# Electric motor and no load speed

#### alpheratz

Joined Nov 10, 2019
17
I find that from mathematical point of view we have no torque when there's no load so the motor needs no current to work. For example a SPM motor , without a current we still have a speed due to the voltage applied and the permanent magnet flux linkage and considering no resistance it's equal to back emf.

$$V = \Lambda_m \omega$$

I'm new into this but what I think is that the motor needs always a current to work because of its physical structure it is a mechanical load itself. Maybe it's negligible compared to the currents it can work at with big loads.
So I don't understand what is this information used for? Is this an approximation?

#### WBahn

Joined Mar 31, 2012
25,550
Is what an approximation? You talk about several different things.

#### MrAl

Joined Jun 17, 2014
7,476
I find that from mathematical point of view we have no torque when there's no load so the motor needs no current to work. For example a SPM motor , without a current we still have a speed due to the voltage applied and the permanent magnet flux linkage and considering no resistance it's equal to back emf.

$$V = \Lambda_m \omega$$

I'm new into this but what I think is that the motor needs always a current to work because of its physical structure it is a mechanical load itself. Maybe it's negligible compared to the currents it can work at with big loads.
So I don't understand what is this information used for? Is this an approximation?
Hi,

Just a quiet note...

Yes a motor always needs current to keep running even with no load. That is mostly because of friction within the motor itself and also the tendency for the armature rotation to be impeded by the air around it, and also if there are any brushes they rub on the commutator (more friction).
The time to full rotation is also impeded by the inertia of the shaft and armature. The higher the current the faster it gets up to speed. Once up to speed however the inertia no longer plays a part in the rotation unless the current is cut off (actually if it is increased or decreased) and then it will keep the armature spinning for some time depending on the mass and the friction. It will eventually come to rest if there is no more current applied.

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