A Rotor Resistance Starter for a wound Induction Motor

[shamooooot]

I am working on an educational model that involves a wound Induction Motor with the following parameters:
 - 3 Phase 400V
 - 1/2Hp
 - 4 Pole
 - 1750rpm
 - Y connection

A Rotor Resistance Starter is required as well and I need help finding out how it can be made/added, as to the best of my knowledge it's not something standard that can be found in wound induction motors nowadays.

[Siwastaja]

Is there any point in this educational exercise? Rotor resistance starting is really a thing of past because it adds so much mechanical complexity (slip rings) for so little gain compared to simpler soft-start circuits: a tad more torque, but still crap efficiency which you can't escape with large slip speeds.

Instead, variable frequency drives actually solve the problem and are able to run the motor at any RPM, producing full torque, while maintaining full efficiency of the motor, and have been low cost mainstream for two decades already.

[Terry Bites]

Them kids need to get to grips with rotating machine therory, so its a good exercise.

[Siwastaja]

Them kids need to get to grips with rotating machine therory, so its a good exercise.

There's some point in this, because to do this exercise you need to dive quite deep into the AC induction motor design theory, basically how the rotor resistance affects the torque vs. slip curve. But I don't think such modelling is part of the usual curriculum, it's quite advanced theme in motor industry R&D, it's like discussing how the amounts of different doping materials in semiconductor designs affect semiconductor parameters, but those who want to use the semiconductors don't need to understand this.

Modern-day AC induction motor design, as far as I know, ignoring some very cheap fan motors etc., tries to minimize the rotor resistance (against cost, of course, so aluminum would be used instead of copper because it's good enough and cheaper); but purposely increasing the resistance beyond the usual cheap-assing would not be a thing anymore because with variable frequency drives, flattening/widening the torque vs. slip curve and moving breakdown torque left on the curve are not sensible targets anymore (and they worsen the efficiency, significantly).

So while I have learned quite some rotating machine theory (and would like to learn more), learning about the equations from rotor resistance to torque characteristics with a motor design is not very high on my list.

But maybe the OP is going to design the motors (not just motor drives / applications for motors) in which case this exercise, while dated, could still be valuable.

[shamooooot]

Thank you guys for the very valuable inputs..

I found out that I can buy the slip rings separately, or I have to find a motor that already have them.

But I am struggling to know the simplest way to make the circuit and which "Selector switch" to use to switch between these resistances. Also how to calculate their values and are their ratings.

Is this schematic viable?

[shamooooot]

I read that:

"You need to know the rotor resistance, and then use a resistor bank that is variable between zero and roughly 3 times the rotor resistance. By the time you have got from zero to 1 x the rotor resistance you will have lost half the torque"

I now wondering about the rating of these resistors..

[Siwastaja]

That picture is wrong. The whole point is that resistors go in series with the rotor windings, not the supply (supply is to stator winings). Which also makes this practically impossible if modifying a normal off-the-shelf motor; the slip rings are the easiest part, but how are you going to put anything in series with the rotor windings when said "windings" are just a block of aluminum, melted into the rotor slots, all shorted together? I might be wrong but I think motors with adjustable rotor resistance need to be designed ground-up to be that way (and that's also why it's such a big compromise, in RUN position you have the added resistance of the thinner connections, slip rings and the relay contacts, sacrificing RUN efficiency.

e.g. see https://circuitglobe.com/wp-content/uploads/2016/01/Starting-of-an-Induction-motor-fig-2-768x354.jpg

[Benta]

^This.
The motor has to have a wound rotor and integrated slip rings for your idea to make sense.
Industry-standard "squirrel cage" induction motors will not support it.
You won't find any motors with wound rotors and slip rings below the 20 kW range, and even those are very rare today.

[bdunham7]

You won't find any motors with wound rotors and slip rings below the 20 kW range, and even those are very rare today.

These motors were used in things like overhead cranes and large conveyor belts in cases where they were large enough that a normal high starting torque motor would be inconvenient or inefficient enough to matter.  They can be centrifugally switched so it might not be totally obvious that you have one. But today I can't imagine finding one as a new product unless it were some extremely large specialty case.  Regular COTS inverter-drives and motors are readily available up to at least 1000HP.  However, I'm sure there are still plenty of 40 year old examples still out there spinning away.  They weren't a bad solution if you could live with a fixed final speed.

https://www.baldor.com/catalog/IDDRPM4410004

And that's just COTS low-voltage (460V in this case).  The Allen Bradley/Rockwell Powerflex VFDs go to 34,000HP @ 4160V and there's probably no real limit.  When you get to this level the power bills are quite substantial so the initial expense isn't much of an obstacle.  Besides that, a VFD system is way more versatile electrically, and mechanically if you engineer everything around that capability in the first place. 

https://www.rockwellautomation.com/en-us/products/hardware/allen-bradley/drives/medium-voltage-ac-drives/powerflex-7000-ac-drive.html