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12th Jan 2021, 02:11 PM #16Senior Member
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................. Did I read your reply right?
the motor spec of 3.9amps is the FLA and does not include starting amps. It is the max amps at full load under running conditions.
3.9 Amps is the maximum current drawn by the motor.
It was not specified when this (condition) will occur.
As the calculated full load current (for the specified wattage of the motor) is much less, I have assumed that the manufacturer of the motor is trying to say that this would be the starting current. Then again it could be the current drawn when the motor stalls. Either way, the conductors supplying the motor need to be of a suitable size.
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13th Jan 2021, 09:35 AM #17Most Valued Member
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All motors are rated at full load...ie the rating stated is at full load running conditions.
as start current times are brief the wiring only needs to handle full load current plus the normal fudge factor. start currents are generally accepted as being some where between 6-7 times full load current.
Ie a 10amp motor at full load would be drawing 60-70amps on start up....thats a big cable size based on your theory.
when in fact it will run on 2.5mm standard 15a amp cable quite comfortably. (assumes length of cable run is not excessive....ie a standard residence where switch board is in opposite side of house)
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13th Jan 2021, 11:49 AM #18Senior Member
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Re-reading post #1 the stated 3.9Amps is probably the stall current. If you multiply 3.9Amps by 240Volts you get 936VA which is about three times the rated output.
Starting current will be as you say. This will produce an increased volt drop on the supply cable (during starting) which means low volts at the motor when starting. Probably not a problem in this case.
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13th Jan 2021, 12:16 PM #19Member: Blue and white apron brigade
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Sorry to nit pic here but the rated current on a motor nameplate is not really the max current a motor can draw. Most motors can draw a lot more current that their rating if they are overloaded. eg my 3HP Table saw can draw up to 17A for about 5s before it trips the thermal overload.
The rated current is usually the current that is drawn by the motor when performing its maximum recommended power output ie under its max recommended load.
My motor testing shows that the current at maximum recommended power output usually coincides or is close to a motors peak power output.
If a motor is pushed beyond this load most of the excess current drawn goes into heating the motor and produces very little "extra" and usually even "less" power
Some quality motors can easily produce around 20% more power than their rating without serious overheating - I guess its the manufacturer just being conservative and maximising a motors lifetime
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14th Jan 2021, 11:09 AM #20Senior Member
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Good answer Bob. Real test with real results always beats theory!
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