Motor Question ???????

in article RcS0e.3327$4% snipped-for-privacy@fe2.news.blueyonder.co.uk, John C at snipped-for-privacy@blueyonder.co.uk wrote on 3/25/05 3:02 AM:

A synchronous moter rotates at a speed in lock step with the power supply frequency. That can be either at the line frequency or at a the line frequency divided by an integer dependent upon how the motor is constructed. Thus, at 60Hz, possible speeds, among others, are 3600, 1800, and 1200 rpm. Other kinds of (induction) motors will run at lower speeds, depending upon the mechanical load, reduced by a factor called the slip.

There are other kinds of ac motors, such as universal motors,that are similar to dc series motors, where such a description does not apply at all.

Bill

Hi John,

The Stator part of both motors is wired with a distributed winding which, when energized with a 3 phase voltage, produces an equal number of pole pairs specific for a particular motor design, which rotate around the motor axis at synchronous speed. This synchronous speed is 3,000 RPM if you only have one pole pair (50 Hz Supply) and drops proportionally with increasing number of pole pairs, (4 poles = 1,500. 6 poles = 1,000 etc)

The difference between the two comes in the design of the rotors

A synchronous motor rotor has distinct magnetic poles wired with coils of copper. These are in "North / South" pairs. These fields are energized with DC Via Sliprings, Causing the rotor to develop a distinct number of pole pairs (1 north & 1 South)

The poles of the rotor "Lock In" with the rotating poles of the stator, (once it is started) and so the motor operates at synchronous speed. Thus "Synchronous motor".

As the motor load changes, the DC Excitation must be changed to suit, so the control is a little more complicated then an unduction motor. These motors tend to develop little or no torque at stand still, so some method must be devised to start them. THe easiest way is to have a kind of limited Squirrel cage built into the tops of the poles, which will make the motor develop some torque to get it started. once the motor is synchronized, the squirrel cage develops no useable torque.

The Asynchronous Motor has the same distributed winding on the stator, producing the rotating magnetic field at synchronous speed.

The difference is in the Rotor. On the asynchronous machine, the rotor is built with a conductive, Shorting ring, at each end of the rotor joined by a series of conductive bars. kind of like a mouse exercise wheel (or Squirrel Wheel) These are imbeded in an iron core ( usually laminated)

The best way to visualize the operation is to assume the motor is stopped and then Energized to get it going. Initially the rotor is stationary, and the rotating magnetic field of the energized stator cuts through the many loops of the bars of the squirrel cage producing an induced Current In the Bars. (along one bar, around a segment of the shorting ring, and back along a second bar, back along another segment of the shorting ring on the other end, and back to the start.

These Induced rotor currents then produce magnetic fields in the iron core, which interact with the Rotating magnetic field poles of the stator, and torque is produced. This torque accelerates the motor rotor and hopefully, It's load.

As the speed of the rotor approaches synchronous speed, the difference in speed between the rotor and the rotating magnetic field poles of the stator, drops to almost nothing and thus the induced currents drop, and with this, the Magnetude of the rotor magnetic field. At synchronous speed there would be no slip, and thus no induced currents and no magnetic field in the rotor ( no torque)

Even on an unloaded motor there will always be some load due to the windage of the rotor and the friction of the bearings, so the motor will not reach synchronous speed but tend to slow to some speed below synchronous. once it slows, there is again, slip and then induced rotor currents and then Rotor field, and then torque, so the net effect is the motor will stabilize at some speed below synchronous, where the developed torque matches the torque of the load. if the mechanical load is increased on the motor, the motor will slow, causing the torque to increase and another stable operating speed is reached where the motor torque matches that of the load.

Because the motor does not operate at synchronous speed it is referred to as an "Asynchronous" machine, But we usually call them "Induction Motors" because of the way the current is induced in the rotor.

THere is much more to this but Hopefully this is enough to get you started.

Regards Tom Grayson

I guess i could have added a few Formula's, and photo's. Or even perhaps clickable links and multi media :o)

Tom