[GreenKeys] Machine speed and VFDs

[Scott Johnson]

I doubt you can do this effectively with a PSC single phase induction motor,
as the capacitive reactance will change with frequency, thereby changing the
phase shift and the current through the phase shifted winding.
Varying from the design frequency and voltage doesn't work well for single
phase motors, and the usual means of varying the voltage only (as is done in
ceiling fans) limits the torque to unusable values in a heavily loaded
motor.

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 Ralph Mowery
Sent: Monday, September 3, 2018 7:54 AM
To: GreenKeys at mailman.qth.net
Subject: Re: [GreenKeys] Machine speed and VFDs


As long as the shaft is turning very close to the rated speed , that is all
that matters.  

I worked with VFDs among other things at work. You just have to be sure the
VFD will have enough power to turn the shaft at a reduced speed.  That is
the motor looses tork as the speed is reduced and may not be able to turn
the shaft.  Some drive and motor combinations work well and some do not at
low speeds.  Also the motor will have a fan on the back of it to cool the
motor.  We had to add external driven fans on some motors as the speed was
reduced very much.  

You may come out better if you gear up at 75 wpm and only have to go down a
small amount, but could go up ok.

I don't recall where I saw it, probably in a back issue of the RTTY Journal
but someone had made an optical interrupter for feedback and made something
similar to a VFD.  Supposed to have worked well.




-----Original Message-----
From: greenkeys-bounces at mailman.qth.net
[mailto:greenkeys-bounces at mailman.qth.net] On Behalf Of Robert Goff
Sent: Sunday, September 02, 2018 9:49 PM
To: GreenKeys at mailman.qth.net
Subject: [GreenKeys] Machine speed and VFDs

This is probably a really stupid question,  BUT:

I was just looking at the gear chart in an old Atlantic Surplus catalog
comparing the gear ratios from 100 speed to 60 speed on a model 28.  It
looks to me like for 60 wpm the 3600 rpm of the motor is being reduced to
525 rpm by the combination of the pinion and the main gear.  For 100 wpm it
is 857 rpm, approximately the ratio of 5:3.

If you had a machine geared only for 100 wpm and you used a freq drive to
reduce the motor speed to 2205 or so, so that the main gear was spinning at
525 rpm, would the machine then operate at 60 wpm?

Has anyone tried this?  Is there something I'm missing?  Frequency drives
for fractional horsepower motors have really gotten pretty cheap, and maybe
easier to track down than 60/67/75/100 gear sets.

Thanks,
de Robert W7MKA

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