Details about separately excited DC motor

[seyyah]

Think of a seperately excited dc motor.

What is the relationsip between the speed and torque?
Without a control can torque stay constant?
With a control (dc drive) can torque stay constant at any speed?
What is the purpose of using gear with a dc motor?
Can we achieve the same goal using a dc drive without a gearless motor like a geared dc motor? (I mean at any speed can we obtain the same torque as in the geared motor with a gearless motor?)

 

[human2003]

Re: dc motor details

i think you should use the eketroda's search engine to fish out my umpteen postings in the past on ac and dc motors and cotrols. your question has about 5 page answer

 

[crono]

Re: dc motor details

Hi,

I try to give some (short) answers....

> What is the relationsip between the speed and torque?
For a given motor Torque curve can be plotted like a family of curve in a Speed/Current graph

> Without a control can torque stay constant?
Torque of course depends on motor load, so....

>With a control (dc drive) can torque stay constant at any speed?
No, for a given motor you have to plot the torque curve to see in wich way torque vary vith speed at a given current.
If you have a currrent limited generator to supply the motor you can have a torque limit (at a given speed).
If you use a current generator speed change with motor load.

>What is the purpose of using gear with a dc motor?
Motor optimization. If you use an increase ratio of course you can have higher speed (and lower Torque). I the opposite way you can have Higher torque with lower speed (gear efficency must also be take in count ).

>Can we achieve the same goal using a dc drive without a gearless motor >like a geared dc motor? (I mean at any speed can we obtain the same >torque as in the geared motor with a gearless motor?)
Of course no, you cannot have the same result in the full range of applications (otherwise why to spend money with gear?).

 

[seyyah]

Re: dc motor details

Ok i'll search the forum human2003 but before it i want to ask a question
to crono. May be this will help me much more.
Say we have a 800rpm 24V dc motor.

I want to run it, say, at 200 rpm.
I have two choices:
1- Using a gear to reduce the speed. This reduction also leads an increae in torque. And assume that this torque is enough for us.
2- I can use a dc drive (an electronic circuit) to reduce the speed to 200 rpm. I want to ask that can i obtain the same torque somehow (using electronic circuit) as in the first case at this speed?

 

[crono]

Re: dc motor details

Hi

If the gear is ideal you have in the first case an increase of a factor 4 in torque.

In the second one if the motor is ideal you can obtain the same torque as at 800 RPM.

Try to supply your motor with a lower voltage in a open loop way and try the torque of your motor it can be useful for test the motor torque. Of course if you can close the loop the speed can be controlled when motor is loaded (and the torque increase)

 

[seyyah]

Re: dc motor details

So in a closed loop system the answer is yes. I can achieve the same result in both of the cases if i use closed loop system.

 

[newborn]

Re: dc motor details

pergatory.mit.edu/2.007/handouts/motorcalc/motorcalc.pdf

your annswer is here

 

[fioreh23]

dc motor details

Hi.
Stephen chapman's electrical machine book has chapters about this.
Regards.

 

[mexx]

Re: dc motor details

hi crono
i just wanna ask a simple question.i am using a 2400 rpm/12v dc motor for my robotic car.what should be the ideal torque/speed ratio for the control . i m using h-bridge for the control of motor