View Full Version : how do i calculate amp draw? and something else

12-15-2005, 02:54 PM
i know how to calculate current draw in a stall situation, but how do i calculate current draw while the motor is running at a certain rpm?

reason being, i am making a powerfeed for a mini-mill, and am stacking (4) 56mm stators together, and i want this motor to be as powerful as possible without exceeding 12a @ 5v while running under load, becuase that is what the only power supply that will fit on the back of the mill nicely that i own will supply.

also, one other question. i had an idea, that will allow me to change the wiring of the motor from wye to delta by flipping a switch, which will allow me to move the mill table rapidly with no load, without compromising the ability of the wye winding to produce the low rpms and torque required for a steady slow cut. do you think this will work? the "b" stands for beginning, the "e" stands for end. the numbers stand for the phase#. the switch is a 3 row on/on toggle. the circle is the motor

12-15-2005, 08:26 PM
to calc amp draw you need to know load. then it is load (Power) = IV.

but you need to factor in fricitional losses and heat losses which is P=I^2R

one way to determine load on your mill would be to use a spring balance and pull on the drive. however the load will have alot to do with the depth of cut and feed rate etc etc...

also, most induatrial motors use the star/delta principle already for that exact reason.

12-15-2005, 09:46 PM
See directly below my name.:D

12-15-2005, 11:54 PM
by load do you mean power in? how about i tell you everything i know about this motor i am creating, and an rpm?

also, it is my intention to use a sturdy (heavy) 1018 steel rotor, which i will make myself as it is cheap, highly magnetic, and is heavy enough to cause a flywheel effect and prevent a stall when i engage the motor (so it has a running start, because brushless motors need to be rotating for their timing systems to work) would using a heavy rotor make much of a difference in amp draw? i know it would during spool up, but it is not like it will require much more power to keep rotating i should think...

12-16-2005, 05:58 PM
i am still curious whether my wire termination switch idea would work...

also, how would i calculate torque?