AC vs DC

[Dennis Alwon]

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?

 

[Strathead]

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?

I will give my supposition which is simplistic and someone with depth can correct it. DC motors push from one side of the stationary part and pull from the other side using magnetic repulsion and attraction. AC can simulate this by running the electricity through the stationary and moving parts in opposite directions without any lag. So that at any moment in time, the two are polar opposites. I imagine that this is only effective with small motors because of the complexity of switching back and forth over multiple poles. Larger and larger DC motors require more poles, I think.

AC motors use the actual fluctuation in magnetism as the current goes from maximum to minimum and the reverses for the same. This fluctuation rotates around one component (stationary or moving) while the other part remains magnetically stable. DC has no fluctuation through zero so it wouldn't work with that technology.

Regarding light bulbs, I wasn't actually aware that they labeled them AC of DC. Don't know why they would. That would be like a DC resistor. So a lamp will work equally on equal electricity. Taking root mean square in to account.

How did I do?

 

[kwired]

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?

With the light bulb - the RMS AC is basically the DC equivalent.

Induction motor principles of operation simply won't work with DC. Same for permanent magnet DC motors.

Universal motors as you mentioned do work with either.

A VFD is essentially putting out DC, but is pulsing it in a manner that it mimics AC current in a way that an AC motor will function. Try connecting other items to it and it may not work so well.

 

[electrofelon]

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.

 

[al hildenbrand]

Hi Dennis,

I think the simplest image comes from a classroom demo that a professor did in my college freshman physics class.

We were studying electricity, and had gotten to the simple three phase induction motor.

The demo was the windings and poles of the body of the three phase motor only. The armature and the motor case ends were completely removed, all he had, on the demo desk in front of us, was the magnetic windings turned so we were looking through the hollow were the armature would go.

He placed a 1" ball bearing in the middle of the inside of the magnetic windings from front to back, just sitting on the bottom, and he turned on the three phase AC power.

The ball bearing, being attracted to the closest strongest magnetic pole, instantly accelerated in that direction, and, as that magnetic pole waned, and the next magnetic pole strengthened, the ball bearing's kinetic energy propelled it onward. The ball bearing whizzed around the inside of the motor's magnetic windings until the power was turned off.

If DC were applied, the ball bearing would freeze in a single spot and not move.

The demo, of course, is showing the moving, or "rotating" magnetic field.

 

[__dan]

The choice of AC or DC is mostly made by the available system supply type. Distribution grid AC is widely available so the equipment is made to match. Three phase AC is ideally suited to rotating machinery and distribution over long distances possible using transformers.

DC motors have the highest starting torque at locked rotor of any engine or motor type. That's why you will see them in electric trains and trolleys. AC power tools use series connected universal AC DC (DC type brushed) motors for the low speed high torque. New brushless ECM motors, electronically commutated, will take over their market because of this very high torque at low speed and variable speed that regular AC motors cannot match.

Corded AC power tools with brushed motors run perfectly fine on 130 volt DC (I've done it). I have wondered why an adapter pack that takes regular power tool cordless batteries and puts them in series to run the corded AC tools on portable DC power is not widely available.

 

[Dennis Alwon]

Thank you guys. My son is in Asheville and he knows more about motors than I do. He is taking class in the engineering area at the community college. We started talking about this and were not sure about it all so I forwarded him the link to this thread.... Thanks

Originally I could not understand why batteries for a standard flashlight had to go in a certain direction. It didn't make sense but then I assumed that the contact point on one end would probably not make contact when the batteries were reversed. :dunce:

 

[Besoeker]

AC has a couple of advantages over DC.
It is easier to switch because it has natural zero crossing points.
And the voltage can be transformed up or down. Generally up for transmission and down for utilities.

Long distance transmission is moving towards DC - lower voltage drop and fewer conductors. It also removes the synchronising requirements. As I recall, Japan has both 50Hz and 60Hz systems. DC doesn't care.

 

[Jraef]

To answer your question on light bulbs though, an INCANDESCENT light bulb (remember those?) if fact does NOT know or care if the voltage is AC or DC.

 

[wwhitney]

an INCANDESCENT light bulb (remember those?) in fact does NOT know or care if the voltage is AC or DC.

With AC power to an incandescent bulb, will a periodic change in light intensity/color be visible on a high speed camera, or is the thermal time constant of the filament long enough to prevent that?

Cheers, Wayne

 

[gar]

180316-1101 EDT

A DC power source I would describe as one that produces only a single polarity output. This does not mean the voltage can not be constantly varying from 0 volts to many other voltages of a single polarity.

However, in this varying voltage case I would rather call it a DC voltage with an AC component.

Sometimes when the variation is not huge the AC component would be called ripple. I could also have a pulsed DC source where the DC voltage is present for only a very small percentage of time. For example, power to a radar transmitter. Like a 1 microsecond pulse every 5 milliseconds. For a target 300 miles away the roundtrip travel time for the transmitted pulse is 2*186,000/300 = 1.24 milliseconds. I believe the pulse rep rate for the Pearl Harbor (1941) radar was 3 milliseconds.

An AC voltage source would be one where the polarity of the source is repetitively changing between + and -. This could be of any waveform.

An AC voltage source might contain a DC component, bad for the input to a transformer. Some phase shift dimmers, probably most by numbers, produce this problem. Ideally you probably want your AC source to have no DC component. You also may want your your AC to be a sine wave rather than some other wiggly waveform.


Lights:

Light produced by heating a material, an incandescent bulb, doesn't care much on how you get heat into the emitting material. The primary consideration is the material thermal time constant compared to the pulse rate of energy input. Ripple of light intensity.

Of secondary importance is bulb lifetime. Possibly shorter on DC.


Gaseous discharge lights to some extent don't care about AC or DC so long as electron emission can occur. However, pulsed DC or AC may require a sufficiently high frequency to avoid flicker. 120 Hz (2 pulses per 60 Hz cycle) is usually high enough.


Basic LED chips are strictly DC. Light is produced only with one direction of current flow. An LED can be driven with AC, but consideration of the LED PIV rating has to be considered. An LED is an approximately constant voltage load, not real constant, but needs to be driven from an approximately constant current source. What may be called an LED light is a considerably different animal than an LED chip.


Motors:

Broadly there are DC, AC-DC, and AC motors.

To get motor rotation there must be one magnetic field pulling or pushing another one.

In some ways a multiphase AC motor is easiest to understand its rotation. With a multiphase motor with fixed in space coils and AC excitation of the coils with phase difference between the coil excitations a magnetic vector can be created that rotates in space. This goes back to the fact that the sum of sine waves of the same frequency and related phase produces a sine wave.

If I have a moderately constant amplitude magnetic vector rotating about an axis in space and I add a PM magnet located about the same axis, then that PM magnet will magnetically couple with the vector and rotate at synchronous speed. Thus, an AC synchronous motor.

You can demonstrate this by putting a PM magnet on one shaft, and a different one on a separate independent shaft.


An AC induction motor works by having a rotor with shorted turns on it. From the rotating magnetic field vector there will be current induced in the shorted rotor coils producing magnet fields that link with the rotating vector. But slip, difference is speed, between the rotating vector and the rotor must exist to induce any current in the rotor coils. Amount of slip increases as load increases because more rotor current is required.

There are wound rotor induction motors so rotor resistance can be changed to change motor characteristics.

An AC synchronous motor has no slip between the rotating vector and the rotor as load is increased. Rather as load increases the lag angle between the rotor shaft and the rotating vector increases.


A true single phase induction or synchronous motor does not have a rotating magnetic vector in space. Rather it has an oscillating, changing amplitude and polarity, single direction in space vector.

This motor must be started in some fashion. Usually some form of a two phase motor, or use a pull rope.

Once started the rotor inertia keeps the rotor rotating, and the pulsating magnetic field kicks the rotor every 1/2 cycle. The same as putting energy into a swing.


A DC motor consists of a fixed magnetic field, either a coil or permanent magnetic, and coils of wire on a rotor. The coils are connected to a commutator, a many pole switch. Current is switched to a coil in synchronization with the rotor position so as to push or pull the rotor. When a particular coil has done its job a different one is switched in. Thus, unidirectional rotation.


A universal motor is a DC motor where the field is AC excited as well as the commutated rotor. So the magnetic fields are always phased to cause rotation in one direction independent of the input AC polarity. In a series motor the armature and field always have the same current and no phase shift between the two.


Possibly more some other time.

.

 

[gar]

180316-1310 EST

wwhitney:

Use a fast enough acting photodetector and a scope and you can see the light ripple.

.

 

[ggunn]

180316-1310 EST

wwhitney:

Use a fast enough acting photodetector and a scope and you can see the light ripple.

.

Not by much, though, and it also depends on the lamp. Incandescence is a thermal property and how much the light dims is dependent on how fast the filament cools between half cycle peaks. Fluorescent and neon lamps shut off pretty quickly, though.

 

[Besoeker]

180316-1310 EST

wwhitney:

Use a fast enough acting photodetector and a scope and you can see the light ripple.

.

My camera has a shutter speed of 1/3000 secs. So, yes if that's what you want to do.

 

[kwired]

With AC power to an incandescent bulb, will a periodic change in light intensity/color be visible on a high speed camera, or is the thermal time constant of the filament long enough to prevent that?

Cheers, Wayne

Not by much, though, and it also depends on the lamp. Incandescence is a thermal property and how much the light dims is dependent on how fast the filament cools between half cycle peaks. Fluorescent and neon lamps shut off pretty quickly, though.

Yes, depends on frequency and how fast the filament cools between half cycles.

Electric discharge lighting (fluorescents and HID's) produce light from an arc through a tube, and with AC current the arc is essentially extinguished and restarted every cycle, but is not noticeable to human eye at 60 Hz, it can give "stroboscopic effect" to rotating machinery and they may not appear to be rotating as fast as they are though.

 

[STucker]

Universal Motor Construction

Universal Motor Construction

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

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

 

[ggunn]

Electric discharge lighting (fluorescents and HID's) produce light from an arc through a tube, and with AC current the arc is essentially extinguished and restarted every cycle, but is not noticeable to human eye at 60 Hz, it can give "stroboscopic effect" to rotating machinery and they may not appear to be rotating as fast as they are though.

And in some cases make them appear to be not moving at all.

 

[STucker]

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

Looks like I linked the original instead of the one with markings, but you get the idea. :roll:

 

[Jraef]

And in some cases make them appear to be not moving at all.

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.

 

[zbang]

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!

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.