Small drives, Big motors
Posted by
Mariss Freimanis
on 2000-09-16 14:20:14 UTC
Hi,
Here are some things that should be taken into account when using
small drives with size 42 or larger step motors.
Like many small step motor drives, the G201 maximum ratings are 80VDC
and 7Amps.
A 7Amp per phase triple stack size 34 delivers a measured 200W on a
dyno. Under the same conditions a double stack size 42 delivers less
than 150W. There are two reasons for this:
1) Size 42 motors have considerably higher detent torque and detent
torque is always a loss that requires power to overcome. This power
subtracts from what is ultimately available at the motor shaft.
2) A size 42 motor has considerably higher inductance than a size 34
motor when both motors have the same phase current rating. This is
because the winding encompasses much more iron. Motor output power is
proportional to V/square root of L, thus a 42 motor produces less
power than a 34 when rated phase currents are the same.
If inductance is normalized instead of rated phase current, then
fewer turns of wire must be used in the size 42 motor. Torque is
proportional to ampere-turns, fewer turns require greater current,
certainly beyond 7Amps.
Approaching it from the other end, normalizing power requires
increasing power supply voltage considerably above 80VDC.
A size 42 motor should be capable of delivering more than 1HP, or
about 800W. If a 7A, 80VDC delivers 200W mechanical, then to scale it
to 800W would require a 7A, 320VDC or a 28A, 80VDC drive. Though you
can drive a 42 motor with a G201, you simply are not getting your
money's worth from a 42 motor. It needs a much more powerful drive.
If more than 200W (1/4 HP) is needed, a DC servomotor is the more
cost-effective way to go.
Mariss
Here are some things that should be taken into account when using
small drives with size 42 or larger step motors.
Like many small step motor drives, the G201 maximum ratings are 80VDC
and 7Amps.
A 7Amp per phase triple stack size 34 delivers a measured 200W on a
dyno. Under the same conditions a double stack size 42 delivers less
than 150W. There are two reasons for this:
1) Size 42 motors have considerably higher detent torque and detent
torque is always a loss that requires power to overcome. This power
subtracts from what is ultimately available at the motor shaft.
2) A size 42 motor has considerably higher inductance than a size 34
motor when both motors have the same phase current rating. This is
because the winding encompasses much more iron. Motor output power is
proportional to V/square root of L, thus a 42 motor produces less
power than a 34 when rated phase currents are the same.
If inductance is normalized instead of rated phase current, then
fewer turns of wire must be used in the size 42 motor. Torque is
proportional to ampere-turns, fewer turns require greater current,
certainly beyond 7Amps.
Approaching it from the other end, normalizing power requires
increasing power supply voltage considerably above 80VDC.
A size 42 motor should be capable of delivering more than 1HP, or
about 800W. If a 7A, 80VDC delivers 200W mechanical, then to scale it
to 800W would require a 7A, 320VDC or a 28A, 80VDC drive. Though you
can drive a 42 motor with a G201, you simply are not getting your
money's worth from a 42 motor. It needs a much more powerful drive.
If more than 200W (1/4 HP) is needed, a DC servomotor is the more
cost-effective way to go.
Mariss