1.) An AC starter has a 12V supply and 6V back EMF. Calculate the starting current and operating current if the resistance is 0.1 ohm.

For this question I used ohms law and got 120V starting and 60V running. Is this correct?

2.) A DC motor has a supply voltage of 220V, 10A and 50 ohms of resistance. Calculate the back EMF.

I used E=Eb+(IaRa) and got 280 back EMF. Is the

 

Q1. Check what the question asks for against your answer.

 

Please check the units in the answer tp #1.

 

I go by post # 3 for the First Question.

The second question is not clear the way it is worded.

I would presume that the Starting Current is 10A, and on Running, it exhibits an equivalant resistance of 50 Ohms.
This works out as the back EMF roughly ..... (sorry, no answer..... homework question)

 

What do you mean sorry no answer, homework question? Also I do not have the answers for these questions, that's why im asking for help. And all the units are correct.

And for the second question, I apologize for the incorrect wording. This is exactly how it is worded =

A D/C motor has a 220V supply & a 10A current. If the resistance is 50 ohms, calculate the back EMF.

 

sorry guys brain fart, 1st question answer is in A and not V.

 

A D/C motor has a 220V supply & a 10A current. If the resistance is 50 ohms, calculate the back EMF.

Shunt or series field ?
Max.

 

It doesn't say..

 

Let's see... 220 volts into a motor with 50 ohms resistance.

4.4 amps is the answer, but now we have a new question.

If back EMF reduces the current a motor takes, then why are we pulling 10 amps in a motor that only takes 4.4 amps?

Can you post a picture of your questions from the paper? Maybe something is being lost in translation.

 

Let's see... 220 volts into a motor with 50 ohms resistance.

4.4 amps is the answer, but now we have a new question.

If back EMF reduces the current a motor takes, then why are we pulling 10 amps in a motor that only takes 4.4 amps?

Can you post a picture of your questions from the paper? Maybe something is being lost in translation.

What did u get for the back EMF value, same as mine? There is no difference in the question and how I wrote it. It's from a sample question PDF I have

 

What do you mean sorry no answer, homework question?

We won't provide answers to homework questions. We say if your answer is correct, and will guide, hint, etc if it's not.

 

Fair enough. Yeah the question is worded oddly

 

 

Hi,

The 'back emf' is the apparent voltage that subtracts internally from the applied voltage.
So if we call that Vemf and the applied voltage Vs and the internal forcing voltage Vf then we have;
Vf=Vs-Vemf

and that is the voltage that appears across the resistance of the motor Rm, so the current is:
If=Vf/Rm

and it looks like you did this because you had 12v in and 6v Vemf and 0.1 ohms so you did:
Vf=12-6
If=Vf/0.1

however, you wrote "60v" which is not a current, 60v is a voltage, so you have to think about it and change the units to what current is actually measured in (which i am sure you know).

For the second question you just change things around a little and compute Vemf instead of Vf. Just a tiny bit of algebra.

 

What do you mean sorry no answer, homework question? Also I do not have the answers for these questions, that's why im asking for help. And all the units are correct.

And for the second question, I apologize for the incorrect wording. This is exactly how it is worded =

A D/C motor has a 220V supply & a 10A current. If the resistance is 50 ohms, calculate the back EMF.

Not possible.
I would presume that the Starting Current is 10A, and on Running, it exhibits an equivalant resistance of 50 Ohms.

Kermit2 has given a Hint.

 

Not possible.
I would presume that the Starting Current is 10A, and on Running, it exhibits an equivalant resistance of 50 Ohms.

Kermit2 has given a Hint.

Hi,

The resistance is usually the resistance of the winding, which limits any current level including the starting current. In fact, that is exactly what is limited by the resistance not the running current. The running current is limited by the supply voltage minus the back emf voltage divided by the resistance,and the back emf is positive too so it makes the total internal operating voltage less so it makes the running current less than the starting current. If the motor stalls, we see the full applied voltage across the resistance:
I=Vs/Ra

and that is the maximum current.

This of course means that we can not get 10 amps with 220v and 50 ohms no matter what we do that is reasonable. Maybe if we run the motor backwards with reversed voltage for a time and then suddenly switch the voltage back to the normal polarity then we might see more current because then the back emf is temporarily in reverse to what it normally is
Ir=(Vs+Vemf)/Ra for a somewhat short time. However, to get 10 amps with 50 ohms we'd need 500 volts and even twice 220 is only 440 volts so that's still not going to do it because the back emf is never as high as twice the supply voltage