Replies

1. 
   philip | Dec 21, 2006 11:08am | #1

   Are those other two also 230volts?

   Philip Marcou
1. 
   woodenfish3 | Dec 21, 2006 10:14pm | #5

   yes, all are 230v motors

   Actually, amp is also a measure of power when the voltage is fixed or nearly fixed. P=IV.  In this case, v is 220 to 240. This is an overall true concept.

   The important question are I believe:

   1. amp at what condition, but I think all (nameplate on motors) are FLA

   2. how the motor handles brief surges of load.

   A motor that has less FLA but handles brief surges of load well can be very adequate for most WWers, I tend to think.

   Edited 12/21/2006 2:16 pm ET by woodenfish3

1. 
   philip | Dec 22, 2006 04:46am | #6

   I am still trying to establish that you are comparing like with like: are all three 2 pole or 4 pole or to put it another way are they all the same rpm?Philip Marcou

2. 
   PeteBradley | Dec 22, 2006 05:04am | #7

   TKanzler has it right.Pete

1. 
   woodenfish3 | Dec 22, 2006 08:17am | #8

   I think TKanzler is right as well in the sense that the Delta motor is more efficient in accumulating power to be released in sudden load surge.

   However, 13 amp FLA vs 18 amp FLA has to be significant under some circumstances. If the 18 amp motor puts out the same or less power all the time then where is the energy going to? There would be tremendous heat buildup. Instantaneously, P is always IV. This is just conservation of energy.

   I think in cutting wood, sudden surge in motor load due to unevenness of the material is inherent. If one cuts extremely uniform wood then FLA would be more important, I tend to think.

   I am happy about buying the Unisaw overall.  

    

    

2. 
   Quickstep | Dec 21, 2006 01:33pm | #2

   Amps are a measure of how much energy is consumed and although it's often used to compare motors, it's not really a measure of power. If measured accurately, horsepower is horsepower is horsepower. In theory, if all things are equal and both companies use the same standard for measuing horsepower, the difference between the amp consumption for the Delta motor and the Jet/Grizzly motor would only indicate that the Delta motor is more efficient.

3. 
   oldusty | Dec 21, 2006 06:49pm | #3

   woodenfish3 ,

               I'm not an expert electrician but if memory serves me correct the the amperage may be the draw at startup not necessarily the running amperage or a reflection of the available power . Maybe the motors even though rated similarly in hp draw a different amount of amps at start up .

                dusty

4. 
   TKanzler | Dec 21, 2006 08:57pm | #4

   The relationship between full-load current and output power is:

   HP = (V x A x pf x eff) ÷ 746

   The big variables between motors are pf (power factor) and eff (efficiency).  These are fixed by the design of a given motor, and will vary from one manufacturer to another, and from one model to another within a product line.

   Power factor is basically the percentage of current that's in phase with the voltage, and not used for magnetization (reactive current).  The reactive part just sort of bounces back and forth between the motor and the utility, and doesn't even register on your residential electric meter, but since it flows through the wiring, it's part of the nameplate current.

   Efficiency is the percentage of the current that's actually converted to output (shaft) power - the rest is converted to heat.

   A 3 hp Marathon-built Unisaw motor has an efficiency of 0.80, and a power factor of 0.98 at rated power.  Those values go down at lower output power, but so does the current.  Plug those in, along with the nameplate voltage (230V, NOT 220 or 240V) and you'll get 2.997 hp.

   The Marathon has such a high power factor because it has a run capacitor, which is in the start winding circuit all the time.  This is very common in larger single-phase motors.  The centrifugal start switch only switches the start capacitor in and out for starting, but the start winding (it's really just a second winding) is in the circuit all the time.  That motor is basically a 2-phase motor, because there are two sets of windings with their magnetic peaks at different times - the times are different because of the capacitor.  It's kind of like a 2-cylinder engine vs a single-cylinder, both turning at the same speed.  Search on RLC circuits if you want to know more about the relationship between voltage and current in resistive and reactive circuit elements, and how inductance and capacitance (and resistance) can change the relationship.

   Motors that don't use a run capacitor, like many 3 hp Baldors, have a higher current draw for the same output power.  Inexpensive Asian import motors seem to have the highest current draw for their rating, but I don't have authoritative information as to why. 

   And just so you know, that Marathon pulls around 80A or so on startup.

   Be seeing you...