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Low frequency adjustable but stable oscillator

v-mwalk

May 20, 2023
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I have a 50KTYZ syncronous 1RPM motor I got for a swimming pace clock. Only, it turns out to be actually 1.1 ish RMP @ 50 hz. Using the 50KTYZ as it is a BIG clock - so lots of torque needed

So, having an electronics background (but not for about 25 years!) I thought, no problem; i'll do a simple 50hz ish adjustable osc circuit with a 240v output.
(the mains stage is simple - lol, and safe :) I'm not messing with mains voltages unknown - just a lot of respect :)

I kinda assumed the circuit would be simple and based on an off-the-shelf chip and a crystal. Crystal for stability and a chip to control dividing down to low frequency and some adjuster - either analogue (a skeleton pot based, to set the freq and then leave it alone) or maybe a 16-bit (for good/fine adjustment resolution) based digital setup.

However, this seems more complex than I thought... Could any of the folks here suggest a starting point? Basically, I'm looking for an output with 50% duty cycle, and adjustable (with fine adjustment) between about 50 and 60hz output and then very stable.
 

v-mwalk

May 20, 2023
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Sort of. But i was thinking more along the lines of a chip, or two, a crystal and the odd resistor and capacitor. Powered from rectified mains and a resistor/zener down to 5v....

My initial thought was a 10Mhz xtal and a chain of dividers. But i need it to be adjustable 55Hz +-10ish. Then i thought of a PLL in the loop and the complexity started to mount. Surely there must be a device or small cheapo circuit to do that?
 

danadak

Feb 19, 2021
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What is power / current level needed for motor ?

Regards, Dana.
 

Harald Kapp

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adjustable (with fine adjustment) between about 50 and 60hz output and then very stable.
When the motor runs 10 % fast on 50 Hz, you'll need less than 50 Hz to slow it down to 1 rpm. Not more.
Anyhow: how can a synchronous motor run too fast? The rpm should be bound to the mains frequency. Is your mains this unstable?

What is power / current level needed for motor ?
6 W - 10 W acc. to different websites.
 

crutschow

May 7, 2021
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I'm also puzzled as to how that synchronous motor is not running at synchronous speed.
Sounds like your 50Hz mains is not generating 50Hz but more like 55Hz.
 

danadak

Feb 19, 2021
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Do synch motors experience slip, such that instantaneous RPM does not track line ?
 

AnalogKid

Jun 10, 2015
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A crystal circuit with that adjustment range is not simple. Two methods that come to mind are phase-locked loop and direct digital syntheses.

Since a squarewave signal is acceptable to you, a starting point might be a CD4060 oscillator/divider with 1% capacitor, a 0.1% resistor, and a trimpot with a matching temperature coefficient.

That gets you a symmetricalsquare wave with the frequency you want. After that, filter the square wave a bit, feed it to an audio power amplifier, and use the output to drive a 50/60 Hz power transformer connected backwards.

ak
 

roughshawd

Jul 13, 2020
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optocoupler time....
just put a little sensor in there and spin it up, theres your hz and everything!
But as far as motor potentiometers are concerned...
what you need is called a rheostat, not a pot.
 

hevans1944

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Jun 21, 2012
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it turns out to be actually 1.1 ish RMP @ 50 hz.
That sounds suspiciously like the speed the motor would run if the line voltage was 60 Hz instead of 50 Hz. It clearly says on the motor that it runs at 1 RPM (Revolutions Per Minute) with 50 Hz 110 VAC power and 1.2 RPM with 60 Hz 110 VAC. See this Amazon page for pictures and specifications of the

CHANCS 50KTYZ Synchronous Low Speed Motor AC 110V 1/1.2RPM Small Electric Motor​

Apologies if this is not the motor you have.

In what country is your swimming pool timer located? Are you absolutely SURE your line voltage frequency is 50 Hz rather than, say, some higher frequency like 60 Hz?
 

hevans1944

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optocoupler time....
just put a little sensor in there and spin it up, theres your hz and everything!
But as far as motor potentiometers are concerned...
what you need is called a rheostat, not a pot.
Your post is not helpful. Synchronous motors do NOT vary their synchronous speed as a result of placing a rheostat in the circuit. The synchronous speed is DETERMINED only by the power line frequency. Inserting "a little sensor in there and spin it up" is not good advice, either with or without an optocoupler. What did you expect to accomplish with that?

This motor has FOUR leads: two with black insulation, one with red insulation, and the other with green insulation (see video on Amazon website). A phase-shifting 0.56 µF non-polarized 630 volt capacitor MUST be installed between the red and green leads. The two remaining black leads are connected together and then connected to one side of the mains line. The other side of the mains is connected to ONE terminal of the capacitor, the choice of which determines the direction of synchronous rotation.
 

AnalogKid

Jun 10, 2015
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That sounds suspiciously like the speed the motor would run if the line voltage was 60 Hz instead of 50 Hz.

Agree. If the motor is running exactly 10% fast, that sounds like a 50 Hz motor running on 60 Hz.
 

bertus

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Nov 8, 2019
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Hello,

Australia seems to use 50Hz @ 240 Volts.

Bertus
 

hevans1944

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Jun 21, 2012
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I have a 50KTYZ syncronous 1RPM motor I got for a swimming pace clock.
If this motor is not running at the desired speed, and you are sincere about building a motor driver, then perhaps you should consider using a stepper motor instead of a synchronous motor. Most largish stepper motors have 200 full-steps per revolution and would therefore require 200/60 = 3-1/3 steps per second to rotate at one revolution per minute. A simple 555 timer circuit with an adjustable frequency of 6.67 Hz ±0.5 Hz followed by a type-D flip-flop to divide the output frequency by two and create a square wave to drive a stepper motor controller would be sufficient. Select temperature-stable resistor and capacitor components for best accuracy.

A few years ago I bought an inexpensive Chinese stepper motor controller (sold by Amazon) and was quite pleased with it. Just add a logic-level oscillator input, plus DC power, and select the motor rotation direction with a logic-level input. Some of these controllers, mine included, also support micro-stepping, so with an appropriately high number of micro-steps per revolution, almost any timing resolution is possible. Or quadruple the 555 timer frequency and use two flip-flops to create two square waves in quadrature phase to each other, followed by MOSFET motor-driver transistors. Lots of example circuits on the Internet will show you how to do this, but I would check the mains frequency FIRST to see if your synchronous motor is being powered with the correct line voltage frequency, i.e., 50 Hz for one RPM rotation speed.

I have been using large stepper motors (Superior Electric brand) since the late 1960s and am amazed at the low level of cost versus high-performance capability of modern PWM (Pulse Width Modulated) MOSFET drivers. Well worth investigating for projects like yours.

Also, I am not here to promote Amazon or any other electronics supplier. I often use Amazon for its convenience and free shipping for Prime members, but you should investigate the option of purchasing (possibly less expensively) from an electronics supplier such as Digi-Key or whatever distributor is available in your area. Not all the electronics "stuff" sold via the Internet is suitable, even for casual or hobby use, so the reputation of the seller is important... especially when ordering on eBay.
 

hevans1944

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Jun 21, 2012
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Hello,

Australia seems to use 50Hz @ 240 Volts.

Bertus
Unfortunately, the location of the OP is (so far) undetermined. If this project is intended for use in Australia, and the synchronous motor is the one I found on the Amazon website, and ALL AC power in Australia is generated at 50 Hz... well, as one of the moderators says (paraphrasing here): @v-mwalk, you're up s#it creek in a canoe without a paddle. Umm... that's a native American water-vessel without a means of propulsion:

1684704269103.png
 
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