Pulley calculations

[Paul Dimaline]

I need to work out the pulley sizes for my TT project, are there any (free) online calculators to use? I did a google but didnt find what I am after, maths not being my strong point!

TIA,
Paul.

[Samo]

Hey Paul,

If this is just about the pulley sizes, so no stiffness and strength;

When you go from for instance a motor to a platter that should
turn you have to change the turning speed:
when your motor turns at 500 rpm
and your platter should only do 33 1/3 rpm
you need a conversion of 500 divided by 33 1/3 = 15
this means that any configuration where the diameter of the platter
pulley is 15 times bigger than the diameter of the motor pully will
work.

diam A / diam B = rpm B / rpm A

hope this answers your question,
Samo

[Paul Dimaline]

Thanks Samo,

I did in fact wonder if it would be as simple as that, seems it is!! Cheers.


Paul.

[AndrewT]

Hi,
if you are using a synchronous motor then it is likely to be running at half, quarter or eighth of the mains frequency. i.e.1500rpm ,750rpm or 375rpm. I think there are some that turn even slower.
There will be a tiny amount of electrical slip and a slightly larger amount of mechanical slip between mains and platter speed. This could be as large as 2% in total and will need to be allowed for to get the platter speed about right.

[Paul Dimaline]

Cheers Andrew.

I am using a Airpax motor from a Pink Triangle, runs at 250 rpm.

[EC8010]

No, there won't be any electrical slip in a synchronous motor - that's why they're called "synchronous". I suspect that you are thinking of an induction motor, which does indeed have a slip speed a little lower than a true synchronous motor. Turntables like the Garrard 301 and 401 have an induction motor with a squirrel cage rotor that has induced magnetism - if they actually ran at the full synchronous speed, there would be no induced field for the rotating field to drive. True synchronous motors use a permanent magnet rotor, so they are very firmly locked to the mains frequency. However, there is mechanical slip in the belt and I'm given to understand that this is not easily predictable (tension, temperature, cleanliness, etc, etc).

Solution? Drive the motor from a synthesised supply of slightly variable (+/- 5%?) frequency and adjust it to give the correct speed. Simple.