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motor harmonic current

motor harmonic current

motor harmonic current

(OP)
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

RE: motor harmonic current

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.

RE: motor harmonic current

wrong it is heating up

RE: motor harmonic current

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?



-----------------------------------

Start each new day with a smile.

Get it over with.

RE: motor harmonic current

(OP)
scottyuk

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

RE: motor harmonic current

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.



-----------------------------------

Start each new day with a smile.

Get it over with.

RE: motor harmonic current

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.

RE: motor harmonic current

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.

RE: motor harmonic current

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.

RE: motor harmonic current

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

RE: motor harmonic current

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.

RE: motor harmonic current

(OP)
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

RE: motor harmonic current

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

RE: motor harmonic current

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

RE: motor harmonic current

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

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.

RE: motor harmonic current

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?



-----------------------------------

Start each new day with a smile.

Get it over with.

RE: motor harmonic current

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.

RE: motor harmonic current

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.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

Electripete

Whilst both of my previous submissions refer to third harmonics (which are zero sequence) there is no reference or implication of three phase.  As the original entry referred to a "cheap and nasty" motor I had rather assumed it to be single phase.

The 3H (or any other triplen harmonic for that matter) will have an I2R heating effect on the winding but will not produce any torque effects as it is zero sequence.

The phase sequence of harmonics basically follow on as follows.

f 2 3 4 5 6 7 8 9 10 11 12
+ - 0 + - 0 + - 0  +  -  0

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

RE: motor harmonic current

Quote (CJFlatters):


<Apr 4, 2004>...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),

I think it is clear the use of the use of the terms "positive sequence", "negative sequence" and "zero sequence" implies three phase. Do you disagree?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

You wrote:
f 2 3 4 5 6 7 8 9 10 11 12
+ - 0 + - 0 + - 0  +  -  0

This is for 3-phase system.
Let the fundamental be I1(t) = cos(w*t + k*delta) where
delta = (2/3)*Pi/w   (120 degrees of fundamental)
k = 0, 1, 2 for phase A, B, C (assuming A,B,C rotation)

The hth harmonic has frequency h*w but still has the same time shift k*delta independent of h.   That time shift corresponds to a bigger phase shift for higher harmonics.  ie a time interval of 120 degrees fundamental corresponds to 240 degrees 2nd harmonic, 360 degrees 3rd harmonic, etc.

Ih(t) = cos(h*w*t + k*delta)
Ih(t) = cos(h*w*t + k*(2/3)*Pi/w)
Ih(t) = cos(h*w*t + h*k*(2/3)*Pi/(w*h))

Phase relationship between the three fundamental currents is  (0,120, 240) (positive sequence)

Phase relationship between the three 2nd harmonic  currents is  (0,240, 480) = (0,-120,-240)  (negative sequence)

Phase relationship between the three 3nd harmonic  currents is  (0,360, 720) = (0,0,0)  (zero sequence)

Phase relationship between the three 4th harmonic currents is  (0,480, 960) = (0,120,240)  (positive sequence)

Phase relationship between the three 5th harmonic currents is  (0,600, 1200) = (0,-120,-240)=  (negative sequence)

I fail to see how we could ever hope to derive any comparable relationship for single phase.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

I made an error in the symbols my proof. Please let me try it again.

Let the fundamental be
I1(t) = cos(w*(t+k*delta)) = cos(w*t+w*k*delta)
where
delta = (2/3)*Pi/w
delta represents a time shift equivalent to 120 degrees of fundamental
k = 0, 1, 2 for phase A, B, C (assuming A,B,C rotation)

The hth harmonic has frequency h*w but still has the same time shift k*delta independent of h.   That time shift corresponds to a bigger phase shift for higher harmonics.  ie a time interval of 120 degrees fundamental corresponds to 240 degrees 2nd harmonic, 360 degrees 3rd harmonic, etc.

Ih(t) = cos(h*w*(t+k*delta))
Ih(t) = cos(h*w*(t + k*(2/3)*Pi/w))
Ih(t) = cos(h*w*t + h*w*k*(2/3)*Pi/(w*h))

Ih(t) has a phase shift of h*k*(2/3)*Pi i.e. h * 120 degrees separation between hth haromic of phases A, B, C.

Phase relationship between the three fundamental currents is  (0,120, 240) (positive sequence)

Phase relationship between the three 2nd harmonic  currents is  (0,240, 480) = (0,-120,-240)  (negative sequence)

Phase relationship between the three 3nd harmonic  currents is  (0,360, 720) = (0,0,0)  (zero sequence)

Phase relationship between the three 4th harmonic currents is  (0,480, 960) = (0,120,240)  (positive sequence)

Phase relationship between the three 5th harmonic currents is  (0,600, 1200) = (0,-120,-240)=  (negative sequence)

Sorry - I didn't mean to belabor the point.  Just wanted to correct my slipup.  btw welcome to the forum cjflatters. I enjoy your posts. Look forward to hearing more.

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Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

Oops - thanks electricpete

Its a fair cop - so used to thinking three phase heavy power that i missed the obvious (the preceding comments would of course apply to 3 phase machines).

_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

RE: motor harmonic current

I wrote:
"Ih(t) = cos(h*w*(t+k*delta))
Ih(t) = cos(h*w*(t + k*(2/3)*Pi/w))
Ih(t) = cos(h*w*t + h*w*k*(2/3)*Pi/(w*h))"

The last equation is incorrect: It should not have any "h" in the denominator as was pointed out by jbartos.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

RE: motor harmonic current

Suggestion: The above posted equations might have shown current magnitudes and the k index to be correct and practically applicable, namely:
Ih(t) = cos(h*(w*t + k*(2/3)*Pi))
Ih(t) = cos(h*w*t + h*k*(2/3)*Pi)"
might be:
Ihk(t) = Ihk,max x cos(h*(w*t + k*(2/3)*Pi))
Ihk(t) = Ihk,max x cos(h*w*t + h*k*(2/3)*Pi)

RE: motor harmonic current

Don't most universal motors have brushes?  I'd think you'd get noise on every harmonic, including the third.  They sure make enough audible noise, and they sure screw up radio transmission, they seem to be rather "broadband" transmitters.

2dye4 -- back to your original question -- yes, harmonic currents will cause the windings to heat above and beyond what you'd get from the fundemental current alone.  As mentioned above, the heating will be based on I-squared R losses, where I is the rms current of the fundemental plus all harmonics.

RE: motor harmonic current

Oh yeah -- regarding zero sequence --

Single-phase systems have no directionality.  They're more like "up-down" rather than "clockwise vs. counterclockwise" (which leads to things like shaded-pole motors, etc).  So "sequence" is meaningless.

That said, this motor could certainly be fed from a 3-phase supply.  And if it was, its 3rd-harmonic currents would contribute to the zero-sequence currents on that system.

So, you're both right!  It's win-win.

RE: motor harmonic current

Suggestion to peebee (Electrical) May 14, 2004 marked ///\\\
Oh yeah -- regarding zero sequence --

That said, this motor could certainly be fed from a 3-phase supply.  And if it was, its 3rd-harmonic currents would contribute to the zero-sequence currents on that system.
///Not only third harmonics, the fundamental and other harmonics too, since the single phase motor creates an unbalanced load which in turn creates the zero-sequence current(s).\\\

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