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taking into account the definition of pull-up torque:

Pull-up Torque: The Pull-up Torque is the minimum torque developed by the electrical motor when it runs from zero to full-load speed (before it reaches the break-down torque point)

When the motor starts and begins to accelerate the torque in general decrease until it reach a low point at a certain speed - the pull-up torque - before the torque increases until it reach the highest torque at a higher speed - the break-down torque - point.

The pull-up torque may be critical for applications that needs power to go through some temporary barriers achieving the working conditions.

I can't find anywhere in this text( and in other articles on the subject) WHY the torque lowers a bit(inflexion point) and then it raises again. What is the mechanical explanation for this?

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This is what Allen Bradley say about pull-up torque: -

enter image description here

Please click on picture to help but in words, the specific sentence is: -

Pull-up Torque is caused by harmonics that result from the stator windings being concentrated in slots. If the windings are uniformly distributed around the stator periphery, Pull-up Torque is greatly reduced. Some motor design curves show no actual Pull-up Torque and follow the dashed line between points A and C.

Info taken from this document entitled

Drive Fundamentals, Drive/Motor Basics, Revision 1.0

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  • \$\begingroup\$ This is exactly what I was looking for. So harmonics are the culprit. Thanks! \$\endgroup\$ – Ernesto Sastre Sep 26 '16 at 18:36
  • \$\begingroup\$ @ErnestoSastre - there is some dispute between what AB say in their document and a seemingly reasonable explanation from Charles Cowie in his answer. Can I ask you a favour - if you find out anything to the contrary please come back and leave a comment and links. I also see some different kinds of answers on-line too so please don't be too hasty and totally rely on this answer until digging a bit more yourself. At least the answers give you a starting point to investigate more and I'd rather have my answer tarred and feathered if the "real true" answer became apparent. \$\endgroup\$ – Andy aka Sep 26 '16 at 18:47
  • \$\begingroup\$ I'm currently double checking the answers, yet what I highlight in this one is that it provides the CAUSE to a phenomenon(the pull up torque). I don't know if a combination of two curves of resistance effects (high and low slip) yielding a different curve that features a pull up torque is enough as an explanation to a cause of it \$\endgroup\$ – Ernesto Sastre Sep 26 '16 at 19:10
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"Squirrel Cage" motor designers can change the characteristics of motors by modifying windings, rotor slot geometries, end ring size, rotor bar and end ring resistances, number of slots,number of rotor bars, amount and type of magnetic steel, etc.

ref http://industrialelectricalco.com/wp-content/uploads/2014/01/nema-abcde-torque-curves.pdf

By reducing the eddy current rotor losses from thick metal squirrel cages, the motor has less iron and less starting torque but more torque for fans at high speed when the slip speed is smaller.

  • Torque increases with speed squared as slip is reduced between rotational coupling of stator fields with moving rotor permeability coupling
  • Torque reduces towards zero at 0 slip speed due to back EMF opposing voltage applied to stator reduces available current to meet load demands as speed increases.

  • The mass of steel and geometry of rotor bars affect these variables where a peak can occur.

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  • \$\begingroup\$ But it stills doesn't explain why the torque lowers. What's the phenomenon that makes that happen? \$\endgroup\$ – Ernesto Sastre Sep 26 '16 at 17:34
  • \$\begingroup\$ Yes, but the pull_up torque phenomenon is never addressed here. Still thanks for taking the time to answer. I marked the answer already \$\endgroup\$ – Ernesto Sastre Sep 26 '16 at 18:38
  • \$\begingroup\$ Pull-up torque is when the rate of torque increase exceeds the rate of decrease below max speed on generally lighter squirrel cages found in fans by reducing slot gaps in stator. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 26 '16 at 18:51
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Most 3-phase induction motors are designed to have a reasonably high locked-rotor torque (130-160%), a high breakdown torque (150-250%) and low slip (1.5-3%). A motor with a high resistance rotor has a high locked-rotor torque, but high slip and no breakdown-torque inflection point. A motor that has a low resistance rotor has low slip and high breakdown torque, but the locked rotor torque is lower than would be desirable.

A rotor bar that is close to the surface of the rotor has a low leakage reactance reactance while a rotor bar that is closer to the shaft has a higher leakage reactance. If the rotor is constructed with two rotor bars, or one elongated bar that extends toward the shaft, the effective rotor resistance is high at locked-rotor and low at the normal operating point. That happens because the rotor frequency is high at high slip and low at low slip. At locked-rotor, the rotor current is forced to flow mostly in the low-reactance part of the rotor bar near the rotor surface. The effective resistance is increased by reducing the bar area in which the current flows.

As the motor accelerates, there is a transition between high rotor-resistance characteristics and low rotor-resistance characteristics. That transition can result in an inflection in the torque vs. speed curve.

There is a Chapman text that explains and illustrates this quite well.

Here is a link.

Additional Information

Pull-up torque is not the name of a special or undesirable phenomenon. It is a standard part of an induction motor performance specification. The NEMA definition is:

Pull-up torque: The minimum torque developed by the motor during the period of acceleration from rest to the speed at which breakdown torque occurs. For motors which do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed. [MG 1-1.48]

For some motors, pull-up torque is equivalent to locked-rotor torque. For others, pull-up torque is less than locked-rotor torque and occurs at a speed between zero and the speed at which breakdown torque occurs. NAMA standards allow pull-up torque to be less than locked-rotor torque and, for some ratings, less than rated torque.

It is clear from the above linked reference and from similar references that rotor bar shape, position in the rotor and material are the primary factors used to shape the torque vs. speed curve of an induction motor.

Harmonic torque is generally minimized by several aspects of motor design. One factor is the selection of the number of rotor bar slots relative the the number of stator winding slots. Another is "skewing" the rotor bar slots at an angel relative to the shaft direction. A third is the distribution of the stator windings to produce a sinusoidal flux wave.

One undesirable effect of harmonic torque is a distinct dip in the torque vs. speed waveform as shown below. That is different from the relatively gentle inflection in the torque curves of some production motors.

enter image description here

The following figure shows the torque vs. speed curves for various double squirrel-cage rotor designs and one deep-bar rotor design. One of the deep-bar curves shows the minimum (or pull-up) torque occurring above the zero-speed (locked-rotor) torque. Note that this is a side effect of raising the stall torque, not a design that lowers the minimum torque. Thus is is not the result of poor design. The various "m" values refer to a constant that is defined to simplify the motor performance equations. The value of m = infinity indicates a motor with no double-cage or deep-bar effects.

Standard motors have sufficiently deep rotor bars to produce a curve above the m = infinity curve but a bit below the m = 2 curve. Sometimes the curve has a minimum torque inflection point below the locked-rotor torque,

enter image description here

Alger describes the effects of harmonic fields using a diagram similar to the above Fig. 204 from Puchstein, Lloyd, Conrad. According to Alger, an acceptable design must minimize such effects. On the other hand, a minimum torque infection point above zero speed is accepted by international motor standards.

To address a comment; "pull-up torque" is not a phenomenon it is a point on an induction motor torque vs. speed curve that is defined by international standard. If there is a minimum point on the curve that is above zero speed and above the minimum set by motor standards, the motor meets standards.

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  • \$\begingroup\$ Yes, but the pull_up torque phenomenon is never addressed here. Anyway thanks for taking the time to answer with such detail . I marked the answer already. The culprit of the low pull up torque are harmonics it seems \$\endgroup\$ – Ernesto Sastre Sep 26 '16 at 18:40
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    \$\begingroup\$ I don't believe this is true. I believe that the harmonic problem produces a more pronounced torque perturbation. I will try to investigate further. \$\endgroup\$ – Charles Cowie Sep 26 '16 at 19:10
  • \$\begingroup\$ @Andy aka: I came across this question today and revised my answer based on Philip Alger. Note that the curve in the Allen-Bradley publication (link now broken) has a locked-rotor torque nearly equal to breakdown torque, That is clearly the curve of a deep-bar or double-cage rotor and does not show any evidence of harmonic torque. I complained to A-B about that but didn't get a response. The material is available elsewhere, but I don't know if any of those attribute it to A-B. \$\endgroup\$ – Charles Cowie Feb 8 at 22:54

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