SQUIRREL-CAGE ROTOR DESIGN..

[powerjunx]

basicaly, rotor of induction motor is somewhat like the secondary circuit of a transformer where the imbedded bars are used as a short-circuited secondary circuit for motors, am i right?
consequently, id known the variations of squirrel-cage rotor for induction motor is for "torque requirement"; but i would like to know what is the significance difference (on torque, speed and slip)of the following:
1.)double cage rotor?
2.)single cage rotor but has a shorting ring imbedded in the middle that seems to have three shorting rings?(in a single shaft)
3.)conical type rotor?
4.)single cage rotor but divided into two in which the through dismantling they were separated?(1 cage has a solid shaft while the other 1 has a hollow shaft and different rotor slot)

anyone could give me some web sites dealing with the design of rotors and its variation?
thank you very much..

regards,
fbjavier

 

[DougMSOE]

Your motor theory is correct except that the rotor is not like, it is a shorted secondary of a transformer of a "transformer". Look at the equivalent circuits of the two they are almost the same.
A double cage rotor is to give the rotor good starting charactoristics but also good running efficiency.
The motor with the 3 shorting rings. This was done for reliability. If rotor bars broke in one section, the motor still had a good chance to start and run.
Conical or skewed rotor? will need more info. Skewed was done to prevent the rotor from magnetically locking or cogging (i.e. poor starting and/or running)torque.
Remember that the rotor bar and cage metallurgy is VERY IMPORTANT!!!
Dual cage same as above (double cage).

I would be very suprised if you will find any web site with this information. When I did motor design work, this info was considered proprietary information. Look in some old motor endineering books; Alexanders induction motor design was one that I used and it is very accurate.

 

[jbartos]

Suggestions:
1. The double cage motor may have a much smaller starting current and locked rotor current.
2. Agree with the above posting.
3. Also, there are smaller vibrations.
4. To increase the rotor resistance/impedance.

 

[Marke]

Hello Fbjavier

The rotor design has a major effect on the starting characteristics of the motor. I have some information at

http://www.lmphotonics.com/m_control.htm

but this is a little limited. Essentially, although we describe the rotor as a shorted winding, it does have both inductance and resistance. The current and the torque during start, are both dependent on the characteristics of the rotor. The current is obviously influenced by the impedance of the rotor. The torque is a fuction of the I^2R of the rotor with the R not necessarily the actual resistance of the rotor, but more the effective resistance of the rotor. The reactance of the rotor is a fuction of the inductance and the frequency of the current flowing in the rotor (= slip) We can influence the inductance by the position within the rotor. A bar on the surface will have a much lower inductance than a bar deeply imbeded into the rotor. The reistance of the rotor is a function of the cross sectional area and the material of the bar. A shallow square bar will exhibit low inductance, and low reactance and will result in a high start current and low start torque (Design A). A deeper bar with a smaller cross section will have a higher inductance and a higher resistance and will exhibit a lower start current and higher start torque (Design B). Deeper bars can often have a smaller cross section due to the increased cooling from the surrounding iron.
If we take two bars of equal cross section in parallel, one shallow and one deep, then during start, the outer bar will have a lower reactance so will carry more current than the inner bar. As the motor reaches full speed, the rotor current frequency approaches 0Hz and the current distribution becomes even. The effective resistance is higher during start than during run. Taking this further, you can use a thin rectangular bar penetrating deep into the rotor to give a higher resistance during start than run. A double cage rotor uses an outer cage of brass bars and an inner cage of copper bars to give a high starting torque at low current but retain good running characteristics. (A high resistance bar results in high full load slip and reduced efficiency) Ideally, you want a high resistance during start, reducing with speed to a very low resistance at full speed, just like a slip ring motor.

The conical rotor is often fitted into a conical stator with a strong spring to act as a brake. When the stator is energised, it centralises the rotor, releasing the brake.
Hope this answers a few questions,
Best regards, Mark Empson

http://www.lmphotonics.com