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'[EE] AC Motor control'
2005\04\28@015415 by Phil Keller

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All,

 I have been asked to look into controlling (turning on and off the
power) to a couple of AC motors.  It is not clear yet what size these
will be but at this time they are estimated to be less that 3/4 Hp.  
This will be used as part of some animation at a fund raiser for a local
charity so it needs to be safe but it does not need to be "production
quality".  As it stands right now the plans are that the motors will be
used to drag a small cart (think of a 1/2 scale roller coaster car,
without people) up a track and then release it at the top where it is
free to roll back down the track.

 I know that the inrush current will be significantly greater than the
nominal running current but that is about where my knowledge ends.

 Lets say for a 1/2 horse power 120v motor:
- What would be the current carrying capabilities necessary for the relay?
- I assume that I will need some form of over voltage protection (like I
would use on a DC motor).  Is that correct?
- What are the issues that I need to be aware of?
- Any web sites that you can point me to?

 Thanks for any pointers you can provide.
Phil



2005\04\28@060411 by Hopkins

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Check this reference out
http://www.nipsco.nisource.com/elecstd/er/er13560.pdf

Looks like your are talking about a single phase motor.

Characteristics for motors very depending on the type of motor ie a
capacitor start, capacitor start and capacitor run etc.

You can also get motor rated fuses that allow a brief surge of current
during start up but give protection for a prolonged current draw as
would happen with a locked rotor.

Google has lots of hits for "Motor start current"


_______________________________________
Roy
Tauranga
New Zealand
_______________________________________



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2005\04\28@081613 by alan smith

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Use a motor contactor/overload designed for this.
Best prices I've seen is at whttp://www.automationdirect.com

Probably need a limit switch to shut the motor off
when it reaches the top of the ride, a start switch to
enable it and of course an emergency stop switch to
shut it all down in case of a problem.  Probably a
second limit switch at the bottom to know its in place
and ready to go.  Put it all in an enclosure, with a
MCP on the front (motor circuit protector or breaker)
and for under a few hundred be ready to roll.



--- Hopkins <spam_OUTrdhopkinsTakeThisOuTspamihug.co.nz> wrote:
> Check this reference out
>
www.nipsco.nisource.com/elecstd/er/er13560.pdf
{Quote hidden}

> --

2005\04\28@114554 by Phil Keller

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Thanks for the reply but I think I may not have asked the right question
or more than likely I do not understand the answers.

What I was thinking of doing was to use a PIC to control the
sequencing.  As a person enters the room and a motion detector signals
the PIC which turns on a spot light and releases the car that is parked
at the top of the tracks.  The car will roll down the tracks, driven by
gravity, and "crashes" into a barricade with the appropriate lights
flashing and sounds (Also controlled by the PIC). After a few seconds
the spot light is turned off and the car is dragged back to the top of
the slope by the motor to await the next trigger.  At the top and the
bottom I will have limit switches to ensure that the car is where I
think it should be.  I will most likely add redundant switches at the
top to ensure that the car is not pulled over the top.

So,
-playing sounds on queue I can do.
-turning on and off lights I can do.

What I do not yet fully understand is how to safely start and stop the
AC motor.  I was thinking of using a relay but the ones that I have are
rated at 15A @ 120v.  Based on the reference that Roy provided (If I am
reading it correctly) that would only be good for a 1/6 hp motor.  It
will be geared down but I doubt that it would be powerful enough to drag
the car back up.

 I have Googled for hours but the sites all assume that you know what
you are doing.  Is there a web site "Motors for Dummies"?
-How do I decide what size motor to use? (Assuming a 50 lb car and a 50%
slope)
-What do I need to do to start and stop the motor safely?
-We are also thinking of using an electronic clutch as the release
mechanism for the cable between the motor and the car.  Then we could
just leave the motor running and use the clutch only.

Any ideas or thoughts are gratefully appreciated.

Phil
alan smith wrote:

{Quote hidden}

>>--

2005\04\28@125313 by Alan B. Pearce

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>  I have Googled for hours but the sites all assume that you know what
>you are doing.  Is there a web site "Motors for Dummies"?
>-How do I decide what size motor to use? (Assuming a 50 lb car and a 50%
>slope)
>-What do I need to do to start and stop the motor safely?
>-We are also thinking of using an electronic clutch as the release
>mechanism for the cable between the motor and the car.  Then we could
>just leave the motor running and use the clutch only.
>
>Any ideas or thoughts are gratefully appreciated.

I guess you are using a motor that large to get the car back up the ramp in
a reasonable time, but it does also bring up the thought, how do you return
the pull hook to the bottom, or do you have an endless chain always going
up?

My thought was to use a winch as fitted to off-road 4WD vehicles to get
themselves out of a bog or awkward spot, but that would probably be too
slow, and the power supply you would need would also be hefty.

At the end of it all, I think you just have to find a big enough contactor
for the motor - any electrical supply warehouse (not DIY shop) should be
able to supply one. You may need a smaller relay driven from the PIC to
activate the contactor.

2005\04\28@132026 by olin_piclist

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Phil Keller wrote:
> -How do I decide what size motor to use? (Assuming a 50 lb car and a 50%
> slope)

Actually the slope has nothing to do with it.  The relevant factors are the
weight of the car, the total height it must be lifted, and how fast it must
get there.  Since you only supplied 1 of the three, I'll make up numbers for
the other two as an example.  The car weighs 50 pounds, which is about 23Kg.
Let's say it needs to be raised up 5 meters in 10 seconds.

The upward force on the car needs to be 23Kg x 9.8N/Kg = 225N.  Times 5
meters lift is 1127 Joules.  Divided by 10 seconds is 113 Watts = .15
horsepower.  Assuming 80% motor efficiency and another 50% effeciency in the
remaining mechanical system, .38 horsepower is needed.  So a 1/2 horsepower
motor can do this just fine.  A 1/3 HP motor is close and maybe OK if the
mechanical efficiency is not as bad as 50% or you're willing to wait a
little longer.  A 1/4 HP motor is enough if you're willing to wait 20
seconds instead of 10.  Of course this all assumes proper gearing is in
place to get the specified lift time at the optimal speed of the motor.


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Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\04\29@004317 by Hopkins

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Check this site out for single phase motor info

http://www.iprocessmart.com/leeson/leeson_singlephase_article.htm


_______________________________________
Roy
Tauranga
New Zealand
_______________________________________



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