS Fundamental voltage to the motor—motor is operating in Voltage Starved mode— and has to slip more to support the mechanical load.

NB! The voltage on the display of the VFD is a calculated value; not a measured value. So it lies!

Don't bother trying to find an instrument to read the FUNDAMENTAL RMS voltage; the slip and output amps will tell you the truth.

However, if you wanted to measure the RMS fundamental output voltage, a FLUKE 43A will certainly do the job. Use it to do a harmonic plot of the VFD output voltage with the VFD operating at 50 Hz output.

The 1st harmonic is the FUNDAMENTAL.

Many VFDs do not produce rated RMS Fundamental voltage at rated frequency output.

Their displays and the display of many digital meters and analog meters capable of displaying the VFD output voltage ...... display TOTAL voltage. Total voltage is the composite of the RMS Fundamental and the Harmonic voltages.

Only the RMS FUNDAMENTAL voltage produces useful output torque from the motor. The harmonics produce heat .... and some of them actually produce a negative torque.

Some of the better quality VFDs inject some 3rd harmonic into the output waveform and / or increase the modulation index slightly inorder to get the RMS Fundamental voltage to be close to or equal to the mains voltage applied to the input of the VFD.

FYI..... before purchasing a VFD, ask the manufacturer what the output RMS Fundamental Voltage is when operating at rated frequency output. Ask him if he will guarantee the value. If you hear tap dance music as he answers your question, take a pass on his product.

#4 AB2005


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Posted 02 November 2006 - 02:16 AM

Dear jOmega,

Thanks for nice answering.
"Don't assume any thing, always check/ask and clear yourself".

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