# Linear Syncronous Motor

1. Dec 4, 2004

### Roshio

Does anyone know how a three phase linear syncronous motor (LSM) work? and if u can help plz put it in the simplist terms possible. I think my brain is starting to melt from all the big words on the net trying to explain this. Thanks :grumpy: :tongue2:

2. Dec 4, 2004

### wire2

To start off I'll describe normal 3 phase AC motor operation.

There's 3 sets of coils in the stator connected to 3 power lines. The 3 lines peak in voltage and current (I'll ignore power factor for this explanation) in sequence, creating a rotating magnetic field. For a 4 pole stator on 60 Hz, the field rotation speed will be 1800 rpm.

The rotor has large aluminum conductors in a laminated steel body. The stator field induces current in the aluminum conductors (by transformer action), which causes high rotor current because the conductors are shorted at the ends. The rotor current makes its own magnetic field, which is stationary when the motor is first started. The rotor field is "pulled" by the stator field and the motor develops torque. It will never catch up to the stator's 1800 rpm because then there's no more transformer action, so no torque.
The difference is called "slip" and a typical actual rotor speed is ~1750 rpm. If the motor is loaded, it slows down, slip is increased and stator current will rise, creating more torque.
Still with me?
Now if we replace the parasitic rotor with one that has copper magnet wire windings and connect it via slip rings to a DC power source, it will have a magnetic field independant of the stator. This rotor field will "lock on" to the 1800 rpm stator field with no slip. (synchronized)

Making it linear just involves "unrolling" both the the stator and rotor to flat.

Permanent magnets can be used to provide a rotor field in place of DC windings.

A side benefit of a synchronous motor is power factor correction. If the DC field is made stronger than what's required for the load,(over exitation), it exhibits a leading power factor. That can be used to compensate for induction motors causing lagging power factor. A power factor other than unity draws more power and incurs a penalty charge by the utility company.

Last edited: Dec 5, 2004