Re: Question for Mariss

Dom,

The voltage rating on a step motor simply means if you apply 3VDC
across that motor's coil, 4 amps of current will flow. In other words
it has 0.75 ohms resistance.

The step motor coil is an inductor (L). Inductance has a property
called inductive reactance in the same way as a resistor has a
property called resistance. Both are measured in Ohms.

What makes inductive reactance different from a resistor is
its "resistance" is proportional to frequency. This means every time
you double frequency (steps per second), the "ohms" double as well.

According to Ohm's Law (I=V/R), it means current is the inverse of
speed. Torque being proportional to current, becomes the inverse of
speed as well.

Going the other way, as speed approaches zero, the inductive
reactance "ohms" also approach zero and current would become
unlimited. This cannot be permitted.

What actually happens is the drive limits current to a programmed
value (4A in this case) and can be considered a "current source".
That simply means it will not permit more than the programmed current
to flow into the motor.

When the motor is stopped, this 4 amps produces an effective voltage
of 3V across the motor. As its speed increases, the effective voltage
across the motor increases as well so as to keep that 4A of current
flowing.

Eventually a speed is reached where this voltage equals the supply
voltage. Beyond this speed the motor's increasing inductive reactance
limits current to less than 4A and torque begins to drop as a result.

At no time does more than the programmed current ever flow thru the
motor. All that happens is the effective voltage across the motor
increases with speed. That voltage times current is the power
absorbed by the motor to be converted into mechanical energy.

Hope that helps you see more clearly what happens.

Mariss

>--- In CAD_CAM_EDM_DRO@y..., NCS <ncs@n...> wrote:
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> 10A x 60 V = 600W or 200W per motor.
> 200W / 3V = 66.6 A per motor.
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> Uh-oh, somethings not right!
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> - Dom