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 Post subject: DC brush motors
PostPosted: Wed Jul 22, 2009 2:29 am 
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Citizen

Joined: Mon Jan 10, 2005 7:56 pm
Posts: 33
I did a little research for DC Brush type motors for someone, so I thought I would fill an empty slot in the forum.

You will need to know.
1. Voltage DC you will be running and the current you have available to the motor in amps.
2. Torque in inch pounds etc.
3. Motor RPM 70% of max RPM of motor at x voltage and 90% of a geared motor.
4. Mounting: yoke, faceplate, rear, insert or moulded in.
5. Speed sensing: internal encoder the best or  shaft mounted magnetic  or photo target sensing.
6. Motor driver:  basic transistor or MOSFET,  digital, reversible or pulse  width modulated.
7. Shaft size and length.
8. Motor size and length.
9. RPM tolerance.
10. Duty cycle: intermittent or continuous.

Size your motor properly, it would be best to get a DC motor of about >2500 -  <4000 RPM  if  your target is 1200 RPM.
Look for bearings on both shafts and a built in encoder. If you need a torque of 2 in. Pounds your motor should have about 5 in. Lbs. stall torque. Be sure to get one that will be manufactured for some time yet and find at least two sources so you can replace them if one source drops out.

American, Japanese and Germans make very good motors.
The Torque is very important, so you need to figure out how much power it takes to drive the load you are driving. A geared motor will hold RPM better if the load fluctuates. The addition of a flywheel will stabilize the RPM more but at a cost of run up/down speed and higher starting current. Starting current is a big factor. Often a small motor will pull 3 to 5 Amps on start up, and more, if connected to a load. You will want a soft start up (slowly ramping up to speed). If your load is heavy you may have rotor lock, not a good thing, as smoke will appear!

Will the load be fixed or will it flex? If flexing is the case you will  need a shaft isolator to isolate the motor from movement of the load.  This is a rubber coupling cog device that goes in between the shaft and the load.
You should be careful about aligning the motor up to the load correctly as poor alignment will cause early bearing failure.
Test your load for balance, or failure to do this will tear things up in a hurry.
If physical motor noise is be a problem, rubber mounting and shielding for  noise may be needed, and some drivers may cause motor whine.
The heat of the motor and driver will not be a factor if runs are short, but may build up on long or continuous running. The motor will run fine warm, but heat will degrade the life. The driver cannot run hot, cooling will have to be added and any fire danger will have to be eliminated in the motor driver area.
Keep the motor area clean, dust and liquid free.
EMI (electrical noise causing radio interference ) from the motor and driver must be controlled with shielding, Capacitors and ferrite attenuators (beads).
A small value cap across the terminals will cut it down but may be a dead short when using high frequency PWM.
Twisting the wires together from the motor or using a choke will work, and ferrite beads are good for high frequency EMI.
Using a reverse diode will protect you from back EMF, as that can be a driver killer.

I am not an expert on this so I will leave this to be corrected or added to by the beloved propeller heads on this forum.

conehead


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 Post subject: Re: DC brush motors
PostPosted: Thu Jul 23, 2009 3:43 am 
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Master

Joined: Mon Aug 18, 2008 1:26 am
Posts: 799
Location: CA bay Area
Thanks for the article, Conehead. I hope you don't mind that I proofread this and made minor corrections. I left the important items intact.
Take care.

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kenjj
http://blog.basicmicro.com/
http://kjennejohn.wordpress.com/


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 Post subject: Re: DC brush motors
PostPosted: Thu Jul 23, 2009 4:47 am 
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Citizen

Joined: Mon Jan 10, 2005 7:56 pm
Posts: 33
Thanks Ken

My proofreader (my Wife) is tired of fixing my mysteakes and punchuation.
It was about time I put something on the forum,.
We use DC motors for many projects but when you have to do a product things get a little more complicated, it has to last and always do the job, I think I opened up the eyes of the Customer who just wanted to put a motor in the device and use it.

The Nano looks like it could take off as a leader product, if I was just starting out I think I would go with them.
I would like to see a Nano book in the near future, and maybe more on the data sheets, low cost entry level package set up for new users, club and School discounts, Microcontrollers are like drugs
sell the first ones low and make your money on the future purchases......
many thanks
conehead


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 Post subject: Re: DC brush motors
PostPosted: Thu Jul 23, 2009 6:51 am 
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Master

Joined: Mon Aug 18, 2008 1:26 am
Posts: 799
Location: CA bay Area
We have hired writers to produce articles and examples for us. The data sheets got major rework and we are still tweaking them. I am now Editor-In-Chief. All I need to do now is finish the new Atom manual and there will be a large cheering crowd.
And we progress...

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kenjj
http://blog.basicmicro.com/
http://kjennejohn.wordpress.com/


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