Hi all,

I have a customer who services institutional and public clocks

The vast majority of their installations have been 30-second 12VDC
solenoid ratchet/pawl mechanisms, for which I make PIC-based
drivers

http://waitingtrain.blogspot.co.nz/2015/05/a-large-gents-turret-slave-movem=
ent.html

They tend to be brass/heavy/old/expensive/eventually wearing out

An advantage of them is that it's easy to single-step or fast-forward
(eg daylight saving) with pushbuttons, you just pulse the solenoid as
required. Because of the ratchet they can't go backwards, electrically
anyway

They're thinking of converting some to 230VAC synchronous motors

https://www.cousinsuk.com/category/movements-clock-mains-synchronous

That does mean someone will be up and down ladders all day when
it comes time to adjust them if I can't sort out electronic control

I have an induction motor speed control design which will generate
three phases of variable frequency (0-55Hz) 230VAC from rectified
mains, using a dsPIC, PWM and an STGIPS20K60 IGBT module.
A single-phase motor is put across any two outputs.

As the clock motors draw < 30mA I'd use two small discrete IGBTs,
with an effective 100Hz 230VAC output. A relay will switch the motor
over to this for fast-forwarding. To advance an hour it would run at
100Hz for 2 hours

I've not investigated yet how easy it is to make them run backwards
(eg there might be a worm, or capacitor/switch [unlikely] (shaded pole
start winding ?)). For my DC units I simply pause the drive for an hour,
and synchronous motors shouldn't get ahead of time anyway

It's been suggested to me though that such a motor will lose torque
at the doubled frequency. Here are a couple of Google results

These doesn't really mention anything at higher frequencies. VFD
seems to be more of an industrial thing, increasing speed to overcome
load inertia etc but not exceeding mains frequency

http://www.engineering.com/ElectronicsDesign/ElectronicsDesignArticles/Arti=
cleID/8154/Changing-an-Induction-Motors-Power-Supply-Frequency-Between-50-a=
nd-60Hz.aspx

http://ecmweb.com/basics/understanding-variable-speed-drives-part-2?page=3D=
2

Some say here that V must be increased to maintain the V-f ratio,
some don't

https://in.answers.yahoo.com/question/index?qid=3D20090706032855AAL5TtK

Does anyone have practical experience of what might actually happen ?

It would be quite an exercise to make a circuit + s/w and I don't
want to waste time on that if the end result is a predictable failure

TIA

Joe=20



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