Re: Gecko and Anaheim

> Mariss;
>
> I have 42's rated at 10A, 80V and 1125 in.lb. Can a triple stack

34 equal

> this (and possibly offer greater acceleration due to lower

inertia)? And

> (here's the kicker) is such a motor less expensive than the 42?

This sounds

> interesting.
>
> Doug Harrison

Doug,

Winding inductance and power supply voltage determine motor output
power. The relationship is:

P = V / square root of L

Where L is motor inductance and V is power supply voltage.

If you don't mind me being a little technical, here's what comes into
consideration. First, a step motor is a constant power motor. If you
double its speed, then torque is cut by half. From the equation, if
you want to double power then you must A; double the power supply
voltage or B; cut inductance by 4. You can do this by going from
series (or full-winding) to parallel (or half-winding).

There is a limit to this however. While power increases
proportionally with supply voltage, iron losses (hysteresis and eddy
currents) increase more rapidly. An empirical determined limit is
your supply voltage should not exceed 20 to 25 times the motor's
rated voltage. Beyond that the motor will be too hot because of iron
losses.

Detent torque is always present and subtracts from the motor's output
power. If you double motor speed, you double the power necessary to
overcome detent torque, which subtracts from what the motor could
have put out.

Now to bring this all together. Size 42 motors I'm sure you have
noticed have much higher detent torque than size 34 or 23 motors.
This is the reason why size 42 motors run out of steam before the
smaller motors do. Second, for the same amperage, a size 42 motor has
much higher inductance and rated voltage than a 34 motor. That means
they come into their own at much higher supply voltages; 160 to 200
VDC. To get the maximum power from a 7A, 80V drive, pick a 7 Amp, 4
volt motor. This will be either a 4 stack 34 or a single stack 42.
For peak power, gear the motor to run just past the speed where the
drive ceases chopping (i.e. current limiting).

Hope that helps; and yes, 34 size motors are much cheaper.

Mariss