[GreenKeys] Machine speed and VFDs

[Scott Johnson]

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

-----Original Message-----
From: greenkeys-bounces at mailman.qth.net <greenkeys-bounces at mailman.qth.net>
On Behalf Of jim at k6ccc.org
Sent: Monday, September 3, 2018 11:03 AM
To: Green Keys <greenkeys at mailman.qth.net>
Subject: Re: [GreenKeys] Machine speed and VFDs

Scott Johnson said (in part):
> I doubt you can do this effectively with a PSC single phase induction
motor,

Unless I'm mistaken, Teletypes used synchronous motors, not induction.

Interesting idea.


73
-----
Jim Walls - K6CCC
jim at k6ccc.org


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