AC vs DC

[kwired]

Looks like universal motors are built just like a DC motor. Two series wound coils connected to the commutators so that no matter the polarity of the current, the torque is still unidirectional.
View attachment 19934

I pulled this image from this article that gives a pretty concise description of Universal Motors: http://www.electricaleasy.com/2014/02/universal-motor-construction-working.html

I added some polarity markings so you can see that when the polarity shifts, the armature will still be pushed in a clockwise direction.

In this diagram, the top part where the commutator connects is always repelled by the like charged N pole and the attracted to the S pole, just like a DC motor. This article does point out that when it operates on AC, there is a performance loss due to reactance, which doesn't come into play when a DC current is applied

Thing that is misleading about that image is the stationary field is not 90 degrees from the rotor field. I don't know what is optimal but at other than 90 degrees it gives better torque for a specific direction. To reverse motor rotation move the brushes to the other side of the 90 degree center. At center who knows which way it will try to run or how much torque it will produce.

 

[Jraef]

Wow. I thought discharge lighting around moving parts was banned by OSHA over 30 years ago, so I wonder how they managed (not inspected?). My friend's machine shop had to have incandescent work lights on all the equipment because the overheads were florescent.

This WAS over 30 years ago...

 

[kwired]

I have had it explained this way: An induction AC motor can be thought of having a rotating magnetic field that is produced by the alternating current. The other half of the motor gets dragged along for the ride.

With DC, since it doesnt alternate, you need to switch the polarity. this is what the commutator does, or it can be done electrically.

I dont understand how those universal motors that can run on ac or dc work.

With induction motor rotating magnetic field is created by currents being out of phase with one another in adjacent windings. Three phase motors is simple, each "phase" is 120 degrees from the others. With single phase motors they have to use capacitors or windings with different inductance, or combinations of both to get desired rotating magnetic fields and essentially have a two phase motor. Even the simple shaded pole motor, the "shading coil" that only gets it's current from induced voltage on it is critical to creating rotation in that motor.

 

[Jraef]

With induction motor rotating magnetic field is created by currents being out of phase with one another in adjacent windings. Three phase motors is simple, each "phase" is 120 degrees from the others. With single phase motors they have to use capacitors or windings with different inductance, or combinations of both to get desired rotating magnetic fields and essentially have a two phase motor. Even the simple shaded pole motor, the "shading coil" that only gets it's current from induced voltage on it is critical to creating rotation in that motor.

Also, on an INDUCTION motor, the rotor is not simply "dragged along for the ride". The expanding and collapsing magnetic fields in the stator induce (i.e. "induction" motor) a voltage and current to flow in the rotor bars, just like in a transformer coil. THAT current flow in the rotor bars creates magnetic fields in the rotor, that are OPPOSITE polarity of the magnetic fields in the stator, and delayed slightly (because if the induction time constant). So that attraction/repulsion of the stator fields and rotor fields is what becomes rotation. Is it being dragged or pushed? It's kind of a chicken and egg thing.

 

[gar]

180316-1709 EDT

ggunn:

Thermal time constant and excitation frequency determine magnitude of visible ripple.

The incandescents are sine wave ripple at 120 Hz.

A 100 W 120 V tungsten filament bulb at about 123 V provides a peak to peak ripple of about 0.5 vs a mean of 4. That is about 4% on an RMS basis.

A 25 W is about double this.

A CREE 9.5 W bulb also has a sine wave ripple. Its RMS ripple is about 16%, or twice the 25 W incandescent.

An 8' Slimline has a very peaked ripple at 120 Hz. Much of the cycle no output.

Back in the late 50s I worked on signalling from a car to home via modulation of the headlight beam. If I remember correctly modulation was about 15 Hz. Car headlights have a long thermal time constant.

.

 

[gar]

180316-1733 EDT

A true single phase motor does not use a rotating magnetic field for run purposes, just for starting.

A capacitor run single phase motor is really a two phase motor under running operation, and on a single phase sourcre has a better power factor than a three phase motor.

A true single phase motor while running has a lot of torque ripple.

.

 

[Ingenieur]

ac vs dc motor
common basic types
the field passing (relative motion) by the armature produces a force ~ armature current or torque
power is delivered to the armature, a focused at the field magnet
the more power the harder the field tries to catch (or repel from) it
the field can be the stationary (stator) or rotating (rotor) part
same for the armature

dc
the field is the stator
the armature is the rotor
relative motion of the field is performed by mechanical commutation

ac/induction
the field is the rotor and is induced (counter emf) by the stator
the armature is the stator
ac current swinging pos to neg povides 'commutation' electrically

ac/synchrnous
the field is the rotor and its current is supplied via slip rings from an external source
dc power supply or a generator mntd on the rotor shaft
the stator is the armature

 

[GoldDigger]

Looks like universal motors are built just like a DC motor. Two series wound coils connected to the commutators so that no matter the polarity of the current, the torque is still unidirectional.

I pulled this image from this article that gives a pretty concise description of Universal Motors: http://www.electricaleasy.com/2014/02/universal-motor-construction-working.html

Just a note about universal motors:
It is true that most are series wound, but with a parallel wound or compound coil arrangement the commutator would still produce the same mechanical phase shift between the rotor and stator fields. The motor rotation does not depend on the polarity of the applied DC. And the only potential downside of applying AC is that the current may be limited by the inductance of the coils and so deliver less torque at locked rotor and a smaller amount of tourqe reduction at normal speed.

A series wound motor has the sometimes unfortunate behavior of accelerating without limit when unloaded. So a DC series wound motor may have a centrifugal governor (typically a simple switch) to prevent overspeed.

 

[JFletcher]

On the lighter side....

When I was 7 years old, I connected a 4.5 volt DC Lego motor to an outlet in the living room. It did not do so well it went poof and burnt my fingers. Sadly, the Grolier encyclopedias that my grandmother got me when I was 5 years old did not have an explanation for this catastrophic failure.

 

[retirede]

On the lighter side....

When I was 7 years old, I connected a 4.5 volt DC Lego motor to an outlet in the living room. It did not do so well it went poof and burnt my fingers. Sadly, the Grolier encyclopedias that my grandmother got me when I was 5 years old did not have an explanation for this catastrophic failure.

So that makes you one of the pioneers who demonstrated the need for TR receptacles!

 

[kwired]

So that makes you one of the pioneers who demonstrated the need for TR receptacles!

TR receptacles just turns away potential future electricians I even did some things like that when I was a kid.

 

[JFletcher]

So that makes you one of the pioneers who demonstrated the need for TR receptacles!

Lol. I'm sure my seven-year-old self could have figured it push on both shutters at the same time to defeat them. Resistant, not tamperproof...

 

[ggunn]

Years and years ago when I was starting out as an electrician at a steel mill, there was a machine (slitter) that had to be serviced from underneath. They installed LPS (Low Pressure Sodium) lights in the pit as an energy savings move in the 70s. But the rotating speed of one part of the machine exactly matched the strobe effect of the LPS lamps, so you couldn't tell if it was rotating or standing still! Made for some hairy close call events, so eventually they gave us DC powered drop lights for working down there. The other thing we found out about the yellow/orange hued LPS lighting is that you can't tell the difference between blood, oil and lube. With all of the steel materials going through that machine being razor sharp, we would sometimes cut ourselves and not notice for a long time, which led to one guy almost bleeding out. The DC drop lights helped with that too.

In a (somewhat) related story, one time at a gig with a now defunct band, I wrote out my set list in red Sharpie on a yellow legal pad. When we hit the stage the lighting guy hit the stage with a red wash and my set list became a blank sheet of paper.

 

[ActionDave]

In a (somewhat) related story, one time at a gig with a now defunct band, I wrote out my set list in red Sharpie on a yellow legal pad. When we hit the stage the lighting guy hit the stage with a red wash and my set list became a blank sheet of paper.

So did you go with Jazz Odessey instead?

 

[ggunn]

So did you go with Jazz Odessey instead?

I'm sorry; that does not compute.

 

[ActionDave]

I'm sorry; that does not compute.

Obscure reference to This is Spinal Tap.

 

[StarCat]

I understand the basic differences between ac and dc but I still find myself thinking why you cannot do certain things or what is the difference. For instance, is there a difference between an AC light Bulb and a DC bulb? I can't see how it would make a difference if the voltages were the same.

It looks like if you put AC current through a DC motor, it could possibly run. If the field winding and armature windings are in series it will work, but if they are wired in parallel it won't work. It looks like universal motors, which run on DC or AC are constructed with the field and armature windings in series.

I have no idea why this is so. Nr do I understand why the reverse would not work, ie, using DC to an ac motor. Any simple explanation for this simpleton?

The truly great and open minded persons out there should give a listen to this show especially and all other subject matter presented by Eric Dollard:
https://www.youtube.com/watch?v=TbnDH-D1o6w

http://ericpdollard.com/

Star

 

[ggunn]

Obscure reference to This is Spinal Tap.

Ah, thanks. I love that movie.

 

[Jraef]

On the lighter side....

When I was 7 years old, I connected a 4.5 volt DC Lego motor to an outlet in the living room. It did not do so well it went poof and burnt my fingers. Sadly, the Grolier encyclopedias that my grandmother got me when I was 5 years old did not have an explanation for this catastrophic failure.

Lol, I did almost the EXACT same thing, albeit with an Erector Set motor. I had made a helicopter and of course, the motor didn’t turn the blades very fast. Convinced myself that if I could get it spinning fast enough it would fly. I had already learned that if I used more batteries in series the motor would spin faster, so if I hooked it up to 120V, it should fly! I got all of the extension cords together, then plugged in the motor. It Immediately started melting the all plastic case and in scrambling to unplug it, the copter fell over on the motor and melted the shag carpet. I gave the carpet a hair cut to try to hide the evidence, but that didn’t really work...
:happysad: