AC ver. DC Motors
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physis
Senior Member
Re: AC ver. DC Motors
From the bottom line construction I've seen in household appliances I would expect "one diode", cheapest ever built, to be the rectifier.
You know that doesn't provide real DC.
Closer (close enough) to DC would be full wave recified. That takes four diodes. Now that same 4.2 watts is 16.8 watts.
Edit: There are the on times that will adjust that down. You could cut that in half right off the top because only two diodes will be on at any given time.
[ March 28, 2005, 05:13 PM: Message edited by: physis ]
From the bottom line construction I've seen in household appliances I would expect "one diode", cheapest ever built, to be the rectifier.
You know that doesn't provide real DC.
Closer (close enough) to DC would be full wave recified. That takes four diodes. Now that same 4.2 watts is 16.8 watts.
Edit: There are the on times that will adjust that down. You could cut that in half right off the top because only two diodes will be on at any given time.
[ March 28, 2005, 05:13 PM: Message edited by: physis ]
Re: AC ver. DC Motors
Single diode drop is about .6 to .7V. Two diodes in series is 1.4V. So disapation is 1.4V X 3A = 4.2 watts. As you noted, the other two are not concucting until the next half-cycle.
I have a Thor 1/4" palm drill that I got from my father about 30 years ago. Great machine. On-OFF Trigger, single direction, very high speed for an electric - has a full wave bridge pill in the handle to feed the universal motor - I suspect that is why it has so much power. Can't buy a new one, can't get major parts. I've had the brushes replaced twice and the rectifier and switch once. But the gear machined in the end of the armature shaft is worn, no new one available. sigh It never ran hot, unless I over loaded it with way too big of a bit.
carl
Ah. I understand. But ...
Single diode drop is about .6 to .7V. Two diodes in series is 1.4V. So disapation is 1.4V X 3A = 4.2 watts. As you noted, the other two are not concucting until the next half-cycle.
I have a Thor 1/4" palm drill that I got from my father about 30 years ago. Great machine. On-OFF Trigger, single direction, very high speed for an electric - has a full wave bridge pill in the handle to feed the universal motor - I suspect that is why it has so much power. Can't buy a new one, can't get major parts. I've had the brushes replaced twice and the rectifier and switch once. But the gear machined in the end of the armature shaft is worn, no new one available. sigh It never ran hot, unless I over loaded it with way too big of a bit.
carl
Re: AC ver. DC Motors
That sounds kind of neat. Let's see they must put the PMs on the rotor, with a wound stationary armature. I had a Mercruiser, 30 years ago, that used that for an alternator. Output was 3phase with a 6 diode bridge and a waste heat, shunt regulator on the DC output. Same thing ought to work as a motor. Just feed the stationary armature with a PWM 3 phase switched supply ... wait a minute, that's a synchronous motor with a fixed field and a VFD :roll: (Please read this as humor - my wife tells me that my humor lacks a lot - something about being radiated with too many EM fields)
Seriously, I do recall an article about using that sort of a driver for high accuracy, speed regulated drives in things like tape drives, hard disks and CD-ROMs. That stuff would definitely be out of my area.
carl
edited to fix spelling
[ May 09, 2005, 07:38 PM: Message edited by: coulter ]
Steve
That sounds kind of neat. Let's see they must put the PMs on the rotor, with a wound stationary armature. I had a Mercruiser, 30 years ago, that used that for an alternator. Output was 3phase with a 6 diode bridge and a waste heat, shunt regulator on the DC output. Same thing ought to work as a motor. Just feed the stationary armature with a PWM 3 phase switched supply ... wait a minute, that's a synchronous motor with a fixed field and a VFD :roll: (Please read this as humor - my wife tells me that my humor lacks a lot - something about being radiated with too many EM fields)
Seriously, I do recall an article about using that sort of a driver for high accuracy, speed regulated drives in things like tape drives, hard disks and CD-ROMs. That stuff would definitely be out of my area.
carl
edited to fix spelling
[ May 09, 2005, 07:38 PM: Message edited by: coulter ]
toddro1971
Member
Re: AC ver. DC Motors
I believe the advantage of DC versus AC in this application is two fold. Cheaper overall and simpler speed control. Remember the main advantage of AC over DC is it can easily be distributed over long distances. No need to distribute long distance in a drill or mixer or whatever. In a DC motor, The higher the armature voltage, the faster the rotation. This relationship is linear to the motor's maximum speed.
So a simple speed pot does the trick. Like in your drill, pull the trigger more it goes faster.
I believe the advantage of DC versus AC in this application is two fold. Cheaper overall and simpler speed control. Remember the main advantage of AC over DC is it can easily be distributed over long distances. No need to distribute long distance in a drill or mixer or whatever. In a DC motor, The higher the armature voltage, the faster the rotation. This relationship is linear to the motor's maximum speed.
So a simple speed pot does the trick. Like in your drill, pull the trigger more it goes faster.
Re: AC ver. DC Motors
Posted by Carl:
Brushes basically do the same thing. So could we say there really is no such thing as a "DC" motor?
Steve
Posted by Carl:
I was kind of thinking the same thing about the "new" dc motors. If the commutation is electronic, doesn't that really just change the DC to AC. So it's basically just a AC motor that runs from DC.
Brushes basically do the same thing. So could we say there really is no such thing as a "DC" motor?
Steve
Re: AC ver. DC Motors
Rattus:
I have to qualify your statement:
And DC is actually more efficient to transmit down long lines. Fewer inductive and capacitive losses, and the constant current makes more efficient use of the wire area.
There are now some DC lines in the US (and a lot in Europe). I know there is one in California, and I think it runs at about 1,000,000 volts.
Steve
Rattus:
I have to qualify your statement:
They do step DC up and down very easily now days (DC converted to AC, stepped up, and converted back to DC). With modern power electronics, it's now very efficient and practical.
And DC is actually more efficient to transmit down long lines. Fewer inductive and capacitive losses, and the constant current makes more efficient use of the wire area.
There are now some DC lines in the US (and a lot in Europe). I know there is one in California, and I think it runs at about 1,000,000 volts.
Steve
physis
Senior Member
Re: AC ver. DC Motors
I've never heard of an electronic commutator. Makes sense there'd be one by now. But yeah, it would pretty much be an inverter.
All of a sudden I have like 50 questions and thoughts about motors. Sometimes I'm fascinated with those things.
That's an interesting way to think of it Steve. Except a real commutator makes pulsed DC. Actually it should make an approximation of a bipolar square wave.
I've never heard of an electronic commutator. Makes sense there'd be one by now. But yeah, it would pretty much be an inverter.
All of a sudden I have like 50 questions and thoughts about motors. Sometimes I'm fascinated with those things.
Re: AC ver. DC Motors
As I recall from my last power class, (1985?) AC HV transmission lines are more efficient up to 600 miles, after that, DC lines get the edge. As I understand, (that's code for "this is not my area of expertise") long AC lines run into some problems with synchronization. Also there are issues with the reactive loading that sucks up a lot of current that heats the wires, but does not transfer power. HV DC lines work pretty well - takes 1/2 the copper, no synchronization or stability issues, no VAR loading issues.
If I remember correctly, one of the first inverters was installed at Niagra Falls in the 1930s. It takes the power from James Bay, Canada and connects it to the grid servicing NY City. James Bay is too far for straight AC transmission, and there were stability issues.
I don't think any of this matters much for 120V. The transmission distances should be very comparable as long as we are sticking to 60Hz.
carl
Yeah, I remember hearing about that. Something about a couple of guys, Steinmetz, working for Edison, and Tesla working for Westinghouse. Are those the ones? (more of my bad humor
As I recall from my last power class, (1985?) AC HV transmission lines are more efficient up to 600 miles, after that, DC lines get the edge. As I understand, (that's code for "this is not my area of expertise") long AC lines run into some problems with synchronization. Also there are issues with the reactive loading that sucks up a lot of current that heats the wires, but does not transfer power. HV DC lines work pretty well - takes 1/2 the copper, no synchronization or stability issues, no VAR loading issues.
If I remember correctly, one of the first inverters was installed at Niagra Falls in the 1930s. It takes the power from James Bay, Canada and connects it to the grid servicing NY City. James Bay is too far for straight AC transmission, and there were stability issues.
I don't think any of this matters much for 120V. The transmission distances should be very comparable as long as we are sticking to 60Hz.
carl
Re: AC ver. DC Motors
The ones I've seen for the brushless DC motors are called "stepper drives". The output is pulsed square waves. So the PM rotor just steps from pole to pole. Of course if one steps a 12 pole motor at 1000Hz, the motor is turning 5,000rpm. And there is no slip. As long as the motor is not reaching pull out torque, the rpm is as close as the frequency.
I agree, fascinating stuff.
carl
Most of the stuff I'm seeing now could easily be described as "pulsed DC". One I'm working on now is a 6600V, 6500hp VFD. It is a pulsed width modulated square wave, 3 phase, pulses go both positive and negative.
The ones I've seen for the brushless DC motors are called "stepper drives". The output is pulsed square waves. So the PM rotor just steps from pole to pole. Of course if one steps a 12 pole motor at 1000Hz, the motor is turning 5,000rpm. And there is no slip. As long as the motor is not reaching pull out torque, the rpm is as close as the frequency.
I agree, fascinating stuff.
carl
physis
Senior Member
Re: AC ver. DC Motors
Carl,
I remember when I just did this stuff for fun. It had to be almost four or five weeks ago. I was building a stepping motor driver when I realized that you might as well not only incriment to the next coil set but also drive the 180? opposing coil set with opposite current. (It just hadn't occurred to me yet) It wasn't long before I found this was standard practice, but at the time I thought I had really come up with something.
I never get to be first.
Carl,
I remember when I just did this stuff for fun. It had to be almost four or five weeks ago. I was building a stepping motor driver when I realized that you might as well not only incriment to the next coil set but also drive the 180? opposing coil set with opposite current. (It just hadn't occurred to me yet) It wasn't long before I found this was standard practice, but at the time I thought I had really come up with something.
I never get to be first.
physis
Senior Member
Re: AC ver. DC Motors
My experience with these things is small stuff, motor sizes you might find in your printer or photo copier. Like Rattus was talking about, permanent magnets. But I've been rather surprized at the torque you can get out of them.
Yeah, I've overdriven a few.
Carl, I just did some math!
How much more than 5,000,000 watts is that motor?
Is it 10,000,000 watts?
Holy cow.
My experience with these things is small stuff, motor sizes you might find in your printer or photo copier. Like Rattus was talking about, permanent magnets. But I've been rather surprized at the torque you can get out of them.
Yeah, I've overdriven a few.
Carl, I just did some math!
How much more than 5,000,000 watts is that motor?
Is it 10,000,000 watts?
Holy cow.
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