Question about single phase AC motors

[Bararontok]

Single phase AC motors generate a rotating magnetic field using two inductors which are turned on and off sequentially. The second inductor is connected to a capacitor to change the phase angle of the electricity delivered to it. When the supply current is turned on, one inductor is turned on while the other is turned off because most of the supply current is used to charge the capacitor connected to the secondary inductor. When the supply current is turned off, the first inductor is also turned off while the capacitor discharges the electrical energy it has stored and this is used to power the second inductor. The constant on and off alternation between one inductor and another produces the changing magnetic field that rotates the rotor.

Is there a single phase AC motor that only uses one inductor to rotate the rotor?

 

[NascentOxygen]

Single phase AC motors generate a rotating magnetic field using two inductors which are turned on and off sequentially.

Possibly the reason you have received no responses is your reference to coils being "turned on and off sequentially." This implies some sort of continuous alternating switching arrangement, which is not correct.

Is there a single phase AC motor that only uses one inductor to rotate the rotor?

Single phase motors can operate with only one winding powered, it's only for starting that they require the auxiliary winding. Do you mean are there induction motors with no auxiliary winding? For low power applications there's the shaded pole motor. http://www.clrwtr.com/Single-Phase-Electric-Motors-Characteristics-Applications.htm

I suppose you could always use a separate second motor to run any motor up to speed...

 

[Bararontok]

The information about the two coils being switched on and off sequentially almost like a two phase motor is found in the following link:

http://en.wikipedia.org/wiki/Induction_motor#Construction

The information also came from a discovery channel documentary about electromechanical clocks that used motors incorporating two inductors being switched on and off sequentially to turn the rotor.

Additionally, the motor that was described in the first post where the second winding is continuously on and out of phase with the main winding could be either a Permanent Split Capacitor Motor or Capacitor Start/Capacitor Run Motor.

Source: http://www.clrwtr.com/Single-Phase-Electric-Motors-Characteristics-Applications.htm

May it also be possible to incorporate a blocking diode into each of the two inductors so that they can be switched on and off sequentially? The diode of the first inductor may permit the passage of the positive cycle to keep it on during this cycle while the diode of the second inductor can do the opposite. The diagram can be found below:

http://img407.imageshack.us/img407/9074/singlephasemotor.png

 

[jim hardy]

I'm confused by the assertions above as to how motors work. I think stepper motor concepts have been mixed up with induction motor concepts. .

I'd suggest reviewing this TI Motor Control Compendium to get vocabulary straightened out. it should answer most of the questions.

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=OLT210201
click the link that says "click here to view presentation"



it's a powerpoint slideshow so be sure you have microsoft's free viewer installed.

oops i just noticed thay have a pdf but i didnt try that one.

 

[Bararontok]

In post #3, a link was already posted to a website about single phase motors that have a secondary winding as a second phase through the use of a run capacitor that changes the winding's phase angle. This secondary winding is not merely a starter winding in these types of motors but is continuously turned on to assist the primary winding in generating the rotating magnetic field.

But the new question is about a design that cannot even be found in the sources provided. Is it possible to use diodes to control two induction windings to make them out of phase instead of using a capacitor?

 

[Bob S]

Is there a single phase AC motor that only uses one inductor to rotate the rotor?

The shaded pole motor has only a single coil; see
http://en.wikipedia.org/wiki/Shaded-pole_motor
Special copper busses on the stator poles produce a phase shift to provide a rotating magnetic field.

 

[Bararontok]

Yes that is correct, but what about the use of diodes to control the alternate switching between two windings in a single phase induction motor? Is such a motor possible?

 

[Bob S]

Yes that is correct, but what about the use of diodes to control the alternate switching between two windings in a single phase induction motor? Is such a motor possible?

No. You have to deal with the stored inductve energy.

 

[NascentOxygen]

The information about the two coils being switched on and off sequentially almost like a two phase motor is found in the following link:

http://en.wikipedia.org/wiki/Induction_motor#Construction

Could you copy the few sentences and paste here, as I can't see it.

May it also be possible to incorporate a blocking diode into each of the two inductors so that they can be switched on and off sequentially?

I'll look closer at it, but diodes, probably not. A diode in series with an inductor is the way to produce a DC component. And the DC component won't do anything useful in an induction motor.

 

[Bararontok]

The lines can be found below:

"Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90 degrees from the single-phase supply and feeds it to the second motor winding."

Here is the diagram for the diode controlled single phase motor so that it can be studied more carefully:

http://img407.imageshack.us/img407/9074/singlephasemotor.png

 

[NascentOxygen]

There is nothing about switching on and off sequentially there. The auxiliary winding can be disconnected once the motor has reached speed, or it can be left connected, according to design.

I'm not dismissing the possibility that someone may invent a new design for an AC motor.

Any DC in the windings represents wasted energy; it generates heat but produces no torque.

 

[Bararontok]

But the auxiliary winding is made out of phase with the primary winding because of the capacitor so that means that the auxiliary winding will not be on at the same time as the main winding if this definition of electronic components being out of phase is correct.

And in the design using diodes to block cycles from the AC source, the inductors will still be powered on at the positive cycle for one inductor and the negative cycle for the other inductor, so each inductor would be running on a mono-cyclic AC current since the current entering each inductor is still fluctuating. After all it is only a DC current if the power level is continuous and constant with respect to time.

 

[Bob S]

The lines can be found below:

"Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90 degrees from the single-phase supply and feeds it to the second motor winding."

Here is the diagram for the diode controlled single phase motor so that it can be studied more carefully:

http://img407.imageshack.us/img407/9074/singlephasemotor.png

...And in the design using diodes to block cycles from the AC source, the inductors will still be powered on at the positive cycle for one inductor and the negative cycle for the other inductor, so each inductor would be running on a mono-cyclic AC current since the current entering each inductor is still fluctuating. After all it is only a DC current if the power level is continuous and constant with respect to time.

AC induction motors have squirrel cages built into the rotor. Any magnetic field in the stator from residual dc currents will create huge eddy current braking.

 

[Bob S]

Here is a picture of a single-phase induction motor with no "starting" coil. It is a repulsion-start motor, made in 1914, with a radial commutator used for shorting some special squirrel cage copper busses inside the rotor. It has a higher starting torque and lower surge current than modern split phase or capacitor start motors.

 

[Bararontok]

Why has this motor become obsolete if it is a better design?

And how does this motor work? Can a circuit diagram be posted in this thread?

AC induction motors have squirrel cages built into the rotor. Any magnetic field in the stator from residual dc currents will create huge eddy current braking.

How can there be residual DC currents in the inductors if they are running on single cycles from an AC current which constantly fluctuates? After all the DC current has to stay on and remain on a constant power level for a given amount of time to be considered a DC current.

 

[Bob S]

Why has this [repulsion start electric] motor become obsolete if it is a better design?

It is about twice as heavy as a modern motor with same HP, and very expensive to build.

And how does this motor work? Can a circuit diagram be posted in this thread?

Look up "Repulsion start electric motor" on web. There are articles discussing theory, and at least three videos showing Century Electric repulsion start electric motors in operation.

How can there be residual DC currents in the inductors [in above posts] if they are running on single cycles from an AC current which constantly fluctuates? After all the DC current has to stay on and remain on a constant power level for a given amount of time to be considered a DC current.

The circuit diagrams in above posts showed half-cycle rectification, which will produce a dc current.

 

[NascentOxygen]

Why has this motor become obsolete if it is a better design?

With a commutator it is going to be expensive to build, and require more maintenance than one with no sliding electrical contacts. (It will be noisy, too, but maybe they lift the brushes after it has run up to speed?)

How can there be residual DC currents in the inductors if they are running on single cycles from an AC current which constantly fluctuates? After all the DC current has to stay on and remain on a constant power level for a given amount of time to be considered a DC current.

If it has an average value which is not = 0, then the voltage contains DC. An average-reading meter will reveal this.

 

[Bararontok]

If it has an average value which is not = 0, then the voltage contains DC. An average-reading meter will reveal this.

But direct current is supposed to remain completely constant over time. If the rectifiers are not connected to a voltage regulating capacitor the output will be either a half wave or full wave alternating current of a single polarity so it is still an alternating current. A list of electric power graphs will be posted below:

http://img69.imageshack.us/img69/2018/electricpowergraphs.png

 

[NascentOxygen]

But direct current is supposed to remain completely constant over time.

No, nothing of the sort. It is never changes "direction" then it is pure DC. It does not have to maintain a constant potential to be DC.

If the rectifiers are not connected to a voltage regulating capacitor the output will be either a half wave or full wave alternating current of a single polarity so it is still an alternating current.

If it never changes polarity, it cannot be alternating. A rectified AC is DC. You could call it fluctuating DC, but it's no longer AC if it never alternates polarity.

 

[Bararontok]

AC induction motors have squirrel cages built into the rotor. Any magnetic field in the stator from residual dc currents will create huge eddy current braking.

What is eddy current braking? How is it caused by DC electricity? Why is it necessary to use an alternating current in the induction winding to avoid this phenomenon?

 

[Bob S]

What is eddy current braking? How is it caused by DC electricity? Why is it necessary to use an alternating current in the induction winding to avoid this phenomenon?

Google "eddy current braking".

 

[Bararontok]

According to the wikipedia article on eddy current brakes, this type of system can use an electromagnet to stop a metallic brake disc from rotating since metals are attracted to magnetic fields.

But how can the constantly reversing current polarity in an electromagnet be used to produce rotation in the rotor if the electromagnetic fields regardless of their polarity will just attract the rotor and keep it stalled? And can this effect be reproduced by splitting the electromagnetic field into two rectifier controlled inductors with the positive polarity going into one inductor and the negative polarity going into another inductor? Or is it really necessary to have one inductor constantly reverse polarity?

 

[Bob S]

According to the wikipedia article on eddy current brakes, this type of system can use an electromagnet to stop a metallic brake disc from rotating since metals are attracted to magnetic fields.

Non-magnetic metallic brake disks. They are not attracted to magnetic fields.

But how can the constantly reversing current polarity in an electromagnet be used to produce rotation in the rotor if the electromagnetic fields regardless of their polarity will just attract the rotor and keep it stalled? And can this effect be reproduced by splitting the electromagnetic field into two rectifier controlled inductors with the positive polarity going into one inductor and the negative polarity going into another inductor? Or is it really necessary to have one inductor constantly reverse polarity?

If only one polarity goes into an inductor, there will be a persistent dc current as well as an ac component. The persistent dc component will generate eddy currents in a motor rotor.

 

[Averagesupernova]

But how can the constantly reversing current polarity in an electromagnet be used to produce rotation in the rotor if the electromagnetic fields regardless of their polarity will just attract the rotor and keep it stalled? And can this effect be reproduced by splitting the electromagnetic field into two rectifier controlled inductors with the positive polarity going into one inductor and the negative polarity going into another inductor? Or is it really necessary to have one inductor constantly reverse polarity?

In a two pole induction motor each pole is the opposite polarity of the other. Why mess with rectifiers? The reason the motor doesn't stall is because at synchronous speed the magnetic field 'looks' to the rotor as if it is rotating.
-
Step 1: The conductor in the rotor gets a 'shove' while next to pole A when the field is growing during part of the AC cycle.
Step 2: The conductor is then swung more out of the field and is 90 degrees from where it started (also not cutting many flux lines) as the field reaches peak.
Step 3: The conductor now is coming back into the field as the field falls and is shoved again while passing pole B.
Step 4: Again, the conductor is swung out of the field and is 90 degrees away from where it started in step 1. Field is about reaching peak again although at the opposite polarity.
Step 5: While the field is falling again, the conductor passes by pole A and the cycle starts over.
-
In order for this to happen, the the rotor must be traveling very close to synchonous speed. You may notice that the direction of the rotor is irrelevant at this point. The direction is determined by the start winding. The rotor couldn't care less which direction it is turning once up to speed. Notice that in order for the rotor to turn the field has to be changing. My description of what parts the field reaches peak may not be 100% accurate but you should get the general idea. Also, keep in mind there has to be 'slip' in order for currents to be induced in the rotor. If there were no rotor currents there would not be any push or pull on the rotor conductors.

 

[Bararontok]

Is the opposite polarity in the inductor necessary to induce rotation? Is it because if it is a constant polarity, the magnetic field will attract the rotor but not turn it? Is it because exposure to a magnetic field magnetizes the rotor so that when the induction winding reverses polarity, the rotor which already has a magnetic field will be repelled by the sudden change in polarity? Does the AC motor work by using changes in polarity to attract and repel the rotor? Is this because the rotor is magnetized with a polarity that is opposite the polarity of the inductor and when the inductor suddenly reverses polarity, the polarity of the rotor will now be the same as the polarity of the inductor and this will cause repulsion?

 

[Averagesupernova]

Ummmmm, the questions just don't end. Bararontok, I am not sure you are thinking about what is going on in an induction motor as you should be. If you were, I don't think you would pose the questions that you are. Think of the rotor as the secondary of a transformer that is short circuited.
-
Any time a wire crosses flux lines a current is induced in this wire. The opposite extremes of such a case are as follows:
1) Open load with nothing connected to the ends of the wire will not produce any mechanical resistance to movement through the magnetic field.
2) Short ciruit the wire (ends connected together) will produce maximum amount of mechanical resistance to the movement through the magnetic field.

Imagine a Honda generator or similar brand that instead of the engine and framework being stationary the whole thing is rotating at synchronous speed of 3600 RMP and the rotor is sitting still. It should work fine other than other obvious technical hurdles like how do you plug into it, etc. etc. With no load, the rotor will have a tendency to sit still. There is no electrical load so there is no mechanical torque between the rotor and the rest of the machine other than from a little friction in the bearings and air drag. Now suppose we plug a heavy load into this generator or even short circuit the output. Now there is a LARGE amount of mechanical torque between the rotor and the rest of the machine. In fact, if we just let go of the rotor, it will spin up and ride along with the rest of the machine. If you understand what I have described, then you should have a pretty good handle on how induction motors work whether they are 3 phase or not.

 

[Bararontok]

Yes, this is correct but it does not explain why a negative polarity must be run through the inductor to produce rotation. It is true that the rotor can be made of a piece of metal or induction winding that will be magnetized when exposed to the magnetic field of an inductor through the principle of mutual inductance but this does not explain how having only a positive polarity in one inductor and running the negative polarity in the other inductor will not produce the rotating magnetic field effect of a conventional induction motor. Of course if only one inductor is being used and it only has a polarized current running through it the rotor will simply be attracted to the inductor and it will not turn.

The original questions of the thread have already been answered and this is the last question though it was only made after the first questions have been answered satisfactorily. The only problem now is understanding why reversing polarity is so important.

In a two pole induction motor each pole is the opposite polarity of the other. Why mess with rectifiers? The reason the motor doesn't stall is because at synchronous speed the magnetic field 'looks' to the rotor as if it is rotating.
-
Step 1: The conductor in the rotor gets a 'shove' while next to pole A when the field is growing during part of the AC cycle.
Step 2: The conductor is then swung more out of the field and is 90 degrees from where it started (also not cutting many flux lines) as the field reaches peak.
Step 3: The conductor now is coming back into the field as the field falls and is shoved again while passing pole B.
Step 4: Again, the conductor is swung out of the field and is 90 degrees away from where it started in step 1. Field is about reaching peak again although at the opposite polarity.
Step 5: While the field is falling again, the conductor passes by pole A and the cycle starts over.
-
In order for this to happen, the the rotor must be traveling very close to synchronous speed. You may notice that the direction of the rotor is irrelevant at this point. The direction is determined by the start winding. The rotor couldn't care less which direction it is turning once up to speed. Notice that in order for the rotor to turn the field has to be changing. My description of what parts the field reaches peak may not be 100% accurate but you should get the general idea. Also, keep in mind there has to be 'slip' in order for currents to be induced in the rotor. If there were no rotor currents there would not be any push or pull on the rotor conductors.

It says in this post that the motor has 2 poles. Are these two poles both connected to a single inductor? It would be better if a diagram or animated GIF of this design is posted here to make it easier to understand the operation of this motor.

 

[Bob S]

....The only problem now is understanding why reversing polarity is so important......

Get a rectifier like the one shown in your post #3, a Variac, and a single-phase fractional HP induction motor. Connect the motor in series with the rectifier and plug it into the Variac, and run the voltage up until the rectified motor current is about 1 amp. Now spin the motor rotor by hand. Do you feel the eddy currents?

Note: Do NOT put the rectifier in series with the Variac input.

 

[Bararontok]

Get a rectifier like the one shown in your post #3, a Variac, and a single-phase fractional HP induction motor. Connect the motor in series with the rectifier and plug it into the Variac, and run the voltage up until the rectified motor current is about 1 amp. Now spin the motor rotor by hand. Do you feel the eddy currents?

Note: Do NOT put the rectifier in series with the Variac input.

Are the eddy currents conducted from the rotor into the motor shaft?

Perhaps the rotor will stall using the circuit diagram employing diodes because each inductor being supplied with only one polarity will act as eddy current breaks being switched on and off in an alternating fashion. Having an eddy current brake on one side and then turning it off and turning on an eddy current brake on the other side will simply keep the rotor in a stalled position and not rotate at all, with the only effect produced being the change of direction of the source of the braking force. The result is nothing more than a braking force that alternates between the 2 inductors. This does not produce a rotating magnetic field, only magnetic fields that are being switched on and off sequentially.

But the question is, how does a constantly reversing polarity in an induction winding produce the rotating magnetic field? Can an animated GIF or diagrams be posted here?

 

[Bob S]

Are the eddy currents conducted from the rotor into the motor shaft?

Perhaps the rotor will stall using the circuit diagram employing diodes because each inductor being supplied with only one polarity will act as eddy current breaks being switched on and off in an alternating fashion. Having an eddy current brake on one side and then turning it off and turning on an eddy current brake on the other side will simply keep the rotor in a stalled position and not rotate at all, with the only effect produced being the change of direction of the source of the braking force. The result is nothing more than a braking force that alternates between the 2 inductors. This does not produce a rotating magnetic field, only magnetic fields that are being switched on and off sequentially.

But the question is, how does a constantly reversing polarity in an induction winding produce the rotating magnetic field? Can an animated GIF or diagrams be posted here?

The best thing you can do is get an old washing machine split phase 1740 RPM (60 Hz) or 1450 RPM (50 Hz) induction motor and take it apart. Note first the squirrel cage on the rotor and the four main coils on the stator. Note also the four smaller starting coils on the stator midway between the main coils. The starting coils have smaller diameter wire and a higher dc resistance than the main coils, so they have a lower L/R time constant. This means that when a voltage is applied to the coils, the current in the starter coils has a phase shift relative to the main coils. This phase shift, plus the location of the starter coils on the stator, produces an apparent rotating dipole field in the rotor. Reversing the connections to the starting coil reverses the rotation of the motor.

 

[Averagesupernova]

Here is a place to start: http://www.google.com/search?q=sing...esult_group&ct=title&resnum=1&ved=0CBcQqwQwAA