AC induction motor

  • #1

Main Question or Discussion Point

What are the factors that affect how much current is drawn into an AC induction motor?
I dont understand how a motor draws more current to start. I get why it would need more to start but I don't get why it would draw current into itself automatically. It may not work like that at all! But I could use a good explanation on this one. I am assuming that the power needed to saturate the core in the windings is greater on starting.?. How do you regulate how much power is delivered to the motor? If I plug a motor right into the socket(mains)of a 15amp circuit what is stopping the motor from drawing more or less or tripping the breaker? What is the power actually used for, I can see how there is power used up as resistance in the wire in the coils but how much power is used to saturate the iron cores(or ferrite whatever)? I have some knowledge of electricity but I need to fill in some blanks here.
I do have a few more questions on induction motors but I think there are enough for right now! Thanks
 

Answers and Replies

  • #2
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Do a search for inrush current and you will see a lot of the answers to your questions.
http://en.wikipedia.org/wiki/Inrush_current
http://ww1.microchip.com/downloads/en/AppNotes/00887a.pdf
http://en.wikipedia.org/wiki/AC_motor
http://lmgtfy.com/?q=ac+induction+motor+inrush


Sorry...I had to do it :)

Inrush is due to lack of back EMF at startup.
Regarding your question about circuit protection, household breakers are typically designed to ignore inrush currents - to an extent anyway (slower reacting).
In situations where this is still a concern, inrush limiting devices (NTCs) are used with motors to prevent this from happening.
 
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  • #3
Thanks, but if you post it I have probably already read it. And it didn't really answer my question thats when I decide I need help from the good people at Physics Forums but I did learn a new word! But really thanks for responding, anyways.. So the inrush current is used to magnetize the cores? I don't really undertstand inrush current still. The description I got was that the inrush current happens because there is little resistance in the circuit because it hasn't heated up yet. I don't get that. I understand how temperature affects conductivity and resistance. But if it was cooled to superconductor temperature then the motor would continue to draw in a ton of current? What about the cores, there is some loss in an inductor right? Where does that loss come from? As I understand an inductor it becomes saturated with a magnetic field(need a little help with this description) by the magnetic field created in the coil wrapping around it. So the current slows as in passed through the inductor until the core is fully saturated than the current continues to flow as if it wasn't there, and then when the circuit is opened and no more current passes through it the magnetic field in the core induces a current in the wire until the magnetic field in the core is gone. Yea so where do you lose current there? And the factors of this loss are what?
I thank you fine people once again.
 
  • #4
Averagesupernova
Science Advisor
Gold Member
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Start over. Clear your mind. Now, think of the stator as a primary winding of a transformer. The rotor is the secondary, and it is short circuited. The stator appears to be generating a rotating magnetic field. The short circuited rotor windings are constantly passing through the field. What is the result? The rotor wants to follow the rotating magnetic field. If the rotor is running at synchronous speed then NO lines are being cut, so no currents are induced in the rotor. When the rotor is stalled upon startup, more windings in the rotor cut through the magnetic field, hence current flows in the rotor, and this is causes more current passing through the stator winding just as in a normal transformer.
 
  • #5
Well I like your answer and the way it was delivered and it is helpful, but what if the rotor is made of PMs? Honestly I am wondering what I designed, I thought it was a brushless dc motor and I asked a question on here about speed controllers. Then I realized it is more of an induction AC motor. Here is what I did: the stator is on the inside, it is made up of 12 windings. The coils are wound in opposite directions and connected in series. The rotor is on the outside and is made up of 12 PMs with opposite polarities facing in N,S,N all around and the whole thing is symmetrical. I am still working on starting it, I was thinking about having 2 'starter coils' closer to two of the coils so that it starts in one direction, and they would be powered by capacitors. Once it got moving then the power would be delivered to the series of coils, one circuit, and an alternating current would power the coils, speed increasing with frequency. Haven't worked out the details and still have a lot to learn but that is what I am working with here. It basically looks like a hard drive motor. So I just need to learn what I can about using this design. Like how long it takes to magnetic the cores. An induction motor must slightly lag behind the frequency delivered right? I would be ideal for them to magnetize and demagnetize(the cores of the windings) instantly but I am pretty sure it doesn't work that way.
 
  • #6
vk6kro
Science Advisor
4,081
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  • #7
Thanks for the reply. I'd like to think I am beyond such animations. I know how it works for the most part I am just looking for details. Specific answers to my specific questions. I would like to know what kind of circuit is used to control such a motor from a battery bank. That is the most important thing right now. The inrush current is really important as well. So is it because the wires haven't heated up increasing resistance? What if you used superconducting wire? Would it continue to draw in all that current. I need an explanation from a PF member. Come on show your stuff and help me out here. AverageSupernova
is that a squirell cage motor you described?
 
  • #8
4,662
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Thanks for the reply. I'd like to think I am beyond such animations. I know how it works for the most part I am just looking for details. Specific answers to my specific questions. I would like to know what kind of circuit is used to control such a motor from a battery bank. That is the most important thing right now. The inrush current is really important as well. So is it because the wires haven't heated up increasing resistance? What if you used superconducting wire? Would it continue to draw in all that current. I need an explanation from a PF member. Come on show your stuff and help me out here. AverageSupernova
is that a squirell cage motor you described?
Induction motors require an AC voltage to operate, and the iron core of a properly designed induction motor is never saturated. As pointed out above, the primary coils and iron are just like a transformer without a secondary. Many many years ago (before college), I sawed through the squirrel cage conductors of a 1/4 horse split-phase induction motor and reassembled it. The magnetic path is the stator iron, the rotor iron (both laminated like a transformer) and two air gaps. When I plugged it in, no inrush current. It drew very little current. Faraday's law of induction works. If you run a 4-pole induction motor on 60 Hz, and monitor the RPM with a laser tachometer, the motor will run at ≈1795 RPM, just slightly below the synchronous RPM.

If you want to run an induction motor on a battery, you need a DC-AC inverter. An induction motor with a clamped rotor is like a transformer with a shorted secondary. An induction motor running at the synchronous RPM is like a transformer with no (or open) secondary.

Bob S
 
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  • #9
Thanks Bob. Thanks everyone. I will keep checking this post if anyone wants to add anything about contolling circuits or ac motors in general that might be useful to this thread, still more info to post. Thanks
 
  • #10
Could the motor design I described be contolled by a variable inductor or capacitor? A variable oscillating circuit powered by DC? Once it gets moving of course. Aren't synchronous ac induction motors like the one I described controlled(or could be controlled rather) in a way that is similar to a radio tuner circuit where the knob controls the variable inductor/capacitor. A constant voltage applied with a change in frequency. I know that it is controlled by frequency but could it be controlled by a knob in that same way?
 
  • #11
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  • #12
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You might consider starting-up and varying the angular velocity of a synchronous motor using some feedback technique to give you positional information. Here's a homebrew example. http://www.geology.smu.edu/~dpa-www/robo/Encoder/pitt_html/encoders.html" [Broken]


I should add that your geometry can still get you into trouble with an equal and even number of stator poles and armature phases. I haven't said this right, but 12 on 12 with spit phase drive could fail. If the poles are aligned and the armature at rest, no matter what voltage is placed on the drive windings no torque is put on the shaft to turn it.
 
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  • #13
The RPM of AC induction and synchronous motors are sometimes controlled with variable frequency drives (solid state inverter circuits) like discussed in

http://en.wikipedia.org/wiki/Variable-frequency_drive

Bob S
Thanks for the post I will have to read more about these.
The reason I asked about controlling it with a knob is I looked at that page and the control interface they described in the article wouldn't work for this application(switches and buttons). I was hoping to simplify the circuitry as well, well actually they don't show the circuit on that page but I have seen others that are really complicated and I don't think it is necessary for what I am doing. If I could control the frequency with a variable inductor it would be a simple as can be, I would imagine. Then again I am not an electrical engineer.
 
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  • #14
You might consider starting-up and varying the angular velocity of a synchronous motor using some feedback technique to give you positional information. Here's a homebrew example. http://www.geology.smu.edu/~dpa-www/robo/Encoder/pitt_html/encoders.html" [Broken]


I should add that your geometry can still get you into trouble with an equal and even number of stator poles and armature phases. I haven't said this right, but 12 on 12 with spit phase drive could fail. If the poles are aligned and the armature at rest, no matter what voltage is placed on the drive windings no torque is put on the shaft to turn it.
Thanks I will have to check that page out later. I was thinking about just having 2 'starter coils' closer to 2 of the 12 coils so that when those are energized with high voltage capacitors they will get me moving in the right direction and then the 12 coils will be energized by a constant voltage varying frequency to get up to whatever speed.
 
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  • #15
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Dear BilPrestonEsq,
AC induction motor is basically a transformer. The stator of induction motor can be considered as a primary of step down transformer while the rotor of induction motor can be considered as a secondary of step down transformer.
Suppose your rotor is at stationary and you apply voltage at the stator winding. This application of voltage on stator winding causes a magnetic field that is rotating at supply frequency(50Hz or 60 Hz). Now the rotating magnetic field is cut by a stationary rotor and hence maximum voltage is generated across the rotor winding. As you know that the rotor windings are short from one end and this short rotor winding causes a maximum current to flow. This flow of current in the rotor winding causes a magnetic field and this rotor magnetic field is out of phase compare to stator magnetic field. This intersection of magnetic fields causes the rotor to rotate.
Now the rotor start to rotate slightly lower than stator rotating magnetic field. The rotor can never catches the rotating magnetic field of stator in induction motor. The rotor always rotate slowly compare to rotating magnetic field of stator. If the rotor speed equals the rotating magnetic field of stator than rotor can not cut the stator magnetic lines of forces and hence no voltage can develop across the rotor winding.
 
  • #16
Thanks for the reply. I realized it is just a syncronous ac motor. The motor I described uses PMs. Sorry for confusing this. I was thinking that it uses "inductors" because of the coils making it a synchronous ac induction motor. Yeah...anyways if anyone can elaborate on a variable oscillating circuit powered by battery bank. I would like to control the motor with a knob like on a radio tuner by varying an inductor or capacitor to change the frequency delivered to the motor. I hope that makes sense! Thank you.
 
  • #17
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Thanks for the reply. I realized it is just a syncronous ac motor. The motor I described uses PMs. Sorry for confusing this. I was thinking that it uses "inductors" because of the coils making it a synchronous ac induction motor. Yeah...anyways if anyone can elaborate on a variable oscillating circuit powered by battery bank. I would like to control the motor with a knob like on a radio tuner by varying an inductor or capacitor to change the frequency delivered to the motor. I hope that makes sense! Thank you.
If this motor has a PM rotor and no brushes, it is probably a BLDC (brushless DC) motor, and the stator coils are probably internally commutated with a Hall Effect sensor monitoring the orientation of the rotor. I have a 24 volt BLDC Hall Effect motor that has 3 leads: + 24 volts, ground, and resistor pot speed control.

Does the motor have a nameplate? How big is it? How many leads does it have?

Bob S
 

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