Re: Is this motor synchronous?/Motor Types
From: Chuck Curran[SMTP:ccurran-at-execpc-dot-com]
Sent: Thursday, January 01, 1998 4:57 PM
To: Tesla List
Subject: Re: Is this motor synchronous?/Motor Types
I thought it might help to offer some specifics on single phase motor types
and typical uses. since we make about 4600 per day, I do have enough
contact to provide certain descriptions relative to single phase motors! A
little more information might make it easier for all when trying to select a
motor for a spark gap, or a cooling fan.
Shaded Pole: This type of motor has low starting torque, low cost, low
efficiency and NO capacitors. It would typically be found running a small
fan, like in the ceiling vent in a bathroom. It would be of little use for
anything associated with Tesla applications other than running a small
PSC (Permanent Split Capacitor): These motors are one notch up the ladder
from a Shaded pole. They will always have a run capacitor in the circuit at
all times. They will have higher efficiency, higher horsepower capability,
50-60% less current draw than an equal shaded pole but limited starting
torque too. They can be used for shaft on fans for example but not a belt
drive because of the poor starting torque.
Split Phase: O.K., here we now have a motor with moderate starting torque
(100-125% of full load) high starting current, NO capacitor, but it will
have a starting centrifugal switch. The switch drops out the start winding
at about 75% of full RPM. Here you can use a belt drive due to the improved
AC Series (Universal): This type is commonly used in your electric drill or
other tools that require a high starting torque and a small size too. It
can run on either AC or DC, resulting in the Universal tag. The field
winding is connected in series with the wound armature through a pair of
carbon brushes and a commutator. You can see speeds over 30,000 RPM with
the universal motor. They are used in most vacuum cleaners sanders, any
portable tool no doubt contains a AC series motor. The means to identify
this type is to simply look for the sparks arcing off the commutator and
brushes when you run it from the 120 VAC wall plug. Or better yet, just
look for the commutator and the pair of carbon brushes.
Capacitor Start: Now we are getting closer to a real motor! This type can
be identified by seeing the presence of both a centrifugal starting switch
and a single capacitor. It is probably one of the most common motors made
in between 1/4 HP and 3 HP. Here you will have high starting torque, up to
300% of full speed torque and at the same time moderate starting current.
This is the type of motor most of us will no doubt find for spark gaps.
This motor will have a single "Dog House" mounted on it that contains the
capacitor. If I were looking for a motor I'd suggest this as a first
choice, however, it will be more expensive. Nice motor to consider for a
synchronous modification too.
Capacitor Start-Capacitor Run: Pretty much identical to the above motor but
seen at horse powers above 3 HP. The run capacitor is added and left in the
circuit at all times to improve the running efficiency.
The capacitor start, capacitor start/capacitor run and the split phase
should not be used with a auto transformer to vary the RPM. If you run it
below the centrifugal switch pull in point, you will very quickly over heat
the start winding and create a rather unpleasant cloud of smoke. That's
really why I chose a universal motor, the Carter many of us bought about 16
months ago. This type can be run at different voltages and change the RPM
One other item that may help clarify things. Many posts have mentioned the
topic of milling flats on the "armature" to create a synchronous AC motor.
Just for information, a more accurate term would be "rotor". An armature is
a associated with a DC motor and contains a winding on a stack of
laminations, while an AC motor uses a rotor which is a stack of laminations
with a series of aluminum bars and end rings cast into/onto the lamination
stack using a high pressure aluminum die caster. Call it whatever you like,
just wanted to share the industry terminology!
From: Tesla List <tesla-at-pupman-dot-com>
To: 'Tesla List' <tesla-at-pupman-dot-com>
Date: Thursday, January 01, 1998 12:22 PM
Subject: Re: Is this motor synchronous?
>Sent: Wednesday, December 31, 1997 12:56 PM
>Subject: Re: Is this motor synchronous?
>In a message dated 97-12-31 03:32:13 EST, you write:
>> My 1725 RPM motor didn't have a cap, although I'm suspecting I might have
>> been successful had I removed less material. How does one determine _by_
>> _observation_ if a motor is a capacitor start/induction run type of motor
>> vs an induction start/ induction run or capacitor start/capacitor run
>> type, short of having a spec sheet?
>> Thanks again for the help.
>> Gary Lau
>I ground my flats for the 1800 rpm motor 1 1/8" wide for a 2 7/8" dia
>armature if I remember correctly (I posted this info in the past). I got
>good results, so I suppose this ratio is OK. I have no idea if there is
>a better ratio. My motor was a 1/4 HP washing machine motor with
>no capacitor. IMO, a .65 amp motor is somewhat weak for use in
>a sync-gap. A 1/25 HP motor is probably a practical minimum unless
>the rotor is tiny.
>Cap start motors have a centrifugul switch inside, as do induction
>start motors. I think cap run motors tend to be small, less than
>1/4 HP. This is a rough guide as I'm not an expert on these various
>Hope this helps,