Motor Speed

[JannaCassandra]

I am planning to use an AC motor which salvaged from Garrard turntable for my DIY turntable. Problem is, i don't know the speed (RPM) of the motor, is there any way i can do to determine the speed?

TIA

[FastEddy]

Usually the speed of an AC motor is intimately interconnected with the frequency of your local power grid = Example: a 60 Htz power grid (as used in most of North America) can spin an AC motor at such a speed as to keep clocks running at the right time with an accuracy of +/- a few minutes per year or better.

I just got a new turntable (from Rega) and its AC motor spins at 300 RPM, quite accurately, and it is in sync with the power grid here in California. The pulleys on the motor shaft drive a rubber band belt that determines the platter speed, 33.33 RPM or 45 RPM ...

You can probably find out what your Garrard motor spins at from an owners' manual ... Find the model number / type and google it ...

[gearheaddruid]

Here is an odd way to measure speed. Put a piece of tape on the shaft with a tab sticking out. Let it hit something while the motor is running. Measure the frequency of the sound and multiply by 60 to get rpm. An audio version of a strobotach.

[Mark Kelly]

If it's out of a Garrard it's possibly not a synchronous motor, the more famous Garrards used shaded pole motors which run about 10% slower than synchronous speed.

An easy method is to take a small DC motor with a known speed / voltage characteristic and couple it to the AC motor shaft. Run the AC motor and measure the open circuit voltage at the terminals of the DC motor. Multiply by the speed / voltage factor (in RPM per volt) and you have the AC motor speed.

This works because any DC motor also acts as a generator. The manufacturer of any DC motor can tell you the speed / voltage factor for the motor.

[Nanook]

er find out the turntable it actually came from, and the platter size where the idler wheel/belt rides

at least you would know the speed assuming correct operation of the motor. The "record the sound approach would seem yo be a good idea if you have a mic and an imput into an audio program

[kevinkr]

Most of the cheaper Garrard's did have 4 pole synchronous motors, and IIRC these ran at 1800rpm.

2 pole synchronous motors run at 3600rpm. (Cheap ceramic cartridge tables only, like many low end/console changers by BSR)

12 pole motors run at 600rpm, 24 pole motors at 300rpm.

2 pole motors don't have phase shift (starting) caps, 4 pole motors generally don't either, 12 and 24 pole motors typically do.

All above values for 60Hz, multiply these numbers by 5/6 to get the 50Hz speeds.

[JannaCassandra]

Thank you for all your inputs.

Any other method?