3-phase electric motors

[N1755L]

Hello all,

I've recently posted to a newsgroup, but was getting practical advice rather than theoretical information. Thought I'd check here see if someone has interest in promoting understanding for its own sake. What I posted is as follows:

Hiking wrote:

> ...
> We have a motor which can be hooked up either in Y configuration, or Delta configuration. Which configuration will give more start torque, and then the most operational torque, and why in each case?
> ...


Never mind guys, I wasn't so much concerned with which configuration to connect for its own sake, the boys here will do trial-and-error and will wire it from there. The guys here don't care why. For myself, I was only interested in the "why", in understanding the "why"s of this situation.

I remember from school many, many years ago that torque was a result of the phase differential between the outer winding (what do you call this one again?) and the induced EMF in the armature, the greater the difference, the more torque you get. As the armature gains speed and nears synchronization with the rotating EMF in the outer core, the torque is reduced conversely. I understand the EMF in the outer core is not really "rotating", but the effect is of a rotating field... and if I understand this correctly, by increasing the frequency of the outer-core field, one could increase the operational speed of the armature, correct? I suspect, however, that this higher frequency would also result in a reduced starting torque capability for the same motor?

But this is in a Squirrel Cage, single-phase induction motor, I have no idea what the situation is with a 3-phase motor... I expect it's the same operational principle, only difference being that there are three outer-core windings.

What I was really hoping to understand is how these dynamics are different with these two different wiring configuration.

If anyone can explain what happens in this motor/system under these two varying configurations, and has the time and desire to explain it, I am all ears and appreciative.

Can you guess I drove my elect teacher nuts? I have to hand it to him, however, he was extremely knowledgeable about the internal, theoretical method of operation of elect motors... good 'ol Mr. O'Brien. Though he seemed to think it was a waste of time for me to understand elect theory to this level, he did humour me and I did understand a lot of it and it still is a big help... the problem being that as soon as I learned it, I ended up going into another field of work and never used/applied, nor thought of electricity since, for over 12 years, so, unfortunately, forgot a lot, and am just trying to refresh my memory.

If you're in a teaching mood, I'd love to learn/understand this subject a whole lot better. Thanks.


Which is why I'm here now at eng-tips.com... perhaps you might know of excellent on-line resources where one can obtain the kind of information I'm looking for?

Thank you.

 

[DRWeig]

Hiya Hiking,

Somebody may shoot me down, but I don't think for a given motor that wye versus delta connection has any effect at all on torque. It's only an accommodation for the type of controller and electrical system.

Put it this way: If I have a 480/3 motor, I can wire it delta or wye. The net effect (current through each winding) is the same, and the phases are still offset by 120 degrees.

With the same currents going thru the same coils, torque and all other properties should be the same.

Delta windings have higher internal voltages to cope with, so the insulation level has to be higher.

Is this kinda what you were looking for?

Old Dave

 

[jghrist]

See

http://www.reliance.com/mtr/mtrthrmn.htm

for basic motor theory.

 

[dpc]

Maybe I don't understand the question, but if a motor designed to run in delta is connected to the same supply in wye, there will be a big difference in torque.

In a delta-connected motor, each winding sees full phase-to-phase voltage (e.g. 480 V). If the same motor is now connected wye, each winding sees only 277 V, and current input will be much less.

Since the torque varies as the square of the voltage, a motor connected wye will produce only 1/3 the torque of the same motor connected in delta. This is the principle behind wye-start, delta-run reduced-voltage motor starting.

If you are talking about two different motors, one designed for wye and one designed for delta, then everything else being equal, I agree the torques would be the same.

 

[DRWeig]

Thanks dbc,

I wasn't very clear in my explanation. I was talking about motor construction options. Should be no performance difference.

For a given motor, though, you said it best. Other way around is where I first experienced it -- motor designed for wye (277 v/winding), hooked up delta (480 V/winding). Smoked it.

Good on ya,

Old Dave

 

[mc5w]

However, high rotor currents when at locked rotor and a little above actually impair torque production. High rotor resistance when starting ( wound rotor, double squirrel cage, deep rotor bars, design D ) actually improves starting torque to some extent. Ideally, what you want is high rotor resistance when starting and low rotor resistance when running. There are several ways to achieve this as mentioned above.

The really big advance was the die cast aluminum squirrel cage for rotors. You could make all kinds of funny shaped deep slot and double squirrel cage designs. Aluminum also has a permeability of 14 ( transformer steel more like 5,000 to 6,000 at 1/2 saturation ) which actually helps by increasing rotor reactance at locked rotor. If you drag a magnet through aluminum machine tool chips the smaller pieces will magnetize and stick to the magnet. I have searched for a dropped steel light fixture part by dragging a magnet through aluminum chips from drilling automotive wheels. The magnet actually collects quite a bit of aluminum!

I have also seen a patent for a wound rotor motor that had the starting resistors on the rotor ( no slip rings ) and used a centrifugal switch to cut out the resistors. A more modern patent used light activated silicon controlled rectifiers on a wound rotor with a amplitude shift keying of the control light.

 

[Marke]

Hello Hiking

The motor should be connected in the format for which it was designed to operate at the preferred line voltage.

When a motor is designed, there is a design voltage and frequency. The motor iron has a maximum flux. If you increase the flux beyond this, you will have a dramatice increase in iron loss and this will cause excess heat and premature motor failure. If you operate at reduced flux, the maximum torqe output from the motor will reduce and the effective rating of the motor is reduced.

If a motor is designed for operation at 400V in delta, then it will be correctly fluxed at that voltage. To get the same flux in the iron with a wye or star connection, you need to increase the line voltage by root three. i.e. run at around 660 volts in wye connection. If you try to operate this motor at rated torque at 400 volt in star, the rotor slip will be increased resulting in excess rotor current and failure.

If you are designing a new motor, you can design it for wye or delta operation at your line voltage and you will see no appreciable difference in performance between a wye designed motor and a delta designed motor operated ath their design voltage.

Best regards,



Mark Empson

http://www.lmphotonics.com