DC motors and Induction motor: What does "no load conditions" mean?

[Pipsqueakalchemist]

TL;DR Summary: So I'm just confused when the question asks me to solve for the no load speed of DC motors and induction motors. Does no load condition mean that the output torque (Tout) is zero? This is what I was assuming so far for both DC and induction motor. Is no load condition the same for both of these motors? I have the imagine of both motors below

Shunted DC motor

Induction motor

 

[Baluncore]

Does no load condition mean that the output torque (Tout) is zero?

Yes.
Consider the RPM of a DC motor, when the back EMF is equal to the applied voltage.
Consider the RPM of an induction motor when the slip is zero.

 

[Pipsqueakalchemist]

Yes.
Consider the RPM of a DC motor, when the back EMF is equal to the applied voltage.
Consider the RPM of an induction motor when the slip is zero

Oh so these are the conditions under no load in DC and induction motor correct?

 

[Baluncore]

Oh so these are the conditions under no load in DC and induction motor correct?

We cannot study for you. When you have examined DC and induction motors sufficiently, you will understand, and will be able to answer to your own question.

 

[Pipsqueakalchemist]

We cannot study for you. When you have examined DC and induction motors sufficiently, you will understand, and will be able to answer to your own question.

Ok after looking over my notes, I believe that there's more than one condition for no load (Tout = 0), one condition is if Emf = Vapplied and hence armature current (Ia) will be zero. But I'm pretty sure that there is a no load condition where Ia isn't zero and there is power delivered to the load, it's just all the power is loss through rotational loss so Tout is still 0. I just want to confirm if my understanding of DC motors are correct

 

[Baluncore]

But I'm pretty sure that there is a no load condition where Ia isn't zero and there is power delivered to the load, it's just all the power is loss through rotational loss so Tout is still 0.

Disconnect the output shaft, so there can be no torque or external mechanical load. The torque then needed to overcome the internal friction and windage losses of the spinning motor must be supplied by Ia. The "no load speed" of a real motor must therefore be slightly less than expected from the equality of back EMF and supply voltage. Ia will not be zero when there is no shaft load on a spinning motor.

If you were to drive the operating motor shaft mechanically, as a generator, then when Ia does become zero, the shaft torque would be providing for the motor internal friction and windage losses. The speed would then be higher than the "no load speed" measured earlier.

 

[Pipsqueakalchemist]

Disconnect the output shaft, so there can be no torque or external mechanical load. The torque then needed to overcome the internal friction and windage losses of the spinning motor must be supplied by Ia. The "no load speed" of a real motor must therefore be slightly less than expected from the equality of back EMF and supply voltage. Ia will not be zero when there is no shaft load on a spinning motor.

If you were to drive the operating motor shaft mechanically, as a generator, then when Ia does become zero, the shaft torque would be providing for the motor internal friction and windage losses. The speed would then be higher than the "no load speed" measured earlier.

Is it correct to say that when EMF = Vsupplied and hence not armature current that this is the max rotor speed, and that when there is no shaft and armature is non zero then this is the no load speed which is slightly less then max moto speed.

 

[Pipsqueakalchemist]

Is it correct to say that when EMF = Vsupplied and hence not armature current that this is the max rotor speed, and that when there is no shaft and armature is non zero then this is the no load speed which is slightly less then max moto speed.

My textbook is very confusing because it refers to the case where EMF = V as no load speed but it also refers to no shaft as no load speed

 

[Baluncore]

V supply is an EMF. To make it clear, refer to the voltage "generated" internally as "back EMF".

There are no internal friction and windage losses in an ideal motor. In an ideal motor, Ia will be zero when Vsupply = back EMF, at the ideal "no load speed".

The "no load speed" is defined as the speed a real DC motor runs, given a particular supply voltage, and there is no external mechanical load being driven by the shaft. The internal losses are not being ignored. Ia is not zero.

The component of Ia that is needed to overcome the internal friction and windage losses, varies as the square of the RPM.

 

[Pipsqueakalchemist]

V supply is an EMF. To make it clear, refer to the voltage "generated" internally as "back EMF".

There are no internal friction and windage losses in an ideal motor. In an ideal motor, Ia will be zero when Vsupply = back EMF, at the ideal "no load speed".

The "no load speed" is defined as the speed a real DC motor runs, given a particular supply voltage, and there is no external mechanical load being driven by the shaft. The internal losses are not being ignored. Ia is not zero.

The component of Ia that is needed to overcome the internal friction and windage losses, varies as the square of the RPM.

I think i understand, so when back EMF = V, this speed is the ideal where losses are ignored and the no shaft speed is when when losses aren't ignored anymore

 

[Pipsqueakalchemist]

I think i understand, so when back EMF = V, this speed is the ideal where losses are ignored and the no shaft speed is when when losses aren't ignored anymore

Also when you mean internal losses, you mean specifically rotational losses. Armature and field loss are still present in the ideal motor case where back emf = Vsupplied