A.C. Compressor (trivia)

[travis martin]

Arch, I just had another thought (!!!)

If the rear evaporator (s) are using expansion valves, which is the case if
we use the units from suburbans etc, then we need the compressor to run all
the time. This will mean installing an expansion valve and POA valve on the
front evaporator. Or do we nead a POA? Mark?

But it means we don't have to worry about cycling the compressor and so a
clutch on the electric motor is back to being unnecessary.

Or that's the current state of my cortexical flatulence.

Ideas?

Travis
- -----Original Message-----
From: Gcbr
To: gmcmotorhome
Date: Friday, April 23, 1999 9:57 PM
Subject: Re: GMC: A.C. Compressor (trivia)

>Travis
>
>If you had an AC clutch on the electric motor you would not have
>the problem with start up of the armature----all you would be starting
>is the outer clutch. Just Theory.
>
>Take Care
>Arch
>
>In a message dated 4/23/99 9:43:50 PM Central Daylight Time,
travism
>writes:
>
>> off) type, then the compressor clutch can be expected to engage at speed.
>> This is going to be hell on clutches and belts if in addition to the
>> compressor the clutch is having to spin up the armature of the electric
>> motor. There isn't any need for an electric clutch for the motor, but a
>> sprag would be great.
>>
>> Travis
>>
>
>

 

[travis martin]

Hi, Arch,

I understand your concern, but I don't think that _in this particular
instance_ it is valid....only because the rated speed of the motor has
nothing at all to do with its physical limitations; it is only the
relationship between the number of poles and line frequency. The same motor
could have been built to run at 3600.

With a gasoline engine, higher rpm means more horsepower. The limit becomes
the physical capability of the components to stay together. That's why my
rich friend's 12 cylinder ferrari makes 440 horsepower on 5 liters of
displacement...at 8500 rpm.

With an electric motor, maximum torque is at zero rpm. There are no
reciprocating parts, and the only moving part is a solid armature. It would
likely handle 20,000 RPM indefinitely if you asked it to; I can't think of
any reason why it shouldn't. Like I say, the nameplate RPM on the motor is
not a design limitation...it's just the speed that will occur at 60 cps line
frequency. Actually, the theoretical speed is 1800 rpm...the 1725 is a
nominal speed factoring in some normal slip. In actual practice, it will
vary with the load on the motor.

Hope you understand that I'm not trying to quarrel; I just think your
concern is not warranted in this one area. I, too, would want to err on the
side of reliability, but don't think that this is even a minor issue here.
Ordinarilly, I'd be concerned about turning something several times its
normal speed, but I believe that the only limiting factor on the induction
motor will be its bearings.

Also, per your earlier observation, we should aim for a higer speed on the
compressor from the motor, so instead of a 1:1 ration from the motor to the
compressor we'll likely need something like 1:1.5
This will have the side benefit of alleviating some of your concern, since
overdriving the compressor will mean underdriving the electric motor when
the 455 is running the compressor.

I really think this idea needs pursuing. This would be a genuine improvement
on the design of the coach.

Travis

- -----Original Message-----
From: Gcbr
To: gmcmotorhome
Date: Friday, April 23, 1999 10:50 PM
Subject: Re: GMC: A.C. Compressor (trivia)

>Travis
>
>I am not worried about cycling the compressor. I am worried about driving
>an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
>It aint cool! bronze bearings are not designed for that. Even if it has
ball
>bearing you are so far over design standards that all hell could break out
at
>any time. If you had an electric clutch then it would not be turning when
the
>455 is driving it. I am going out of bounds here but here I go. You can
push
>the limits---------yes you can. You cant take a motor that was designed to
run
>at 1725 and run it at 8000 rpm. I will promise you something will fail that
>far out of limits. Yes, I know I am sounding like Wes again. Sorry but not
>sorry.
>
>Take Care
>Arch
>
>In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

>
>>
>> But it means we don't have to worry about cycling the compressor and so
a
>> clutch on the electric motor is back to being unnecessary.
>>
>> Or that's the current state of my cortexical flatulence.
>>
>> Ideas?
>>
>
>

 

[mark grady]

I know almost nothing about this topic, so I'll ask these questions:

Would the quality of the balance for an armature that normally
operates at 1750 (or even 3600) be adequate at 8000?

[It's not real, real close, but I can tell you that you can't start a gas
turbine engine with a hydraulic motor.]

Would the centrifugal weights on the start switch inside the motor withstand
the increased speed (assuming a single phase motor here) without self
destructing?

Would the shading (which provides reluctance) in the armature affect its
ability to withstand the increased centrifugal forces of 4.5 to 2.6 times
over speed?

Since we need a fairly high HP motor to drive the compressor, (2HP ?) does
the increased mass of this size motor pose a safety risk if it self
destructs?

As an alternative, how about just driving another A-6 with the electric
motor?
They're cheap, and the oil distribution problem is solved since they have
identical
characteristics. The internals (reed valves, etc.) would easily withstand
the pressure.

You also get to size your drive and driven pulleys to their optimum speed.
There are different A-6 pulleys, with different weight inertia rings.

Just thinking out loud.

Mark

> -----Original Message-----
> From: owner-gmcmotorhome
> [mailtowner-gmcmotorhome]On Behalf Of Travis Martin
> Sent: Friday, April 23, 1999 11:09 PM
> To: gmcmotorhome
> Subject: Re: GMC: A.C. Compressor (trivia)
>
>
> Hi, Arch,
>
> I understand your concern, but I don't think that _in this particular
> instance_ it is valid....only because the rated speed of the motor has
> nothing at all to do with its physical limitations; it is only the
> relationship between the number of poles and line frequency. The
> same motor
> could have been built to run at 3600.
>
> With a gasoline engine, higher rpm means more horsepower. The
> limit becomes
> the physical capability of the components to stay together. That's why my
> rich friend's 12 cylinder ferrari makes 440 horsepower on 5 liters of
> displacement...at 8500 rpm.
>
> With an electric motor, maximum torque is at zero rpm. There are no
> reciprocating parts, and the only moving part is a solid
> armature. It would
> likely handle 20,000 RPM indefinitely if you asked it to; I can't think of
> any reason why it shouldn't. Like I say, the nameplate RPM on the motor is
> not a design limitation...it's just the speed that will occur at
> 60 cps line
> frequency. Actually, the theoretical speed is 1800 rpm...the 1725 is a
> nominal speed factoring in some normal slip. In actual practice, it will
> vary with the load on the motor.
>
> Hope you understand that I'm not trying to quarrel; I just think your
> concern is not warranted in this one area. I, too, would want to
> err on the
> side of reliability, but don't think that this is even a minor issue here.
> Ordinarilly, I'd be concerned about turning something several times its
> normal speed, but I believe that the only limiting factor on the induction
> motor will be its bearings.
>
> Also, per your earlier observation, we should aim for a higer speed on the
> compressor from the motor, so instead of a 1:1 ration from the
> motor to the
> compressor we'll likely need something like 1:1.5
> This will have the side benefit of alleviating some of your concern, since
> overdriving the compressor will mean underdriving the electric motor when
> the 455 is running the compressor.
>
> I really think this idea needs pursuing. This would be a genuine
> improvement
> on the design of the coach.
>
> Travis
>
> -----Original Message-----
> From: Gcbr
> To: gmcmotorhome
> Date: Friday, April 23, 1999 10:50 PM
> Subject: Re: GMC: A.C. Compressor (trivia)
>
>
> >Travis
> >
> >I am not worried about cycling the compressor. I am worried about driving
> >an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
> >It aint cool! bronze bearings are not designed for that. Even if it has
> ball
> >bearing you are so far over design standards that all hell could
> break out
> at
> >any time. If you had an electric clutch then it would not be turning when
> the
> >455 is driving it. I am going out of bounds here but here I go. You can
> push
> >the limits---------yes you can. You cant take a motor that was
> designed to
> run
> >at 1725 and run it at 8000 rpm. I will promise you something
> will fail that
> >far out of limits. Yes, I know I am sounding like Wes again.
> Sorry but not
> >sorry.
> >
> >Take Care
> >Arch
> >
> >In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

> >
> >>
> >> But it means we don't have to worry about cycling the
> compressor and so
> a
> >> clutch on the electric motor is back to being unnecessary.
> >>
> >> Or that's the current state of my cortexical flatulence.
> >>
> >> Ideas?
> >>
> >
> >
>

 

[richard bachert]

>Arch, I just had another thought (!!!)
(SNIP)
>Or that's the current state of my cortexical flatulence.
Travis: "cortexical flatulance?" Is that like a mind fart?
Dick in Atlanta ROFLAO

>Ideas?
>
>Travis
>-----Original Message-----
>From: Gcbr
>To: gmcmotorhome
>Date: Friday, April 23, 1999 9:57 PM
>Subject: Re: GMC: A.C. Compressor (trivia)
>
>
>>Travis
>>
>>If you had an AC clutch on the electric motor you would not have
>>the problem with start up of the armature----all you would be starting
>>is the outer clutch. Just Theory.
>>
>>Take Care
>>Arch
>>
>>In a message dated 4/23/99 9:43:50 PM Central Daylight Time,
>travism
>>writes:
>>
>>> off) type, then the compressor clutch can be expected to engage at speed.
>>> This is going to be hell on clutches and belts if in addition to the
>>> compressor the clutch is having to spin up the armature of the electric
>>> motor. There isn't any need for an electric clutch for the motor, but a
>>> sprag would be great.
>>>
>>> Travis
>>>
>>
>>
>
>

 

[richard bachert]

Parts of that motor COULD replicate a fragmentation grenade. There's
usually a throw-out weight actuated centrifugal starting switch in those 120
V, single phase motors. The weights will probably beat you home from your
first high speed shake-down run. If you plan to run the thing at 6 or 7
times rated speed, build a scatter shield around it. But make it easy to
remove: My guess is that you'll have to take it off very soon after
installing it.
Dick 75 PB in Atlanta

>Travis
>
>I am not worried about cycling the compressor. I am worried about driving
>an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
>It aint cool! bronze bearings are not designed for that. Even if it has ball
>bearing you are so far over design standards that all hell could break out at
>any time. If you had an electric clutch then it would not be turning when the
>455 is driving it. I am going out of bounds here but here I go. You can push
>the limits---------yes you can. You cant take a motor that was designed to run
>at 1725 and run it at 8000 rpm. I will promise you something will fail that
>far out of limits. Yes, I know I am sounding like Wes again. Sorry but not
>sorry.
>
>Take Care
>Arch
>
>In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

>
>>
>> But it means we don't have to worry about cycling the compressor and so a
>> clutch on the electric motor is back to being unnecessary.
>>
>> Or that's the current state of my cortexical flatulence.
>>
>> Ideas?
>>
>
>

 

[travis martin]

Uncle!

My gut feel is that the shading, since it more or less fills its space and
has no room to move would not be a problem, and the weights on a single
phase motor, assuming we used this starting method instead of an external
switch like hermetics use, could probably be accommodated, though I'd be
more comfortable not using the centrifigal switch.......but the upshot of it
is that this is really not necessary. A second compressor is probably the
superior way to go about it.

I believe that with careful choice of pulley sizes that the electric motor
driving the single compressor concept could be made to work without problem,
But I also think that the hermetic, if we can solve the oiling problem, is
far superior, if only because it will be quieter, easier to mount, and more
reliable. An open motor driving a second A-6 would possibly be an acceptable
compromise, but I don't see it as the ultimate solution as it will require
more room, a less reliable drive system, more complicated mounting, etc. But
even with all that, if that were the only way this could be done, it would
still be a big improvement over the roof units.

So keep thinking and brainstorming. I've gotten to know Arch well enough by
reading his posts for a few months to think he will agree that all this
second guessing and trying to anticipate the problems before they occur will
improve the end result. I know I'd rather have the pitfalls pointed out
before starting the voyage. But it is still worthwhile to attempt.

Possibly the best bet is to see if one of the group can "breadboard" this
project without making modifications to anyone's coach until the basics can
be settled. Any ideas on this? Might be easier to just try it on a coach; I
don't know. We have a volunteer "guinea pig" at least.

The pioneers are the guys with the arrows in their backs, but they're also
the winners a lot of the times too.

Like I said, keep thinking...this one is worth pursuing.

Travis

- -----Original Message-----
From: Mark Grady
To: gmcmotorhome
Date: Friday, April 23, 1999 11:46 PM
Subject: RE: GMC: A.C. Compressor (trivia)

>I know almost nothing about this topic, so I'll ask these questions:
>
>Would the quality of the balance for an armature that normally
>operates at 1750 (or even 3600) be adequate at 8000?
>
>[It's not real, real close, but I can tell you that you can't start a gas
>turbine engine with a hydraulic motor.]
>
>Would the centrifugal weights on the start switch inside the motor
withstand
>the increased speed (assuming a single phase motor here) without self
>destructing?
>
>Would the shading (which provides reluctance) in the armature affect its
>ability to withstand the increased centrifugal forces of 4.5 to 2.6 times
>over speed?
>
>Since we need a fairly high HP motor to drive the compressor, (2HP ?) does
>the increased mass of this size motor pose a safety risk if it self
>destructs?
>
>As an alternative, how about just driving another A-6 with the electric
>motor?
>They're cheap, and the oil distribution problem is solved since they have
>identical
>characteristics. The internals (reed valves, etc.) would easily withstand
>the pressure.
>
>You also get to size your drive and driven pulleys to their optimum speed.
>There are different A-6 pulleys, with different weight inertia rings.
>
>Just thinking out loud.
>
>Mark
>
>
>
>> -----Original Message-----
>> From: owner-gmcmotorhome
>> [mailtowner-gmcmotorhome]On Behalf Of Travis Martin
>> Sent: Friday, April 23, 1999 11:09 PM
>> To: gmcmotorhome
>> Subject: Re: GMC: A.C. Compressor (trivia)
>>
>>
>> Hi, Arch,
>>
>> I understand your concern, but I don't think that _in this particular
>> instance_ it is valid....only because the rated speed of the motor has
>> nothing at all to do with its physical limitations; it is only the
>> relationship between the number of poles and line frequency. The
>> same motor
>> could have been built to run at 3600.
>>
>> With a gasoline engine, higher rpm means more horsepower. The
>> limit becomes
>> the physical capability of the components to stay together. That's why my
>> rich friend's 12 cylinder ferrari makes 440 horsepower on 5 liters of
>> displacement...at 8500 rpm.
>>
>> With an electric motor, maximum torque is at zero rpm. There are no
>> reciprocating parts, and the only moving part is a solid
>> armature. It would
>> likely handle 20,000 RPM indefinitely if you asked it to; I can't think
of
>> any reason why it shouldn't. Like I say, the nameplate RPM on the motor
is
>> not a design limitation...it's just the speed that will occur at
>> 60 cps line
>> frequency. Actually, the theoretical speed is 1800 rpm...the 1725 is a
>> nominal speed factoring in some normal slip. In actual practice, it will
>> vary with the load on the motor.
>>
>> Hope you understand that I'm not trying to quarrel; I just think your
>> concern is not warranted in this one area. I, too, would want to
>> err on the
>> side of reliability, but don't think that this is even a minor issue
here.
>> Ordinarilly, I'd be concerned about turning something several times its
>> normal speed, but I believe that the only limiting factor on the
induction
>> motor will be its bearings.
>>
>> Also, per your earlier observation, we should aim for a higer speed on
the
>> compressor from the motor, so instead of a 1:1 ration from the
>> motor to the
>> compressor we'll likely need something like 1:1.5
>> This will have the side benefit of alleviating some of your concern,
since
>> overdriving the compressor will mean underdriving the electric motor when
>> the 455 is running the compressor.
>>
>> I really think this idea needs pursuing. This would be a genuine
>> improvement
>> on the design of the coach.
>>
>> Travis
>>
>> -----Original Message-----
>> From: Gcbr
>> To: gmcmotorhome
>> Date: Friday, April 23, 1999 10:50 PM
>> Subject: Re: GMC: A.C. Compressor (trivia)
>>
>>
>> >Travis
>> >
>> >I am not worried about cycling the compressor. I am worried about
driving
>> >an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
>> >It aint cool! bronze bearings are not designed for that. Even if it has
>> ball
>> >bearing you are so far over design standards that all hell could
>> break out
>> at
>> >any time. If you had an electric clutch then it would not be turning
when
>> the
>> >455 is driving it. I am going out of bounds here but here I go. You can
>> push
>> >the limits---------yes you can. You cant take a motor that was
>> designed to
>> run
>> >at 1725 and run it at 8000 rpm. I will promise you something
>> will fail that
>> >far out of limits. Yes, I know I am sounding like Wes again.
>> Sorry but not
>> >sorry.
>> >
>> >Take Care
>> >Arch
>> >
>> >In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

>> >
>> >>
>> >> But it means we don't have to worry about cycling the
>> compressor and so
>> a
>> >> clutch on the electric motor is back to being unnecessary.
>> >>
>> >> Or that's the current state of my cortexical flatulence.
>> >>
>> >> Ideas?
>> >>
>> >
>> >
>>
>

 

[thomas g. warner]

Some of your assumptions are not valid for instance "It would likely handle
20,000 RPM indefinitely if you asked it to". The use and maximum RPM of the
motor is constrained by the method of winding, the material used, ie;
aluminum of copper, diameter and weight of the components, and the binding
material of the wires. Motors will indeed self construct if the design
limitations are exceeded. I can not imagine any sizable electric motor that
will not at 20,000RPMs.

>Hi, Arch,
>
>I understand your concern, but I don't think that _in this particular
>instance_ it is valid....only because the rated speed of the motor has
>nothing at all to do with its physical limitations; it is only the
>relationship between the number of poles and line frequency. The same motor
>could have been built to run at 3600.
>
>With a gasoline engine, higher rpm means more horsepower. The limit becomes
>the physical capability of the components to stay together. That's why my
>rich friend's 12 cylinder ferrari makes 440 horsepower on 5 liters of
>displacement...at 8500 rpm.
>
>With an electric motor, maximum torque is at zero rpm. There are no
>reciprocating parts, and the only moving part is a solid armature. It would
>likely handle 20,000 RPM indefinitely if you asked it to; I can't think of
>any reason why it shouldn't. Like I say, the nameplate RPM on the motor is
>not a design limitation...it's just the speed that will occur at 60 cps line
>frequency. Actually, the theoretical speed is 1800 rpm...the 1725 is a
>nominal speed factoring in some normal slip. In actual practice, it will
>vary with the load on the motor.
>
>Hope you understand that I'm not trying to quarrel; I just think your
>concern is not warranted in this one area. I, too, would want to err on the
>side of reliability, but don't think that this is even a minor issue here.
>Ordinarilly, I'd be concerned about turning something several times its
>normal speed, but I believe that the only limiting factor on the induction
>motor will be its bearings.
>
>Also, per your earlier observation, we should aim for a higer speed on the
>compressor from the motor, so instead of a 1:1 ration from the motor to the
>compressor we'll likely need something like 1:1.5
>This will have the side benefit of alleviating some of your concern, since
>overdriving the compressor will mean underdriving the electric motor when
>the 455 is running the compressor.
>
>I really think this idea needs pursuing. This would be a genuine improvement
>on the design of the coach.
>
>Travis
>
>-----Original Message-----
>From: Gcbr
>To: gmcmotorhome
>Date: Friday, April 23, 1999 10:50 PM
>Subject: Re: GMC: A.C. Compressor (trivia)
>
>
>>Travis
>>
>>I am not worried about cycling the compressor. I am worried about driving
>>an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
>>It aint cool! bronze bearings are not designed for that. Even if it has
>ball
>>bearing you are so far over design standards that all hell could break out
>at
>>any time. If you had an electric clutch then it would not be turning when
>the
>>455 is driving it. I am going out of bounds here but here I go. You can
>push
>>the limits---------yes you can. You cant take a motor that was designed to
>run
>>at 1725 and run it at 8000 rpm. I will promise you something will fail that
>>far out of limits. Yes, I know I am sounding like Wes again. Sorry but not
>>sorry.
>>
>>Take Care
>>Arch
>>
>>In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

>>
>>>
>>> But it means we don't have to worry about cycling the compressor and so
>a
>>> clutch on the electric motor is back to being unnecessary.
>>>
>>> Or that's the current state of my cortexical flatulence.
>>>
>>> Ideas?
>>>
>>
>>
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach

 

[travis martin]

Yes, of course you're right--anything will fail at some speed. My point was
that the nameplate speed of an electric motor has absolutely nothing to do
with its design limitations--it can't exceed 1800 RPM in its usual intended
role. It is, of course, reasonable to assume that its designers, knowing its
intended speed, would not have wasted more expensive materials or
construction methods to withstand speeds significantly greater than that
which it will be running when hooked to a 60 cycle power line. The 1725 (or
1800) RPM number is neither a maximum nor a minimum speed; it is just the
speed at which it will run if supplied with the correct power.

The reason I threw the 20,000 number out is that the particular motor we are
talking about is an induction motor with a squirrel cage shaded pole
armature. It doesn't have a commutator nor does it have windings in the
usual sense. There are shading windings that are pressed into the armature.
The armature is about 3 inches in diameter. I haven't tried it and likely
won't since it's purely academic, but I really do think that it would handle
20,000 RPM; why shouldn't it? It is very nearly a solid mass of relatively
small diameter.

Keep in mind that automotive alternators and generators are normally
overdriven about 4:1 and survive speeds well in excess of 20,000. I well
recall generators in the good old hot rod days failing from overwinding...in
almost all cases, the failure was at the commutator--usually slinging the
solder out.

I feel like I'm arguing just to be arguing and don't want to alienate any of
you, so I'll fall off on the subject. As I've said, it's purely speculation.

Very warmest regards,

Travis

- -----Original Message-----
From: Thomas G. Warner
To: gmcmotorhome
Date: Saturday, April 24, 1999 8:22 AM
Subject: Re: GMC: A.C. Compressor (trivia)

>Some of your assumptions are not valid for instance "It would likely handle
>20,000 RPM indefinitely if you asked it to". The use and maximum RPM of
the
>motor is constrained by the method of winding, the material used, ie;
>aluminum of copper, diameter and weight of the components, and the binding
>material of the wires. Motors will indeed self construct if the design
>limitations are exceeded. I can not imagine any sizable electric motor
that
>will not at 20,000RPMs.
>
>

>>Hi, Arch,
>>
>>I understand your concern, but I don't think that _in this particular
>>instance_ it is valid....only because the rated speed of the motor has
>>nothing at all to do with its physical limitations; it is only the
>>relationship between the number of poles and line frequency. The same
motor
>>could have been built to run at 3600.
>>
>>With a gasoline engine, higher rpm means more horsepower. The limit
becomes
>>the physical capability of the components to stay together. That's why my
>>rich friend's 12 cylinder ferrari makes 440 horsepower on 5 liters of
>>displacement...at 8500 rpm.
>>
>>With an electric motor, maximum torque is at zero rpm. There are no
>>reciprocating parts, and the only moving part is a solid armature. It
would
>>likely handle 20,000 RPM indefinitely if you asked it to; I can't think of
>>any reason why it shouldn't. Like I say, the nameplate RPM on the motor is
>>not a design limitation...it's just the speed that will occur at 60 cps
line
>>frequency. Actually, the theoretical speed is 1800 rpm...the 1725 is a
>>nominal speed factoring in some normal slip. In actual practice, it will
>>vary with the load on the motor.
>>
>>Hope you understand that I'm not trying to quarrel; I just think your
>>concern is not warranted in this one area. I, too, would want to err on
the
>>side of reliability, but don't think that this is even a minor issue here.
>>Ordinarilly, I'd be concerned about turning something several times its
>>normal speed, but I believe that the only limiting factor on the induction
>>motor will be its bearings.
>>
>>Also, per your earlier observation, we should aim for a higer speed on the
>>compressor from the motor, so instead of a 1:1 ration from the motor to
the
>>compressor we'll likely need something like 1:1.5
>>This will have the side benefit of alleviating some of your concern, since
>>overdriving the compressor will mean underdriving the electric motor when
>>the 455 is running the compressor.
>>
>>I really think this idea needs pursuing. This would be a genuine
improvement
>>on the design of the coach.
>>
>>Travis
>>
>>-----Original Message-----
>>From: Gcbr
>>To: gmcmotorhome
>>Date: Friday, April 23, 1999 10:50 PM
>>Subject: Re: GMC: A.C. Compressor (trivia)
>>
>>
>>>Travis
>>>
>>>I am not worried about cycling the compressor. I am worried about driving
>>>an electric motor that is designed to go 1725 rpm at 4000 or 8000 rpm.
>>>It aint cool! bronze bearings are not designed for that. Even if it has
>>ball
>>>bearing you are so far over design standards that all hell could break
out
>>at
>>>any time. If you had an electric clutch then it would not be turning when
>>the
>>>455 is driving it. I am going out of bounds here but here I go. You can
>>push
>>>the limits---------yes you can. You cant take a motor that was designed
to
>>run
>>>at 1725 and run it at 8000 rpm. I will promise you something will fail
that
>>>far out of limits. Yes, I know I am sounding like Wes again. Sorry but
not
>>>sorry.
>>>
>>>Take Care
>>>Arch
>>>
>>>In a message dated 4/23/99 10:14:59 PM Central Daylight Time,

>>>
>>>>
>>>> But it means we don't have to worry about cycling the compressor and
so
>>a
>>>> clutch on the electric motor is back to being unnecessary.
>>>>
>>>> Or that's the current state of my cortexical flatulence.
>>>>
>>>> Ideas?
>>>>
>>>
>>>
>>
>>
>>
>Tom & Marg Warner
>Vernon Center NY
>1976 palmbeach
>
>

 

[donald w. miller]

The exact rpm an electric motor becomes a hand grenade is not a
consideration for me because I don't plan to find out. It's a simple matter
to manually engage the electric motor upon arrival at a campground if I
don't locate a suitable clutch arrangement.

Weight and complexity of a second compressor is also unattractive. KISS
applies here since one compressor can be made to do it all. My entire
project is based upon goals to minimize weight, noise and maintenance
complexity.

External driven compressors are more efficient because the condenser does
not have to get rid of the motor heat. Many yachts have removed their
factory installed two compressor refrigeration units due to oiling and other
reliability problems.

Our two rooftop units are rated at 24,500 Btu combined. Refrigeration
calculations indicate the A-6 compressor at 1800 rpm pumps enough freon for
36,000 btu at the compressor output. Compressor volumetric efficiency was
assumed to be 85% since I didn't have this data available. It varies with
speed and these compressors are more efficient at the slower speeds. There
are other system losses but the A-6 at 1800 rpm with reasonable condensers
and evaporators should exceed the output of the two rooftop units.

The crank pulley is 8" and A/C pulley is 6" so compressor turns at 1.33
times engine speed or about 1330 rpm with engine idling at 1,000 rpm.

A weak link with all RV A/C systems is often inadequate campground power. A
2 hp dual capacitor motor was selected for it draws less current per hp than
other common single phase motors. 19 to 22 amps is typical at 120 volts.

Condenser capacity directly effects how much horsepower is needed per btu.
A system using 1.3 hp per ton with 110 degree compressor discharge
temperature will need 1.7 hp per ton with 150 degree compressor discharge.
This shows the importance of adequate condenser capacity to make this
concept work properly.

There will be more heat given up ahead of the radiator so engine cooling
becomes more important. A poorly tuned engine puts more heat into the
radiator so engine tuning is another thing to take into account.

Projects such as this one need clear thinking and need to be well thought
out from an overall systems viewpoint. It usually takes me a few revisions
to correct what I failed to recognize initially.

Don Miller
75 Glennbrook project
Shenandoah Valley of Virginia

 

[mark grady]

Don --

Excellent post. I see your point, drive the A-6 shaft with a second pulley
and just slip the belt off before running the engine, interlock the clutch
so it doesn't try to turn the engine over when on 'shore power'.

Unknown fact about boats minimizing compressors, but it makes sense. I also
thought about the hermetic angle solution by using oil separators. I think
your match on the characteristics of the A-6 is very good. That makes me
want all the more to use it as the driven compressor.

I'll see what I can find in aftermarket evaps, and I've got a real bad case
of junk yard yearning. I hope my brother in law isn't busy this weekend.

Cavemen hunted game, I'm hoping to paint a picture of a Suburban rear evap
on the walls of my garage by Monday. As our Alpha leader says: Power to the
'net! This is such a good idea I don't know why it didn't come up sooner.
The remaining questions are if the theory can be turned into production.

Mark

> -----Original Message-----
> From: owner-gmcmotorhome
> [mailtowner-gmcmotorhome]On Behalf Of Donald W.
> Miller
> Sent: Saturday, April 24, 1999 10:31 AM
> To: gmcmotorhome
> Subject: Re: GMC: A.C. Compressor (trivia)
>
>
>
> The exact rpm an electric motor becomes a hand grenade is not a
> consideration for me because I don't plan to find out. It's a
> simple matter
> to manually engage the electric motor upon arrival at a campground if I
> don't locate a suitable clutch arrangement.
>
> Weight and complexity of a second compressor is also unattractive. KISS
> applies here since one compressor can be made to do it all. My entire
> project is based upon goals to minimize weight, noise and maintenance
> complexity.
>
> External driven compressors are more efficient because the condenser does
> not have to get rid of the motor heat. Many yachts have removed their
> factory installed two compressor refrigeration units due to
> oiling and other
> reliability problems.
>
> Our two rooftop units are rated at 24,500 Btu combined. Refrigeration
> calculations indicate the A-6 compressor at 1800 rpm pumps enough
> freon for
> 36,000 btu at the compressor output. Compressor volumetric efficiency was
> assumed to be 85% since I didn't have this data available. It varies with
> speed and these compressors are more efficient at the slower
> speeds. There
> are other system losses but the A-6 at 1800 rpm with reasonable condensers
> and evaporators should exceed the output of the two rooftop units.
>
> The crank pulley is 8" and A/C pulley is 6" so compressor turns at 1.33
> times engine speed or about 1330 rpm with engine idling at 1,000 rpm.
>
> A weak link with all RV A/C systems is often inadequate
> campground power. A
> 2 hp dual capacitor motor was selected for it draws less current
> per hp than
> other common single phase motors. 19 to 22 amps is typical at 120 volts.
>
> Condenser capacity directly effects how much horsepower is needed per btu.
> A system using 1.3 hp per ton with 110 degree compressor discharge
> temperature will need 1.7 hp per ton with 150 degree compressor discharge.
> This shows the importance of adequate condenser capacity to make this
> concept work properly.
>
> There will be more heat given up ahead of the radiator so engine cooling
> becomes more important. A poorly tuned engine puts more heat into the
> radiator so engine tuning is another thing to take into account.
>
> Projects such as this one need clear thinking and need to be well thought
> out from an overall systems viewpoint. It usually takes me a few
> revisions
> to correct what I failed to recognize initially.
>
>
> Don Miller
> 75 Glennbrook project
> Shenandoah Valley of Virginia
>
>
>
>
>

 

[travis martin]

Hi, Arch,

You're right, of course; I used the wrong term. I did not mean a shaded pole
motor as in fan motor; I was actually referring to the armature.

I was thinking in terms of using a start winding controlled by an external
start relay like hermetics do so as to not use the centrifigal start switch.

I think that all the signs so far point to a single A-6 compressor driven by
an electric motor when the 455 is asleep. Just a matter of deciding how best
to couple it. An overrunning (sprag) clutch sounds like the best bet to me
if one can be had. That way it would be completely transparent...when the
motor is energized, it runs the compressor. Otherwise, it just sits there.
Probably ought to go to the Thomas Register and start researching sources
for clutch parts. Everything else ought to be pretty straight
forward...evaporators from suburbans, electric motors laying around
everywhere. BTW, a 3450 rpm motor is quite a bit cheaper than a 1725 and
might be just the ticket. They are lighter too.

Travis

- -----Original Message-----
From: Gcbr
To: gmcmotorhome
Date: Saturday, April 24, 1999 9:27 PM
Subject: Re: GMC: A.C. Compressor (trivia)

>Travis
>
>I hate to keep throwing cold water on your idea but here I go again.
>I dont think a shaded pole motor will start under the load of an AC.
>The hermetics you have mentioned all have a start winding and a
>run winding to make them work. Not only that they have a capacitor
>to give them a big kick too. A fan motor wont start if you just touch
>it. Touch a motor with a start winding and capacitor-----you are going
>to lose something. Sure hope all of this is helping.
>
>Take Care
>Arch 76 GB IL
>
>In a message dated 4/24/99 9:13:41 AM Central Daylight Time,
travism
>writes:
>
>>
>> The reason I threw the 20,000 number out is that the particular motor we
>are
>> talking about is an induction motor with a squirrel cage shaded pole
>> armature. It doesn't have a commutator nor does it have windings in the
>> usual sense. There are shading windings that are pressed into the
armature.
>> The armature is about 3 inches in diameter. I haven't tried it and
likely
>> won't since it's purely academic, but I really do think that it would
>handle
>> 20,000 RPM; why shouldn't it? It is very nearly a solid mass of
relatively
>> small diameter.
>>
>