Motor speed control

On 6 Jun 2006 18:21:19 -0700, (E-Mail Removed) wrote:

>One respondant mentioned duty cycle. I kind like that, don't bother
>rectifying anything. Just use an SCR. I could trigger it from a single
>stage flip flop then to get the 30 Hertz. Heck I could even do 20 Hertz
>with flip flops, and just cut the duty cycle accordingly.

A BELT drive fan? The sears I was talking about was direct drive . .
.. Why not just change the size of the pulley, or put an adjustable
speed pulley in - that's a low cost easy solution even if you to
change the belt size.

I used the words duty cycle, but as an analogy to make a distinction
between RMS and square wave power consumption, and again in making an
ersatz sine wave for a VFD.

You can't just cut down the duty cycle or use SCR/Triac type
controllers for FHP Induction motors. They overheat, unless the
pulses approximate a sine wave - the two efficient ways are with
stepped voltages (usually six levels, plus and minus - and relatively
complicated to pull off) or varying the widths of several pulses for
each half cycle of the sine wave (still relatively complicated IMHO -
but better than dealing with voltage levels)

Then there's that pesky starting winding - you're driving a fan. A
fan (unless it is seriously broken) turns freely with no real
resistance (unlike wash machines with transmissions seals and gears to
get moving, compressors, or most anything else . . .) until the fan
gets up to speed - torque required tracks shaft speed.

Run the motor at half speed and the starting winding will kick back in
even if you start it at full speed.

DC brush, or "universal" AC/DC motors are ideal for speed control
applications - but the longevity of the brushes is a limiting factor
and one reason they are seldom used to drive fans. (also noisy,
create sparks and ozone, radio interference)

We had this centrifuge at work - big one horse brush type motor with a
variable transformer to change the speed. Probably dated back to the
40's. The transformer burned out one day and they wanted $400 for a
new one. I went to Grainger's and got a high power lamp dimmer for
$20 - no problems.

A VFD drive that uses single phase power and drives a three phase
motor is probably the ideal if cost is less important than longevity,
excellent speed control, and efficiency.

Plenty of appliance repair/junk dealers around with two speed wash
machine motors - and a new one might cost $50- $70.


----== Posted via Newsfeeds.Com - Unlimited-Unrestricted-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----

I agree much with Default, yet there is one general rule for an
induction motor: They are not variable speed. You can selec
different speeds based upon the wound poles, but to cause the moto
to slip excessively is not an operable condition. The only way t
safely adjust the speed of any induction motor (aside fro
shaded-pole) is to reduce frequency and voltage amplitude on
measured scale. Shaded-pole motrs are horribly inefficient, but mor
tolerant of moderate overloads

Once the stator field loses grip on the squirrel-cage, the losses ar
converted to heat in the stator as a result of decrease
inductive-reluctance in the stator, which results in increase
current. The heat will be turned on the resistive value of th
windings rather than the inductive properties that normally contro
the current, and hence it will overheat, meaning the motor will los
it's inductive properties and become closer to an unwound length o
wire to the line current...

If you are intending on varying the speed other than preset values fo
an induction motor that are avaliable, you best either use a DC powe
motor driven by SCR's such as a treadmill motor, or a brushless D
motor, which is phenomenally-more expensive and rarely worth th
project

My enthusiastic advice here is to either repair or replace the motor
or convert yourself to DC. Then and only then will
duty-cycle/amplitude modulation be useful for adjusting the speed

In any case I advise anyone to get speed controllers from discarde
treadmills, they are the diamonds in the rough for anyone workin
with anything DC-compatible, replacing the lab variac for D
applications. Just be sure to salvage or at least know the po
(varistor, not weed ya goofball) value for it, as guessing can b
hazardous to the controller. I now have one rated 28 amps D
145+V-out, detuned to work on a standard 115VAC circuit at 1
amps...The motors couldn't be any more ideal for any applicatio
anyway, so grab those too

Gold I tells ya....gold....grab them like they are the last powe
supply on Earth, because most of the cost is in these parts anyway..

"Electromotive Guru" <(E-Mail Removed)> wrote in
message news:449b44e7$1$70924$(E-Mail Removed) reenews.net...
>I agree much with Default, yet there is one general rule for any
> induction motor: They are not variable speed. You can select
> different speeds based upon the wound poles, but to cause the motor
> to slip excessively is not an operable condition. The only way to
> safely adjust the speed of any induction motor (aside from
> shaded-pole) is to reduce frequency and voltage amplitude on a
> measured scale.

Am I right in thinking the RMS voltage should be proportional to the
frequency when this is done?

"mc" <(E-Mail Removed)> wrote in message
news:qIJmg.5160$(E-Mail Removed)...
> "Electromotive Guru" <(E-Mail Removed)> wrote in
> message news:449b44e7$1$70924$(E-Mail Removed) reenews.net...
>>I agree much with Default, yet there is one general rule for any
>> induction motor: They are not variable speed. You can select
>> different speeds based upon the wound poles, but to cause the motor
>> to slip excessively is not an operable condition. The only way to
>> safely adjust the speed of any induction motor (aside from
>> shaded-pole) is to reduce frequency and voltage amplitude on a
>> measured scale.
>
> Am I right in thinking the RMS voltage should be proportional to the
> frequency when this is done?
>
Yes, the voltage and frequency are usually reduced. To vary the speed of AC
motors a device called an inverter is used. Internally it will convert the
AC to a DC voltage and then convert it back to an AC type of voltage that
the frequency and voltage are matched to the motor load. I say AC type
because most of them do not put out a sine wave but odd looking waveforms.
Some use several steps in the output to step up to the maximum voltage and
then back down to 0 and then the same for the negative half cycle. We use
some where I work that put out a very odd waveform and normal volt and amp
meters will not work correctly on them. They use something called a
carrier wave that is operating at from around 5 to 15 kiloHz. No where the
normal 60 Hz or less you would normally think of in the US.

Here is a link to the type I am thinking of.

http://www.yaskawa.com/site/products...Drives~J7.html

> mcwrote
Am I right in thinking the RMS voltage should be proportional to th

> frequency when this is done

Yes, it should be, however the formula for this varies by the moto
itself and it's physical construction. What you need to do if yo
intend to try this is to carefully watch current as you alter th
frequency, however there is only so far you can go before th
reactance of the stator core breaks down and the stator fails t
respond proportionally because magnetic saturation has a sort-o
flywheel effect....yes, electrons have mass too...

As mentioned before, only a switchless motor will respond to an
resonable degree because most induction motors are not meant to var
their speeds. When you lower the frequency, you affect the magneti
flux of each laminate in the stator and rotor cores, as well a
overall efficiency and it's power factor. Lower-frequency motors hav
thicker plates because they can respond to the lower frequency an
saturate more slowly

The best way to perform this is to find yourself a 400Hz inductio
motor (see aircraft service) made to run at a comparable speed a
what you are replacing, and then work from there. Be sure to find th
highest service factor for this motor that you can find, 1.35 is a
ideal, with class H insulation (preferably class B rise with clas
F-H insulation)..

While it may be entertaining to try varying the speed of an inductio
motor, it is far from preferred practice. If you really want to var
speed while taking advantage of the non-contact design of a
induction motor, go find a synchronous/stepper motor to run from, a
this will respond exactly as you would want it to up to a much wide
range than the standard induction motor, the more phases the better
If you really wanna get trick, use a Hall-Effect motor/circuit
alhough a bit excessive for a simple fan...

Still above and beyond is the use of a DC power motor. Brus
replacement is not nearly the factor that many think it is, as prope
maintenance/operation can provide over a decade of service. If brushe
are that much a biog deal, opt for a brushless DC motor..

If ya wanna try something fun, take an alternator and short it'
rotor's slip-rings and run it as a low-impedance 3-phase inductio
motor after you remove all the DC components from the circuit. Ove
80 amps is usually wound delta, so keep his in mind..

Whatever you do, best of luck to ya, but I still strongly recommen
the DC power motor, like the treadmill motor