Telescope motor (no audio troubleshoot for once)
« on: June 28, 2020, 09:23:08 AM »
Hello all

I extract a faulty motor from one of my father newton telescope
An old Berger motor that stop to turn.

For what I can say and after partial disassembly, it's a synchronous motor, single phase, with 4 wire to outside world.
It consist of two successive stator around the axis with 8 coil each that are in series, and a rotor looking more or less like a cross so probably 4 poles ? two permanent magnet at 90° ?

I suspect the motor was previously wired the wrong way as only one stator assembly was used, and not both with a capacitor to ensure to proper rotation direction by delaying one of the stator current supply.
My father always have to connect/disconnect the motor to the main when it start the wrong way, (I suppose one time over two depending of the 50Hz sin direction at the connection instant)

Now, the motor stop turning and I find the fault, one of the 8 coil in series at the connected stator assembly have open circuit.
I connect the second stator to the 220/50 and the motor turn.

I'm tempted to short this coil an wire the motor properly, two stator with the requested capacitor to phase delay both stator and ensure constant rotation direction, as proper torque.

Due to the 8 coils I suppose the motor axis turn at 50Hz /8 which is 6.25 rpm
And I suppose the only issue will be a half torque lose (I have two stator) on time over 8 (at the "dead" coil 45° position)
The working stator measure about 6k ohm DC for 8 coils, obviously each coil measure aprox 750ohm (as the 7 remaining at dead stator)

What do you think ? Am I right ? a least for the global understanding.
Can I try this fix by shorting one coil?

Best
Zam


moamps

Re: Telescope motor (no audio troubleshoot for once)
« Reply #1 on: June 28, 2020, 01:24:35 PM »
According to your description it is single phase asynchronous (induction) motor which has 2 stators, one directly connected to mains, second via a capacitor in order to generate rotation field. Rotor has just shorted turns.  If you have two pairs of stator coils, maybe it is used to switch direction.
RPM is little bit lower than 120xf/N, where N is poles number.

For a smooth rotation all windings should work.

Re: Telescope motor (no audio troubleshoot for once)
« Reply #2 on: June 28, 2020, 06:01:48 PM »
so this is asynchronous ?!

I have precisions since I disassemble more today as reviewing my math...
I now think the motor axis run at 50hz/4 meaning 12,5 rev/sec
the gear box show 1:12,5 stamped label on it (I don't saw before)
meaning 12.5/12.5=1 turn/sec
so this correlate the motor label at 60U/min
anyway the rotation speed is not an issue and obviously selected at start to have a prefect match with earth rotation.

moamps, sorry but I don't get your formula, because I don't know what it mean... 120xf ?

not sure about poles N, I extract the rotor today (see attached) and to me it's 4 poles ?
Interesting thing is that rotor magnet metal guide have half 45° offset between them (22.5°)
so yes you can change direction depending at how you connect capacitor (which stator get wave first)

in this particular use case direction don't have to be changed ! except if the earth decide to flip rotation  ;D

but basically you confirm that shorting a coil won't change speed rotation ?

the motor have 60t/min at output (via integrated gearbox) but the whole system have more wheels including giant brass one at "time axis" (not sure how you say in English) to establish the proper tracking.
Just to say that maybe this is enough to smooth the torque variation with 1 over 16 coil shorted ?
(the motor output at 60t/min is converted to 1t/day ....)

Do we agree that what I want to achieve won't change the RPM ?

Best
Zam

moamps

Re: Telescope motor (no audio troubleshoot for once)
« Reply #3 on: June 28, 2020, 06:31:03 PM »
so this is asynchronous ?!
I'm not sure now. If the rotor has permanent magnets, it is synchronous. If not, it is asynchronous.
Quote
... I don't know what it mean... 120xf ?
120 x frequency / number of poles
In your case, 120x50/8=750, after a gear ratio it is 750/12.5=60 RPM
Quote
but basically you confirm that shorting a coil won't change speed rotation ?
You will change the speed according the formula if one pole is missing. And IIRC the poles should be in pairs..
I don't know how much it is important for you.

Re: Telescope motor (no audio troubleshoot for once)
« Reply #4 on: June 28, 2020, 07:38:10 PM »
I'm not sure now. If the rotor has permanent magnets, it is synchronous. If not, it is asynchronous.

If I follow your "if" this motor is synchronous

You will change the speed according the formula if one pole is missing. And IIRC the poles should be in pairs..

That's exactly the point I try to understand, I will short one of the coil in the pairs, not both.
Will this slow the speed? or reduce torque !?
Knowing second stator coils pairs will "push-pull" too (at pi/2 ?) like a 2 phase motor.

I don't know how much it is important for you.

Speed is the most important, if my attempt change RPM speed this is a no go, and we have to change the motor  :-\

Best
Zam

moamps

Re: Telescope motor (no audio troubleshoot for once)
« Reply #5 on: June 29, 2020, 03:33:39 AM »
That's exactly the point I try to understand, I will short one of the coil in the pairs, not both.
Will this slow the speed? or reduce torque !?

I'm not expert in that field, but think that this depends of how much the motor is loaded.
If the motor is heavily loaded, it will loose torque, lower speed temporarily and maybe fall from sync.  Have you  contact some repair shops? Like this one from neighboring Slovenia.
https://matris.eu/
Maybe they can fix it.

Re: Telescope motor (no audio troubleshoot for once)
« Reply #6 on: June 29, 2020, 05:40:43 AM »
Hell moamps

I'm not expert in that field, but think that this depends of how much the motor is loaded.
If the motor is heavily loaded, it will loose torque, lower speed temporarily and maybe fall from sync.  Have you  contact some repair shops? Like this one from neighboring Slovenia.
https://matris.eu/
Maybe they can fix it.

Ok, I'll see what happen

Extracting and repair the dead coils if of course the best solution, but the whole stator assembly is more or less melted in varnish or epoxy, this is a complicated task  :-\

I think if I can't fix it we will change for modern microcontrolled motor

Thanks for your help

Best
Zam


 

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