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Home > Archive > Electrical Engineering > July 2005 > Starting single phase motors
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Starting single phase motors
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| JohnR66 2005-07-02, 12:25 pm |
| I often see two types of motors used in heavy appliances. One with a
capacitor and one with the starting relay. The relay has a very thick
winding connected in series with the main winding that holds the phase
winding in the circuit until the motor comes up to speed and the phase
winding is switched out of the circuit. (In the application with the relay,
I'm not sure if the term *phase winding* is corect.)
I believe the capacitor types are called PSC (permanent split capacitor). In
either case I see a field wound with finer wire that is offset (angular
displacement) from the main field coil. This phase winding is used to
produce starting torque. In either case, removing the capacitor or starting
relay, the motor will hum and not spin (draws heavy current and can
overheat). Manually spinning the shaft in either direction gets it started
in the direction turned.
On PSC motors, removing the capacitor while it is running does not change
the speed, but current draw is increased by several percent.
My question is: Can the motor that starts with the current relay be adapted
to use a capacitor or is the angular displacement if the phase/starting
winding different from the PSC motors?
Thanks, John
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| BFoelsch 2005-07-02, 12:25 pm |
|
"JohnR66" <nospam@att.net> wrote in message
news:Rxwxe.379905$cg1.294720@bgtnsc04-news.ops.worldnet.att.net...
quote:
> I often see two types of motors used in heavy appliances. One with a
> capacitor and one with the starting relay. The relay has a very thick
> winding connected in series with the main winding that holds the phase
> winding in the circuit until the motor comes up to speed and the phase
> winding is switched out of the circuit. (In the application with the
relay,
quote:
> I'm not sure if the term *phase winding* is corect.)
> I believe the capacitor types are called PSC (permanent split capacitor).
In
quote:
> either case I see a field wound with finer wire that is offset (angular
> displacement) from the main field coil. This phase winding is used to
> produce starting torque. In either case, removing the capacitor or
starting
quote:
> relay, the motor will hum and not spin (draws heavy current and can
> overheat). Manually spinning the shaft in either direction gets it started
> in the direction turned.
>
> On PSC motors, removing the capacitor while it is running does not change
> the speed, but current draw is increased by several percent.
>
> My question is: Can the motor that starts with the current relay be
adapted
quote:
> to use a capacitor or is the angular displacement if the phase/starting
> winding different from the PSC motors?
>
> Thanks, John
It can be done, sort-of...
You have outlined two types of single phase motor starting. Both of these
techniques are suitable for light-duty starts. The fundamental problem with
changing a split-phase to a PSC is that the starting winding in the
split-phase is not designed for continuous duty. It is intentionally wound
with small wire to achieve a high resistance. This high resistance causes a
lot of heating, but only for the few seconds when it is actually used.You
could select a capacitor small enough that it permits only a small current
to flow, one which would not overheat the winding, but the starting torque
would be lower than intended. The extra phase shift due to the introduction
of the capacitor will tend to increase the starting torque somewhat. It may
be satisfactory for your application.
What are you trying to do? Eliminate the relay? What is your application? If
you don't need high starting torque you can probably pull it off.
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| Bob Eldred 2005-07-02, 11:25 pm |
|
"JohnR66" <nospam@att.net> wrote in message
news:Rxwxe.379905$cg1.294720@bgtnsc04-news.ops.worldnet.att.net...
quote:
> I often see two types of motors used in heavy appliances. One with a
> capacitor and one with the starting relay. The relay has a very thick
> winding connected in series with the main winding that holds the phase
> winding in the circuit until the motor comes up to speed and the phase
> winding is switched out of the circuit. (In the application with the
relay,
quote:
> I'm not sure if the term *phase winding* is corect.)
> I believe the capacitor types are called PSC (permanent split capacitor).
In
quote:
> either case I see a field wound with finer wire that is offset (angular
> displacement) from the main field coil. This phase winding is used to
> produce starting torque. In either case, removing the capacitor or
starting
quote:
> relay, the motor will hum and not spin (draws heavy current and can
> overheat). Manually spinning the shaft in either direction gets it started
> in the direction turned.
>
> On PSC motors, removing the capacitor while it is running does not change
> the speed, but current draw is increased by several percent.
>
> My question is: Can the motor that starts with the current relay be
adapted
quote:
> to use a capacitor or is the angular displacement if the phase/starting
> winding different from the PSC motors?
>
> Thanks, John
Motors that do not use a capacitor are usually termed "split phase" and get
the reqired phase shift for rotation from inductance and resistance instead
of capacitance. Placing a capacitace in the inductance circuit (start
winding) would defeat the original design of the motor and might even cancel
the starting torque if the capacitace was a certain value. However, there
would be a certain vector sum of capacitace, inductance and resistance that
could allow the motor to operate. One would have to do calculations and
experiments on a specific motor to determine this operating point. It may be
possible to get a higher running torque without excessive overheating by
doing this. Is that your goal? On the other hand it might burn up a given
motor if the dissipation and heating are not considered. In otherwords, it
takes some measurements and mathematics to answer your question.
Bob
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| JohnR66 2005-07-05, 11:25 pm |
| "BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
news:Nu6dnbxneOSCAVvfRVn-oA@giganews.com...
quote:
>
> "JohnR66" <nospam@att.net> wrote in message
> news:Rxwxe.379905$cg1.294720@bgtnsc04-news.ops.worldnet.att.net...
> relay,
> In
> starting
> adapted
>
> It can be done, sort-of...
>
> You have outlined two types of single phase motor starting. Both of these
> techniques are suitable for light-duty starts. The fundamental problem
> with
> changing a split-phase to a PSC is that the starting winding in the
> split-phase is not designed for continuous duty. It is intentionally wound
> with small wire to achieve a high resistance. This high resistance causes
> a
> lot of heating, but only for the few seconds when it is actually used.You
> could select a capacitor small enough that it permits only a small current
> to flow, one which would not overheat the winding, but the starting torque
> would be lower than intended. The extra phase shift due to the
> introduction
> of the capacitor will tend to increase the starting torque somewhat. It
> may
> be satisfactory for your application.
>
> What are you trying to do? Eliminate the relay? What is your application?
> If
> you don't need high starting torque you can probably pull it off.
>
>
This motor is scavenged from an old washing machine. I was experimenting
with it - taking stalled and no load current measurements. I was curious to
see what a capacitor would do for starting (haven't tried yet). Starting, it
draws around 20 amps, running it draws 5. AFAICT, the starting windings
looks to be at wound 45 Deg from the main windings.
John
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| Don Kelly 2005-07-06, 4:25 am |
| "JohnR66" <nospam@att.net> wrote in message
news:VpCye.390493$cg1.72611@bgtnsc04-news.ops.worldnet.att.net...
quote:
> "BFoelsch" <BFoelsch@comcast.ditch.this.net> wrote in message
> news:Nu6dnbxneOSCAVvfRVn-oA@giganews.com...
capacitor).[vbcol=seagreen]
change[vbcol=seagreen]
these[vbcol=seagreen]
wound[vbcol=seagreen]
causes[vbcol=seagreen]
used.You[vbcol=seagreen]
current[vbcol=seagreen]
torque[vbcol=seagreen]
application?[vbcol=seagreen]
> This motor is scavenged from an old washing machine. I was experimenting
> with it - taking stalled and no load current measurements. I was curious
to
quote:
> see what a capacitor would do for starting (haven't tried yet). Starting,
it
quote:
> draws around 20 amps, running it draws 5. AFAICT, the starting windings
> looks to be at wound 45 Deg from the main windings.
> John
>
--------------------
The winding of the motor with the starter are designed for that use. The
windings of a split capacitor motor are also designed for that use. There
are motors with a single capacitor and a centrifugal switch (this appears to
be your starting relay) and again they are designed for the purpose. While
it is possible to use a capacitor, you still might not be able to eliminate
the switch.
Bob Eldred is right on.
--
Don Kelly
dhky@peeshaw.ca
remove the urine to answer
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