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Topic Title: Dc motors
Topic Summary: why do we change ac to dc?
Created On: 15 January 2013 11:37 AM
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 15 January 2013 11:37 AM
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andrewmac

Posts: 237
Joined: 14 April 2004

Can someone explian why:

we still use Dc motors

why we use a commutator to convert the alternating emf to dc i.e why not just use the ac?

thanks
 15 January 2013 11:44 AM
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OMS

Posts: 19549
Joined: 23 March 2004

Speed control and positional control is easy with DC.

They are usually more energy efficient in a given application

the "power density" is better

That should be a start for you

regards

OMS

-------------------------
Failure is always an option
 15 January 2013 12:19 PM
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broadgage

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Until fairly recently it was much easier to control the speed of a DC motor than an AC one, and DC units were often prefered for variable speed applications.

Now that variable speed AC inverter drives are cheap and relatively reliable, I suspect that use of DC motors will decline, though they will be needed for some niche applications.
 15 January 2013 12:34 PM
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John Peckham

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Yes when I was a lad DC motors were fitted to high speed lifts for speed control and starting torque reasons. A standard question on Post Office engineers promotion boards as a prelude to being asked to draw a block diagram of a Ward Leonard VV system and explain how it works. I could do it then but not know. I go in to lift motor rooms now on new lift installs and see control panels a fraction of the sizes I used to see and AC motors that appear smaller than I have in one of my old Black and Decker drills. Nice smooth starting, stopping and speed control with VF drives. Now where did I put my rose tinted glasses?

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John Peckham

http://www.astutetechnicalservices.co.uk/
 15 January 2013 12:37 PM
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Jaymack

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Originally posted by: andrewmac
we still use Dc motors

Not so much nowadays though, they are more expensive and require more maintenance on the commutator and brushes. Before VSD's they were more prevalent in industrial applications such rolling mill drives and crane controls in steelworks etc. On overhead cranes etc., some were of the "mill motor design" or hinged coffin type where the "stator" with the field windings could simply be unbolted in situ, in order to lift out the armature, since this was the most common source of failure.
D.C. series motors are used in high torque applications, the downside is that they will runaway to self destruction if the load is removed, i.e. they are not recommended for belt drives. D.C. shunt motors are used for speed control by varyng the field current, i.e. field weakening raises the speed.

why we use a commutator to convert the alternating emf to dc i.e why not just use the ac?

The armatures in D.C. motors have A.C. voltage and current, the commutator is required to convert these to D.C. for the least expensive control.

Regards
 15 January 2013 01:07 PM
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Jaymack

Posts: 4622
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Originally posted by: John Peckham

A standard question on Post Office engineers promotion boards as a prelude to being asked to draw a block diagram of a Ward Leonard VV system and explain how it works.

They were common in the steel industry for large rolling mill drives in the steel industry. For starting the large generating sets, the windings of a pony motor were connected in series with the windings of a synchronous motor, then a 3 phase CB was used to short out the synchronous windings when near syncronous speed.

In one mill in Scotland, when starting up after one weekend, a cat that had been sleeping in the overhang of the windings for one D.C. generator; caused a fire shutting down the mill. We scoured the country and resurrected some mercury arc rectifiers as a temporary measure until the motor could be rewound. The first time they had been used for rolling mills!

Nostalgia ain't what it used to be!

Regards
 15 January 2013 01:18 PM
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iainross2

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Probably not applicable to most applications, but DC Motors are used in the Rail Industry to drive points and switches, as they are immune to stray AC currents that may be induced by the 25kV overhead traction.

The risk being that an induced current starts to move the points and derails an oncoming train.

As previously mentioned, the high starting torque is another benefit.

Iain
 15 January 2013 03:23 PM
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andrewmac

Posts: 237
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Superb - thanks all.

Regarding commutation:

"the thin piece of insulator passes under the brushes reversing the direction of the induced e.m.f so that it is no longer alternating but unidirectional."

how does this cause a reverse? Is the AC sine wave just being restarted each time i.e. it is not allowed to be sinusoidal, hence it is d.c.
 15 January 2013 04:13 PM
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gkenyon

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Originally posted by: andrewmac

Superb - thanks all.



Regarding commutation:



"the thin piece of insulator passes under the brushes reversing the direction of the induced e.m.f so that it is no longer alternating but unidirectional."



how does this cause a reverse? Is the AC sine wave just being restarted each time i.e. it is not allowed to be sinusoidal, hence it is d.c.
This has a good illistration of how it works:

http://en.wikipedia.org/wiki/Brushed_DC_electric_motor

Just to get this right, we do NOT use d.c. to power these "d.c. motors" in appliances. It is simply that some types of self-exciting brushed d.c. motors can be used on a.c. systems simply because the commutator swaps the polarity of the rotor winding with respect the stator winding (and hence controls the polarity of the magnetic field of the rotor winding with respect to the stator winding, keeping the rotation going).


I think it will be some time before self-exciting d.c. brushed motors are replaced in smaller appliances (vacuum cleaners, mixers), because of their relative cost, simplicity and weight, compared with other motor technologies.

Similarly in washing machines, where speed control is required, and the load torque varies quite a bit for a number of reasons, even throughout part of a wash, at the moment makes the self-exciting d.c. brushed motor the lead contender, again due to costs. Capacitor-run induction motors were been tried with limited success on some makes in the 1980s but by 1990s we were back on the d.c. types. Not sure on the viability of cost-effective permanent-magnet a.c. motors in such consumer applications at the present time.

Tumble-dryers use capacitor-run single-phase induction motors - difference is this is a constant speed application.

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Eur Ing Graham Kenyon CEng MIET TechIOSH
 18 January 2013 02:36 PM
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andrewmac

Posts: 237
Joined: 14 April 2004

Apologies,

Still not grasping how or why the 'break' caused by the insulating segments causes a unidirectional output.

Can someone try to explain this to me...
 18 January 2013 06:40 PM
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Legh

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Originally posted by: andrewmac

Apologies,

Still not grasping how or why the 'break' caused by the insulating segments causes a unidirectional output.

Can someone try to explain this to me...

Ok,

Start right back to first principles

1/ Look at Maxwells Cork screw rule for currents and the resulting flux generated by a current flowing through a conductor

2/ Apply this to the amature coils (use one coil as an example)

3/ Apply Flemings Left Hand Rule for motors

4/ The interaction of the two fluxes causes movement

5/ This works for both DC and AC voltages applied to the commutator

6/ Sometimes called a Universal Motor because it can be operated on both DC and AC

7/ The commutator segments are there to separate each coil so that as the motor turns and energizing the next pair of segments on the commutaor

8/ We'll leave out armature reaction until I've read a little more

(Courtesy of Mr Ladybird)

It will become clear, believe me, Just keep going........

Legh

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http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
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