I don’t think I’ve ever had access to so much motor knowledge in one place before, so I have a theory question I’d like to ask.

When you remove the iron core from a coil winding, be it a solenoid, a relay or a motor, the current goes up. When the core is in place I think the winding flux is manifested in the core as organized work for the coil. I consider the rotor in an unloaded motor running near to synchronous speed to have its core in place to the winding. When load is added it results in slip, which is analogous to pulling the core away from the winding. Am I making a proper assumption with that statement?

So when the core is removed, what are the mechanisms or forces at work which cause the rise in current through the winding? Are the winding currents and resulting magnetic flux in the winding turns counteracting one anther in a negative way?

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So when the core is removed, what are the mechanisms or forces at work which cause the rise in current through the winding? Are the winding currents and resulting magnetic flux in the winding turns counteracting one anther in a negative way?

What prevents current from flowing in an ac coil is the inductance. When the core is removed, you have an air-core coil and the inductance is very low. This allows higher current to flow. When the core is inserted, inductance increases, current decreases.

There are a number of ways to explain motor operation. I can't think of any way to relate it to the analogy to removal/adding of a core. (others are welcome to try). As you increase slip, you increase the voltage induced in the rotor, which increases the rotor current. Increasing rotor current increases stator current in the same way that increasing transformer secondary current causes an increase in transformer primary current.

Since you confine this to an induction motor running at near-synchronous speed, you have to consider reverse EMF. The stator indeed is an electromagnet with a given inductance. So is the rotor. And when the rotor is excited by the induced currents, it generates a magnetic field of its own which is of opposite polarity to the inducing voltage in the stator. This opposition to the changing/rotating fields in the stator act like a reverse voltage, or "electromotive force" (EMF). The reverse EMF bucks the applied voltage in the stator and holds the current down. As the motor is loaded up and rotor slippage increases, the rotor's reverse EMF gets more out of phase with the stator and allows the current to increase. Ultimately, at max slip (locked rotor) you have almost no effective reverse EMF and the motor's current is limited only by internal losses to about 6x-10x of FLA.
In an ideal motor with superconducting windings, lossless cores, frictionless bearings, running in a vacuum with zero slip, unloaded current would be at or near zero.

Transformers do the same thing. If you leave the secondary windings of a transformer open (unloaded), the secondary current is zero and the primary current is quite low, representing only the transformer's built-in losses. As you load up the transformer's secondary, the reduced impedance of the secondary (as a system with its load) allows more primary current to flow until you either pop a fuse or get to core saturation, which is that point when the magnetic core cannot be magnetized any further no matter how much current you force through the coils.

Motors are kind of a transformer with a spinning secondary.

At least this is what I remember from EE-101 almost 40 years ago....

This message has been edited. Last edited by: Gary H.,