It has been shown that the value of the phasing capacitor on a synchronous motor is very important to prevent vibration when running.
However there seems to be a wide variation in the values used by manufacturers and it is left up to perfectionist owners or aftermarket companies to make the final selection mainly by trial and error.
An example of this variation would be the Thorens TD 160 motor which can be found supplied with 0.15uF or 0.33uF as original equipment.
Surely there must be a more scientific way of selecting this value based on the motors measured parameters, the supplied voltage and the Hz frequency.
A typical Thorens motor with 16 poles is designed to run on 115 volts ac.
The two coils measure 4.6k ohms each with an inductance of 7 Henry.
Is it possible to calculate the capacitor value required based on this data ?
However there seems to be a wide variation in the values used by manufacturers and it is left up to perfectionist owners or aftermarket companies to make the final selection mainly by trial and error.
An example of this variation would be the Thorens TD 160 motor which can be found supplied with 0.15uF or 0.33uF as original equipment.
Surely there must be a more scientific way of selecting this value based on the motors measured parameters, the supplied voltage and the Hz frequency.
A typical Thorens motor with 16 poles is designed to run on 115 volts ac.
The two coils measure 4.6k ohms each with an inductance of 7 Henry.
Is it possible to calculate the capacitor value required based on this data ?
I would say you cannot calculate the run capacitor value because the motor's mechanical load is part of it (phase-shift). Especially with two-speeds like the Thorens.
Empirically, I would line-trigger an oscilloscope and look for the 90 degree phase shift, at both loads (speeds); the cap value is probably a compromise.
The cap different values are probably 50/60Hz and 110V/220V.
I show: For TD-160, TD-165, TD-166 0.33uF; TD-145, TD-145 MkIII 0.15uF
I use Wurth X2 caps with longer leads 275VAC 10% polypropylene.
Empirically, I would line-trigger an oscilloscope and look for the 90 degree phase shift, at both loads (speeds); the cap value is probably a compromise.
The cap different values are probably 50/60Hz and 110V/220V.
I show: For TD-160, TD-165, TD-166 0.33uF; TD-145, TD-145 MkIII 0.15uF
I use Wurth X2 caps with longer leads 275VAC 10% polypropylene.
Yes I guess using a scope would be the best way, if I had one.
This series of TD turntables all use a 115 volt motor, with a dropping resistor for 230 volt application.
The motor is single speed only being synchronous, the two speeds are achieved by moving the belt on the double pulley. 50 cycle and 60 cycle use different diameter pulleys.
Cheers.
Of course you can. After all it is plain engineering. But it is much easier to do it by compass and straight-edge construction, which leads to a capacitance value of 0.37 µF @ 50 Hz or 60 Hz for the OP's motor (Rdc = 4k6, L = 7 H), if it's armature is stalled. An idling motor would require some lower capacitance. The next E12 standard value of 0.33 µF is close enough.
Most probably the TD145 motor has some other specs, hence needing another capacitance value.
Best regards!
Most probably the TD145 motor has some other specs, hence needing another capacitance value.
Best regards!
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I would be interested to see your compass and straight-edge solution to this problem, since it is so easy. It isn't obvious that winding inductance with stalled armature is useful for this problem. It is also not obvious what the desired steady state conditions would be...such as maximum motor torque, minimum VA, etc.?
I might say that no information about this small ac motor has been supplied here. Is this a 4 pole motor? And even so, would each winding be identical and actually be placed exactly at 90 degrees? Obviously, measurements should be taken under normal load.
The OP asks why various motors are supplied with different run capacitors. This assumes that all the motors are the same and have identical drive configurations and thus the same load. I'm not sure this is the case.
Working backwards, I would like to examine the case where a particular motor exhibits vibration. Perhaps the capacitor value and motor measurements would tell us something. Various part values are commonly adjusted during the production cycle of a product, using using information from "field testing". Cost considerations may also be responsible for a particular choice of part value.
Finally, it would be a good idea to confirm that the original run capacitors are in good condition and actually have the stated value without excessive leakage.
RA
Motors are specified with a guaranteed torque at and very little else. There will be considerable variation in iron permeability and permanent magnet strength through material variations over the years. The better turntable manufacturers probably tested each motor batch for best compromise capacitor value.
Modelling these AC motors, the stator characteristics you can measure (L, R) but the rotor (L,R) and air-gap is difficult to measure. There's no magnets.
TD-160 (Pabst) motor pics I see V-shaped poles so torque-ripple is less of a worry. I have a couple TD-165 motors I could play with but no way to measure torque ripple.
TD-160 (Pabst) motor pics I see V-shaped poles so torque-ripple is less of a worry. I have a couple TD-165 motors I could play with but no way to measure torque ripple.
Well after much faffing about using the good old trial and error method I have arrived at what appears to be the perfect capacitor selection for my particular motor.
As mentioned earlier this is a 16 pole 115 volt synchronous Thorens motor.
I should mention it is being powered by a DIY Geddon with 100v output stepped down to 90v with a 1.5k 5W resistor.
The perfect capacitor value was 0.2uF achieved by paralleling two 0.1uF x2 caps.
This provided 90volts on the directly fed winding and 92volts on the winding being fed by the capacitor. The voltage measured across both windings is 128volts.
To confirm that these windings are 90 degrees apart it has been confirmed that the measured voltage of the individual windings should be multiplied by 1.41.
91 x 1.41 = 128. !!!!!
Close enough for me.
And other than a very faint hum from the motor it is running beautifully smooth and quiet.
Thanks for the input guys.
As mentioned earlier this is a 16 pole 115 volt synchronous Thorens motor.
I should mention it is being powered by a DIY Geddon with 100v output stepped down to 90v with a 1.5k 5W resistor.
The perfect capacitor value was 0.2uF achieved by paralleling two 0.1uF x2 caps.
This provided 90volts on the directly fed winding and 92volts on the winding being fed by the capacitor. The voltage measured across both windings is 128volts.
To confirm that these windings are 90 degrees apart it has been confirmed that the measured voltage of the individual windings should be multiplied by 1.41.
91 x 1.41 = 128. !!!!!
Close enough for me.
And other than a very faint hum from the motor it is running beautifully smooth and quiet.
Thanks for the input guys.
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