Author 
Topic 

ron_o
Moderator
United Kingdom
1052 Posts 
Posted  June 01 2014 : 17:31:06

I recently "found" an old DC permanent magnet motor from a 12 volt cordless drill in my junk box. Which i wanted to test to see if it was suitable to power a small device
Back EMF of a permanent magnet DC motor.
1) measure the no load speed of the motor at it's stated working voltage
2) measure the DC resistance of the motor
results were 8500 rpm and 0.55 ohms , stated working voltage 12 volts
Pi = 3.14159
3) convert rpm to radians per second
w = rpm x 2 x Pi / 60 = 8500 x 2 x Pi / 60 = 53407 / 60 = 890 rads/s
Under no load conditions V = E so E = 12 volts
w = E / Kt0
Note Kt0 is a constant for this motor only
so Kt0 = E / w = 12 / 890 = 0.01348
Stall Torque.... since the motor is not rotating no BEMF is being generated so the current limiting factor is the DC resistance of the winding and brush contacts resistance
I = V / R so stall current is 12 / 0.55 = 21.8181 amps
The Stall Torque = Kt0 x I(amps) = 0.01348 x 21.8181 amps = 0.2941Nm
To calculate the speed of the motor when it is loaded by a torque of 0.15 Nm ( the purpose of this calculation )
I(load current) = Torque / Kt0 = 0.15 / 0.01348 = 11.1275 amps
E(bemf) = V  ( I(load current ) x R( motor resistance )
E(bemf) = 12  ( 11.1275 x 0.55 ) = 12  6.120 ) = 5.88 volts
Convert E(bemf) to rads/s
w = e(bemf) / Kt0 = 5.88 / 0.01348 = 436.2 rads/s
Convert rads/s to rpm
rpm = 60 x 436.2 / 2 x Pi = 4165 rpm
Which when actually tested gave a reading of 4152 rpm
ron


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olddawgsrule
Advanced Member
USA
1434 Posts 
Posted  June 02 2014 : 16:37:36

Two questions;
"To calculate the speed of the motor when it is loaded by a torque of 0.15 Nm (the purpose of this calculation)"
Nm stands for?
Where did the .15Nm come from?
Old in age, not in mind, so 'Teach me something new'!



ron_o
Moderator
United Kingdom
1052 Posts 
Posted  June 02 2014 : 18:30:41

Hi ODR
Nm stands for Newton metre and the 0.15 Nm figure was a torque load that the unit i wanted to drive required to turn it.
The calculation was performed to see if the small DC motor was strong enough , rather than make up a mounting plate etc only to find the motor was not up to the task.
ron 


olddawgsrule
Advanced Member
USA
1434 Posts 
Posted  June 03 2014 : 15:42:00

I'll have to add this to my spreadsheet collection for further reference.
I also learn as I build the sheet. Gives me a chance to break it down part by part.
Can you do this in reverse? Looking to figure footpounds (?) of water required to turn the motor.
Old in age, not in mind, so 'Teach me something new'!



ron_o
Moderator
United Kingdom
1052 Posts 
Posted  June 03 2014 : 17:00:10

Hi ODR
The Nm value can be readily converted into foot/pounds :
Inch Pounds ( in/lbs ) = Nm x 8.85
Foot Pounds ( ft/lbs ) = Nm x 0.7376
hope that helps
ron 


ron_o
Moderator
United Kingdom
1052 Posts 
Posted  June 03 2014 : 17:06:00

Hi again ODR
Are you trying to work out the amount of force / torque required to turn a generator to provide a required amount of electrical energy ?
ron 


olddawgsrule
Advanced Member
USA
1434 Posts 
Posted  June 06 2014 : 17:33:14

Yes that's what I'm up to.
I'm about to meet up with my Cousin and pass along 3 DC motors I've collected. I still don't believe they'd be good for a wind genie application, but his Hydro project I do believe they'd fit in nicely.
It would be nice to calculate the force required as you've done here with your situation, before going into the build.
I hesitate to ask, you do so much.. If you have time If your so willing
He has a beaver pond uphill of the house. I'm heading up to help survey the area and attempt to determine the possibilities. It's in the new plot he bought, uphill from his and hear it's 2 acre plus. Sounds like it has possibilities.
Old in age, not in mind, so 'Teach me something new'!

Edited by  olddawgsrule on June 06 2014 17:54:40 


ron_o
Moderator
United Kingdom
1052 Posts 
Posted  June 07 2014 : 17:48:41

Hi ODR
Well it sounds like a worthwhile project ( so no need to hesitate to ask ) so lets see what we can do.
I will need the details of the three motors ( volts and amps + rpm ).
By beaver pond am i right to assume a dam and that a head of water exists .... if so what is the difference in height between the two water levels ?
It will also be necessary ( at some point ) to establish the volume of water flow per unit time. This information will allow gearing and wheel bucket dimensions to be calculated.
I will start a new topic in the "WATER POWER" section of the forum
ron 



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