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Anonymous Poster

Universal motors

11/22/2009 11:22 AM

If a universal motor runs on both AC or DC, why is my universal device labeled as AC?

Universal motors do not depend on frequency, correct?

I only ask because If the device speed is affected by frequency,(which i assume AC motors are)? ..it's a good thing to know before potentially burning out the motor while using overseas.

Any help appreciated

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#1

Re: Universal motors

11/22/2009 12:54 PM

Universal motors are really DC motors, but the design of the DC windings is such that applying AC to it has the same effect as applyng DC. So if you feed it AC, the motor itself essentially converts it to DC, if you feed it DC, it just runs the motor. Voltage is what is important, not frequency. Speed is controlled by altering the voltage (or having multiple steps of field coils, like a multiple speed blender). Just make sure when you run it over seas, you are powering it at the same voltage level it was designed for.

The potential pitfall is if your device has other electrical systems associated with it, as many do now because we like "high tech" toys. If so, then it will depend on where the power comes from to run those extras. If there is a SMPS built in which has it's own rectifier and maybe a voltage switch (i.e. 115 / 230V), then it should be OK. If the extras are powered by a cheap internal power supply, it may not be able to track the different frequency. A nameplate should say f it is 50/60Hz.

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#2
In reply to #1

Re: Universal motors

11/22/2009 2:10 PM

Exactly correct JRaef. Some manufacturers anticipate that their universal DC motor will likely be given an AC source. So the current rating, power, torque, unloaded RPM numbers and any other parameters specified will be including the inductance for the frequency of the source they used for testing. By running this type of motor with a voltage and frequency other than the one tested will typically not produce a catastrophic failure, within reasonable limits. For example, I would expect a 200VAC 50Hz universal motor to run just as well at 240VAC 60Hz. But a 120VAC 60Hz motor connected to a strictly 120VDC source will draw more current and produce more torque etc. than the 60Hz rating since inductance drops out of the current limiting equation with a DC source. Likely this higher current will not overheat the motor but one would have to know more information to make a more confident assessment. Connecting a 120VAC motor to 480VAC of any combination of frequency ratings will just be inviting trouble.

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#3

Re: Universal motors

11/22/2009 2:45 PM

Hi, thanks for the replies guys.

I've just searched everywhere on the device, and box. The only information is Volts (120) and watts. No hertz. Do i take this as a good sign?

I know the voltage won't be a problem overseas as i'll use a converter. It's the hertz i'm worried about. Any change in the frequency, i'd rather not use it.

Initially i thought it to be ok given that the motor is 'universal'. The only thing that led me to believe it may not run well is the 'AC' label. ..Or is this not important?

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#4
In reply to #3

Re: Universal motors

11/22/2009 4:04 PM

Pls be sure Universal motors are dependent with the frequency.

In fact they are the serial connected motors, built for driven by AC.

Certainly they can run when DC voltage is applied. But their behaviour is difference.

Their stator and rotor windings have inductance to resist against the AC power source. All the manufacturer calculations are based on this total impedance in AC source.

When they are fed with DC, there is no inductance, therefore no impedance.

Therefore the total resistance in DC is lower against the time it is working with the same voltage in AC.

This means the motor will draw much current in DC and turns faster. and more powerful of course.

But this extra power is out of manufacturer's considerations. The motor heats, creates more sparks from the brushes.

İn the event of DC, the supply voltage must be lowered accordingly to match the same current with A.C. (Or a serial resistance may be connected to limit the current)

But eventually if your question is the matter of 50 Hz - 60 Hz. difference at the overseas, do not worry, . The power de-rating in 60 Hz is non remarkable..

Kindest Regards

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Anonymous Poster
#5

Re: Universal motors

11/22/2009 9:24 PM

http://www.phys.unsw.edu.au/hsc/hsc/electric_motors2.html

http://mounttaylor.com/t120-spr09/reference/ib10-25-universal-motors.html

http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee2/bdeee2_1-4.aspx

The reactance (absence of) or the voltage drop is compensated by the back emf.

Look at the warnings "Never run the universal motors with low load or zero load, even in the starting" as we tend to do with normal AC motors "the runaway speed may be uncontrolled and may damage the motor" - this is essencially due to no current limiting.

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#7
In reply to #5

Re: Universal motors

11/23/2009 1:21 AM

50 odd years ago electric drills etc ran on AC or DC and there were many towns (and ships)still on DC. AC replaced DC with progress.

As power tool prices fell the switch cost became a significant matter, and customer need shrank, so a cheaper ac only switch went into later production tools.

AC arcs at the switch contacts are quenched at the next zero crossing of current, the energy stored in the motor's indductance makes hot DC arcs longer and they persist like an arc lamp. Just check catalogues, for switch AC & DC ratings and prices.

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#11
In reply to #5

Re: Universal motors

11/23/2009 12:44 PM

You wrote:-

Look at the warnings "Never run the universal motors with low load or zero load, even in the starting" as we tend to do with normal AC motors "the runaway speed may be uncontrolled and may damage the motor" - this is essentially due to no current limiting.

I disagree with this statement to a degree, it applies only to one type of AC/DC motor, not to all.

It depends upon whether the field coils are in series (then it is true) with the armature or parallel (then not true!)....

Please read here:-

http://www.tpub.com/content/doe/h1011v2/css/h1011v2_114.htm

or

http://zone.ni.com/devzone/cda/ph/p/id/54

I hope this helps with the understanding of the way the two motors differ. It does not matter if they are fed AC or DC, the effects are the same....

There is a further, I believe less often used nowadays, but I could be wrong on that point, type of DC/AC motor that is called a compound and it has both a series and a parallel (or shunt) winding/coils......it has generally the good characteristics of both motor types, but this does depend on the "balance" between the types. Not a subject for here!!!

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#12
In reply to #11

Re: Universal motors

11/23/2009 2:32 PM

Dear Andy

As far as I know the universal motors are a type of serial motors. In other words, they are all serial.

Electric drills, some kitchenwares like mixers, blenders, sewing machine motors are the examples for universal motors.

Serial motors reach uncontrolled high speeds when not loaded, and therefore damage their rotor windings and their armature sections due the huge centrifugal forces.

Shunt (Parallel) and compound motors are out of universal classification.

Pls. let me know if I am wrong.

Regards

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#14
In reply to #12

Re: Universal motors

11/23/2009 6:46 PM

It simply depends on the job the motor is required to do do. If there are times when no load or even almost no load is on the motor, it will overspeed if serial.

Size is also a factor here....both series and shunt are made and used for various jobs.....

A further problem of serial motors is that the speed varies dramatically with load, so for many jobs that is unacceptable.....

A fan has a very steady load moving air, this would be a good use for a serial motor....

A mains drill though would be difficult to use with a serial motor as it would speed up dramatically till drilling starts.....then it would slow down dramatically.....so that would be a good use for a shunt motor....

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#16
In reply to #14

Re: Universal motors

11/24/2009 1:48 AM

Drills are all serial motors.

Remove the covers of any of them to prove this yourself. You will see Rotor and stator are serial via the brushes.

Shunts are parallel motors, of which rotor and stator are parallel connected.

I did not see a common use of shunt (Parallel connected) motors in household units.

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#18
In reply to #16

Re: Universal motors

11/24/2009 4:36 AM

The drills that I have (at least the ones I have taken apart for maintenance over the years) are shunt,......and a really old one from B&D (that still works well, I bought it in 1973 or 4), two fixed speeds only, I have had it apart several times, is also shunt. The speed also dose not vary worth a tad........unless you change gear!!!

Where are you in the world, maybe other manufacturers do things differently where you are......?

To test if shunt of serial (for anyone who does not how), measure with an Ohmmeter between the pins on the mains plug (not the earth if one is fitted!), then you open the drill up and place a piece of paper under one brush so that it cannot touch the commutator, and measure again.

Open circuit means serial. Still a circuit, but with a higher resistance than before, then its a shunt motor......

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#19
In reply to #18

Re: Universal motors

11/24/2009 6:44 AM

Oh impossible.

The ones I have just replaced the brushes recently. A Bosh, 850 Watt Drill hammer, and te other is a Japanese Makita.

As soon as I remove the brush, mains plug side is open. No conductivity at all.

This is pointing a serial connection.

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#21
In reply to #19

Re: Universal motors

11/24/2009 8:13 AM

Yes it is.

But one Skylark does not make a summer.......

I expect different manufacturers and/or age play a distinct role.....all my electric drills (except the rechargeable ones) are old......I use mostly rechargeable for any work around the house.....

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#22
In reply to #21

Re: Universal motors

11/24/2009 9:45 AM

Startup torque is high at the serial motors. Even when they triggered under full load, the speed climbs up easily. This is the main reason, the hand drills, some similar household appliances are equipped with serial motors.

Of course obtaining a constant speed at those motors is rather difficult. As you mentioned before, their speed is very dependent with the momentary load.

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#20
In reply to #18

Re: Universal motors

11/24/2009 8:09 AM

I forgot to add, press the trigger when measuring.

If there is no connection at all, there will be a PWM unit (electronics) blocking the signal path, then you need to open up the trigger grip and measure AFTER the electronics.....

My apologies for forgetting this important part.....

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#23
In reply to #16

Re: Universal motors

11/25/2009 1:31 PM

I agree with you and believe the reason for domestic el appliencies being serial motors is hasard of fire. Lost of contact anywhere - and there is no current flow altogether.

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#15
In reply to #11

Re: Universal motors

11/23/2009 10:28 PM

Yes you are absolutely right. Missed the shunt excited universal motors (and the other one - DC motor converted through the rectifiers to be an AC motor)

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#17
In reply to #15

Re: Universal motors

11/24/2009 4:21 AM

I wanted to keep it simple and easily understood..

You have still missed several more motor types..........do you know what they are?

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#27
In reply to #5

Re: Universal motors

11/26/2009 4:25 AM

The reactance (absence of) or the voltage drop is compensated by the back emf.

Good Point.

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#6

Re: Universal motors

11/23/2009 12:54 AM

I would like to give an example of what works and doesn't.

I have an 30 year old electric drill that is on or off trigger. It is a universal motor. It works fine on DC.

I have a new electric drill with a variable speed trigger. It has a universal motor also. BUT on DC the variable speed triggher fried, and now it is either on or off. Bummer.

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#8
In reply to #6

Re: Universal motors

11/23/2009 1:38 AM

Quoted "DC the variable speed triggher fried, and now it is either on or off"

This is very normal at DC. because the speed controllers are composed of thyristors or most commonly triacs. They work like the light dimmers.

They need Alternating voltage to work, of which pass 100 times from zero voltage level. Otherwise they don't cut off and continue conducting until you switch off manually..

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#10
In reply to #8

Re: Universal motors

11/23/2009 11:04 AM

(Thanks --- I already new that. -- I was trying to give an example of why an apparently universal motor (appliance) may be labelled AC. Other circuitry may limit its utilization.)

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#9
In reply to #6

Re: Universal motors

11/23/2009 9:30 AM

That is because the electronic circuit works on AC only even though the motor is still an universal one. The main compnent in most variable speed controlls is a triac which has to have alternate cycles to turn on and off properly.

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#13

Re: Universal motors

11/23/2009 2:52 PM

By the way, what size motor are we talking about?

What is the application?

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#24

Re: Universal motors

11/25/2009 7:49 PM

If a universal motor runs on both AC or DC, why is my universal device labeled as AC?

Universal motors do not depend on frequency, correct?

I only ask because If the device speed is affected by frequency,(which i assume AC motors are)? ..it's a good thing to know before potentially burning out the motor while using overseas.

Any help appreciated

Hi, Motors are classified as either AC MACHINES OR DC MACHINES, We are dealing with DC machines. DC machines are manufactured and tested as DC machines.However with the comutator design, it is possible to run this DC machine with AC.

Types of DC machines are as follows:

(1) SERIES MACHINE. This machine is also called a universal machine. The name UNIVERSAL was given to this machine because the machine will operate satisfactorily with a high or low voltage and also with a high or low frequency. Although high or low voltage and frequency will affect machine performance, the machine WILL STILL OPERATE SATISFACTORILY.

The field coil is connected in SERIES with the armature, in such a manner that their magnetic fields are cumulative.Thus as the load increases, the current increases,as the current increases the magnetic fields of the armature and stator increases, this results in increased torque. This process repeats itself continuously the machine will operate slower and slower but will not stall under increasing load, until the machine comes to a complete stop and burns itself up.

By design, the commutator allows current to flow to the armature in one direction only. So the armature always sees DC current. The commutator is also called a mechanical rectifier.Hence if you apply AC to this machine,the field coil sees AC and the rotor sees DC.

The speed of a series machine (universal machine) in theory is infinite on no load.However,by design there are bearing friction loads and windage loads.These loads are relatively small,and the machine can still overspeed and destroy itself. Strapping down the wires on the stator and by using epoxy varnish help to avoid self destruction, but if the temperature rises moderately high,the epoxy may become spongy and the machine will self destruct.

The series machine is used on all drills for the torque reasons above. The shunt machine WILL STALL when drilling even the smallest of holes and shunt machines are not suited for drills,elevators etc.

One cr4 member mentioned his drill is a shunt.He needs to pull the machine apart,and physically trace the connections.Be aware of any R/C or R/L/C radio intereferancecircuits connected across the armature.

SHUNT MACHINES

COMPOUND MACHINES

CUMULATIVE SHORT SHUNT MACHINE

NON CUMULATIVE SHORT SHUNT MACHINE

SHORT SHUNT MACHINE

LONG SHUNT MACHINE

CUMULATIVE LONG SHUNT MACHINE

NON CUMULATIVE LONG SHUNT MACHINE

REGARDS

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#25
In reply to #24

Re: Universal motors

11/26/2009 2:37 AM

I will do just that the next time it (one of them, I have several for different jobs) needs pulling apart. Good idea.

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#26
In reply to #24

Re: Universal motors

11/26/2009 3:30 AM

Thanks Taslim. Good explanation.

I only want to draw your attention on the point , a reminder, of which I believe known by everybody.

Even universal machines can work both AC and DC,

The exact DC machines are built as solid core at the stators. (Say as salient poles.) They are not built of thin laminated sheet metals. There is no need them during working with only DC case. No matter whether they are serial or shunt , or compound connected.

They cannot work with AC, due the high Fucoault losses.

Of course they cannot be considered in the coverage of universal motors.

Coming to the other matter, "AC power, is rectified at the carbon brushes, " It is only a simulation.

In fact the power is not exactly rectified. It's still AC. The current, of which flowing in the rotor and the stator windings are still AC.

Carbon brushes are only achieving the pole configuration between stator and rotor windings to match, to be the same pole to push each other. It's only a commutator.

Kindest Regards.

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#28
In reply to #26

Re: Universal motors

11/27/2009 12:10 AM

Hi Nezihozfirat,

Before I give an answer to your contribution, I must correct an error in my last contribution. I said in my contribution that straps are placed on the stator....this should be ROTOR. I am sorry for this mistake.

I, will try to explain further what I said and I will explain the contradictory points that you raised.

NOTE CAREFULLY WHAT I AM SAYING.

ALL DC MACHINES CONNECTED IN SERIES ARE KNOWN AS A UNIVERSAL MOTORS FOR REASONS I GAVE PREVIOUSLY. THERE ARE NO DC MACHINES BUILT TO DATE, WHERE THE STATOR IS A SOLID CORE. SALIENT POLES ARE POLE SHOES AND THEY ARE LAMINATED. THERE ARE HOWEVER, UNIVERSAL MOTORS WHERE THE FIELD COILS IN THE STATOR ARE REPLACED WITH A PERMAMENI MAGNET,BUT THIS DOES NOT NEGATE THE FACT THAT IT IS STILL A UNIVERSAL MOTOR.

PLEASE UNDERSTAND THIS:

THERE ARE NO AND I MEAN NO EXAMPLES WHERE AN ELECTRICAL COIL IS WOUND ON A SOLID SOFT IRON CORE!!!!!!!SO BE IT A MOTOR ROTOR, A MOTOR STATOR, AN ARMATURE, A FIELD COIL, A CONTACTOR COIL, A RELAY,A TRANSFORMER.(THE REASON FOR THIS IS INCREASED FLUX DENSITY CAN ONLY BE ACHIVED BY LAMINATIONS)

THE UNIVERSAL MACHINE WITH A FIELD COIL OR A PERMANENT MAGNET CAN WORK WITH EITHER AC OR DC. THE ONLY DIFFERANCE IS THAT WITH A FIELD COIL UNIVERSAL MACHINE IF YOU REVERSE THE POLARITY OF THE INPUT, THE ROTOR SPINS IN THE SAME DIRECTION, WHEREAS IN A PERMANENT MAGNET UNIVERSAL MACHINE IF YOU REVERSE THE POLARITY OF THE INPUT, THE MACHINE SPINS ON THE OPPOSITE DIRECTION.

PLEASE FORGIVE ME. I NEVER SAID "AC power is rectified at the carbon brushes" NOR THAT I IMPLIED THAT. WHAT I SAID OR IMPLIED IS THAT AC current is being rectified at the ........YOU CANNOT RECTIFY POWER !

THIS IS NOT A SIMULATION. SURELY YOU HAVE NOT SCOPE OUT THE WAVEFORMS AT THE FIELD COILS AND ARMATURE WHEN A UNIVERSAL MACHINE IS RUNNING ON NO LOAD AND ON LOAD. IF YOU DO THIS YOU WOULD SEE A SINE WAVE ACROSS THE STATOR WINDINGS AND A DC WAVEFORM ACROSS THE ARMATURE.(IF YOU MISTAKENLY SEE A SQUARE WAVE ACROSS THE ARMATURE, THIS ACTUALLY A DC. JUST LOOK FOR YOUR 0 RFEFRENCE POINT.) IF YOU GET THE OPPORTUNITY MEASURE THESE WAVEFORM PLEASE SHIFT THE BRUSHES FROM THE MAGNETIC NEUTRAL PLANE TO THE GEOMETRIC NEUTRAL PLANE. LOOK AT THE CURRENT DRAW DIFFERENCE AND ALSO THE INCREASED HEAVY SPARKING OF THE BRUSHES.

HEY, YOU CANNOT RECTIFY POWER. COMMUTATOR OR COMMUTATING MEANS REVERSE OR REVERSING CURRENT, YOU CAN ONLY RECTIFY CURRENT.YOUR FINAL PARAGRAPH SAID "to be the same pole to push each other." How can you achieve this when your incoming current is CHANGING POLES " AC CURRENT." If your incoming current is constantly changing poles (120 hz), How can you achieve "to be the SAME POLE to push each other? The answer lies in COMMUTATION (THIS IS FOR ANOTHER FORUM)

I may have answered a bit firm, but the standard of this forum must and shall be maintained by all of us.

Please be guided,

Regards,

Taslim.

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#29
In reply to #28

Re: Universal motors

11/27/2009 2:45 AM

Answering firm is this, shouting/insulting is THIS, and this is quoting from other source.

1. There are no DC machines built to date where stator is a solid core- wrong statement. Stators are usually solid soft iron in case it is a rotating coil type machine.

2. The purpose of lamination is not to increase flux density, rather they reduce by reducing available iron area for the same volume. The purpose of lamination is to cut down the eddy current loss.

3. I am not aware of PM universal motor, may be they exist. But the principle of Universal machine does not make that simple, but then who knows.

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#32
In reply to #29

Re: Universal motors

11/27/2009 9:07 AM

Its seldom that I agree with a guest, but you are right on all counts as far as I can tell......

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#36
In reply to #32

Re: Universal motors

11/27/2009 11:11 AM

I am member on a mission, hence anonymous.

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#37
In reply to #32

Re: Universal motors

11/27/2009 11:13 AM

Sorry closed the post in between

in original avatar, we have agreed as well as disgreed in a few posts , so don't feel bad about agreeing with this guest who is not

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#38
In reply to #37

Re: Universal motors

11/27/2009 1:30 PM

Thats as it should be in a normal world.

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#30
In reply to #28

Re: Universal motors

11/27/2009 8:30 AM

Dear Taslim

There may be a confussion.

DC serial machines, (Of which are designed to work only with DC) are equipped with Solid Iron stator fields. But only the stators, not the rotors. (I am in doubt if they can be included in the Universal motors classification.)

The rotors are pole changing.Therefore they must be made of laminated sheets to reduce the eddy currents. This is common at serial, or compound (External excitation field) DC motors.

Above mentioned (DC) motors cannot be connected to alternating current. Can you include them into Universal Motors class?

The second point. Permanent magnet poles.

Any of universal motor can not be equipped with permanent magnet. A serial motor means Stator and Rotor field windings are connected serial.

Permanent magnet pole is different. Permanent magnet poles can only be used at DC motors. I am sure you know this.

Third point. Yes the carbon brushes at the commutator are changing the poles of the rotor windings, not dependent with the mains frequency. No matter if 50-60 or 120 Hz, or even 0 hertz at DC.

Since rotor and stator windings are under the same half wave (Because they are series connected) their pole configuration according to each other can be changed almost 5000- 1000 times per minute. (Similar they are working like a DC motor)

An then the same thing occurs in the other half wave. The poles of the rotor changes 5000-1000 times depending to the speed of the motor. It is obvious the poles of the rotor and the stator at the same angle must be the same to push each other. (N to N or S to S). This is the work of the commutator, no matter it is fed with DC or AC.

I have never seen a square wave form at any point of the motor.

If this is in DC case, There is no interruption of the current at the carbon commutators of which can create big sparks. The current is continuous.

Regards

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Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
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Good Answers: 200
#33
In reply to #30

Re: Universal motors

11/27/2009 9:11 AM

I am pleased to see that I am not the only one who agrees that Taslim does not know much about ANY sort of motor....

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Power-User

Join Date: Oct 2009
Location: Istanbul. European side
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#35
In reply to #33

Re: Universal motors

11/27/2009 10:14 AM

Dear Andy

I'm afraid I am in the same opinion.

I agree with you..

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Guru
Popular Science - Weaponology - New Member Safety - ESD - New Member Hobbies - Fishing - New Member

Join Date: Sep 2006
Location: Near Frankfurt am Main, Germany. 50.390866N, 8.884827E
Posts: 17996
Good Answers: 200
#31
In reply to #28

Re: Universal motors

11/27/2009 9:06 AM

I have seldom seen such a statement as yours that answers nothing in a way that can be understood....sorry to be so negative.... Don't give up the day job!

I am interested in hearing more about the universal motor that has magnets that you mentioned, does it have some electronics that you neglected to mention? Do you know of a web page that you can guide me to where I can read more about it? I am really interested in learning more....

Have a great day anyway.

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Power-User

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#34
In reply to #28

Re: Universal motors

11/27/2009 10:11 AM

Dear Taslim

Both of below statements are yours. One statement tells the opposite of the other one.

By design, the commutator allows current to flow to the armature in one direction only. So the armature always sees DC current. The commutator is also called a mechanical rectifier.Hence if you apply AC to this machine,the field coil sees AC and the rotor sees DC.

I NEVER SAID "AC power is rectified at the carbon brushes" NOR THAT I IMPLIED THAT. WHAT I SAID OR IMPLIED IS THAT AC current is being rectified at the ........YOU CANNOT RECTIFY POWER

Kindest Regards

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Andy Germany (11); Anonymous Poster (6); garth (1); GW (3); JRaef (1); nezihozfirat (11); redfred (1); Taslim (2); Yuri B. (1); Zaf (1)

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