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

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

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

(OP)
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

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

3
If you increase frequency and keep voltage magnitude the same, then the no-load current will decrease, and any harmonic content of the  no-load current present at 50hz may also decrease.

1 - Why does current decrease:
In the linear model the impedance limiting the no-load current is

I = V/Z = V / [X1+Xm] = V /[j*2*Pi*f(L1+Lm)]

where X1 & L1 are associated with stator leakage reactance/inductance  and Lm and Xm are magnetizing reactance/inductance.

2 - Why does harmonic content decrease:
Because your taking the core further away from saturation.

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

(OP)
Thanks Electricpete!

With only the standard motor manufactures data (nameplate, PF, Eff etc.) is there any method to estimate the value of the increased current?

joe

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

Suggestion: The no-load motor run also includes losses by Eddy Currents and Magnetization Losses (Steinmetz Law). Both losses will be higher for higher frequency, i.e. 60Hz.

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

Hi, not to mention that the fan on the motor will increase the load on the motor and hence the current.

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

The current will very nearly go to 5/6 as predicted by the above formula.

Yes, there will also be change in the losses, which affects the resistive component of the current. But since the current is primarily inductive (vs resistive), the small change in resistive component associated with the losses will have even smaller effect on total current.

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

Question to electricpete (Electrical)Nov 10, 2002 marked ///\\\
2 - Why does harmonic content decrease:
Because your taking the core further away from saturation.
///Please, could you clarify this in terms of voltage V increase across the jXm parallel branch on no-load conditions?\\\

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

For no-load conditions, I would choose an approximate equiavelent circuit for fundamental power frequency which has only two impedances: j*2*Pi*L1 and j*2*Pi*Lm.

Fundamental voltage accross magnetizing branch is
Vm = (j*2*Pi*Lm) / (j*2*Pi*L1 + j*2*Pi*Lm) = Lm/(L1+Lm).

That fundamental voltage magnitude does not change as we increase frequency.  But the fundamental flux does change (decrease) since
Phi ~ Integral(Vm)dt ~ Vm/(2*Pi*f)
f increase => Phi decrease

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

Suggestion: Please, would you clarify:
Fundamental voltage across magnetizing branch is
Vm = (j*2*Pi*Lm) / (j*2*Pi*L1 + j*2*Pi*Lm) = Lm/(L1+Lm).
in terms of voltage on the right hand side. I do not see voltage there and dimensions on the equation sides do not match, namely:
the left side of equation has Voltage dimension [Volts] and the right hand side has dimensions [per unit]. This is not acceptable in engineering and design (i.e. it is a nonsense).

Also, if the no load voltage increases across the parallel Xm branch, then the increased voltage on no load is bringing the operating point further into nonlinear B-H curve causing the current Im in Xm to be nonlinear with spikes. An oscilloscope may help to evidence it.

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

jbartos - I have assumed the applied voltage is 1 (per unit representation).

Fundamental no-load voltage across the magnetizing branch does not increase with increasing frequency. That is what I showed with the equivalent circuit:
Vm p.u.= Lm/(L1+Lm) has no dependence on f.

Increasing frequency with constant voltage under no-load conditions moves the core further away from saturation and current harmonic content will decrease or remain the same.  

Decreasing frequency with constant voltage under no-load conditions moves the core further into saturation and current harmonic content will decrease.  

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

Suggestion to electricpete (Electrical) Nov 12, 2002 marked ///\\\
jbartos - I have assumed the applied voltage is 1 (per unit representation).
Fundamental no-load voltage across the magnetizing branch does not increase with increasing frequency.
///Never disputed.\\\
 That is what I showed with the equivalent circuit:
Vm p.u.= Lm/(L1+Lm) has no dependence on f.
///Agreed. Never disputed. However, this equation presentation is mathematically objectionable since the voltage variable is missing on the right hand side. It is supposed to be shown there as a variable, e.g. Vt or 1.0. My claim is that Vm on no load increased and it may have increased into saturation region of the nonlinear Lm causing current spikes.\\\

Increasing frequency with constant voltage under no-load conditions moves the core further away from saturation and current harmonic content will decrease or remain the same.  
///Please, could you prove it or provide some references.
Notice, that Vm=constant=jXm x Im for fundamental wave in Xm linear region. For harmonics: Vm= Xm(Im), i.e. Vm is a nonlinear function of Im over nonlinear characteristic called Xm.\\\
Decreasing frequency with constant voltage under no-load conditions moves the core further into saturation and current harmonic content will decrease.

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

I aggree with electricpete on this one. Increasing the frequency for a fixed voltage will reduce the no load current and the harmonic content. Reducing the frequency with a constant voltage, increases the current and the harmonic content. A relatively small reduction in frequency at fixed voltage will result in iron saturation and a rapid increase in current.

As the frequency increases at a constant voltage, the motor operates at a higher speed, but with reducing torque.
Best regards,

Mark Empson
http://www.lmphotonics.com

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

Suggestion: I disagree with Marke's and ElectriPete's postings. There is also the Steinmetz Equation that pertains to remagnetization. So far, it has been downplayed in the above postings.
The Pr power loss due to the remagnetization is proportional to the frequency raised to power exponent 1.6 (actually range 1.4 to 1.8, 1.6 is the average). This Pr increases with frequency faster than Xm=j x 2 x pi x f since here the exponent at frequency f is equal to 1.0. That is why some manufacturers impose a limit on the inverter motor RPM to prevent the motor from overheating and damage.

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

jbartos - As I have addressed on 11/10, core losses do increase with increasing frequency.  But that will produce a very small increase in resistive current which will be overcome by the decrease in inductive current, remembering that the total current is primarily inductive.

Regarding harmonic current, the non-linearity of magnetizing current results from saturation. As you increase frequency you move further away from saturation and reduce harmonic content.

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

jbartos
You are correct, the core losses for a constant flux density, will increase with increasing frequency.

Additionally, core losses with fixed frequency will reduce with reducing flux density.
In the case of an unloaded induction motor driven from a constant voltage but increased frequency, we will have an increase in "remagnetisation losses" per unit of flux, but the flux will reduce due to the increased frequency into an inductive load.

If we look at the total current vector at no load, it is made up of reactive current (primarily magnetising current) and resistive current (loss current).

The power factor of an unloaded motor is typically 0.1 to 0.2 depending on losses, design etc. This shows that the reactive component of the current is significantly higher than the resistive component and therefore the variation in the resistive current component has a small influence in the total current measured relative to the effect of the change in the reactive current component.
Best regards,

Mark Empson
http://www.lmphotonics.com

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

Comment to electricpete (Electrical) Nov 10, 2002
If you increase frequency and keep voltage magnitude the same, then the no-load current will decrease, and any harmonic content of the no-load current present at 50hz may also decrease.
///I am essentially questioning the how well this idea is posed. If the B-H curve (or V-I) curve of magnetic material is considered and relationships stated below:\\\

1 - Why does current decrease:
In the linear model the impedance limiting the no-load current is

I = V/Z = V / [X1+Xm] = V /[j*2*Pi*f(L1+Lm)]

where X1 & L1 are associated with stator leakage reactance/inductance  and Lm and Xm are magnetizing reactance/inductance.
///with V=constant, L1=constant and assuming L1 negligible for this topic, B-H curve holds, and Lm=constant. These constants constrain the I current to constant by means of B-H curve (or V-I curve) relationship. Supposing that this is so, then varying frequency is causing this topic being mathematically ill-posed. This is why I inquired about any references or laboratory test that would confirm this.
Of course, the f frequency can be varied; however, something must be unconstrained in the above relationships. Assuming that B-H curve relationship stays, then the Voltage V cannot be constant to produce well-posed physical relationship and physical realizebility. Please, have you had it reviewed by any Professional Mathematician?\\\

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

jbartos -
I = V/Z = V / [X1+Xm] = V /[j*2*Pi*f(L1+Lm)]
is the linear model to demonstrate that the no-load current magnitude decreases when frequency increases.  It comes directly from the basic induction motor equivalent circuit with s=1 => R2/s >> (Xm) and => load branch current is neglected.  This linear model is valid for the linear range of the b-h curve.

You wrote: "These constants constrain the I current to constant by means of B-H curve (or V-I curve) relationship. Supposing that this is so, then varying frequency is causing this topic being mathematically ill-posed"  Apparently you believe that a fixed B-H curve implies a fixed frequency-indepdent relationship between voltage and current magnitude.  You are wrong in this belief.  The relationship between voltage and B is frequency-dependent.  In the linear range for sinusoidal v and B that relationship is |v|~dB/dt~2*pi*f*|B|.  This gives rise to the frequency-dependent inductive reactance terms in the equivalent circuit and the frequency-dependent relationship between voltage and current.
 
You wrote:
This is why I inquired about any references or laboratory test that would confirm this.
I don't need a lab tests  to prove my statement that induction motor no-load current magnitude and harmonic content will decrease when we increase frequency. It is self-evident from basic principles.  You are welcome to run a lab test if you'd like.

you wrote:
Please, have you had it reviewed by any Professional Mathematician
I don't find that to be necessary.  This is basic knowledge for any electrical engineer familiar with motors.

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

Comment to the previous posting: Thank you for adding the
|v|~dB/dt~2*pi*f*|B|
to your earlier posting:
"2 - Why does harmonic content decrease:
Because your taking the core further away from saturation."
However, your clarification just confirms that the voltage across the parallel magnetizing branch must vary with varying frequency. It cannot be constant as stipulated in above postings. The constant voltage |v| actually makes the topic or problem ill-posed as I mentioned when it comes to frequency variations. Please, would you prove or evidence where I was "wrong" as you indicated in the previous posting.

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

Comment to the previous posting: Thank you for adding the
|v|~dB/dt~2*pi*f*|B|
to your earlier posting:
"2 - Why does harmonic content decrease:
Because your taking the core further away from saturation."
However, your clarification just confirms that the voltage across the parallel magnetizing branch must vary with varying frequency.


No. I have assumed constant applied voltage and neglected leakage reactance.  This results in constant magnetizing branch voltage.  Therefore from the above relationship |B| changes with f, not |V|.

The constant voltage |v| actually makes the topic or problem ill-posed as I mentioned when it comes to frequency variations. Please, would you prove or evidence where I was "wrong" as you indicated in the previous posting.

Well, I think I have already answered that.  I believe you were mistaken when you wrote:

with V=constant, L1=constant and assuming L1 negligible for this topic, B-H curve holds, and Lm=constant. These constants constrain the I current to constant by means of B-H curve (or V-I curve) relationship. Supposing that this is so, then varying frequency is causing this topic being mathematically ill-posed. ....
Of course, the f frequency can be varied; however, something must be unconstrained in the above relationships. Assuming that B-H curve relationship stays, then the Voltage V cannot be constant to produce well-posed physical relationship and physical realizebility

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.  More specifically |V| establishes |B| establishes |H| establishes |Imagnetizing|, with each of these relationships independent of frequency by your logic.

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

So in summary .... increasing frequency with applied voltage magnitude constant causes no-load current magnitude to decrease and any harmonic content in that current to decrease.   I don't believe I'm going out on a limb with that statement.

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

Suggestion: Reference:
1. L.J. Giacoletto "Electronics Designers' Handbook," Second Edition, McGraw-Hill Book Company, 1977,
1a. Page 2-87 Section 2.4b Hysteresis Effects,
1b. Page 2-90 Section 2.4c Soft Magnetic Materials,
Reference 1 indicates in 1b that the flux density B decreases by a factor 1/e in a skin depth, Xdelta (refers to Table 2.18).  Thus for utilization of magnetic material Xthickness < Xdelta.
Else, if H(t)=Hp x sinwt, then
B(t)=Bw x sin(wt-d) + B3w x sin(3wt) + ...
This means that B has higher harmonic components.
The hysteresis losses (the area of Rayleigh loop) increases as the cube of the peak magnetic field excitation.
C.P. Steinmetz hysteresis loop energy Wh=eta x f x Bp**1.6
Eddy Currents Losses We=[(pi x r x f x Bmax)**2]/(4 x ro)
(Notice that if frequency increases from 60Hz to 600Hz, the Eddy Current Losses increase 100 times, e.g. from 1kW to 100kW).
With increasing frequency the Eddy current and magnetization losses will increase more rapidly, Ir through the branch of this losses will increase more rapidly than the decrease of the current due to any reduction of B due to skin depth, Im', since the B is reduced by a coefficient 1/e only, e=2.71.. . The parallel branch current Im = Ir + jIm'. However, the form of B-H curve (or V-I) curve of the magnetic material stays since it is characteristic to the magnetic material, e.g. soft iron.
Please, notice that if the frequency does not increase sufficiently enough, i.e. Xthickness < Xdelta, the Bmax=Bpeak=Bp stays approximately constant. So that the current Im' does not decrease much, if at all.

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

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.

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

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.\\\

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

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?

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

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 "Classical Electrodynamics," Second Edition, John Wiley & Sons, 1975, page 15 "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." The epsilon and mu are tensors. This is linked to properties of matter at high frequencies.\\\

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

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)

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

(OP)
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.


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

skrab
I think this rather proves the point??

Mark Empson
http://www.lmphotonics.com

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

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

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