Projecta IM2000 Instruções de Operação descarregar pdf (Página 26)

Hybrid stepping motors combine the features of the PM stepping motors with

the features of another type of stepping motor called a variable reluctance

motor (VR). VR motors are low torque and load capacity motors which are

typically used in instrumentation. The IM483 cannot be used with VR motors

as they have no permanent magnet.

On hybrid motors, the phases are wound on toothed segments of the stator

assembly. The rotor consists of a permanent magnet with a toothed outer

surface which allows precision motion accurate to within ± 3 percent. Hybrid

stepping motors are available with step angles varying from 0.45° to 15° with

1.8° being the most commonly used. Torque capacity in hybrid steppers ranges

from 5 - 8000 ounce-inches. Because of their smaller step angles, hybrid

motors have a higher degree of suitability in applications where precise load

positioning and smooth motion is required.

Sizing a Motor for Your System

The IM483 is a bipolar driver which works equally well with both bipolar and

unipolar motors (i.e. 8 and 4 lead motors, and 6 lead center tapped motors).

To maintain a given set motor current, the IM483 chops the voltage using a

constant 20kHz chopping frequency and a varying duty cycle. Duty cycles that

exceed 50% can cause unstable chopping. This characteristic is directly related

to the motor’s winding inductance. In order to avoid this situation, it is

necessary to choose a motor with a low winding inductance. The lower the

winding inductance, the higher the step rate possible.

Winding Inductance

Since the IM483 is a constant current source, it is not necessary to use a motor

that is rated at the same voltage as the supply voltage. What is important is that

the IM483 is set to the motor’s rated current. See Section 7: Interfacing and

Controlling the IM483 for more details.

As was discussed in the previous section, Power Supply Requirements, the higher

the voltage used the faster the current can flow through the motor windings.

This in turn means a higher step rate, or motor speed. Care should be taken not

to exceed the maximum voltage of the driver. Therefore, in choosing a motor

for a system design, the best performance for a specified torque is a motor with

the lowest possible winding inductance used in conjunction with highest

possible driver voltage.

The winding inductance will determine the motor type and wiring configuration

best suited for your system. While the equation used to size a motor for your

system is quite simple, several factors fall into play at this point.

The winding inductance of a motor is rated in milliHenrys (mH) per Phase. The

amount of inductance will depend on the wiring configuration of the motor.

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