DIY 4 Phase Sinewave Generator for Turntable Motor Drive

I built a similar controller based on work done by @ralpfcooke. It is driving a Papst motor from an Empire table - not sure if it's the KLZ tho. I used different digital amps (TDA7294 I think - would have to check). I needed a small resistor in series with each transformer so the amps saw something other than a inductive load. I used small value, 1 ohm, 3 watt and it works fine. Possibly 100 ohm is too large.

As I read what I've written, I find there may be too man "ifs" to be helpful but I wish you good luck. the information from Ralph is embedded in the following thread:

3 Phase Class D amp for DIY BLDC motor Drive

I started off with 5watt 1.2ohm in series. They got scorchy-hot in no time.
Had two 8ohm 100W for dummy loads so just bought another to try.
I realise they are not 100W until heat sinked but Shirley they should cope with this not very big motor. ...

Would love to know how to build a transformerless mosfet output.
Is that unpopular because of high DC involved and/or does the transformer help in other ways?

I will experiment further with less Ohms and also try clamping the 100w to some Heavy Metal...
 
Measuring the volt drop across the resistor should allow you to calculate the current and the power dissipation.

Will do that now.
Just found a photo of the same motor with 4.5W printed on the side...
 

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Hi iWozza,
My suspicion is that this is not right.
Well then, you're wrong! :)

This is how it works. A resistor turns energy into heat, that is how it works. How much heat depends on you. These power resistors have a mounting surface and holes used to bolt them to a large heat sink in order to transfer the heat into the air so that the temperature rise stays within safe limits. It would seem to me that the resistance is too high so that the resistor is dissipating most of the energy.

Depending on the shape of the current waveform and its crest factor, you might be able to read it with a meter. Knowing the application, many cheaper meters will read far too low.

-Chris
 
7V drop across resistors.


"It would seem to me that the resistance is too high so that the resistor is dissipating most of the energy." That's my feeling too. Wasted energy.


"Depending on the shape of the current waveform and its crest factor, you might be able to read it with a meter. Knowing the application, many cheaper meters will read far too low." Not sure what you mean. It's supposed to be sinewave. My meter was about £20
 
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Hi iWozza,
Normally you need to be in the $400 ~ $600 rang on a hand held meter to get the crest factor and frequency ranges required to read non-sine type wave forms with any accuracy. Fluke or Keysight.

Your current draw may not be a sine wave, that depends on how your motor draws current even though you may be delivering a sine wave voltage. That's why I have current probes for my scope and meter.

-Chris
 
7V drop across resistors.


"It would seem to me that the resistance is too high so that the resistor is dissipating most of the energy." That's my feeling too. Wasted energy.


"Depending on the shape of the current waveform and its crest factor, you might be able to read it with a meter. Knowing the application, many cheaper meters will read far too low." Not sure what you mean. It's supposed to be sinewave. My meter was about £20


When I was investigating an 'optimum' way of connecting the amplifiers to the toroidal output transformers I found that using NTC thermistors worked best (simplest).
I experimented with several types, and found that NTC 5-D9 (5ohm, 3A) work best for my setup, which has 3 X 50VA transformers ( 24 - 240V ).

The NTC thermistors work almost perfectly, with no more than 0.5V drop when hot.
The only very small downside occurs if you switch the amps off and back again within a few seconds, when the thermistors are still hot, the amps can lockup. Even then the amps reset themselves a few seconds later and the motor spins up normally.
 
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When I was investigating an 'optimum' way of connecting the amplifiers to the toroidal output transformers I found that using NTC thermistors worked best (simplest).
I experimented with several types, and found that NTC 5-D9 (5ohm, 3A) work best for my setup, which has 3 X 50VA transformers ( 24 - 240V ).

The NTC thermistors work almost perfectly, with no more than 0.5V drop when hot.
The only very small downside occurs if you switch the amps off and back again within a few seconds, when the thermistors are still hot, the amps can lockup. Even then the amps reset themselves a few seconds later and the motor spins up normally.

Hi Ralf,
The thermistor route sounds good.
I just experimented a bit.
a) No resistors: the Amps kept cutting out but the motor kind of picked up speed hinting at start up being the trouble.
b) Tried 1.2 Ohm 5W resistors. Only 2.5V loss and tons more torque on motor up to 190V. Very Hot so tried 2 x 1.2 in parallel 1.4V drop. More torque & over 200V at motor.
c) Then put 3 x 1.2 ohm in parallel. Bingo!
Only about 1V voltage drop, 210V+ at motor, dangerous torque and a sensible heat at the resistors...
Now motor quite audible so can try to turn Amps down.

Will order some Thermistors!

Have 3 of these motors but not sure if they are Synchronous...?
Should get me a strobe.
Wondered if they could be like hysteresis with a bit of slip...

They come from a Dynachord Echoing desk so you wouldn't want an echo effect that had wonky speed me speculates...


Thanks everybody for helping me work it out :)
 
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I'm seeing 117VAC with motor off and 110V with it on, seems reasonable to me. I do notice an audible ticking-like noise at high preamp gain, a little less at 45 rpm. Shouldn't the filter cap on the transformer output block that? I haven't yet grounded everything, so hope it goes away when I build it into a case.
 

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The PWM frequency on the TDA7492 amps is ~350kHz, so it would not be audible and yes, it does get filtered out by the inductance of the transformer and the cap across the output.

It will most likely quiet down when you put things in a shielded case, shorted the leads and ground everything properly.
 
Thank you for your prompt response gentlemen.
I am not tech savvy and I have never undertaken a diy project like this.
Would it be advisable to opt for a turnkey system like the Sota Total Eclipse which I understand was designed by Bill?
I have a VPI Aries with the old PLC.
Alternatively, I can get it built by a professional technician in Sydney.
 
VPI Aries- Nice table.

PLC- Nice boat anchor. Seriously, get rid of it; it puts out 140VAC ringing square waves. Same basic circuit as the old '80s power inverters that were used to run a drill or a soldering iron in the field. Not suited for TT use.

The DIY controller and AA motors will get you 80-90% of the way there. The SOTA solution will get you 100% there in a small package. I would say it looks better than DIY, but some of the guys here have done amazing jobs on the case work so that might not be true. DIY is less expensive.