motor harmonic current

[2dye4]

I have a motor to evaluate that draws significant
third harmonic current. It is a low cost universal motor
that is placed in a appliance. Does harmonic current
make the windings heat as fast as regular current?
I don't think so because the frequency is different.
Am I right or left

2dye4

 

[jbartos]

Comment: The harmonic current creates heat losses in the iron magnetic circuit due to Eddy Currents heat (Watt) losses and Steinmetz Law of remagnetization (Watt) losses. Also, the harmonic current causes higher leakages through the dielectric or the distributed capacitance since
-jXc=-j/(2*PI*f*C) in Ohms is decreasing with the increasing frequency or higher harmonic current. The higher leakages increase the RI^2 Watt losses.

 

[advidana]

wrong it is heating up

 

[ScottyUK]

2dye4,

Why does your motor draw significant third harmonic? Are you measuring the harmonic spectrum with little or no load on the motor? If you get those results with the motor under load, the iron of the core may be starting to saturate - not a particularly good condition to be in, and one that will certainly help warm things up. Is it a cheap and nasty motor, or from a reputable supplier?





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Get it over with.

 

[2dye4]

scottyuk

It is a cheap mass produced motor from a reputable
supplier. For cost sensitive application

 

[ScottyUK]

Cheap motors are often cheap because there is almost enough iron in the core, and the copper of the windings is almost thick enough. I suspect this is the source of your harmonic problem.

In answer to your original question, yes, harmonics will add to your problems, but you will also have a high magnetising current heating the winding, and your iron losses will be higher than ideal. All the problems relate to the motor design.





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Start each new day with a smile.

Get it over with.

 

[aolalde]

The rms current of any harmonic will produce heat in the windings ( I^2)*R
The harmonics having high frequency will increase eddy current losses and hysteresis loop losses on the magnetic circuit.
The torque developed by harmonics could break the motor rotor.

Normally the total rms content of harmonics is small as compared to the base frequency.

 

[jbartos]

Suggestion: Sometimes, motors come incorrectly designed, e.g. coil-span or chording factor should take care of the third harmonics that is supposed to disappear, when the winding coil is properly pitched.
Reference:
M.G. Say "Alternating Current Machines," John Wiley & Sons, 1978,
Page 97 Coil-span and chording factor.

 

[CJFlatters]

I am not a motor designer / expert, but can appreciate your comments regarding the potential for harmonic currents in cheapo mass produced motors.

I must correct the response given by 'aolalde' above states that the currents produced will produce torques which will break the motor (I have no experience of harmonic torques breaking a motor - does anyone else) ?

A zero phase sequence (third harmonic) current cannot produce a torque - It will only cause I2R losses in the windings and eddy current losses in the frame.

Positive sequence harmonics will produce accelerating torques, negative sequence harmonics will produce deccelerating torques (contra-rotating), all these will place stresses on the electrical and mechanical performance of the machine.



Obviously any triplen harmonic will produce I2R (heat) losses in the windings

_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.

 

[jbartos]

Comment on the previous posting: Harmonics can cause motor vibrations that can in turn damage the motor.

 

[CJFlatters]

If there are other harmonics present (eg positive or negative phase sequence components), then harmonic torques will be produced which, I fully accept will produce pulsating torques resulting in mechanical stresses and almost certainly , a degree of vibration.

However, the only reference made to harmonics in the original query refers to third harmonics which will only have I^2R heating losses as they are ZERO sequence.


_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.

 

[2dye4]

Just a question.

How can current harmonics that result from B/H material
non linearities cause motor torque pulsations.

My understanding is that the magnetic field developed is
forced to be sinusoidal as a function of line voltage.

The harmonic current does not change the magnetic flux
density B .

Please comment
thanks

 

[aolalde]

Thanks for your correction. I meant “torque pulsations or torque ripple”.
Induction motors respond to harmonic voltages by producing harmonic fluxes and consequently harmonic torques. Thus losses are increased and heating is the result.
Hopefully motor designers care reducing harmonics, so the negative effects are reduced to acceptable limits. Please visit:

http://www.wempec.wisc.edu/reports/2002/2002_12.PDF

 

[charlierod]

How can motor draw significantly amounts of third harmonic current?, is it grounded?

 

[electricpete]

I agree with Charlie, in theory an ungrounded wye motor is not capable of drawing 3rd harmonic current assuming all three phase currents are "symmetrical".

If the phase currents are displaced by a time interval equal to 120 degrees on a fundamental (60hz basis), that time interval is 240 degrees on 2nd harmonic (120hz) basis and 360 degrees on 3rd harmonic (180 hz) basis. The 3rd harmonics components are therefore in-phase with each other. They cannot sum to zero at the neutral unless the 3rd harmonic of each phase is zero.

There may be assymetries which allow 3rd harmonics to flow, but I believe in general it is 5th, 7th, 11, 13th etc that are the most prominent harmonics in motor current.

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[jbartos]

Comment: The motor electrical equivalent circuit magnetization branch is nonlinear mainly due to the nonlinear magnetization reactance that depends on B-H curve. The form of the B-H curve happens to be such that it enables the generation of odd harmonics. The Fourier Series Theory classifies it "odd nonlinear function."
The 5th,7th, etc. harmonics have impact on the motor Torque Speed characteristics.

 

[jbartos]

Suggestion: Visit

http://ocw.mit.edu/NR/rdonlyres/Ele...8C-7FF2-4CB9-86A9-4FB31FBA2C6E/0/chapter8.pdf

for: Belt Harmonics, 5th and 7th

http://www.microchip.com/download/appnote/devspec/17cxx/motion/00887a.pdf

for: harmonics associated with motors

http://www.engnetbase.com/ejournals/books/book_summary/toc.asp?id=519

for: Induction Machine Handbook
etc. for more info

 

[ScottyUK]

E-Pete,

The OP states that the motor is a universal type, which suggests single phase to me. Triplen cancellation would not occur on a single phase circuit. Is there such a thing as a three-phase universal motor?





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Start each new day with a smile.

Get it over with.

 

[aolalde]

EPete;

The original post states: “I have a motor to evaluate that draws significant third harmonic current. It is a low cost universal motor that is placed in an appliance."

Universal Motors are connected to a single phase supply.

My opinion is that magnetic circuit saturation and constant radial air gap make the magnetic flux distribution became close to a rectangular wave instead of a sinus wave.

 

[electricpete]

I agree my comments apply to 3-phase and not relevant to the original post.

There was mention by CJFlatters of heating from zero sequence (implying 3-phase) 3rd harmonic current and I believe that is not normally the case.

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