Difference Between AC and DC motor

" The basic construction difference between ac and dc motor is that dc motor has direct conduction current supply to rotor and ac motor has inductive supply to rotor- no direct conduct. A motor needs three basic things-A magnetic field, a current carrying conductor, harnessing the created force on current carrying conductor due to magnetic field in ways to create rotary motion hence generate torque.

DC motor has- Armature that rotates, & Field winding that stays stand still. Direct current is supplied to rotating armature by means of Brush arrangement.Field winding may be in series or shunt with armature and fed with dc supply to create flux.So dc supply goes to both armature and field winding

On the other hand Ac (Induction) motor has- Stator that stays stand still and provides rotating field, rotor that rotates. Only AC is supplied to stator which generates rotating magnetic field, then by Faraday's laws of electromagnetic induction this rotating magnetic field induce EMF in rotor circuit, thus produce current in it.

This no direct conduction between rotor and stator, applies only in case of induction motor. But for ac synchronous motor, separate dc power is supplied to rotor. AS rotor rotates, to supply dc power to it brush, slip ring arrangement is provided."

http://eblogbd.com/basic-construction-difference-between-ac-and-dc-motor/

There are many different designs of motors, and it's not possible to come up with a general rule. You have to understand how they work.

https://en.wikipedia.org/wiki/Electric_motor

NO!

1. It is the current requirements of the motor that determine the wire size, not whether it is AC or DC. Most people are more familiar with low voltage DC motors, which do commonly have heavier wires than similar sized AC motors, but that is because the AC motors are commonly running on a higher voltage. Power is Voltage multiplied by Current, so if two motors of the same power run on different voltages, the one running on the higher voltage can use a smaller wire due to the lower current.

2. It is the voltage that determines the danger to humans. There are lots of DC motors that run on low voltage, like all motors in ordinary cars, but there are also DC motors that run on several hundred volts, like the main motors that turn the table/chuck on older Vertical Turret Lathes.

In fact, for a given (peak) voltage, DC is actually more dangerous than AC, since the AC actually turns off once every half cycle, giving the muscles a small chance to pull away, while the DC is continuous, with no time for muscles to relax.

You can be just as dead touching the terminals of a high voltage DC motor as touching the terminals of a similar AC motor.

Sorry I disagree.

The first two points you posted absolutely perfect, my compliments!

But the rest not!!! Sorry.

AC tends to "vibrate" the muscles in a position (my words and experiences of many AC shocks over many years, not very technical), but you basically stay "stuck" on AC power, you simply cannot let go.......

DC tends to cause muscles to "shorten" in a single direction (again my experiences of thousands of 220 VDC shocks, it was my "test tool", two fingers same hand over a blown or removed fuse! If you want to know how, PM me!), which can often pull a finger or arm away from the power.

Several text books I have read over the years on the subject have been written by authors with absolutely no experience of DC shocks, and are simply wrong and totally misleading at best......this is far better understood nowadays I feel, though the AC and DC electrocution experiments were made in the 19th Century!!

Look here:-

Electrocution

There you will find a very important and VERY correct sentence:-

Currents as low as 30 mA AC or 300-500 mA DC applied to the body surface can cause fibrillation.

Quite a difference in value.......

This link here is also very good on the subject:-

Electric_shock

Where you can read the following:-

At around 10 milliamperes, AC current passing through the arm of a 68-kilogram (150 lb) human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.

Notice that DC was NOT mentioned in that sentence.....

Also:-

The comparison between the dangers of alternating current at typical power transmission frequences (i.e., 50 or 60 Hz), and direct current has been a subject of debate ever since the War of Currents in the 1880s.

Animal experiments conducted during this time suggested that alternating current was about twice as dangerous as direct current per unit of current flow (or per unit of applied voltage).

The only DC injuries I was ever close to, but not myself directly involved, were all skin burns and "Arc eye".....none lethal.....but very painful.....

My Father saved someone just after WW2, who tested wrongly a very high DC voltage source and basically vaporised the cable he was holding in his hand.....but he recovered with severe scars to his hands and arms, it did not even cause his heart to stop.

If I can be of any further assistance, just ask, I have had extensive, mostly accidental, the AC ones anyway, shock experience....It is better understood nowadays, but as I said, some text books, even University reading material is accepted by many, but is simply wrong!!

We have had this same discussion on CR4 many times by the way over the last 10 years or so!! It does not go away!!

Happy New Year!!

It has been many years since I was taught what I stated (and believed); I won't argue with your research.

I too have been shocked many times by AC, mostly by 120V and 240V systems, but a couple of lucky (because there was high resistance in the circuit) circumstances at significantly higher voltages. It has been many years since my one incidence of being shocked by something around 300VDC, and that was far more painful than any of the AC shocks. I have come in contact with CRT anodes quite a few times (always unintentional, and most of those were small 9" CRTs (early Mac computers)), but that current never lasted long enough to do significant damage.

I have to agree with you, those high voltage CRT shocks ARE painful, one of mine even left a small hole through the skin that did not bleed (cauterised?), it took months to finally heal up!!

Its other effects were gone in a flash (Pun intended! ) no brain "stopping", you just removed your hand FAST and said "OW!" or something like that.....

But it did not have anywhere near the pain and effect of one 440 VAC shock I had on HMS Puma, my brain stopped is the only way I can put it. If my legs had not been trapped between the side of the ship and the large bit of pipe I was sitting on, I might have fallen 20 feet!

You are simply "out of it!!" for several long seconds or forever!!!

My shock was through both hands down to my sweaty a**e, on the pipe. Left and right of the heart, BUT NOT OVER/THROUGH THE HEART!! Close enough though.

It was almost like lying on one hand of God, while he was enthusiastically clapping with the other.....You never, ever forget it!! My whole body seemed to ache for several hours afterwards...imagined or fact I could not now say.....

Have a great day!!

If you have to check a circuit without any other means available, use the back side of your hand. your muscle group will pull your hand away from the circuit. Don't do this if you are unsure of the voltage! 120 vac can bite, 480 vac Will kill.

I agree. Be safe!

When working on AC I have a uselful "non contact" tester, cheap and its also a great screwdriver. It can also look for leaks of rf radiation from a microwave too.....

Or I use neon screwdriver, or a multimeter if my screwdrivers are not handy....

But as you may have read, I have 3 RCDs installed, one on each phase coming into my house!!!

Thats the best safety equipment that I know of in any house or workshop, wherever....works for the whole family and the pets....I have tripped a 220 VAC RCD in the office years ago with my forefinger, unintentionally, but it still hurts!!

Glad to read you made it through that incident.

Thanks, but I think you would have to be pretty sick in some way, bad heart maybe or having a pacemaker (which is also a bad heart!!) before that shock that you receive when a working RCD is online, would do you any real harm.

Current wise its quite low and more importantly, time wise very short!! But it still smarts!!

I am amazed still that so few country electrical codes it seems, require them "everywhere", and that people here, who have been reading about them online, seemingly still cannot comprehend just how good they are for the WHOLE house, garden and workshop etc.....and can save "nosy" children (I had several shocks as a child, my tiny fingers could easily fit inside the then still common/dangerous/old fashioned 15 amp UK sockets!), pets let alone adults.......

Even here its not 100% coverage....and the few electricians I have consulted appear to simply not have a clue!!

I had mine installed by a "Master" electrician and he took half a day before they operated as they should, because he had NOT "un-bundled" the neutrals....I had to tell him!!! DUUUHHHH!! You cannot have neutral current in the neutral from another phase.....it seems obvious to me, but it wasn't to him!!!

That problem, few private houses in the UK and USA will have!!

Here is a video, made in a 50Hz 240 VAC region (you can see that by the 13 amp plugs and sockets), but in fact everything he actually says applies to anywhere, 120 VAC or 240 VAC:-

http://www.youtube.com/watch?v=q219oqv15h8

There is a clear single phase wiring diagram that could be useful to most people in single phase situations here:-

http://www.electricaltechnology.org/2013/05/wiring-of-distribution-board-with-rcd.html

Where this clear and simple diagram can be seen:-

Some may wish to use more of them to prevent say a whole house suddenly being plunged into darkness.

Naturally, there are several reasons for complete loss of mains, cable failure, unpaid utility bill, lightning strike and and, so only having one RCD is really not a problem if proper planning has been made...

.If only one RCD is installed, there are small modern battery operated lights that can be placed in various sockets around the house, which will switch on automatically if power is lost for ANY reason whatsoever.

Like these:-

http://www.ebay.co.uk/itm/2PK-Rechargeable-20-LED-Motion-Sensor-Safety-NIGHT-Light-Emergency-Torch-Flashli-/301742790502?hash=item4641459766:g:DWUAAOSwpDdVE3iB

I wish you a happy new year.

Andy, you said

I have tripped a 220 VAC RCD in the office years ago with my forefinger, unintentionally, but it still hurts!!

A bit OT but it needs to be said. We have covered this in a previous post. But in my case it was a deliberate act as part of a bravado pioneering sales demo (but stupid with hindsight) to show an RCD would trip on body current. It did - and I am still here to talk about it.

But never again - I got an almighty poke that I can still 'feel' to this day.

I can only say what you did was brave and painful, but not dangerous (I assume you used the test button to make sure that it was functioning correctly before doing that!!)

It is recommended to test at least once a year, which is a pain in the a**e to do......

Mine was a training class I was giving on tape units with exposed mains connections, the pupil was not supposed to have it plugged in and I did not check, well I did, but not in the recommended way!!!! IT WAS PLUGGED IN.......DUUHHH!!!

Well Andy, we live and learn, if you don't mind the living pun.

As an insight to my idea of touching the live wire to test the RCD, it might help to explain that in the very early days of RCD's when I was pioneering the sales of them in Bristol UK (1960's), the whole emphasis was on improved personal 'safety' over the usual voltage or current earth leakage trips that helped a great deal, but was not necessarily 'safe' compared to RCD's that were promoted on the basis of 'prevention of electric shock' - which I took literally - and to be honest I had no reason not to believe it.

Well of course they didn't prevent shock - as I found out the painful way - but they did provide protection against death from shock - QED.

PS: I've still got my sales demo model - I use it in the circuit feeding my narrowboat - and it still works - and within today's time and trip current limits.

I like your post!!!

Especially :-

Well of course they didn't prevent shock - as I found out the painful way - but they did provide protection against death from shock - QED.

May I modify it to:-

Well of course they did ALMOST prevent shock - as I found out the painful way - but they did provide protection against death from shock - QED.

I agree, its still quite painful, in spite of being for so short a time....

But you and I here appear to be (almost?) the only ones who believe in them!!!

I think a great sales pitch of yours then could have been something like " Buy one or I get you to test one!!"

I found it VERY convincing to get the shock, although I already had them installed in the UK and in my house here, BEFORE the "event!"

My Father, an electrical Engineer himself, installed my "first one" in 1975 in my house, built in around 1450 AD.....It was the second oldest house in the village, but with the MOST modern electrical system as we ripped out all the old stuff and did it all new.....

Have a great day.

As others have stated look for the name plate. If it isn't easy to read try a magnifying glass, pencil rubbing, wash the plate or other "detective ways" to read the label. Often times the label can be read easier by changing the direction of the lighting such as can be done with a LED flashlight.

A simple, but not always conclusive way to determine is to look at the color of the input power leads or the wires connected to the screw terminals. DC motors usually (but not all the time) use black and red wires. The black is for the negative (-) connection and the red for the positive (+) connection. This is just like the battery and alternator on a car. AC motors usually (but not always) use a black and a white wire or dual black wires for each lead from the power source.

Good Luck, Old Salt

You have a few good and correct answers here and also some crap ones.

This is mainly because you are here Anonymously. Fix that first and you will usually get better quality answers.....

Basically, if you have to ask us, you haven't got any idea at all yourself, so even following the good advice here, you are still not going to be 100% right all the time....so is it worth it?

Best of luck!!

Apart from the nameplate, it's not likely to be possible to tell the difference (with guaranteed certainty) just by looks alone. An internal inspection will be required, and even with experience in motors it is still doubtful to be sure - but there are clues that others had said, so in a few cases you could get it right.

Put another, for the motor experts, would you be able to down the local scrap yard and pick out a motor of the correct voltage and power - and be certain it is the right one?

...just by looks alone....you are allowed a screwdriver and spanner to take covers off to look inside....but no meters....but you can assume the motors are in working order - redundant rather than broken.

Dear friend,

Several Members have given different answers. I want to say that answers - that ,

1. Have a look at the Starter of the Motor. AC starters are different from DC starters.

2. Look out for capacitors. Ac Motor will have capacitors and DC motor will not have

3. Look out the main power cable - AC motor will have 3 phase supply- each phase color will be different - R, Y, B (old Standards) and DC Motor will have single color (though multi cable to suit the current value)

4. Look out for any spark in the commutator/brush through inspection hole. If spark is observed it is indication that commutator. Some times spark will not be seen.

DHAYANANDHAN.S

2 & 3 don't compute.

Sorry, but this post of yours will not help him much, let me explain why (and I have not covered even all motor types either!):-

1. Have a look at the Starter of the Motor. AC starters are different from DC starters.

What if the motor is say at the scrap heap with no starter anywhere near, remember YOU ONLY HAVE THE MOTOR TO LOOK AT!!

No help there!

2. Look out for capacitors. Ac Motor will have capacitors and DC motor will not have

3 phase AC induction motors need generally no capacitors that I know of, neither do, as you said, DC motors....Finding a cap might tell you that it is single phase!! But that is all.....

No help there!

3. Look out the main power cable - AC motor will have 3 phase supply- each phase color will be different - R, Y, B (old Standards) and DC Motor will have single color (though multi cable to suit the current value)

Some AC motors, for example single phase induction, universal and synchronous and some others, only need two wires plus ground!!! No 3 phase supply! Also some DC motors are polarity sensitive.....

No help there!

4. Look out for any spark in the commutator/brush through inspection hole. If spark is observed it is indication that commutator. Some times spark will not be seen.

And what does that tell you? It could still be an AC universal motor or a DC motor (plus some other rare types I have never even heard of!!).....not much help unless you consider the physical design as well, by looking inside!! Some DC motor types are brushless nowadays.....many in fact!!

Some modern brushless DC motors have 3 fields, for an amateur, he might believe that it is 3 phase AC, even by looking inside!!!

Some VFD motors look to be AC 3 phase, but are driven at varying speeds from a DC supply!!!

Also, you are only allowed to LOOK!!! Not power it up and see it running!!

No help there!

It may be possible, to make a fairly simple formula with questions and answers, that ask for certain obvious parameters and depending upon the answer you cross off, lead you to a correct solution. I have never tried it as I am not even aware of all motor types, only the ones I have worked on....

But even then, there could be problems of voltage where no name plate is visible...as both AC and DC motors have been made over the years in many different voltages......

Personally, I do believe that I could maybe with a normal examination - opening of covers and looking in with a torch (sorry flashlight for the picky people!), looking at the connector block etc., decide what type of motor it is.

But I do believe that I could be seriously mislead as to the correct voltage rating without a meter.

Maybe even with one!!

I feel that its not a simple job to decide for ANY motor type at all, just by looking at it, what it is in all details, but I could be wrong.....been wrong before!!

Happy New Year!

Lots of advice to point you in the right direction - but #9 in point 2 says DC motors do not have capacitors.

In principle I agree - but unless you know how to recognise a capacitor and know about relative sizes and how they are connected - you need to be careful because spark suppresion capacitors could be fitted to DC motors to eliminate radio interference.

Good point, which I completely missed, thanks.

My business is to retrofit powder metering feeders and frequently convert DC motor to AC. Sometimes it is difficult to get close enough to read the motor nameplate, or the nameplate is unreadable.

Most of the motors I retrofit are fractional to 3HP. The quickest way to determine is look for brush covers that identify a DC motor (see picture circled in red). However some DC motors are brushless but this is not common in my industry.

Normally what I've noted between the two motors in the industry I've been in for so many years, all ac motors have a ribbed casing while dc motors have cylinder stators not ribbed and some are rectangular shaped with external cooling and provided with glass to see the brushgear and normally provided for in case arcing may arise . Though nameplate is the best tag can tell what nature is the motor . I thank you.

PS to my previous post.

As an alternative way of approaching the problem you will appreciate the difficulty of telling AC motors from DC motors if you compare it to telling whether a car has a diesel or petrol engine just by external appearances.

LOL!!

Well put.

You can add electric and gas to the petrol and diesel nowadays!!

Your idea works fine!!!

This specification guide on Engineering360 talks about the differences:

http://www.globalspec.com/learnmore/motion_controls/motors/ac_motors

There will be lot more differences in connections. In three phase AC motor, there will be three connections whereas in single phase AC motor will have phase and neutral marked on it. In case of DC motor, clearly there will be marking stating positive and negative terminals as change in connections will change the direction of rotation. Above all there will be name plate on every motor indicating vital characteristics of the motor.

You wrote:-

.........stating positive and negative terminals as change in connections will change the direction of rotation.

This is not true as an across the board statement.

Only certain, mainly but not limited to, small motors may change direction when changing the supply polarity. Generally these will be motors will be, but not limited to, ones with permanent magnets providing the stator field.

Some brush less motors will actually be damaged for example with a switched supply. They can ONLY have their polarity connected in one direction. They can be reversed usually with some form of electronic/sitched control.

Some DC motors with wound fields connected in parallel or serial (or both!) to the commutator, will simply not change direction.

This post is incomplete in the fine detail, it is only intended as a "heads up!"

You may find the following informative:-

DC_motor_types

If I can assist in any way please ask.

In addition to Andy's very correct post: If you have not yet had to deal with a motor whose name plate was missing or unreadable, or never had one, then you haven't worked very long with motors. Many small motors have only a part number and perhaps a date code. The first two motors I happened to look at are marked: 5630 6535 3 82.

I'm pretty sure the 3 82 is the date code. Can you tell me whether they are AC or DC? Looking up that number on Google brought up Men's twill, Portuguese and Australian phones, and a Marathon, among others.

It is very common for AC motors to have 2, 3, 4, 6, or 9 connections, commonly with one more for ground. Usually these connections are color coded or numbered, but in old motors the colors frequently become indistinguishable, and the numbers get knocked off or unreadable. A multiple speed motor will have extra connections, so it is quite possible for a single phase AC or a DC motor to have 3, 4, 5, 6 or more connections.

Then there are stepper motors that can be considered AC or switched DC, depending on your point of view, and can have 3, 4, 6, and probably other numbers of connections, with or without ground...

To summarize: you can't generalize!