Curious effect when testing motor - ?

[rrinker]

 We've been over this one before. Simplified version: power in = power out (minus loss in the transformer/power supply). Power - volts times amps. So with an ideal power supply, no losses, 120V at 1 amp when reduced to 12V would suppyl 10 amps. Or put it another way, drawing 10 amps on the 12V side would make it draw 1 amp from your 120V wall outlet.

Note I said SIMPLFIED explanation, this is not technically 100% accurate but no way would drawing 17 amps on the 12V side draw over 15 amps on the house circuit. Not even close. if this were true it would be impossible to use a modern computer that can draw 15 amps on the 3.3V side for the CPU and another 40-50 amps on the 12V side for the video card.

                                        --Randy

[CSX Robert]

locoi1sa

...17 amps would be blowing your 15 amp breakers in you fuse panel..

17 amps at 12 volts wouldn't.  17 amps at 12 volts would only draw about 1.7 amps at 120 volts(it would be more, due to the inefficieny of the transformer, but nowhere near 17).

[locoi1sa]

  Take your meter off auto range. 17 amps would be blowing your 15 amp breakers in you fuse panel. Also I doubt your DC power supply is putting out 17 amps unless your using one of the transformers from a pole. Try to get a hold of an old analog meter with the needle. A DMM is sometimes too sensitive to minor changes and will bounce around  and an accurate reading is hard.

        Pete

[Hamltnblue]

What are you using for a power source?  make sure it isn't a pulsed power supply. 

Also how high does your meter go?  17 amps sounds a bit high for most multi-meters.  Even the good ole Simpson only goes to 10 amps.

I have a feeling that your meter auto ranged and showed 17ma or 170ma.

[rrinker]

 Just as a test to verify no other problems, connect the meter in the ohms setting across the motor and slowly rotate it to connect each brush to the commutator sectiosn and check for a shorted winding - at no point should you ever get a 0 reading, if you do there is either a shorted commutator segment or a shorted winding, often caused if the motor ever overheated.

                           --Randy

 

[jeffrey-wimberly]

tbdanny

I'm guessing that it's the motor's back-EMF interfering with the multimeter - would this be correct?

Sounds about right to me. It takes electricity to make the motor turn but as the the armature turns within the field of the magnets it also creates electricity and this can do strange things to power readings.

[tbdanny]

Hi all,

I've just been testing the current draw on a brand new mashima motor, and I noticed a rather curious effect.  When the throttle (straight DC) was set to somewhere in the 9-12 volt range, the current reading went off the scale - I think the highest reading was 17 amps.  But once I grabbed the shaft to stall the motor, it dropped back down to 0.25 amps.  This also happened on another motor I was testing.

I'm guessing that it's the motor's back-EMF interfering with the multimeter - would this be correct?

Cheers,

tbdanny