Servo and closed loop stepper motor have similar construction and share the same fundamental operating principle. Both motor types incorporate a rotor with permanent magnets and a stator with coiled windings … and both are operated by energizing or applying a dc voltage to the stator windings. That then causes the rotor to move. However, this is where the similarities between servo and stepper motors end.
Drive methods for stepper motors
Stepper motors have 50 to 100 poles and are
In contrast, servo motors have between four
and 12 poles and are three-phase devices.
What is more, stepper motor driver generate sine waves with a frequency that changes with speed … but with an amplitude that is constant.
Servo drives, on the other hand, produce
sine waves with variable frequency and amplitude — allowing them to control
both speed and torque.
Control methods for stepper motors
Traditional stepper motors move when they
receive a command to advance a certain number of pulses, which correlate to a
distance. Steppers are considered open-loop systems because they lack a
feedback mechanism to verify that the target position has been reached. Servo
motors also move on receipt of a command signal from their controller. In
contrast to the open-loop operation of stepper motor systems, servo motors are
closed-loop systems, with built-in encoders that continuously communicate back
to the controller, which makes any needed adjustments to ensure the target
position is reached.
stepper motors eliminate many of the disadvantages of traditional open-loop
stepper systems, making them similar in performance to servo motors. But servo
motors outperform even closed-loop steppers in applications that require high
speed, high torque at high speed, or the ability to handle changing loads.
HIGH POWER DENSITY Our industrial-grade NEMA 34 motors are more powerful than similarly sized motors because of the combination of fully sintered, high temperature,Neodymium-Iron-Boron magnets and an optimized stator design. This means that you can match existing performance specifications with smaller (and less expensive) motors or get more powerwithout increasing motor size.
GREATER CONTINUOUS CAPACITY Several innovations combine to provide more continuous motor performance.The ribbed motor housing and internal construction provide a better thermal path from the stator to free-air and the front mounting face, resulting in significantly more thermal conductivity than other motors. Additionally, the FEA-optimized electromagnetic design ensures that this motor is a more efficient converter of electrical to mechanical power (i.e., high motor constant) which means that less motor power is lost to extraneous heating.
LOW DETENT = SMOOTHER MOTION Special attention was paid to the interaction of the rotor’s magnetic field with the stator’s tooth design with regard to detent (or cogging) torque. Low detent torque enables the smoothest motion possible–which minimizes stepping motor heating and improves the motor’s continuous capacity. The magnetic design also assures a sinusoidal torque constant for minimization of torque ripple when using sinusoidal vector drive.
FEA DESIGNED SHAFTS Finite element analysis is used to reduce stress concentrations on machined areas of the shaft (such as the fillet where the shaft exits the bearing assembly). This allows the use of oversized bearings without reducing shaft strength. The result is rugged, load-bearing capability.
The simplest is just to connect DC to each
winding in turn, via switches (FETs, driver ICs). And in that case, use 4.8V
(5V – switch losses) as you confirmed from current and resistance. This is fine
at low and medium speeds.
If you need maximum performance, you’ll
find the motor’s inductance attenuates short pulses, so running the motor
faster reduces its torque. You can overcome this with a more complex stepper
driver, supplying pulses at the recommended 12-24V, to maintain current and torque
at higher speeds.
Each pulse is maintained at a high voltage
for long enough to build the rated current in the phase, then it should reduce
in voltage to the safe level of 4.8V for the remainder of a slow pulse or
steady state. This reduction in voltage can either be timed, or achieved by
monitoring and limiting the drive current.
So both voltage ratings can be correct :
4.8V continuous, and 12-24V for an optional boost to high speed performance.