[GreenKeys] Machine speed and VFDs

[Bruce Gentry]

The motors ARE synchronous. The rotor has a hard steel outer shell that 
mimicks a squirrel cage winding to start the motor as an induction, but 
becomes permanently magnetized by the rotating field as synchronous 
speed is approached. Once it locks in to synchronous speed, the 
mangetism becomes very strong and the motor continues as synchronous. 
The main disadvantage to this type of motor is it's inefficiency, that 
is why they run so hot.  Athough the motor does turn the correct RPM 
quite faithfully, the instantanious speed  varies  above and below 
during each revolution. Think of a race car making  laps on a one mile 
course at precisely 30 second intervals. The average speed is 120, but 
it will be going 150 in the straightaways and 90 on the corners, but 
always exactly 30 seconds per lap.  The flywheel action of the fan  
helps reduce this. That is why the machine will be very noisy if the fan 
is missing, as well as the motor overheating.


      Bruce Gentry, KA2IVY



As far as I know, they are induction motors, permanent split capacitor.
That's what I have in my model 28s.  The Governed motors are in another
class, being brush-type machines.  In order for a motor to run a true
synchronous speed, it must either have a DC- excited revolving field rotor,
or a permanent magnet rotor.  Induction motors will run at a fairly constant
speed with a prescribed amount or "slip" if they are designed and specified
properly, typically a 4 pole 60 Hz motor will run around 1750 RPM lightly
loaded (versus 1800 RPM for a synchronous machine).  The so-called BLDC
(brushless DC) motor is really a synchronous machine, having a PM rotor with
a three phase stator, driven by a three phase inverter driver.  Single phase
induction motors can be made to run at precise speed by driving them with a
variable voltage/variable frequency signal, and using hall effect sensors to
feed back actual speed. Obviously that technology did not exist in the
industrial market when the model 28 was in vogue.

Scott V. Johnson W7SVJ
5111 E. Sharon Dr.
Scottsdale, AZ 85254-3636
H (602) 953-5779
C (480) 550-2358
scottjohnson1 at cox.net
scott.johnson at ieee.org


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