Class D amplifier as a VFD..?

[handcut]

Hello,


I realise this is a slightly bizarre question and not strictly audio-related, but bear with me...


I'm running two disc recording lathes from the 40's. They're rim driven by 1/8th HP single-phase motors at 1800rpm, 115Vac, 50Hz (I'm the UK, using a step-down tx). No matter what I do, I can't seem to get the speed bang-on for some reason, so I've been thinking about using a VFD to dial it in. Someone suggested using a high-power class D amplifier with a function generator at the input as a cheap alternative. I know nothing about class D amps and I'm assuming that a motor will be significantly higher impedance than the 2, 4 or 8 ohm the amplifier was designed to operate with, so I guess my question is: what potental problems do people see with this set-up??

Thanks in advance...

 

[mterbekke]

Is that motor and drive system properly maintaned, lubricated etc? I`m taking the pulley is of the right diameter and you actually get to 115 VAC RMS at the motor? Are there any capacitors and resistors used and are those checked?

I`m not seeing any particular problems in driving it with a Class D amp, and to be safe and steer clear of strange load currents you could put a resistor in series (e.g. a few ohms, 1 or 2).

What I`m not really sure of is the voltage you need to get out of the amp.

115 VAC into 8 ohm would mean you`re looking for a 1650 Watt amp, and since many Class D amps get advertised with their power rating at a load of 4 ohm, you`re even looking for a 3300 Watt class D amp.

If the advertised amp isn`t bridged already, you`d have no problem finding a suitable amp, but I`m afraid many are..

1940`s I think I saw them once, they`re really big and heavy machines. Would be great if you could get them back alive 🙂

 

[handcut]

Hello mterbekke,

Thanks for your reply. I got a couple of those specs wrong (from memory), here are the correct ones:

Volts: 115
Current: 1A
HP:1/25
Frequency: 50/60Hz
RPM: 1500-1800



Voltage across the terminals of the main winding is 114Vac. There is a 3.75uF run cap that tests good.



The wow & flutter is actually very low, just the rpm is a little off - 997Hz instead of 1kHz. Maybe 0.3% is within spec for a 1940's rim drive turntable, but I would still like to get it perfect if I can. .

 

[drMordor]

I do this for my friend.
50Hz generator - TPA3255 amp in PBTL - 100w transformer.

 

[Mello4]

Are you using 60Hz 115VAC induction motor at 50Hz?

60Hz motor running on 50Hz power supply or vice versa

What Determines the Rotating Speed of a Motor?

 

[mterbekke]

I do this for my friend.
50Hz generator - TPA3255 amp in PBTL - 100w transformer.

Hi DrMordor, in the datasheet it says the maximum power supply voltage is 51 volts, bridging it is equivalent to 102 volts, a mere 72 VAC. I don`t think this is sufficient, or do you have a clever trick up your sleeve 🙂

 

[arg_]

I have a similar requirement for a turntable motor and I consider the possibility to get an off-the-shelf VFD, such as this, as a more compact and practical solution

 

[mterbekke]

Looks great!

 

[handcut]

Are you using 60Hz 115VAC induction motor at 50Hz?

No, it's a 50Hz 115VAC sychronous motor

I have a similar requirement for a turntable motor and I consider the possibility to get an off-the-shelf VFD, such as this, as a more compact and practical solution

Maybe, but I have a lot of function generators and class-d amplifiers are very cheap.

I do this for my friend.
50Hz generator - TPA3255 amp in PBTL - 100w transformer.

Nice!

 

[drMordor]

Hi DrMordor, in the datasheet it says the maximum power supply voltage is 51 volts, bridging it is equivalent to 102 volts, a mere 72 VAC. I don`t think this is sufficient, or do you have a clever trick up your sleeve 🙂

The transformer is normal))) is connected by the secondary winding to the output of the amplifier, therefore, on the primary winding of 230 volts, the frequency and voltage can be adjusted.