In article , Johnny B Good johnny-b-
scribeth thus
On Mon, 13 May 2019 21:13:27 +0100, tony sayer wrote:


Do the secondaries need to be isolated from each other? If not then a
10-0-10 would be close enough to 9V and in series give 20V. I'm not sure
if there are many transformers with this output. 9-0-9 used to be more
common.

Yes they do one drives the motors at AC as best I can tell, the other 9
volt supply the microprocessor and display..


Maybe the cheapest auto-transformer is a standard one with 2 x 110V
primaries. I expect a true autotransformer with the same rating will be
smaller.


Thats what is in use at this moment. Pending the compact 230 volt step-
down..

If you opt for 2 transformers remember they don't have to be so large.
The rotator power is also likely to be intermittent and that can be
reflected in the transformer rating.

Indeed..

As you've surmised, the rotator motor is an ac only type (a universal ac/
dc motor will be unsuited on account of the interference produced by the
commutator/brush gear).

Don't think that would be a problem anyway your not normally using the
aerial/s whilst rotating them.


It will be a 120v 60Hz induction motor but the thing you have to
remember when running such motors off a 50Hz supply is that this makes
them a 100v 50Hz induction motor. You have to scale the voltage down in
proportion to the scaled down frequency to avoid excessive current draw
along with the real possibility of core saturation which will exacerbate
the increased I sq R losses in the motor windings.

I'm not a 100% sure it is AC it seems it might be. Haven't got it going
as yet but hopefully soon, do appreciate the 50/60 Hz bit it won't be
running that long or much either with the intended use.


TNP is in my killfile so I don't know what type of output his suggested
"compact 230 volt stepdown converter" is. If it's just a 9v smpsu to
power the electronics part, that won't be a problem but if it's a 230vac
to 110vdc converter that'll just fry everything.

It is AFAIK an AC one look like a small plug mounted type
autotransformer

It seems to me unlikely that a compact switching 240vac to 110vac
converter would be cheaper than a 50VA rated 240 to 100 volt
autotransformer, especially one made up from a readily available (or to
hand) 30 to 50 VA 240v split primary to 20v secondary transformer which
you can use to get 120v minus the 20 volt secondary connected in 'buck'
to get the desired 100v 50Hz motor supply.

Anyway, I just thought it might help to remind you about the need to
scale the motor voltage requirement with the reduction of frequency when
driving a 120v 60Hz motor off a 50Hz supply.

All noted I'll let everyone know how it goes when i get a aroundtuit!..

--
Tony Sayer


Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.