On 31/10/2020 08:47:56, The Natural Philosopher wrote:
On 31/10/2020 08:34, Harry Bloomfield wrote:
It happens that Brian Gaff (Sofa) formulated :
If he had one why bother with the device, unless its some kind of
slow starter circuit to lessen the load at start up.

Maybe to be able to vary the motor speed?

Or he might have access to 3-ph, though no 3-ph on the site where the
motor is.

All electric motors are in the end synchronous AC motors. In a typical
DC motor or 'universal' AC/DC motor the synchronous AC is created by a
mechanical commutator. This commutator may be replaced at some gain in
efficiency by an electronic inverter, either controlled by a sensor on
the motor or by sensing voltage and current in the windings.

Of all the motors you choose to compare the induction motor is an
entirely different beast. It relies on a rotating field, and a rotor
where currents are induced in empathy with the field, that effective
drag the rotor up in speed towards that indicated by the rotating field.

There is therefore no need for a commutator. It is never quite
synchronous in that there is always an element of slip.

A normal synchronous 3 phase mains AC motor suffers from the inability
to deliver decent torque at anything except its rated RPM. The use of a
variable frequencyÂ* electronic inverter removes this limitation.

A deep bar rotor can overcome much of the poor starting torque limitation.
https://www.electrical4u.com/deep-ba...duction-motor/
Though I've not seen many good explanations. Some even fail to mention
skin resistance.

Crawling/cogging is just as an important feature in these motors; where
the rotor locks to a sub-harmonic of the rotating field frequency.
https://www.electrical4u.com/crawlin...duction-motor/