May 18, 2007, 12:09 PM Registered User Joined Dec 2006 317 Posts 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
 May 18, 2007, 01:10 PM Registered User Joined Dec 2006 317 Posts 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?
 May 18, 2007, 02:51 PM Scratch builder USA, PA, Telford Joined Apr 2004 1,392 Posts 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.
 May 18, 2007, 02:59 PM Scratch builder USA, PA, Telford Joined Apr 2004 1,392 Posts This site shows everything very well. Click on "understanding the graphs" http://www.peakeff.com/
 May 18, 2007, 03:18 PM Registered User Joined Dec 2006 317 Posts 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. Last edited by dogshome; May 18, 2007 at 03:31 PM.
 May 18, 2007, 03:29 PM Registered User East Anglia, UK Joined Sep 2002 29,713 Posts 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.
 May 18, 2007, 03:38 PM Registered User Joined Dec 2006 317 Posts 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!
 May 21, 2007, 11:22 AM Registered User Joined Nov 2006 39 Posts 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
 May 21, 2007, 12:10 PM Registered User Beaverdam Creek, VA Joined Aug 2005 2,322 Posts 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!
 May 21, 2007, 12:20 PM Flying motor mount master San Jose, California, United States Joined Oct 2004 8,745 Posts 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.
May 21, 2007, 09:03 PM
Registered User
Beaverdam Creek, VA
Joined Aug 2005
2,322 Posts
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!
Last edited by BeavrdamElectric; May 21, 2007 at 09:16 PM.
Jun 07, 2007, 05:25 AM

South Africa, GP, Pretoria
Joined Jul 2003
600 Posts
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
Jun 07, 2007, 07:00 AM
Registered User
East Anglia, UK
Joined Sep 2002
29,713 Posts
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?
 Jun 07, 2007, 10:07 AM Registered User Joined Dec 2006 317 Posts 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
Jun 07, 2007, 11:00 AM
Registered User
The Northeast Kingdom, Vermont
Joined Jun 2004
5,866 Posts
Quote:
 Originally Posted by vintage1 Are we talking about prayer wheels?