# Meaning of “with respect to rotor and stator”

In the following example, when it says “with respect to stator” and “with respect to rotor”, did they mean the rotor part of the synchronous generator in both cases?

Example: The excitation field in the synchronous generator is stationary with respect to the rotor. When the rotor rotates at synchronous speed, the excitation field rotates with respect to stator.

• This question appears to be off-topic because it is not about electrical / electronic design. – Chetan Bhargava Mar 28 '14 at 5:39
• Electric motor's are part of electronics – Michael Mar 28 '14 at 6:59
• Why do electrical engineering questions get such short shrift in a forum titled ..."Electrical Engineering" in rather large letters? – Brian Drummond Mar 28 '14 at 10:03

From what I understood from your question you want to know what is "with respect" to refers to in a synchronous motor.

This means relative velocity. If two object are moving at same speed it appears like one object is stationary with respect to other. The phrase "excitation field in the synchronous generator is stationary with respect to the rotor" means that both field and rotor have same speed.

For example let angular velocity of electric field is 40 rps. for synchronous motor, rotor is synchronous to electric field. Therefore speed of rotor is also 40 rps.

Here speed electric field with respect to stator is 40 rps. But speed of electric field with respect to rotor is 0 rps.

• @suaad let me know if you need more clarification in my answer. – tollin jose Mar 28 '14 at 7:13

With respect to stator: does not rotate.

With respect to rotor: rotates with the rotor

excitation field in the synchronous generator is stationary with respect to the rotor.

This says that the electric field rotates around with the rotor. In fact leading the rotor into it's advancing position.

When rotor rotates at synchronous speed the excitation field rotates with respect to stator.

It's likely referring to a brush-less DC motor (BLDC), that the excitation field from the magnets that are physically attached to the rotor.

The electric drive field also rotates synchronous to the rotor; it must lead the rotor magnets by a fixed angle ahead of them. As long as the angle is fixed, the excitation in the stator is synchronous to the rotor.

If the stator is not driven correctly, the rotor and electric drive can slip out of synchronization; essentially the field is not properly positioned to pull the rotor around to it's target position and speed.

• "this is confusing " ... not really, "synchronous speed" is widely used to mean "the speed at which synchronous mode holds". He also means the excitation field, not the drive field : in a BLDC motor the excitation field is provided by magnets fixed to the rotor which cannot be other than stationary wrt the rotor. (In a classic synchronous motor or generator this would be generated by a DC field communicated to the rotor by slip rings) – Brian Drummond Mar 28 '14 at 10:07

Lets clarify this with example of a induction motor(Referred as IM) for easy understanding.

Things to know before-

1.Synchronous speed (Ns) is 120*f/P.

2.Relative speed concept-Speed of object when seen from other reference.

3.IM has a stator(static) and rotor(rotating or static).

4.Stator magnetic field - Field generated by stator.

5.Rotor magnetic field -Field generated by induced currents in rotor.

In case of induction motor during normal operation.

Stator magnetic field speed = Ns.

Rotor magnetic field speed =Ns.

Speed of Rotor magnetic field w.r.t Stator M.F or vice versa =Ns-Ns=0.

Speed of stator = 0 (static).

speed of Rotor = N (actual speed).

speed of stator field w.r.t rotor= Ns-N.

In general speed of X w.r.t Y , we just compute by = Vx-Vy