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Old Jun 15, 2002, 08:30 PM
Crashez-vous sans stress!
Dan H.'s Avatar
Chicago
Joined Nov 2001
374 Posts
Bench runs are bad?

I recently saw in FAQ or newsletter some advice cautioning strongly against running a motor without load/prop (this makes sense to me) but also against bench-running a motor/prop for more than a few seconds, lest you damage the motor. This part didn't make sense to me, until I thought of the prop being in a stalled state from operating at zero airspeed and thus doing more work/drawing more amps. Is this the concern and is it a real one?
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Old Jun 15, 2002, 08:37 PM
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poobs's Avatar
oakdale, ny, usa
Joined Aug 2000
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IMHO

It is a real concern if you care. If for example you are using an old speed 400 to run some type of test then it does not - at least in my case.

If you are running an expensive motor with a marginall (large) prop, you are just taking life out of your investment.

Basically short runs if you need them in order to measure amps etc should be ok.
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Old Jun 15, 2002, 10:05 PM
Registered User
Folsom,Ca,USA
Joined Apr 2001
992 Posts
Bad news

Think about it.

Maximum load on motor, = max current draw and no airflow to speak of.

Almost guaranteed to cook motor and /or esc.

Things get hot quickly without the airflow from flight.

Short runs, ( less that 30 secs ) to check current don't hurt.

Try it an feel how quickly things get almost too hot to touch.

With no prop, on some hot motors it could over speed and throw windings!

All in all, nothing good to say about it.
And why? Whats the goal??

Dave
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Old Jun 15, 2002, 10:18 PM
Been There! Done That!
boomerace's Avatar
Eugene, Oregon, United States
Joined Sep 2001
19,187 Posts
My son cooked his first motor in the TM I let him use in short order because I forgot to tell him not to run it without airspeed! It went like "Dad why won't my TM fly anymore ??? It was working fine last night when I used it to run the battery down after I finished flying!!"
boomer
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Old Jun 15, 2002, 10:41 PM
Crashez-vous sans stress!
Dan H.'s Avatar
Chicago
Joined Nov 2001
374 Posts
I've been bench-running a Mag Mayhem Reverse for short periods just to check throttle range and for EM interference & range check on system, observe brush & comm wear, see if the bearings I installed are ok, etc. Castle Pegasus 35 (35A) ESC and 8xCP1300SCR or 8x800AR, 6x5 Aeronaut carbon prop.

The motor can and ESC have been hardly warmer than room temperature after running a minute or two at various throttle settings. Only the CP1300 pack got too warm, almost hot. 800AR was just warm. Lack of cooling from slipstream seems to be a non-issue for the motor and ESC. This setup should not draw more than 28 amps in flight. Is the load from a stalled prop greater or less than non-stalled?
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Old Jun 15, 2002, 10:50 PM
Registered User
Colonial Heights, Virginia, USA
Joined Mar 2001
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since the normal use for a static motor run is to check everything out or get a quick motor amp reading, either of which should not exceed 20 to 30 seconds, i've not seen any damage done by this...however, this is not to be confused with an extended run to finish off a battery pack,etc , which probably would overheat the motor to serious damage .......kw
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Old Jun 16, 2002, 03:04 AM
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N. Staffs, UK
Joined Jan 1997
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Static is indeed max. current draw, props unload in flight. But normal runs to measure current rpms etc will not usually cause problems.

The thing you really should not do as a bench run is try to see how long the total motor run time is, i.e. run the battery out. First it proves nothing because you don't fly at that current and second it kills motors and sometimes ESCs and batteries too.

Steve
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Old Jun 16, 2002, 07:35 AM
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ScottS's Avatar
Houston,TX
Joined Mar 2000
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Short runs to check things out should not be a problem. The problem is when people, normally new to electrics, decide to "see how long it will fly" by charging the battery fully then doing a full discharge on the ground under static conditions where the current is high and cooling airflow is low. Then after cooking the power sytem they wonder why it doesn't fly very well when they really go fly it.
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Old Jun 16, 2002, 05:35 PM
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Peter W's Avatar
London
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Scott, i did exaclty THAT with my glider, though mine was at half till bec cut in.....

Pete
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Old Jun 16, 2002, 10:24 PM
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Joined Dec 2001
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If you really must do bench runs...put a fan in front of the plane, turn it on to the highest setting - there's your airflow.

~Matt Segal
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Old Jun 16, 2002, 10:38 PM
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rpage53's Avatar
Victoria, BC, Canada
Joined Apr 2001
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Quote:
Originally posted by Dan H.
Is the load from a stalled prop greater or less than non-stalled?
A stalled prop draws less current. Many high power setups with square props (like a 16X16) draw considerably more current during the climb than on the bench.
Running a motor without a load is bad since all the current is turned to heat. This seems to be the source of the "never bench test" rumour.

Don't let the motor get too warm -- doesn't matter whether you are on the bench or not. Most electric motors were never intended for aircraft and do just fine in cars, drills and vacuum cleaners. Bench testing is fine, done properly.

Rick.
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Old Jun 19, 2002, 12:30 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,425 Posts
There is a difference between (1) a "bench" test where the motor is in a plane with no or little airflow and (2) a bench test where the motor is mounted on an arm in a static test rig and is completely out in the open in the airflow generated by the prop. Use caution, monitor motor temp (as Rick says) and you should be ok with the second type -- within reason.
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Old Jun 19, 2002, 04:20 PM
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Peter W's Avatar
London
Joined Jul 2001
1,034 Posts
so when prophanging, isn't that bad? i know it's for a few secs, but what if you were good enough to continuosly do it?

Pete
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