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Rosco Bodine
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[*] posted on 2-2-2006 at 02:54


A question for the circuit gurus , please take a look
at the attached file and tell me if the modernized
and hexfet adapted " emitter follower " I have
drawn in the schematic will work as I am thinking it will .

On rare occasion I have been reading and have
encountered the term " electronic rheostat " used
in various ways for no specific circuit which of course
is never illustrated either . But what I have supposed
is that such a device would work similarly as do the
resistance wire rheostats , limiting current by their
IR heat loss , and voltage drop across the semiconductor
when it is operating in the active region .

Anyway , there are uses for these simpler circuits where the noisy waveforms and ripple generated by PWM and other switching type power supplies is unacceptable .

I have been thinking about possible ways of speed / power controlling permanent split capacitor asynchronous AC motors
such as 1/10 horsepower and less , using off the shelf standard motors , and not the more expensive inverter duty
motors as are required for PWM variable frequency drives .
The simple circuit I have been contemplating could work
well open loop and it could also very easily have the feedback loop closed with some modification and added components . Anyway it looks to me like the basic circuit should work , but I haven't tried it yet .

Take a look and tell me what you think .

Variable AC Voltage Regulator.jpg - 41kB
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[*] posted on 2-2-2006 at 04:32


Likely the mosfet will blow when the gate goes negative with respect to the source.
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Rosco Bodine
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[*] posted on 2-2-2006 at 09:22


Quote:
Originally posted by IrC
Likely the mosfet will blow when the gate goes negative with respect to the source.


I don't see how that would happen since the source has to follow the gate , the same as does the emitter have to follow the base in an NPN emitter follower . The source will
be at the same voltage as the gate which is Vref.
The idea is that a greatly current amplified Vref. appear on
the right hand side of the AC load , with the rest of the voltage above Vref . being dropped across the mosfet being
forced to act as a power resistor .

Hmmmm , please analyze and elaborate .

Please talk me through it by the half cycle ,
because I don't see it looking straight at it :D

Edit : see attachment for the mosfet modus operandi

[Edited on 2-2-2006 by Rosco Bodine]

Attachment: mosfet used as source follower.pdf (93kB)
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Rosco Bodine
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[*] posted on 2-2-2006 at 16:40


Quote:
Originally posted by IrC
Likely the mosfet will blow when the gate goes negative with respect to the source.



For a few hours I have been studying the proposed circuit
and I still don't see the problem . However should you prove to be correct because of something I'm not seeing in terms of direct or parasitic paths , I have been thinking about how to remedy the problem of the barbecued mosfet so that volunteer mosfet number two has a happier time of it .
What I am thinking is to self-bias the mosfet from a local
supply derived from its own DC rails across the power bridge rectifier , using an optocoupler to control the gate , and using
Vref. to drive the transmitter side of the optocoupler . The ultimate voltage would probably be 6 volts less across the AC load , but the mosfet would be bulletproofed . I am thinking a 400 volt rated mosfet minimum , 500 volts even better , for the same reason as the 370 volt capacitor rating if you have a PSC motor as the load , the peak to peak doubling of voltage that can happen because of the capacitor .

Edit : I have been retracing the paths using different colored highlighters for the two halves of the cycle , and now I see the problem . It wouldn't hurt the mosfet , but neither would the circuit provide fullwave regulation . When the bottom AC rail is negative going the mosfet can't function as a source follower but would be in saturation at any of the higher Vref. settings . The result would a baseline shift for the AC through the load with regulation only upwards from 50% and an asymmetrical output to the load . So ....back to the drawing board :D

I can do better and I will sketch this hopefully better idea
and post it here .

The alternative idea which probably has a better chance of working , involves a ganged pot directly between the AC rails with each separate Vref. from the wipers driving the gates of two separate N channel mosfets each having their drain connected to the respective AC rail and the source of each mosfet connected to opposite ends of the AC load . The mosfets operate sequentially as source followers during the half cycles when their drain is positive , with the return diode in the non-conducting mosfet completing the path through the load to the rail which is negative on that half cycle . The ganged pots are connected between the AC rails in the way that the wiper of each moves towards rail voltage of the respective mosfet whose gate is supplied increasing Vref. as the pot is turned clockwise . This should provide a balanced circuit with a symmetrical AC output and no baseline shift and using fewer parts than the other idea , as well as dividing the heat dissipation between two mosfets when the voltage is set for lower levels . I have never seen this circuit I have in mind , but it seems so simple
it would have to work .....I hope :D



[Edited on 3-2-2006 by Rosco Bodine]
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[*] posted on 2-2-2006 at 18:38


At the very least, toss on a protection resistor and 12V zener diode, yeesh!

Tim




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Rosco Bodine
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[*] posted on 2-2-2006 at 19:23


See my edit above , I found the problem and
thought of a better circuit at about the same time :D
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[*] posted on 2-2-2006 at 19:48


Having the gate more negative than the source is not a problem - its a trick often used to provide noise immunity to a mosfet thats supposed to be off.
The problem is the likelyhood of exceeding the gate/source breakdown voltage - which will fry your fet instantly.
The other issue is heat - you're running the fet linear, with potentially larfe current and large voltage across it. The poor thing could end up trying to dump 50W or more.




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Rosco Bodine
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[*] posted on 2-2-2006 at 20:56


Here's the simpler circuit that I am thinking
would be sure to work . Any criticisms or
comments or suggestions are welcome .


With regards to the breakdown voltage ,
I thought it was customary practice that
the manufacturers put zener protection
on the gates . I have seen the zener on
manufacturers data sheets , and sometimes
the return diode is a zener too , something
would have to be a higher rated zener than
the max allowable drain to source voltage .

One thing which would probably not hurt is
to parallel an external return diode across the
mosfet to split the heat dissipation from the
internal diode which occurs on each half cycle .
Yeah running any semiconductors in that
active region where they are in fact semiconducting
and acting as resistors is going to produce a whole
lot of heat , so big old heatsinks would be a good idea ,
even a fan cooled CPU heatsink would probably
be a good solution for this circuit when operating
at medium output voltages where the mosfets are
dumping as much heat as the motor .

[Edited on 3-2-2006 by Rosco Bodine]

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[*] posted on 3-2-2006 at 15:53


I've got a couple of questions:
Wouldn't an autotransformer (Variac (tm)) work a lot better?
Is there a specific reason to take an analog-ish mosfet approach?

Comments:
It will probably self-destruct releasing the magic smoke inside the transistors.
Sam Goldwasser and his friends have published the circuits inside of commercial
dimmers, motor controls, etc. See sci.electronics.repair.
Each application (motor control, etc.) mentioned in the thread probably
requires a different circuit design and approach if active devices are used.

What follows is a random rant about the subtleties of power circuits.
The FET manufacturers have extensive application notes and sample circuits to copy.
They're not simple - they can't be and still work reliably. Many of the
sample circuits include a sample PCB layout which can be copied as well.
This saves enormous grief.

If the intent is to make a low electrical noise attenuator for line power,
a transformer or autotransformer is the best power/cost for an amateur.
If you happen to have (for example) Crown (tm) power amplifier and a signal generator,
you can make clean sine waves. This costs more $$ unless you have the equipment already.
The third way (more exotic) is to get Texas Instruments' pulse density modulation
power amplifier modules and wire them up (carefully), using a correctly sized reconstruction
filter on the output.

Using today's power MOSFETs as analog devices is treacherous.
They oscillate, overheat, and generally are rebellious unless you've studied a lot of electronics.
They are not designed to be used "partly on" and they are not characterized for that.

In any case attaching the gate to a pot connected to the power line is
a recipe for much smoke. At least connect series resistors and parallel zeners
to make sure that the gate never exceeds + or - 15 volts with respect to the source.
Even instantaneous spikes will kill the FET. The built in zener diodes (if present) have
extremely limited power handling capability. Be sure that the zener diode power
limits are never exceeded.

There's a further problem here:
most power FETs can work well into high megahertz frequencies, and the parasitic
inductances and capacitances of the leads & nearby components frequently cause
oscillations which can destroy the FET quickly. The gate must be driven from a low
impedance (usually in the 1-100 ohm range) to prevent oscillation. This usually requires
an active circuit to drive the gate.

If anyone makes it this far and wants to know more, I'll deliver more prime bull.
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Rosco Bodine
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[*] posted on 3-2-2006 at 21:24


@densest

Look at the attached file showing the evolution of the
circuit design which I have been contemplating , and
consider why certain advantages are provided over
a variac , such as being able to set a minimum voltage
where the adjustment begins to effect adding power to the motor . The use of control circuitry also provides locations
where logic input is possible to be applied to make the circuit behave differently in response to sensed conditions .

For example when the circuit is first powered up , the existing
voltage setting could optionally be over-ridden by external signal specifying some selected number of complete cycles of full voltage to the motor to provide extra starting torque ,
and when the counted number of full voltage cycles has
completed , the circuit resumes operation at its setpoint voltage just after the motor breakaway into rotation .

What I am looking at is simple ways of improving the performance particularly of off the shelf 1/10 hp and less permanent split capacitor AC motors , using something
better for the task than triac based sine wave choppers .
The trapezoidal waveform from the circuit I have attached
is more nearly zero crossing with its waveform being continuous to less than 2 volts of zero on each half cycle
before dropout . This should be a very quiet circuit in
terms of any unwanted harmonic effects or spikes .

IGBT's could be used as easily as MOSFET's .

Edit : I may have the left side of the range adjustment pot
drawn wrong , and moving the wrong direction .
I just drew this and am looking at it . I think
its coil probably needs to be above ......yeah I'll have to redraw it and repost the corrected schematic . I have made
some changes and really should redraw the entire schematic
in a better layout . Long day .

I'm too tired to redraw the whole thing so forgive the smudges from my erasures and corrections . Here's the
corrected schematic for where I am now .

[Edited on 4-2-2006 by Rosco Bodine]

Variable AC Voltage Supply for AC Motor.jpg - 84kB
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[*] posted on 3-2-2006 at 22:09


@balanced_world: for temperature control (resistive heater) a triac driven by an asymmetrical diac (to prevent "snap on") is cheap, effective, and simple. If you need electrical quiet, use a cycle skipping zero crossing triac chopper which only delivers full cycles to the load. Thermal inertia takes care of temperature fluctuations.

@rosco: (please excuse typos - a cat is holding one hand hostage)
Sorry if I'm pedantic about this. I've been studying motor drives and power transmission recently. I'm assuming that your intended applications are not for large scale production, so scrounging good equipment on EBay, etc., can cut your costs a lot.

Your motor control application is reasonable. Is this intended for existing motors or new ones? What's the intended speed range (i.e. if 100 is "full speed", do you want 1-300 or 5-120?) Do you want constant torque, constant power, or ?? over the speed range? Remember that as speed decreases you must increase torque (magnet strength) to maintain power, and electromagnets fry if you saturate the steel.

For old motors or existing ones: remember stiction at startup or low RPM. At low voltages, the power and torque of AC motors is pretty low. Some of the chopper drives, ridiculously simple as they are, do provide more power as the load increases even at low speeds. If your goal is to control the rotor speed, you must have load feedback.

For a new design: consider a three phase motor. It inherently is easier to drive at full power or torque over a wide speed range. Rather than changing the voltage alone, changing the frequency as well gives you a more control. The capacitor phase shift in a 1PH motor doesn't like a wide frequency range. It's inherently simpler (no startup circuits, no cap to blow up...) Some new washing machines use IGBT choppers to drive a 3PH motor for all the washer functions with only a belt connecting the tub and the motor. No more transmissions to break.

If you derate them (like using a 1HP motor at 1/3 HP), "motor drive rating" doesn't matter very much. Saves a lot of money.

For a new design: consider a permanent magnet DC motor if you need high torque at low speed.

In any case, IGBTs and FETs are much happier used in a H-bridge configuration when driving an inductive load.

I'm looking forward to seeing your design.
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[*] posted on 3-2-2006 at 23:04


@densest

For reasons of economy and simplicity and practicality
for the application , the alternatives you suggest are
not valid . I want to control a single phase 120 Volt
60 cycle off the shelf permanent split capacitor motor
of 1/10 horsepower or less , using most desirably the
inherent abilty of a quadratic load to self regulate
the speed of the motor , without the use of closed loop
digital speed regulation . If the power and motor is
well matched to the load , particulaly a dual quadratic
load on a dual shaft motor where say 20% to 30% of
the motors torque is diverted into a parasitic quadratic
load like a squirrel cage cooling fan or a hysteresis disc ,
or even static field of DC injection into the stator itself ,
an asynchronous motor being driven under these conditions
will self speed regulate acceptably well at a fixed supply voltage . If the workload decreases on the motor , its
speed increase is limited by the parasitic load increase ,
and vice versa . And the response is instantaneous in
such a self-regulation scheme where feedback is an inherent
property of a quadratic load being exploited as a parasitic
load . I hope that made sense . This regulation effect
should occur best in the speed range from about 120 rpm to 1200 rpm for a four pole PSC motor where the torque speed
slope is linear . The shaft horsepower output should still be more than double at the same wattage for a PSC motor run
in such a scheme as is gotten from a shaded pole motor ,
and the speed regulation better , as well as the starting torque and entire low speed range torque .

I really can't understand the fascination of people with variable frequency drives for asynhronous motors which will never be run up to synchronous speed where the effciency
would be realized . And for a 75 watt motor the hardware investment is hardly worth consideration in the same sense as would be understandable for a 75Kw motor in a factory .
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[*] posted on 3-2-2006 at 23:54
Zero crossing firing and cycle skipping


@Densest,

I would like to know what you think about using a "cycle skipping zero crossing Triac chopper" in a Power Supply.

Transformer Specs.(Philips): 24 Volts and 40 Amps.

This is not the usual 1000 Watt Magnetron type Transformer kind of garbage. At least 4 Magnetron type Transformers would fit this Core, and the leads are about 8 mm diameter and made of solid Copper wire connected to M10 Brass studs. At 40 Amps, this "Monster" stays ice cold which makes me believe that 40 Amps is a rather conservative rating from the guy who sold it to me.

I would like to keep the "Bridge-Caps" voltage about 5 Volts (or even less) above the regulating Transistors output Voltage, to give them regulation space.

By using Triacs in the primary leads and firing them in the Sin Cycle, not only "bumps" the transformer, but also produces lots of Harmonics and heat in the Core. The Transformer efficiency drops drastically.

"Zero crossing firing and cycle skipping" seems to be a very elegant solution. I have 10, 25, 50 and 100 Amp zero crossing Solid-State relays from "Crouzet" and "Gunther" etc.. They are internally Optic-Coupled, and only need 3-32 Volts to trigger them and can switch 24-280 Volts AC. Even if they are fired during a Sin Cycle, they still take the first zero crossing.

[Edited on 4-2-2006 by Lambda]
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Rosco Bodine
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[*] posted on 4-2-2006 at 00:13


Lambda ,

Be careful with cycle skipping or people
will say the real truth is that you are
performing burst modulation :D

Page 10 on the attached file may interest you .

[Edited on 4-2-2006 by Rosco Bodine]

Attachment: Triac Inductive Load Control Circuits.pdf (92kB)
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[*] posted on 4-2-2006 at 03:46


@ rosco

The circuit you have in mind controlling a series
connected AC load using the DC side of a bridge rectifier
does work but has many limitations such as non linear operation and not of much use except between 80-120 volt. The simplicity can't be beat except perhaps by a triac like the inductive rated "quadracs" made up to 40A
800V by Teccor.

This may well be adequate for your application and can be simply implemented using a duty cycle modulated transistor on the DC side. This circuit puts a lot of noise on the AC line if that matters.

Motorola had patented a circuit like this years ago but it never recieved much attention or use. The unit I built some years ago for a 3phase 20A 480V application used
a PWM duty cycle controlled bipolar transistor on each phase and worked well enough.
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[*] posted on 4-2-2006 at 10:43


The latest corrected revision of the mosfet circuit I am
considering is first attached at the end of my fourth
post above counting this post as one . Please take a
look . I think this particular circuit will work fine and
can be matched to the characteristic of a particular motor
in a particular application by using different value
power resistors for the fixed idle current value to the
motor , to supply just sufficient power that the motor
will barely turn , windmilling under light load . The value for these fixed resistors will have to be selected by first doing a rheostat test with the motor and 4 diodes ( two in series forward biased paralleled with two more in series reverse biased ) and measuring the resistance value needed for setting the idle current . These power resistors will divert much of the heat dissipation from the mosfets when the adjustable voltage is at minimum , reducing the heatsinking requirement . The redundant return diode I have added externally across the mosfet drain to source will also halve the return diode dissipation which would otherwise be there in the mosfet . The added zeners are likely to be 15 volt and are redundant gate protection . Candidate mosfets are Hitachi 2SK1837 , overkill for sure , but the overkill translates to ruggedness at little extra cost above using a minimal device for the mission critical component and having a failure
later from too little transient immunity .

I think I will reattach the schematic file here since I am further describing the circuit here in more detail and it
may be lost in the posts above departure to other related
circuits .

There is an idea which I have wondered if it has ever been
explored by the industry regarding the possible use of
many CdS photoresistor elements paralleled as regions
to form a " power photoresistor based optocoupler " ,
a " power vactrol " if you are familiar with the signal level
vactrol optocoupler by Perkin Elmer . Such devices are
high voltage AC photoresistors which might be arrayed
in sufficient number in parallel , deposited directly on a
heatsinkable substrate and driven by high output LEDS ,
to form a high current capable full wave line voltage AC controller , driven by TTL voltage levels , and passing
the AC power with the waveform completely unchanged
except in terms of controlling its voltage . Has anyone
heard of such a device ?

Anyway the revised mosfet circuit from the earlier discussion
is reattached here . I will likely prototype and test this
particular revision attached below since it seems workable in visualization of how it should work . Now if only Murphy's Law will not discover what I may have missed , it should be all right :D

I am going out on a limb here , but there is always the possibility when anyone is drawing a schematic for a
dedicated circuit for a niche application , that what they
design is entirely novel , since if they could find the circuit
already published in an application note as a commonly known circuit , then there would be no need to invent it
themselves in the first place . So it is just possible that
what I have done is propose an entirely new circuit in
one of those necessity is the mother of invention scenarios ,
and the fact it is a simple circuit in no way rules that out .
So if anybody should run across this circuit already published somewhere , please let me know since it also likely that often these sort of things turn out to be that in the lands where no wheel was to be found , it was reinvented by one genius after another who are all cousins of mine :D

[Edited on 4-2-2006 by Rosco Bodine]

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[*] posted on 4-2-2006 at 11:20


Erm... *ANY*!!! *AC* motor will really highly NOT appeciate a "light dimmer" approach. This will ONLY work on universal type motors (the kind that you can use a dimmer switch on anyway).

Other than that, the circuit ought to work as shown, not counting the blown junctions of course (I hope you have a good way to dissipate all that wasted power).

Tim

[Edited on 2-4-2006 by 12AX7]




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[*] posted on 4-2-2006 at 11:47


Quote:
Originally posted by 12AX7
Erm... *ANY*!!! *AC* motor will really highly NOT appeciate a "light dimmer" approach. This will ONLY work on universal type motors (the kind that you can use a dimmer switch on anyway).


Hmmm , there are dimmers of the sort we all know ,
and then there are other different circuits having different waveforms , like what I am illustrating , and indeed this should work for PSC and shaded pole motors better than
triac based wave choppers or series rheostat circuits which
are often used even though their behavior is counterintuitive
in that the inherent feedback to motor current increase under loading goes in the wrong direction , reenforcing the
speed decrease towards a stall . And then there's the buzzing created by switching harmonics of triac circuits which
cause stator hum and insulation breakdown over time .
What I propose should address those deficiencies and that
is the motivation .

Quote:

Other than that, the circuit ought to work as shown, not counting the blown junctions of course (I hope you have a good way to dissipate all that wasted power).

Tim


Exactly where do you expect blown junctions and why ?
And who cares about dumping heat and " wasted power " from smoothly controlling a sub-fractional horsepower motor
when you don't get something for nothing in this case ,
and the wasted heat amounts to about what is dumped
from your computers CPU heatsink right now .....that's
wasted power too , but very necessary to get done
what business is at hand . All in all the waste is less than
what is dumped by the average desk lamp , and that waste
goes away completely as the voltage setting is raised to
maximum . The heat dumping only occurs at the lower
voltage settings .


When it comes to the matter of active closed loop speed regulation , there is a method which is fairly simple to implement to provide for fine control of the speed when
there is not a great variation of the load , perhaps in the range of plus or minus 10% . The method involves eddy
current braking being applied as a parasitic load to a motor
to increase its slip and reduce its stable rpms at a given voltage . If you have two iron core inductors with their
airgaps widened sufficiently that the opposite edges of an aluminum disk on the motor shaft is travelling through the airgaps during rotation , the magnetic field through the gap induces eddy currents in the disc and produces a torque loading on the motor which varies quadratically with rpms or with the strength of the field which may be varied by the current through the electromagnets . There is some inherent
speed regulation present with this sort of parasitic load ,
and the effect can be increased by active control of the current through the electromagnets , which can decrease or
increase the torque load on the motor which varies its
slip speed and rpms accordingly , even if the voltage to the motor is held constant . This is similar to the scenario where
a car with an automatic transmission is stopped on a steep uphill grade and a little throttle is applied sufficient for the car
to roll uphill , but a little pressure on the brake pedal holds
the vehicle in position ......yes power is being " wasted " but
for very good reasons which the situation requires , a balancing of forces which allows the vehicle to only roll forward when the traffic light changes and pressure on the brake is released , with no chance of rolling backwards and
smashing the grille of the police car stopped behind :D

Now that's regulation ! :D

[Edited on 4-2-2006 by Rosco Bodine]
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[*] posted on 4-2-2006 at 15:09


Quote:
Originally posted by Rosco Bodine
or series rheostat circuits which are often used even though their behavior is counterintuitive
in that the inherent feedback to motor current increase under loading goes in the wrong direction , reenforcing the
speed decrease towards a stall.


Yeah, you can reduce the RPM of an AC motor by reducing voltage, thus limiting power by the DC resistance of the winding and running it on the stall slope (relatively constant torque), but that's a really shitty way to do it.

Quote:
And then there's the buzzing created by switching harmonics of triac circuits which
cause stator hum and insulation breakdown over time .
What I propose should address those deficiencies and that
is the motivation.


Proper snubbing can prevent this, although in general triac control of any inductive load is irresponsible anyway.

Quote:
Quote:

Other than that, the circuit ought to work as shown, not counting the blown junctions of course (I hope you have a good way to dissipate all that wasted power).


Exactly where do you expect blown junctions and why ?


*Points at schematic* That transistor, and that transistor.

Quote:
And who cares about dumping heat and " wasted power "


Well, I suppose you wouldn't understand what engineering is about, so it's a bit of a lost cause on you anyway...

Quote:
from smoothly controlling a sub-fractional horsepower motor


It might be rated for 0.6A at 120V. At half voltage (assuming a resistive response), that's 0.3A at 60V = 18W in each (9W per transistor). That can be easily dissipated, but using this on any heavier motor will quickly cost you.

Oh, and don't forget short circuit protection. Transistors don't appreciate 160V peak at 50A. (Don't say it won't happen, if it can it will. A fuse is NOT fast enough to protect silicon!)

Quote:
When it comes to the matter of active closed loop speed regulation , there is a method which is fairly simple to implement to provide for fine control of the speed when
there is not a great variation of the load , perhaps in the range of plus or minus 10% . The method involves eddy
current braking being applied as a parasitic load to a motor
to increase its slip and reduce its stable rpms at a given voltage.


Holy crap, I suppose you also run your heating and air conditioning at the same time, too?!

If you're going to do *breaking* in the first place, skip the whole voltage control and use the low torque output of the motor itself... it'll waste less power, too! (All shaded pole motors are rated as "impedance protected".)

Quote:
If you have two iron core inductors with their
airgaps widened sufficiently that the opposite edges of an aluminum disk on the motor shaft is travelling through the airgaps during rotation , the magnetic field through the gap induces eddy currents in the disc and produces a torque loading on the motor which varies quadratically with rpms or with the strength of the field which may be varied by the current through the electromagnets.


Ahem.. force (torque) is proportional to RPM linearly, not squared. Since *power* is force times rate, the power varies as a squared law.

Quote:
There is some inherent
speed regulation present with this sort of parasitic load ,
and the effect can be increased by active control of the current through the electromagnets


I wonder, do you also keep your car's gas pedal floored and control your speed down the road with the brakes?

Tim




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Rosco Bodine
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[*] posted on 4-2-2006 at 15:53


Taken to extremes of absurdity , any rationale
including engineering principles which have validity
in a qualified sense and limited application can
be made into a farce , which is what you are doing .

The small rotor mass and momentum of small motors
involves different considerations for how to do things
best , than is directly applicable to much larger motors .

You are a purist when it comes to engineering .
I am a pragmatist , get the job done with what
works best within economic reason , but not
necessarily the cheapest or dumbest POS
that can be made .

If mosfets were never intended nor designed to be
run in DC linear mode , then why in the hell do the
manufacturers data sheets specify their performance
operating in exactly that fashion as one of their
parameters ? They will not fry so long as they are
operated within limits .

And short circuit protection is optional but can be added .
Voltage sensing across a low value resistor in series with the load can be used like a meter shunt as an input to a comparator , and a reference for current limit set on the other input , with the comparator output used to drive optocouplers clamping the gates to their rails at below their minimum on voltages . But with your vast knowledge of engineering , I'm sure I don't need to explain this to you .

I know how to drive , and can power brake even a
manual transmission vehicle on a hill by using my right heel on the brake pedal and the toes of the same foot on the gas while the left foot eases out the clutch ....
but I have never found this useful while simply cruising
down the road .

What you were saying about eddy current braking being used entirely for speed control is indeed possible but it
is generally done by letting the motor run at full speed
and using an eddy current clutch to couple to the load ,
they may call it a hysteresis clutch in some circles . It
works like a fan clutch on an automobile radiator , and they
may even make some of those using an eddy current clutch ,
but most of them are miniature hydraulic torque converters
which are thermostatically engaged or disengaged or
throttled in between by a little spiral pilot valve operated
by a bimetal coil which responds to the temperature of the airflow from the radiator .

With regards to quadratic loads , the torque requirement appears as the speed squared , and the power requirement
as the speed cubed . Somehow we aren't on the same page . And the linear slope for the torque speed characteristic is the shallow slope upwards from locked rotor
to about 75% of synchronous speed of 1800 for a 4 pole motor , 1260 rpm or so . The steep part of the slope is from
about 95% of synchronous back down to about 80% , not
much of a speed range for adjustment where the motor is
working in the range of its maximum output . So the
low and gradual slope is exactly where speed must be
controlled if the purpose for such a speed control is the
drive for a magnetic stirrer , which is what I am doing .
Does my strategy now make more sense ?


[Edited on 5-2-2006 by Rosco Bodine]
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[*] posted on 4-2-2006 at 16:39


Quote:
Originally posted by Rosco Bodine
If mosfets were never intended nor designed to be
run in DC linear mode , then why in the hell do the
manufacturers data sheets specify their performance
operating in exactly that fashion as one of their
parameters ? They will not fry so long as they are
operated within limits .


I don't like wasting power. Linear region means volts times amps. You can certainly make a wonderful amplifier using the full linear region, but if you tack on a PWM and integrator, you can go from 33% to 95% efficiency in one fell swoop! Class D has plenty of volts and amps, but not together, so there's very little waste.

FYI: because of the favorable characteristics, many FETs are only rated for pulse or switching applications. That doesn't mean they won't work in the linear range, it just means they weren't intended for it. This isn't new: vacuum tubes designed for TV sweep or radar pulse applications have few linear operating points listed.

Tim




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Rosco Bodine
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[*] posted on 4-2-2006 at 17:09


Hey I don't like wasting power either , but there
is a time for practicality versus pure power utilization
economy , and for a small motor it isn't worth the expense and grief of hardware complexity to save a few watts
when they can be more easily just dumped as the cost of doing business in the particular application . With light
motors , cogging and hunting oscillations are inevitable
using pure brute force digital closed loop speed control
at low speeds . So it is far better to try to match the
motor to the task , and exploit every inherent speed regulation method which can be used , before ever getting
to the point of using closed loop feedback for fine control .
The idea is that the thing should run pretty stable ,
and perhaps stable enough , even without any feedback
from digital controllers . Well designed equipment where
the components are well matched can do the job very well
open loop , and many times that is exactly the way equipment of the old days was made , mechanically governed like steam engine throttles , before any digital
circuitry was ever invented . Some of these scenarios
are really quite ingenious and work very well . There is
a trap involved in getting overly technical on a simple device where it isn't necessary if there is a simpler and
possibly better way , even if it does waste half the power
of a 75 watt device at its least efficent point of operation .
For the price of 37 Watts I'll take simplicity and reliability
and economy any day .
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[*] posted on 4-2-2006 at 19:54


Bah... Dig out a switchmode power supply chip, a pair of MOSFETs, rig a ferrite transformer (all of ten minutes winding one) with DC restorer for gate drive, rectify the line voltage then chop it, add a filter and you've got a PWM class D amplifier, 95% efficient or better, and if you generate your own drive signal you can get any frequency, amplitude or waveform you want. :D

For something this small, pffbt... but anything over a half horsepower, this is how it's done.

Tim




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Rosco Bodine
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[*] posted on 4-2-2006 at 20:43


For those really critical range hood ventilator fan
speed settings when only the best will do ......

make this little gem the powerhandler and the manufacturer
will throw in the dedicated workstation with user friendly
interface for free :D

http://www.ballantinelabs.com/bli1620a.htm

Then again , when it's a five dollar motor ......
there just has to be an easier way :D

Actually my first thought on this project was to open up
an inexpensive uninterruptable power supply and
see if it would be possible to easily adjust the voltage
output from the inverter .

The output would already be well filtered sinewave
so it would simply be a matter of being able to adjust the output voltage . The input side is already there for keeping the battery charged , but of course the battery isn't needed for the use to which the UPS power stage would be adapted .

[Edited on 5-2-2006 by Rosco Bodine]
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[*] posted on 4-2-2006 at 23:21


Modern circuit design has gone away from analog power circuits for a number of reasons, like size, weight, cost, and reliability. Choppers are widely used to drive motors because they are very simple and do a "reasonable" job for low cost.

There's a "simple" way to make sine waves digitally at reasonably arbitrary voltages from a fixed supply. It uses sneaky math to work, but the circuit is simple. More or less.

Get a microcontroller with one digital output and several digital inputs or one analog input. If the inputs are all digital, this requires log(2) of the number of discrete voltage steps. The microcontroller should be capable of switching the output at (say) 256X the desired output frequency while sampling the inputs at (say) 10x per second. For this application jitter is not too important.

Compute the Fourier transform of the desired waveforms, which for sine outputs is a single nonzero value. Compute the reverse Fourier transform at the desired sample rate and quantize to 1 bit using a good random number generator for dither. Burn tables into the microcontroller for the desired amplitudes. Program the microcontroller to output the table values (1 or 0) depending on the control inputs.

Connect the output of the micro to a high-voltage mosfet high/low driver pair. Invert the output and connect it to another driver pair. Make an "H" bridge of mosfets - the output is the crossbar. If desired, compute a 3 or 4 section passive LC Bessel filter for the output of the bridge. At least put an L -> C for 12dB/octave at 2X output frequency or so and some snubbers (choose your favorite).

There are a couple of options for the input (AC line).

1) Brute force: Get the DC buss from a bridge rectifier and cap. Ripple doesn't matter much. Output voltage is proportional to input voltage.

2) Sneaky: Sync the micro to the input line zero crossings and apply a sine function as a correction to the quantization during table generation. Rectify the line with a bridge rectifier. No filter cap needed at all. Output voltage is proportional to input AC voltage. This is the closest I think practical to a "just step down the input" circuit.

3) Drive the DC buss from an off-the-shelf PFC circuit. You get regulation and universal input for free.

Total parts count: 20-40, with most in the PFC version. All but the power components (4/5 FETs, diodes, inductor(s), filter caps) could be tiny little things on a tiny little PCB.

None of this addresses the problem of speed control. Dissipative speed control is possible but inelegant. Monitoring the reverse EMF of the motor is pretty easy given this approach, but it would add another (say) 10 parts and some microcontroller inputs. Response times could be < 5 output cycles or so.

Efficiency should be > 80%, RFI should be easily controllable, response quick, and cost < $30 for parts. If you want cheaper use a universal motor and an SCR.

Possibilities: for lower speeds changing the output frequency to a lower value would allow the motor to run closer to its synchronous frequency and therefore run cooler. That approach would require input version (3) since the output would no longer be synchronous with the power line. Of course this approach is limited because the motor phase shift capacitor becomes less effective with lower frequency excitation.

A three-phase version would always run the motor at its synchronous frequency, but it is probably impractical for < 200W motors unless you have a cheap supply of synchros, which would work beautifully.
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