Effects of 50/60Hz freq on 3phase motor CURRENT 3

[skrab]

Hi
Most posts regarding the performance of 50hz motors running on 60hz power and vice/versa have been concerned with the most important motor characteristics such as motor speed, torque output and efficiency.

I would like to know the effect on no-load CURRENT when the voltage is the same but the frequency is different than nameplate. Specifically, what would be the change in the no-load running current of small (1-5hp range) motors designed for 380V 50Hz if run on 380V 60Hz. Any and all information/explanations would be greatly appreciated.

Thanks in advance

 

[electricpete]

Magnetizing current is roughly inversely proportional to frequency and resistive core loss current is roughly proportional to frequency squared.

For the frequency change from 50hz to 60hz the 20% decrease in magnetizing current will be much more significant than the 40% increase in resistive core loss current.

It is true that if you increase frequency farther than at some point the f^2 increase in core loss would eventually overtakes the 1/f decrease in excitation current. That is apparently why you bring up 600hz. But it has nothing to do with the discussion of going from 50hz to 60hz.

 

[jbartos]

Comment on electricpete (Electrical)Nov 18, 2002 marked ///\\The logic you are using is that |V| is constant and |V|-|I| curve arising from the B-H curve is unchanged with changing frequency, therefore I must be constant with changing frequency.
///This comment pertained to the mathematic relationships shown there, not to the physics that may have been tacitly assumed and in subsequent postings revealed.\\ More specifically |V| establishes |B| establishes |H| establishes |Imagnetizing|, with each of these relationships independent of frequency by your logic.
///This statements were linked to the mathematics shown not to the actual physics of time varying magnetic properties.\\
As I have discussed above the relationship between |V| and |B| is not frequency independent. |B| ~ |V|/f as shown above. The statement which I believe I have shown to be wrong was ...These constants constrain the I current to constant
///B does vary with frequency. One can further write:
|V|/f ~ |B| ~ mu(H) x H ~ constant x mu(I) x I
where mu(H) is nonlinear function of H and mu(I) is nonlinear function of I. See Reference:
1. Dwight E. Gray, Ph.D. "American Institute of Physics Handbook," 2nd Edition, McGraw-Hill Book Company, 1972,
page 5-41 equation (5b-37): B=mu(H) x H

Normally on V-I characteristics that are valid within a certain frequency range, f1<f<f2, one is sending input either voltage V(t)=|V| x sin(2xpixfxt) or current I(t)=|I| x sin(2 x pi x f x t) within that frequency range and V-I stays constant, i.e. independent of frequency, i.g. some electronic devices. If the B-H curve has its V-I form independent in certain frequency range f1<f<f2, then for the for v(t)=|V| x sin(2 x pi x f x t) one must obtain i(t)= |Io| + |I1| x sin(2 x pi x f x t) + |I3| x sin(3 x 2 x pi x f x t) + ...,
since the i(t) is a nonlinear function of v(t). To have the V-I relationship dependent on the frequency f, one is supposed to know that relationship. This is what I have been seeking in my postings in this thread; however, nothing has been shown or any reference has presented yet.\\

 

[electricpete]

If you want to solve the non-linear system you are welcome to. I don’t believe that is necessary in order to prove my statement that increasing frequency with applied voltage magnitude constant causes no-load current magnitude to decrease and any harmonic content in that current to decrease.

Solve the system first neglecting leakage reactance. It can easily be shown that for this linear assumption the current decreases with increasing frequency (Imagnetizing = V/Z = V / [X1+Xm] = V /[j*2*Pi*f(L1+Lm)]) AND the flux decreases (flux is proportinal to magnetizing current).

What is the effect of adding non-linear B-H characteristics? Since flux decreases, the core is further away from saturation (at higher frequency) and current harmonics will decrease. There is absolutely no reason that considering the non-linear B-H characteristics could somehow cause the no-load magnetizing current magnitude or harmonic content to exhibit an increasing trend with increasing frequency over this range (50hz to 60hz).

I am hoping there will be and end in sight. Please tell me, do you honestly believe that no-load magnetizing current magnitude and/or harmonic content increases when we change frequency from 50hz to 60hz?

 

[jbartos]

Suggestion to the previous posting marked ///\\I am hoping there will be and end in sight. Please tell me, do you honestly believe that no-load magnetizing current magnitude and/or harmonic content increases when we change frequency from 50hz to 60hz?
///As an engineer, I have to go by evidence not by believe. The belief is a privilege of some other professions, e.g. entertainers. I have not seen any reference, such as graph, paper, laboratory result or solid proof in this thread yet. I have some more evidence that the B=mu x H has the mu function of frequency, i.e. mu(f). However, this mu dependence on frequency is for much higher frequencies than 50 or 60Hz. See Reference:
1. John David Jackson &quot;Classical Electrodynamics,&quot; Second Edition, John Wiley & Sons, 1975, page 15 &quot;For visible light or electromagnetic radiation of longer wavelength it is often permissible to neglect the nonlocality in space. Then epsilon-alfa-beta and mu'-alfa-beta are functions of only frequency.&quot; The epsilon and mu are tensors. This is linked to properties of matter at high frequencies.\\

 

[jbartos]

Comment to electricpete (Electrical) Nov 18, 2002 marked ///\\Comment to the previous posting: Thank you for adding the
|v|~dB/dt~2*pi*f*|B|
///This relationship needs some clarifications. Namely, Faraday and Lenz are pioneers of:
e = - N x d Flux / dt
which is often elaborated further to:
Eav = 4 x N x Fluxmax / T
and
FormFactor=Erms/Eav
or
Erms = FormFactor x Eav = FormFactor x 4 x N x Fluxmax / T = pi x 4 x N x Fluxmax / (T x sqr2 x 2) = 4.44 x f x N x Fluxmax,
where
N is number or turns
f is frequency
T is time period = 1/f
FormFactor = pi / (sqrt2 x 2) = 1.11
Now, the Fluxmax = Bmax x Area
The area is dependent on frequency. It was not mentioned above, where there was |B| ~ |V|/f. This is where one tends to conclude that |B| only is dependent on frequency f since the Area is missing, and actually the area is dependent on frequency f much more then |B|, meant Bmax.
Finally,
e = - N x d Flux(t) / dt = 2 x pi x f x N x Bmax x Area x sin(2 x pi x f x t) = Emax x sin(2 x pi x f x t)

 

[skrab]

I originally posted the question pertaining to the change in current draw because of actual test report results I was being asked about.

I apologize for not including this information in the original post, I was merely looking for an explanation of why no load current decreases when frequency is 60Hz vs 50Hz. There is absolutely no doubt (as electricpete has stated) that current will decrease when the voltage is the same but the frequency is increased to 60Hz.

Here are the test readings I encountered:
Motor is 2 winding, 2-speed,4/12 pole, 380-3-50Hz, NEMA design D motor, 4/1.3HP, Namplate FLA (7.1/9.7)
- 50Hz 379V no-load test results:
low speed winding = 9.7 amp avg.
high speed winding = 5.1 amp avg.
- 60Hz 376V no-load test results:
low speed winding = 6.9 amp avg.
high speed winding = 4.1 amp avg.
Note: These readings are not controlled lab data. The 50Hz readings are from the on-site certification team. The 60Hz readings came from our manufacturing plant's shop tests. Additionally, there were a total of 32 motors from 2 separate manufacturing lots. All the readings were very consistent.

Thanks again to all who replied.

 

[Marke]

skrab
I think this rather proves the point?? Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Question: Please, are current peak values and waveforms available or just the current average values?