# dc motor theory question??? Maths

1. ## dc motor theory question??? Maths

Hello,

I have a few questions which I cannot be sure I'm calculating right.

Here is what I have:

Motor speed 500rpm
Back emf 115v
Supply voltage 200v
Motor torque 40nm

I have to work out..
Armature current drawn
Armature resistance
Shaft power
What would be the rpm if the torque increased to 60nm

I have calculated
18.21amp
275.57mOhms
2.8HP
And by ratio 333rpm as I do not have flux.

All assuming 100 percent efficiency.

Can anyone find fault with my answers??

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2. ## Re: dc motor theory question??? Maths

I really do not know where to start.

But you have calculated somehow; tell us the steps.

Once we know the steps we can comment.

How to convert the motor torque 40nm to watts?

Is it a DC motor with 200v DC working voltage)? Anatomically, all motors are AC- because of commutation is needed for DC- but that is a different story anyway.

Do you assume that the load presented by the rotor is zero?

Shaft power should be the same as the motor power because we have no idea about the losses (both copper and iron losses).

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3. ## Re: dc motor theory question??? Maths

Please, find attached my solution.

About the shaft powet, it is Ps=w*T, that is Ps=52.4*40 = 2096W or 2.8HP
if the torque is 60 Nm we will have 32.6*60=1956W or 2.6 HP

I did the approximation that "k" is the same for both 40 Nm and 60 Nm torque

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4. ## Re: dc motor theory question??? Maths

Ia =( wt)÷eb (52.4x40)÷115 = 18.2amp

5. ## Re: dc motor theory question??? Maths

Originally Posted by Harry.f123
Hello,

I have a few questions which I cannot be sure I'm calculating right.

Here is what I have:

Motor speed 500rpm
Back emf 115v
Supply voltage 200v
Motor torque 40nm

I have to work out..
Armature current drawn
Armature resistance
Shaft power
What would be the rpm if the torque increased to 60nm

I have calculated
18.21amp
275.57mOhms
2.8HP
And by ratio 333rpm as I do not have flux.

All assuming 100 percent efficiency.

Can anyone find fault with my answers??
I know how to DC motor calcs, no problems there, but first some questions. Where did you get the data that you "have"? Is this your measured data from a real motor? How did you measure the back emf?

Or is this a hypothetical question that you have been given as an engineering assignment or similar?

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