Replies

1. 
   Andrew | May 11, 2003 01:13am | #1

   Not necessarily. Some motors are more efficient than others, so they draw close to the theoretical requirements. Others need to draw more amps to compensate for their lack of efficiency. This is assuming that the motors being compared are both rated by NEMA standards. However, there could be other reasons. As you see on air compressors and shop vacs, some companies use "peak hp" which is just a #### rating to make their product look better. For a difference of a few amps it is more than likely that their is a difference in motor efficiency though.

1. 
   Pierre42g | May 11, 2003 01:27am | #2

   Thank you Andrew, it's more clear for me for now, I'll check for the efficiency.

   Peter

1. 
   user-135888 | May 12, 2003 08:33am | #5

   Also Have you noticed the same motor will draw more amps when under a heavier load.

   Q. why do large sanders have amp meters.

   A. when a motor is getting upto speed it is drawing more power than when it is at full speed WITH NO DRAG. When a machine dives into a piece of wood the draw goes up.

   I don't know how you come up with a rating on a motor, but I think it's a average of the motor at full speed or with a "CERTAIN" amount of "RESISTANCE" applied.

   PS see the "shop lights flicker when switched on" subject.

   john

   Edited 5/12/2003 1:42:46 AM ET by john

1. 
   Pierre42g | May 12, 2003 01:57pm | #6

   "when a motor is getting upto speed it is drawing more power than when it is at full speed WITH NO DRAG."

   You're right John, it's probably the reason why a few machines comes with a 3 HP instead a 5 HP (1 phase) because it's drawing too much amp!

   Peter

2. 
   TKanzler | May 11, 2003 03:36am | #3

   Besides efficiency, there's also power factor (PF).  That's the percentage of full-load current that isn't just bouncing back and forth, out of phase with the voltage, and not contributing anything.  All induction machines have less than unity PF, including transformers.  A Delta Unisaw with 3 hp Marathon has a run capacitor (in addition to the start cap.), and has a PF of around .98 (with efficiency of .80, both at rated load; these values drop like a stone for all motors as the load is reduced).  Full load current (FLA) is 12.4A, and it produces a real 3 hp, per the above mentioned NEMA standard.  The 3 hp Baldor (with the Unisaw frame), on the other hand, has a PF of around .87, and efficiency of .72.  It draws 15.5 FLA, and it produces the same 3 hp as the Marathon.  Using the Baldor as an example,

   (hp * 746)/(eff * pf * V) = A 

   (3 * 746)/(.72 * .87 * 230) = 15.5A 

   The moral of the story is you can't judge the output of an induction motor by the current at full load.  If it has a real NEMA nameplate, the rated hp is real.

   Be seeing you...

1. 
   Pierre42g | May 11, 2003 02:33pm | #4

   Thank you for the information Tom, it will help me to understand the specifications of each motor!

   Pierre

3. 
   DouglasABaker | May 12, 2003 07:01pm | #7

   I seem to recall that a good "average" to use when trying to convert A to hp is 7.5A/hp (@ 120V).  Be careful using this as more than a general guide - at higher voltages and amperages, other factors come into play)

   This is a reasonable way to figure out how much hp that Craftsman, which claims to be 5hp on a 15A circuit really is - obviously they are using some "different" measuring system (peak, stalled, etc).

   In general, ampere ratings are far more reliable than HP ratings, and can be used to compare relative motor "power".  So, it is probably safe to say that Yes, the 18A is more powerful than the 14A

   d-

1. 
   Pierre42g | May 16, 2003 05:50am | #8

   Thank you for your information DM!

   Peter