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Old May 18, 2007, 12:09 PM
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Help!
Permanent magnet DC/Brushless formulae

I thought I knew how DC motors worked

That is:- a shunt wound DC motor will run at its nominal speed at its rated voltage with no load. The speed being related to the back EMF equaling the applied voltage, less friction etc.

So, I also thought that permanent magnet motors were the same. The motor would run at it's rated speed at it's rated voltage i.e. an amount of RPM per Volt.

From that, I figured that a brushless motor was basically a brushed DC motor except the commutation was now done electrically rather than mechanically. So, a 1,000RPM per Volt motor would run at 10,000 RPM at 10 Volts.

Does that all sound OK so far?


Now if you apply a load, then the motor slows down, back EMF is reduced and current is drawn proportionally to the load.

Is that still generally right?


Now, my colleague at work says that for permanent magnet motors, there is a series motor effect or other effect that means that the speed of a loaded (propped) DC or brushless motor will suddenly increase or "go through the roof" if the load is removed. I don't think that's right, but I'm not sure.

Is there a big difference in speed from an unloaded motor to a 'normally' loaded one and if there is, why is that?

If someone has a permanent magnet motor formula or helpful link please step forward
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Old May 18, 2007, 01:10 PM
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I think I just worked it out.

The Industrial DC machines we (used to use) at work have compound compensating windings with a relatively low armature resistance. You expect them to operate in a constant voltage:constant RPM sort of way.

The little DC buggy motor works some point midway between (0 RPM and it's stalled current) and (KV RPM and nearly zero current). Because the stalled current is volts over armature resistance, it isn't 'that' high. Maybe 25 Amps on a buggy motor with 12V. An industrial 250 horse motor would try to be pulling 10,000 Amps if it was ever subjected to full voltage (500V) when stalled, because it has a similar armature resistance, but lots more volts.

So I think the buggy motor is sitting much further down it's RPM range when loaded in order for the voltage to overcome the armature resistance. The big industrial one has more inherant stability (due to it's relatively lower armature resistance) plus it has a compensating winding to improve that even more.


Please excuse rambling - does anyone agree with all this?
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Old May 18, 2007, 02:51 PM
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It sounds like you are on the right track. Speed, torque, and amps are all linear and proportional on the types of motors we use, brushed or brushless. Increasing the load (bigger prop) causes the speed to drop and the amps to increase. Removing the extra load will cause the speed to increase again and amps will drop accordingly. No prop at all will simply allow the motor to run at it's no load speed for that voltage, and will draw just enough amps to overcome the inherent losses. Our motors operate most efficiently at around 75% of no load speed. Max. power out occurs down around 50% of no load speed. Efficiency is poor at this point, 50% or less.
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Old May 18, 2007, 02:59 PM
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This site shows everything very well. Click on "understanding the graphs"
http://www.peakeff.com/
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Old May 18, 2007, 03:18 PM
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Very helpful answer and link (now bookmarked)

I put my CF 2822 in on 3 lipo and 8*6 and it comes out just about where I measured with my ammeter

Hmm, might put my old ECO8 motor details in, probably explain why it gets too hot. It'll still fly like a dog even if I tune that one in

I think what was throwing us was that we're used to big industrial machines that would never run much below 100% speed against voltage. If you had a 250 horse motor running 75% speed at 100% voltage, it would quickly destroy itself without any limit and/or protection.
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Old May 18, 2007, 03:29 PM
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Some machines are wound with electromagnets, and these can indeed overspeed dramatically on no load..I can;r remember why, but something drifts in in terms of shunt wound motors being able to generate their won voltage for the field coils..or is it series wound? Mists of time mate.

Anyway its not relevant to brushless PM magnet motors which are, as you say, simply brushed DC motors with electronic commutators.
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Old May 18, 2007, 03:38 PM
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That'll be the series wound motor then The field and armature are in series which gives a huge saturated field under load, but a tiny field with no load.

No load = no field = no back EMF = massive speed and ultimately destruction.
Big load = lots of field = lots of torque.

They give masses of torque at zero speed and are used for car starter motors and big industrial processes i.e. turning 30 Ton steel slabs. They normally fit some sort of centrifugal switch to disconnect the motor to stop it destroying itself it if the transmission breaks.

Thanks also!
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Old May 21, 2007, 11:22 AM
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About VOLTAGE and RPM?
Hi Everybody,
About dc current
I have two dc adapter one is 500mah and 5v (500mahx5voltage=2500watt)
and second one is 2500mah and 1v (2500mahx1voltage=2500watt)

The both of adapter watt is same but mah and voltage is different
Can i get same rotation per minute(Rpm Dc motor) in different mah and different voltage but watt should be same or i am wrong?
And how i can get same (RPM) in different mah and different voltage?
------------------------------...
Convert Table
Convert AMPS to Watts:
Watts = Voltage x Amps
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Old May 21, 2007, 12:10 PM
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It all has to do with the Kv of the motor. A motor wound for 1000 Kv will perform (roughly) the same on 5V as the same motor wound for 5000 Kv will perform on 1V.

The higher Kv motor will have 1/5 the number of turns but the amps will be 5x higher so the torque is the same. It only needs to generate 1/5 of the back EMF so the RPMs are the same.

Good Luck!
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Old May 21, 2007, 12:20 PM
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Brushed motors for the most part have NO feedback mechanism (EMF)

Start em up, spin them without a care and hope that your communers
last.
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Old May 21, 2007, 09:03 PM
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Quote:
Originally Posted by fly_boy99
Brushed motors for the most part have NO feedback mechanism (EMF)

Start em up, spin them without a care and hope that your communers
last.
Yes they do, they work the same way. Kv, Rm, and Io describe a motor whether the commutation is mechanical or electronic. RPM and current draw work the same with a brushed or brushless motor.

Any permanent magnet brushed motor will function as a generator, that's back EMF. There could be no Kv rating without it.

Maybe you're thinking about the feedback on the inactive phase wire that tells a brushless controller when to switch phases - that's not back EMF.

Good Luck!
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Old Jun 07, 2007, 05:25 AM
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Quote:
Originally Posted by raazray
About VOLTAGE and RPM?
Hi Everybody,
About dc current
I have two dc adapter one is 500mah and 5v (500mahx5voltage=2500watt)
and second one is 2500mah and 1v (2500mahx1voltage=2500watt)
mAh means "milliampere-hour" or 0.001Ah.

Therefore, 500mAh = 0.5Ah and 0.5Ax5v=2.5Watt
Similarly, 2500mAh = 2.5Ah and 2.5Ax1V=2.5Watt
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Old Jun 07, 2007, 07:00 AM
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Quote:
Originally Posted by fly_boy99
Start em up, spin them without a care and hope that your communers
last.
Are we talking about prayer wheels?
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Old Jun 07, 2007, 10:07 AM
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And MVA means MegaVoltAmpere

We're just looking at a job with a 2.5MVA transformer and a 4000A air circuit breaker. It'll have some 3~400KW variable speed drives and motors on it too

Sometimes I like my work. There's something satisfying about charging up an ACB, hearing the clunk, the buzz from the busbars, the whistle and rumble from big bits of kit being started....It's like steam engines for electricians
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Old Jun 07, 2007, 11:00 AM
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The Northeast Kingdom, Vermont
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Quote:
Originally Posted by vintage1
Are we talking about prayer wheels?
Oooohm, Ooooohm Bad Karma
Pete
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