RE: Starting a Teletype sync motor question

• To: tesla-at-pupman-dot-com
• Subject: RE: Starting a Teletype sync motor question
• From: Gary Lau 02-Jun-1999 0624 <lau-at-hdecad.ENET.dec-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
• Date: Wed, 02 Jun 1999 20:49:03 -0600
• Approved: twftesla-at-uswest-dot-net
• Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

>Original Poster: FutureT-at-aol-dot-com 
>
>I'm using a 50Hz, 3000rpm 1/12th HP Teletype sync motor on 60Hz,
>for 3600rpm in a rotary sync gap.  I tried connecting a 7uF to 30uF,
>cap across the blue and red wire for starting, but it won't start this
>way.  Yellow and blue are connected to the line.  I then touched the
>red and blue wire together with no cap for starting, this works better,
>but the motor still often needs a push to get going, and it makes
>clanking noises as the speed ramps up.  Once it reaches max
>speed, I disconnect the red wire, then it locks up fine.  I've been 
>using this rather awkward method for starting it.
>
>I realize that Teletype motors usually use an external relay block
>for starting, but different motors seem to be wired a little differently?
>
>I wonder if the 60Hz on a 50Hz motor makes the starting a little
>more difficult?
>
>Does anyone have any suggestions or knowledge of the correct
>way to start this type of motor?  Or maybe the motor has an internal
>defect?
>
>Thanks,
>John Freau

Hi John:

My teletype motor uses a cap marked "88-108 MFD, 125VAC 60 cycles", a
curious rating.  It measures 137 uF on my c-meter.  It seems to be very
lightweight for it's capacitance compared with other motor starting caps
I've seen, perhaps because it's only rated for very low duty cycles.
Maybe the cap you're using is too small?

Regards, Gary Lau
Waltham, MA USA

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