Physics of motor startup
Physics of motor startup
(OP)
I am trying to come up with a satisfactory description of the physics involved with a typical full-voltage induction motor startup. Assume the motor is a typical induction motor driving a constant torque load that is full load at rated RPM. Basically, what happens with the voltage, stator current and flux, rotor current and flux, rotor RPM, etc.
Something like this: (Please correct and add to this...)
1. Contactor closes
1. Applied voltage tries to drive current into stator windings
3. Rising flux in stator windings results in current opposing applied voltage, with no net current flowing.
4. Rising stator flux induces current in rotor
5. Rising current in rotor produces flux opposing stator flux
6. Opposing fluxes produce torque - if the torque is high enough to overcome load inertia, rotor rotates.
7. As flux continues to build in the stator and rotor, motor accelerates.
8. System reaches steady state.
I'm trying to get down into the details of this, so if you want to be technical, please do so. I'm trying to understand those first few critical cycles. If it depends on motor parameters, assume typical values. Where does the inrush fit into this? I would love to see a graph of the values listed above for the first 10 cycles of a start, and then another for the first 10 seconds until the motor reaches steady state.
One of the things I have been trying to get at is the answer to this question:
Why, when resistance is added to the rotor, does the starting torque increase? I know it increases the power factor, but does this somehow change the angle of the flux in the rotor (physical angle or phase angle?) thus producing more directly opposing fields and therefore more force?
For the record I think this stuff is fun.
Tom
Something like this: (Please correct and add to this...)
1. Contactor closes
1. Applied voltage tries to drive current into stator windings
3. Rising flux in stator windings results in current opposing applied voltage, with no net current flowing.
4. Rising stator flux induces current in rotor
5. Rising current in rotor produces flux opposing stator flux
6. Opposing fluxes produce torque - if the torque is high enough to overcome load inertia, rotor rotates.
7. As flux continues to build in the stator and rotor, motor accelerates.
8. System reaches steady state.
I'm trying to get down into the details of this, so if you want to be technical, please do so. I'm trying to understand those first few critical cycles. If it depends on motor parameters, assume typical values. Where does the inrush fit into this? I would love to see a graph of the values listed above for the first 10 cycles of a start, and then another for the first 10 seconds until the motor reaches steady state.
One of the things I have been trying to get at is the answer to this question:
Why, when resistance is added to the rotor, does the starting torque increase? I know it increases the power factor, but does this somehow change the angle of the flux in the rotor (physical angle or phase angle?) thus producing more directly opposing fields and therefore more force?
For the record I think this stuff is fun.
Tom





RE: Physics of motor startup
One way is the motor equivalent circuit which can be derived from an understanding of essential principles of an induction motor.
Once you know the motor equivalent circuit, circuit analysis can predict for you current as a function of speed and torque as a function of speed. Reviewing these results with a firm understanding of the basis of the equivalent circuit reveals the relative importance of all the underlying factors.
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RE: Physics of motor startup
One is the locked rotor current which we can determine from the equivalent circuit assuming zero speed. AC current starts at locked rotor current and then begins to decrease toward FLA as motor gets close to operating speed.
Another is the exponentially-decaying dc offset which occurs whenever we suddenly apply a voltage to an inductive/resistive circuit. It results in a maximum possible peak total current (ac plus dc) of 2*sqrt(2) times the rms locked rotor current. This component decays away within the first few cycles.
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RE: Physics of motor startup
At low speed the slip is very high. This creates a high voltage in the rotor at high frequency. The high votlage is responsible for the relatively high current. The high frequency increases the impedance of the rotor leakage reactance so that rotor appears to have a very low power factor. Therefore even though the current is ~5x FLA, the torque is lower than 5x FLT due to the low power factor. As we increase speed just a little within this low-speed range, the dominant effect is reduction of that rotor leakage reactance which generally slightly increase torque and decreases current with increasing speed (but still very low speed <0.5*sync speed).
At very high speed (approx operating speed) the slip is very low. This creates a much lower voltage in the rotor and much less significant role of the rotor leakage reactance now close to zero so rotor resistance is a dominant impedance. If we increase speed just a little the dominant effect is decrease in induced voltage so torque decreases and current decrease.
So at low speed the dominant feature (with changes in speed) is leakage reactance, which explains the shape of current and torque vs speed curves in this region.
At high speed the dominant feature (with changes in speed) is change in rotor induced votlage, which explains the shape of current and torque vs speed curves in this region.
In between, there is a mixture and the curve slowly changes from one shape to the other.
Once you understand torque vs speed and current vs speed characteristics for motor, you can combine it with torque vs speed curve of your load to predict how the motor acts during startup.
If you want details on how to predict torque and current from equivalent circuit let me know, it can be solved analytically.
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RE: Physics of motor startup
Here is what you are looking for. Don't let the title put you off. Chapter 32 will answer in detail any and all questions you have about 3 ph. motors.
Delmars Standard Textbook of Electricity-Second Edition by Stephen L. Herman
Can be had in used/good cond. from just about any used bookseller on the web for about 25 or 30 bucks and you will use it 'till you wear the cover off.(I own 2 copies)
Regards
Mike
RE: Physics of motor startup
Changing the rotor resistance in a WRIM can be worked into the description that e-pete posted, especially the part where he describes the change in rotor induced voltage affecting the torque speed curve. His description was generalized for all Induction Motors. Synchronous motors are close, but not the same. By the way, when external resistance is added, torque is reduced, not increased.
When you mentioned "increasing power factor", you were referring to the characteristics of a Synchronous Motor. DC voltage is applied to the SM rotor (through the slip rings) to alter the field strength, which can be used to manipulate the power factor one way or the other. That action is not the same as what is happening in a WRIM.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
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VermontPE (Electrical)
(OP)
jraef,

CJCPE (Electrical)
In "Electric Machinery" (Fitzgerald, Kingsley, Umans), torque is determined by analyzing a motor's equivalent circuit. Torque is maximum when the power delivered to (rotor resistance)/slip is maximum. Using the impedance matching principle, that occurs when Rres/s is equal to the other impedance in the equivalent circuit (all in series). For Rres/s = Zs as Rres increases, s must also increases. That is why the peak torque occurs at higher values of slip as the rotor resistance increases. The analysis also shows that the actual value of peak torque remains constant as the torque -speed curve is stretched towards the left in the figure that you posted.
VermontPE (Electrical)
(OP)
Here are some more possible mechanisms...ignore the wound rotor and imagine different designs of bars in a squirrel cage motor.
CJCPE (Electrical)
Tom
davidbeach (Electrical)
CJCPE, your first statement only applies to single cage designs. In dual cage designs different bars can have different properties, and carry differing currents dependent on the operating conditions.
VermontPE (Electrical)
(OP)
A concept not yet discussed is the idea of resonance. Does changing the rotor resistance change the resonant frequency of the rotor's impedance, matching (or unmatching as the case may be) the frequency of the rotating field with the resonant frequency of the rotor?
jraef (Electrical)
You are right, I missed the word "starting" when I read your question. Oops
electricpete (Electrical)
Regarding the comment on resonance - there are no significant capacitive elements in an induction motor equivalent circuit model at power frequency or below, so I don't see any possibility of circuit resonance unless there is an external connected capacitance.
cswilson (Electrical)
I'm late to the thread, as I've been traveling, but I think I can contribute some insight going back to my days of trying to figure out how these darned things worked...
VermontPE (Electrical)
(OP)
cswilson
Marke (Electrical)
Hello VermontPE
bam55 (Electrical)
Does all this mean that at no load starting for an say 100HP induction squirral cage motor -the inrush is going to be low or negligable( Won't dim the lights)?
electricpete (Electrical)
No. Locked rotor current magnitude (and also any dc offset) are indepdent of load. Load can affect the duration of the starting current (not the magnitude).
bam55 (Electrical)
Just to deviate a little further to get to it: Is there any point in putting a soft start for this motor which only starts at no load? (The application is a magnetic coupled adjustable speed drive controlled fan which starts it at no load) Is there not a spike whether I have a soft start or not?
DickDV (Electrical)
Thanks alot, guys! I've learned more in this short thread about motor rotor behavior than in the last several years of questioning around.
electricpete (Electrical)
I believe my post of 5 Oct 05 17:32 adddressed that (2nd paragraph) among other things.
electricpete (Electrical)
Maybe I should add (if it's not obvious) that as speed increases in this region, the decrease in reactance (caused by decrease in frequency seen by rotor) causes an increase in power factor.
DickDV (Electrical)
electricpete, if I read your post of 5Oct05 correctly, you say that with slight increases in speed away from locked rotor, the torque goes up.
electricpete (Electrical)
The curve I'm describing would increase continuously from 0 speed until breakdown torque speed, then decrease continously from breakdown-torque-speed to syncronous speed at which point it is 0.
electricpete (Electrical)
The type of curve you are talking about is here:
electricpete (Electrical)
If you use the linear equivalent circuit and neglect the magnetizing reactance, the torque-speed curve will have torque increasing continuously as speed increases from 0 speed to breakdown-torque-speed.
electricpete (Electrical)
Typo Correction:
electricpete (Electrical)
If the math above is to tedious, just follow it to the point of
DickDV (Electrical)
electricpete, I surely appreciate the thought and effort you've been willing to put into these questions.
waross (Electrical)
Hello
electricpete (Electrical)
On the subject of the possible "dip" (minimum) in torque speed curve between locked rotor and breakdwon torque:
electricpete (Electrical)
Does anyone else have thoughts on the cause of the "dip" in the torque speed curve discussed above?
cswilson (Electrical)
I can't make any of my equations produce the "dip" either, analytically or numerically. Still looking, trying to see if there are any higher-order effects.
waross (Electrical)
I have a couple of thoughts. Thinking about the torque curves of wound rotor motors, low slip motors and high slip motors such as are used on punch presses and shears. Would a crossover effect from the high resistance cage and the low resistance cage in a dual squirrel cage motor explain a torque dip?
DickDV (Electrical)
I sure appreciate your efforts to try to bring understanding to this.
electricpete (Electrical)
You are right Dick. The high currents during starting cause high leakage flux which can cause saturation in some parts of the motor such as the core teeth. As current decreases during the start, the saturation decreases and the effective leakage reactance would tend to decrease the current and the power factor. This tends to decrease the torque (depending on how it compares with the other effects which tend to oincrease torque with speed).
electricpete (Electrical)
A correction to my first paragraph (in bold):
amptramp (Electrical)
Just for fun!
//literatu re.rockwel lautomatio n.com/idc/ groups/lit erature/do cuments/br /150-br008 _-en-p.pdf
ww.masterc ontrols.co m/EngInfo/ Articles/W ooddall/TA _RWood.htm
electricpete (Electrical)
On page 2-16 page of amptrap's Allen-Bradley link, we see the following:
Read the Eng-Tips Site Policies at FAQ731-376
RE: Physics of motor startup
Every graph I look at for wound rotor induction motors shows the torque peak moving from high speed to low speed with increasing rotor resistance. If the resistance is low, the startup torque is low. If the resistance is high, the startup torque is high. What is the physics of this? By physics I mean, where does the charge move? What fields are caused in the stator and rotor and how do they behave? Increasing R in the rotor offsets the inductance and pushes the phase of the current closer to the phase of the voltage for more delivered power. I think. At least at startup. Once the motor is running if the resistance is left connected to the rotor coils it will result in reduced torque, probably because when the rotor is moving it gets itself back in phase with the rotating field and then the resistance just limits the current in the rotor.
RE: Physics of motor startup
The maximum locked rotor torque occurs when the peak torque point occurs at 100% slip. Further increasing the rotor resistance puts the peak torque point in the negative speed (braking) region and reduces the locked rotor torque.
"Circuits Devices and Systems" (Ralph Smith), a more general text, discusses the increase in starting torque in terms of increasing power factor using a similar equivalent circuit. Increasing power factor also moves the angle between the rotor field and the stator field closer to 90 degrees.
RE: Physics of motor startup
Some of the rotor bars are in regions where the effective field is very nearly zero. Increasing the resistance of the bars can force more of the total current to flow in the bars in higher regions of field, producing more emf and therefore more torque. This may be a load of...I'm not sure.
There are 2 components to moving charges in a rotor. The changing magnetic field places forces on the charges. One is the motion of the charge in the wire (in this case a rotor bar), the second is the motion of the wire (bar) itself (which conatins the charge). Higher resistance in the bars means that the charges cannot move through the bars as easily, meaning that of the total forces more is applied to moving the rotor, thus more torque.
Not sure the validity of these concepts.
Tom
RE: Physics of motor startup
I think that your first proposed mechanism is invalid because the rotor conductors are distributed uniformly and any increase in rotor resistance would effect the current proportionally everywhere.
With regard to your second proposed mechanism: The magnetic field produced by the stator does not change. A field of constant amplitude rotates at the synchronous speed, RPM = 120 X frequency / poles. The field is stronger at its center and weaker at the leading and trailing edges. Rotor current is caused to flow by the relative motion of the rotor conductors moving backwards through the field because of slip. The frequency of the rotor current is rotorfreq = slip x poles / 120. The rotor current causes a magnetic field that rotates forward with respect to the rotor at RPM = 120 x rotorfreq / poles. To determine the speed of the rotor field with respect to the stator, you add the speed with respect to the rotor to the rotor speed. That gives you a magnetic field in the rotor that rotates at exactly the same speed as the stator field but at about a 90 degree angle. The torque is produced by the force between the stator field and the rotor field. It is the relative magnitudes of the two fields and the angle between them that determines the torque.
Chuck
RE: Physics of motor startup
RE: Physics of motor startup
RE: Physics of motor startup
RE: Physics of motor startup
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RE: Physics of motor startup
The current (and hence flux-linkage) response of the rotor to the stator voltage is just that of a first-order low-pass filter with an L/R time constant. Sketch out both the gain and phase bode plots as a function of slip frequency for this. On the gain plot, the magnitude is a roughly constant 1/R at low frequencies, then above the "break frequency" (R/L in rad/sec), it falls off with a slope of -1 on a log/log plot.
The phase plot shows near 0 degree lag at low frequency, passing through -45 at the break frequency, and asymptotically approaching -90 at higher frequencies.
Next, you may want to redo the Bode plots with the axes plotted linearly, not as log plots. You may also want to mirror the plot so that slip frequency increases to the left, because in the way we are used to looking at these motor plots, with a constant-frequency input, the slip is higher at lower speeds.
The torque generated is simply proportional to the magnitude of the rotor response multiplied by the sine of the lag angle. That's really all there is to it. Sure, there are some minor higher-order effects, but this catches the fundamental issues. I used a simple Excel spreadsheet, and could create the classic induction motor curves very easily.
At low slip frequencies, the lag angle and its sine increase faster than the magnitude decreases, so torque increases with slip. However, as you continue to increase slip frequency, the angle, and especially its sine, start levelling off, while the magnitude continues to decrease. The frequency of the torque peak is directly related to the R/L break frequency.
Therefore, as you increase the rotor resistance, you increase the (slip) frequency of the torque peak. Each of the torque/speed plots you show is fundamentally the same -- the ones with higher resistance are just "stretched out". Their magnitude would also fall off with increasing slip if that were plotted (this would get you into negative velocities).
Curt
RE: Physics of motor startup
Finally what I have been trying to get to. Thanks!
Tom
RE: Physics of motor startup
Sort of correct, if you take a square bar and position it at the surface of the rotor, it will have a fixed value of resistance and inductance. If you position the same bar deeper into the rotor, it will have the same resistance (provided that the material and crossectional area are the same) but it will have a higher reactance.
Now take two bars of the same square cross section and position one at the surface of the rotor, and the other deeper down into the rotor, and connect them in parallel. During start, the freqeuncy of the rotor current is dependent on the slip. At high slip, the effect of the rhigh reactance of the inner bar, is to concentrate most of the current in the outer bar. The effective resistance is higher than under almost zero slip conditions where the current is more evenly distributed.
If we take a single thin rotor bar positioned on the radius of the rotor with the outer edge near to the surface and the inner edge towards the shaft, and we compare this bar with a square bare at the rotor surface with equal cross sectional area, both bars will exhibit the same DC resistance and perform with similar full load slip characteristics. The start characteristics will be markedly different however because the current distribution in the bar during start, will be a concentration towards the outer edge resulting in a higher power dissipation in the bar and a resulting higher start torque.
Mark Empson
http://www.lmphotonics.com
RE: Physics of motor startup
RE: Physics of motor startup
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RE: Physics of motor startup
RE: Physics of motor startup
The only part I still don't have an understanding of now is why a NEMA B torque curve, for example, shows decreasing torque from locked rotor to pull-up torque. If I've picked up correctly on the above discussion, then I don't see any explanation for that part of the curve.
Anyone care to comment on that.
And, thanks again to all who posted.
RE: Physics of motor startup
Let me know if you don't agree.
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RE: Physics of motor startup
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RE: Physics of motor startup
The NEMA B curve goes down in that region. Am I missing something here?
RE: Physics of motor startup
This is pretty typical of most of the motor curves I have seen. I have seen in textbooks the kind of curve you describe... necessary to show the "minimum pullup torque". I'm not sure offhand exactly what would drive that behavior.
One thought - the torque speed curve can be 100% defined by the equivalent circuit parameters. You can find the speed of breakdown torque by solving dT/ds=0 and d^T/ds^2 <0. That always happens. If there is also a combination of parameters to give another point dT/ds=0, this time with d^T/ds^2 >0, that would be your minimum between start and breakdown torque...again usually doesn’t happen. If I get a chance I may check the equation to see what characteristics drive that particular unusual behavior.
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RE: Physics of motor startup
http://tristate.apogee.net/mnd/mfcttor.asp
(different than what I described).
I think there may be a couple of effects at work in that initial dip.
The discussion so far focused really on power aspects. I said in this speed range, the dominant effect was reactance was going down. Based on this you do expect to see an monotonic increase in power output within this range. But power output is the product of torque times speed. When we go from 5% speed to 10% speed that is a dramatic increase in speed, so even with power output increasing within this range we might see some decrease in torque. Maybe there is a more direct way to get disussion of torque without going through power as an intermediate...still thinking.
A minor effect may also be deep bar effect which tends to increase rotor resistance and therefore increase torque at high slip frequencies.
I'll think some more but welcome any other comments.
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RE: Physics of motor startup
The proof:
Z:=R1+R2/s + I*(X1 + X2);
All the remaining quantities represent magnitudes:
I1:= V/((R1+R2/s)^2+(X1+X2)^2)^(1/2)
P_SHP:=I1^2*R2*(1-s)/s
P_SHP := V^2/((R1+R2/s)^2+(X1+X2)^2)*R2*(1-s)/s
T:=P_SHP/w
where w = wsync*(1-s)
T := V^2/((R1+R2/s)^2+(X1+X2)^2)*R2/s/wsync
dT_ds := 2*V^2/((R1+R2/s)^2+(X1+X2)^2)^2*R2^2/s^3/wsync*(R1+R2/s)-V^2/((R1+R2/s)^2+(X1+X2)^2)*R2/s^2/wsync
simplify:
dT_ds := Numerator / Denominator
Numerator = -V^2*R2*(-R2^2+R1^2*s^2+s^2*X1^2+2*s^2*X1*X2+s^2*X2^2)
Denominator = (R1^2*s^2+2*R1*s*R2+R2^2+s^2*X1^2+2*s^2*X1*X2+s^2*X2^2)^2*wsync
Solve for numerator = 0
s_Tmax := (+/-) R2 / sqrt(R1^2+X1^2+2*X1*X2+X2^2)*R2
If anyone is really interested I can post these equations in graphics format (a little easier to read).
Exclusing the negative value, there is only one point where dT/ds=0 and that is at breakdown torque (under the assumptions given at the beginning).
There are motors that act differently than this, with an initial dip in torque and then increase as shown here:
http://tristate.apogee.net/mnd/mfcttor.asp
I did notice after my previous posts that our power plant circulating water motor acts that way. It is a large slow speed motor.
I conclude it must be one of the two factors not included in the above model:
1 - non-linearity associated with the fact that rotor resistance is higher at high slip.
2 - the existence of magnetizing branch in the equivalent circuit which was not modeled above.
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RE: Physics of motor startup
"s_Tmax := (+/-) R2 / sqrt(R1^2+X1^2+2*X1*X2+X2^2)*R2"
should have been
"s_Tmax := (+/-) R2 / sqrt(R1^2+X1^2+2*X1*X2+X2^2)"
I jumped over the fact that there is only one positive value where dT/ds=0. That means only one maximum or minimum of the T vs s curve. Since we know there is a maximum, there can be no minimum for positive s.
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RE: Physics of motor startup
T := V^2/((R1+R2/s)^2+(X1+X2)^2)*R2/s/wsync
Plot this function in excel using positive values for all the paramters. You will find that no matter what values you choose, this function T will never have a minimum for positive values of s.
(Once again there are two items not included in this model mentioned above)
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RE: Physics of motor startup
If you check out the NEMA A, B, C, & E torque speed characteristics, all of them have a minimum point somewhere around 20-25% of base speed.
As I believe you have concluded, the equivalent circuit that you have analysed doesn't provide any explanation for this behavior and is therefore not entirely valid for operation in this range of high slip.
The reasons for this behavior remain a mystery to me.
Thanks again for your work on this difficult subject
RE: Physics of motor startup
A couple of points on your original post. Maybe semantics or possibly I have misunderstood something.
But;
3. Rising flux in stator windings results in current opposing applied voltage, with no net current flowing.
"Rising flux in stator windings results in current opposing applied voltage,"
Possibly more accurate if stated as <Rising flux in stator windings results in voltage opposing applied voltage,>
"net current flowing"
Possibly more accurate if stated as <back EMF (voltage opposing applied voltage) acts to reduce line current.
6. Opposing fluxes produce torque - if the torque is high enough to overcome load inertia, rotor rotates.
At standstill the opposing fluxes produce force not torque. The torque is produced when the flux rises in the adjacent coil fed from another phase. This also explains electricpetes statment that the torque is zero at standstill.
if the torque is high enough to overcome load inertia, rotor rotates.
Possibly more accurate if stated as <if the torque is high enough to overcome static friction, rotor rotates.>
respectfully
RE: Physics of motor startup
I scavenged around and found 4 manufacturer's torque speed curves for specific motors. Two had the dip and two didn't.
The discussion above is not flawed by the assumptions. It is a proof by contradiction. If assuming A and B leads to C, then experimental observation of not(C) leads us to conclude the cause is either not(A) or not(B).
So my conclusion as stated , the cause of the dip is either:
1 - non-linearity associated with the deep bar effect which causes the rotor resistance to be higher at high slip.
2 - the existence of magnetizing branch in the equivalent circuit causes voltage drop across X1.
#2 may be a little weak in terms of a cause... you have to view it in terms of the model...doesn't lend much undestanding. I tend to think a thorough analysis would rule out #2 and leave us only with #1. If I have time (probably not in the near future), I'll study it a little closer.
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RE: Physics of motor startup
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RE: Physics of motor startup
RE: Physics of motor startup
respectfully
RE: Physics of motor startup
I, for no apparent reason, have always just passed it off as magnetic saturation occurring probably in the rotor but why it would get worse as the speed goes up from zero would beg an explanation that I certainly don't have.
Thanks again guys. If nothing further comes from this thread, I've still learned a lot.
RE: Physics of motor startup
I think the other big effect is the deep bar effect. Just to elaborate a little more, the torque equation has R2 as a factor in the numerator and as one of several terms in teh denominator. T := V^2/((R1+R2/s)^2+(X1+X2)^2)*R2/s/wsync
For large slip, the R2/s term in the denominotor is much smaller than the X1+X2 terms in the denominator, so the denominator dependence on R2 is small and the torque for a given value of s is roughly proportional to R2 (based on the numberator. So with R2 decreasing as deep bar effect lessens during startup, this provides another effect which may tend to decrease the torque.
The magnetizing reactance neglected above I don't think has any big role. I don't have any proof but I don't think so.
There can surely be a lot more things that the designers do to shape the torque speed curve that I don't know about. At least I'm learning something here.
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RE: Physics of motor startup
"...The high currents during starting cause high leakage flux which can cause saturation in some parts of the motor such as the core teeth. As current decreases during the start, the saturation decreases and the effective leakage reactance increases which would tend to decrease the current and the power factor. This tends to decrease the torque (depending on how it compares with the other effects which tend to increase torque with speed).
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RE: Physics of motor startup
An old standby from Allen-Bradley:
http:
On Wye-Delta starting but does a good job explaining starting current components and residual flux:
http://w
RE: Physics of motor startup
"As the motor begins to accelerate [from locked rotor], the torque drops off, reaching a minimum value called pullup torque. Pullup torque is caused by harmonics which result from windings being concentrated in the slots. If the windings are uniformly distributed around the periphery pull-up torque is greatlyl reduced. Some motor design curves show no pull-up torque and follow the dashed line [continuously increase from locked-rotor to breakdown torque]"
So apparently another possible cause of the dip is harmonic effects. That rings true to me although I don't understand it.
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