Hello, I have a Thorens TD 124 and lately have become obsessed with feeding the correct and consistent voltage to the table from my apartment where the voltage drops in the evenings. As you know the motor is sensitive to the mains and a sudden drop could possibly slow down the rpm of the motor. With guidance from a few people I have looked at the RCA Viz Isolation transformers, picked up a WP-28 but that is not the optimum solution as there is no variable for adjusting voltage. Does anyone know of any other isolation transformers that could be used with a Thorens that can offer an isolation mains and be able to adjust voltage, hence speed. I have seen a brand called Tenma on eboy and was wondering if this would be a suitable substitute to the RCA Isotap WP's that are around.
Overkill on current, but should work.
Tenma 3.5A Isolated Variable AC Power Supply | 72-1097 (721097) | Tenma
Thanks, figured as much. Purchased a RCA Viz WP28 but I have now moved to the stage where I would like to adjust the voltage. Will keep my eye open for a WP-27A.
If and I think the TD124 has an induction motor, a 50% variation in supply voltage will have no effect as it relies on the frequency to keep the revs right. I bought one in 1966, a MKII, for £42. That was four weeks wages!
why isolating? why not a non-isolating variac? However, what happens when the mains supply voltage rises if you don't turn the power down?
You are correct a change does not affect the RPM's by very much but it does seem to affect the torque of the motor by quite a bit which is why I would like to keep the voltage at optimum.
Once the motor has got the turntable up to speed, there is hardly any torque required to keep it going as there is almost no friction.
Fair enough but Schopper offers what appears to be an isolation transformer with a variable voltage dial and wouldn't an isolation transformer with a good transformer and shielding offer some sort of defense from polluted mains? Have also noticed that Hanze hifi are offering something similar to shield the motor from any mains noise interference. So leaving aside the speed issue vs voltage wouldn't an isolation transformer offer the same thing at a reduced price?
Unfortunately variations in mains voltage does have an effect on speed in shaded pole induction motors, but unless it is constantly varying you can adjust it out.
Frequency, voltage and load are the three variables that influence pole slipping in an induction motor, voltage is the least sensitive parameter, but a 50% reduction in line voltage will definitely result in insufficient torque to maintain speed under load in this particular motor.
An isolation transformer won't help, something that regulates voltage and produces a good sinewave could. I published the design for such a project here over a year ago. A small PSA power station would work even better, and perhaps could be acquired for a smaller fortune used.
Hi,
It doesn't for shaded pole synchronous motors.
They are tied to the frequency with a slip angle.
If the voltage is too low the slip angle becomes
large enough to allow the poles to start slipping.
rgds, sreten.
Hello kevinkr and thanks for your reply, so would not an isolation transformer with a variable voltage selector dial to accurately get the proper voltage not work? What do you mean by PSA (Power Servo Amp or Power Switiching ***) and can you recommend one ? thank you
Torque decreases with decreasing line voltage, running slightly below synchronous speed unloaded, these motors are always loaded to the extent that they slip significantly due to an eddy current brake that is used to vary motor load in order to effect speed control of +/- a few % (In a properly set up TD-124 that number in theory is +/- 3%, never seen it). I have two 124 and a 401 that use induction motors with eddy current brakes. I have found all of these motors to be somewhat sensitive to line voltage.
One more question
I assume you are talking about the power re generators and not their power conditioners.
I assume you are talking about the power re generators and not their power conditioners.
Yes, the power regenerators (aka power station) perhaps you can find a good used one for a reasonable price.
Yes they do come up
fir sale on canuckaudiomart or usaudiomart but they sell quite quickly. Keep my eyes peeled. Thanks again for all your help. Regards
Nick l.
fir sale on canuckaudiomart or usaudiomart but they sell quite quickly. Keep my eyes peeled. Thanks again for all your help. Regards
Nick l.
That's the theory, Jon - but in practice, it seems to work out differently. Though whether 'torque' is the right word ... I don't know.
Let me explain ... I am using a prototype of a very sophisticated AC motor speed controller which should finally be released in its commercial form before the end of 2015.
As a speed controller, it allows you to adjust the Hz fed to the AC motor - so you can adjust the platter speed. And as you can adjust Hz to 3dp ... you can very accurately set the speed. 😀
A 2nd adjustment is the phase between the pairs of Premotec motor wires This affects the vibration of the motor - set it correctly (not 90 deg!) and you can hardly feel any motor vibration when holding the motor in the palm of your hand.
Doing this enables you to take advantage of the 3rd adjustment the 'Number9' speed controller offers - the voltage fed to the motor. Conventional wisdom - used by Linn's "Lingo" for example - is that once the platter is up to speed, you can reduce the voltage fed to the motor (down to 80v for the LP12's 110v Premotec) to "just keep it going" ... and take advantage of the lower vibrations in the motor.
However, when a few of us - including the designer of the speed controller - had a listening session to hear the result of feeding ever higher voltages to a 24v Premotec (the model Rega uses) ... we found that SQ got better as we upped the voltage! 😱
Now the motor gets warmer as the voltage increases - as well as vibrations increasing - but having solved the vibration problem through optimising the phase, how warm the motor gets is basically the only limiting factor to setting the voltage. So I am feeding 32v to the (24v) Premotec and I assure you, the sound is much better at that higher voltage than it is at 24v.
My understanding was that increasing the voltage increases the torque of the motor ... but maybe a more correct statement would be "increases the potential torque"?
Regards,
Andy
If the motor has an eddy current brake to load and slow the motor then it must be asynchronous - runs a little slower than synchronous speed.
A synchronous motor runs at synchronous speed unless it is stalled - then it would skip one (or more) poles - a very noticeable effect! You cannot adjust the speed of a synchronous motor except by altering the power supply frequency.
A synchronous motor runs at synchronous speed unless it is stalled - then it would skip one (or more) poles - a very noticeable effect! You cannot adjust the speed of a synchronous motor except by altering the power supply frequency.
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