Resistance and torque? what relation?

[gauthamtechie]

While trying to understand the concepts of Induction motor and the specifics, I read that resistance(a good/High value i.e.) is required so that good amount of torque is produced, to start the rotor..

Apart from the mathematics, is there any imaginative or analogy-type approach you could use to explain how resistance is good for torque(torque I understand as twisting force proportional to perpendicular distance) ..And now I thought good current means good torque but a resistance reduces current right? Please clarify

 

[Ratch]

gauthamtechie,

You have got to give us more information than just saying words to the effect. "I read it somewhere". Can you post a reference either by link or some jpg(s) of your text that aver this?

Ratch

 

[gauthamtechie]

I'm unable to find the link source which led me to ask this question, but I found a similar source:
**broken link removed**

I quote the second paragraph, "It can be seen from the plots that a high rotor resistance will provide a high starting torque, leading to rapid acceleration of the mechanical system. This is desirable since during starting the stator current is significantly above the rated current. Short acceleration reduce the stress on the power system caused by high currents."

This link: http://www.allaboutcircuits.com/vol_2/chpt_13/8.html quotes, "Placing resistance in series with the rotor windings not only decreases start current, locked rotor current (LRC), but also increases the starting torque, locked rotor torque (LRT). "

So does this mean currents and torque aren't proportional and what does resistance do here? I seem to have misunderstood some concepts here..

 

[FvM]

In fact the quoted literature gives a much more specific direction than indicated in your vague question: "Placing resistance in series with the rotor windings"

The rotor windings point matters. Unfortunately it's only possible with slip-ring induction motors. With standard inductions motors there's no way to access the rotor winding.

A short hint can be found here. https://en.wikipedia.org/wiki/Wound_rotor_motor
For a full explanation, you should refer to the theory of AC machines in a good electrical power engineering text book.

It should be added, that slip ring motors and resistor starters have been mostly abandoned by VFD inverters since 10 or 20 years.

 

[gauthamtechie]

ok may I infer that Torque and current aren't directly proportional? Because the rotor resistance decrease the current but increases torque. I see from the source I posted(allaboutcicuits), that the resistance increase helps in shifting the occurrence of peak torque to the start. So maybe I shouldn't always see "torque proportional to current", given this Torque Vs speed characteristics

 

[FvM]

Torque is approximately proportional to current for DC motors with constant excitation, e.g. PMDC. It generally isn't for induction (AC) motors.

Without retelling the AC motor theory, torque is generated by a product of stator and rotor field. In an asynchronous machine, the rotor field is induced by the stator at a different frequency according to the motor speed.

 

[yamato96]

the electric bicycle guys at endless-sphere forums have covered the torque aspect of induction motors to the exaustion. have a look there.

 

[FvM]

Electric bicycle motors are powered by VFD inverters and can be easily operated at maximum torque.

 

[gauthamtechie]

thank you for your replies. I'm trying to get a picture in my mind but some pieces are missing in the puzzle.. During Machines class long ago, we had this diagram of phasors as shown in this link https://www.allaboutcircuits.com/vol_2/chpt_13/9.html . I once interpreted them but now seem to have lost it and what do those arrows and phases represent?.. Also in the page, can you explain what this means: "The sum of these two phasors is a phasor stationary in space, but alternating polarity in time. Thus, no starting torque is developed. "
Any analogies to explain?