A Doubt About Series Motor

You tell us!

After all it is a homework question which you need to answer!

Have fun!

...subject to the character of the loads being driven? After all, an untethered DC series-wound motor will accelerate to destruction, Boss!

thanks,

i think the speed is 4N?

Why?

If there were no friction, a DC motor would accelerate until the back emf equals the supply voltage. Two motors in series would each see 1/2 the supply voltage. So, in parallel they should run at speed 2*N.

What about the mechanical load?

I love poorly written trick questions.

The same current [I] is being drawn from the supply with the motors wired in series and parallel. Unless the voltage from the supply is changed the current from the supply should change. So this implies that this [ideal] supply is a current source and not a voltage source. Driving a DC motor with an ideal current source will cause constant acceleration, the speed of motors will never be a fixed value N.

There is one condition that meets the given facts, the identical motors are internally broken with an open circuit. |I|=|N|=0

But sir,

The answer is '4N', why this possible?

please give me the reason?

Earlier, the answer <...4N...> was an opinion only. Now it has hardened to a statement. The forum needs to understand how this has developed. The reason is that there is a total lack of information about what these motors are turning in the posts so far.

If the current is constant whether the motors are wired in series or parallel would suggest the voltage has to shift and in this case reduce. With a shift in Voltage the motor would be subject to the same power in either series or parallel configuration therefore the speed would be the same subject to all the usual sorts of mechanical type issues.

I think what is required as an answers is, the speeds will be virtually identical.

...especially if they are on the same motor bogie and there is no slip between the wheelset and the rail and both wheelsets are the same diameter.

I do remember watching the wheel slip light glowing on D212 one of the old EE Whistlers. I suppose those motors might have been at different speeds.

Is that kettle still on?

The only reason for a variation is that the motors are not balanced, not equal with bearing resistance and brush contacts, therfore there should be a slight difference in speed. And if they are on a train boggie, there will be some drag on one motor.

If you're studying to be an electrical engineer, one of the really fun things to do is actually try this. You can pick up series motors pretty cheap, and it is for your education. Oft times, the two in series, like an old man and his wife, will drive each other crazy, one running away, then the other. Ask yourself why.

If you hook them in parallel, the answer depends on how much money they cost. If you paid less than about $2 US, they'll limit up around 12 or 15 grand. If you paid $400 each, you'll get more fireworks than the time grandma revved the International 3/4 ton up to 7000 before popping the clutch.

But, try it. Learning is good fun, and doing is one of the best ways to learn. Let us know what you learn.

Cheers

Vt = k*Φ*ω + I*r + dI/dt *L

assume dI/dt = 0 (steady state)

this is a series motor, so if I supply is constant in both cases, then the Ia in motor becomes 1/2 in the second case so Φ is approx 1/2. Assume supply voltage is constant, and assume I*r is negligible, so in second case Vt of motor has doubled, so

2Vt = k*Φ/2*ω₂ + 0 + 0

so to balance this equation ω₂ = 4 * ω₁

The problem is the assumptions for Φ and negligible I*R losses are not realistic in the real world - so it appears to be another poorly exam question by a know-it-all college professor who has never been in the real world.