Nominal voltage [Volts]
Is the voltage applied to both phase windings that will not overheat the motor. The motor develops nominal holding torque using this voltage.

Nominal current per phase (both phases ON) [A]
Is the current level supplied to both phase windings that will not overheat the motor. The motor develops the nominal holding torque when energized this way.

Phase resistance ¹⁾
Phase winding resistance at 20 °C; tolerance is ±12%.

Phase inductance [mH]

Inductance of the phase windings, measured at 1 kHz.
Back-EMF ¹⁾ [V/k 1000step/s]

Amplitude of the back-EMF at 1000 steps/s. It is one of the factors which reduce the provided torque at higher speed.
Holding torque, at nominal current [mNm]   Is the amplitude of the torque the motor generates with both phases energized in voltage or current mode.
Holding torque, at 2 x nominal current [mNm] Is the amplitude of the torque the motor generates with both phases energized with 2 x nominal current. There is no risk of motor damage due to their magnetic
design. However, to limit heat development the boost current should be applied only for short periods during critical sections of the motion cycle.

Step angle [degr.]
Number of angular degrees the motor moves per full-step.
Step angle accuracy [%]
Percentage of a full step by which the unloaded motor with identical currents in both phases will be off from any calculated fullstep position. This error does not cumulate.
Residual torque ¹⁾ [mNm]
Torque needed to rotate rotor by outside torque when no phase winding is energized. Residual torque is useful to hold a position without any current to save battery life or to reduce heat.
Rotor inertia [kgm2]
This value represents the inertia of the complete rotor.
Resonance frequency (at no load) [Hz]
Is the step rate at which the unload motor will show rotor resonance. It is recommended to start with a frequency above this frequency or to use half-, micro-step to operate outside this frequency. The resonance frequency changes with the addition of inertial loads.
Electrical time constant [ms]
Is the time needed to establish 67% of the max. possible phase current under a given operation point. It is one of the factors which reduce the provided torque at higher speed.
Ambient temperature range [°C]
Temperatures at which the motor can operate.
Winding temperature tolerated max. [°C]
Maximum temperature supported by the winding and the magnets.
Thermal resistance winding-ambient air [°C/W]
The gradient at which the motor winding temperature increases per Watt of power losses generated in the motor. Additional cooling surface is reducing it.
Thermal time constant [s]
Time needed to reach 67% of the fi nal winding temperature. Adding cooling surfaces reduces the thermal resistance but will increase the thermal time constant.
Shaft bearings
Offered are either self lubricating sintered bronze bearings or 2 preloaded ball bearings. The ball bearing preload is assured by a spring washer assembled at the rear bearing.
Shaft load, max. radial [N]
The fi gure is representing for all bearing types the recommended maximally supported radial load.
Shaft load, max. axial [N]
The figure is representing for all bearing types the recommended maximally supported axial load. The load handling capability of ball-bearings is higher than the set preload.
The rotor can be pulled without risk of damage to the motor by about 0,2 mm.
Shaft play max., radial [μm]
The clearance between shaft and bearing tested with the indicated force to move the shaft.
Shaft play max., axial [μm]
Represents the axial play tested with the indicated force.

Isolation test voltage ¹⁾ [VDC]
Is the test voltage for isolation test between housing and phase windings.
Motor dimensions [mm]
The values provide a rapid view about the motor housing diameter and length as well as the standard shaft diameter.
Weight [g]
Is the motor weight in grams.
¹⁾ these parameters are measured during fi nal inspection on 100 % of the products delivered.

Stepper Motor Selection

The selection of a stepper motor requires the use of published torque speed curves based on the load parameters. mIt is not possible to verify the motor selection mathematically without the use of the curves.
To select a motor the following parameters must be known:

• Motion profi le
• Load friction and inertia
• Required resolution
• Available space
• Available power supply voltage

1. Defi nition of the load parameters at the motor shaft

The target of this step is to determine a motion profile needed to move the motion angle in the given time frame and to calculate the motor torque over the entire cycle using the application load parameters such as friction and load inertia. The motion and torque profi les of the movement used in this example are shown below: Depending on the motor size suitable for the application it is required to recompute the torque parameters with the motor inertia as well. In the present case it is assumed that a motor with an outside diameter of maximum 15 mm is suitable and the data has been computed with the inertia of the AM1524.

2. Verification of the motor operation.

The highest torque/speed point for this application is found at the end of the acceleration phase. The top speed is then n = 5000 rpm, the torque is M = 1 mNm. Using these parameters you can transfer the point into the torque speed curves of the motor as shown here with the AM1524 curves for a current mode drive. It is not possible to use the full torque of the motor: a safety factor of 30% is requested. The shown example assures that the motor will correctly fulfill the requested application conditions. In case that no solution is found, it is possible to adapt the load parameters seen by the motor by the use of a reduction gearhead. The demonstrated method does not specify the differences between the two published torque speed curves, one for voltage mode and one for current mode (which was used as the solution for the application example). The difference is mainly linked to the performance one may get from the motor. Whereas the voltage mode is offering good performance at low speed the torque will decrease rapidly, the current mode allows higher speed performance as the constant current mode drive (the current is controlled by a chip related control loop) which allows to apply a higher voltage to the motor phases. Voltage mode is the best choice for application with supply voltage below 10 V mainly due to the availability of suitable driver chips. In voltage mode, the motor winding must have a nominal voltage equal to the power supply to get the best performances. The moment the voltage is higher than 10 V a current mode driver will be the better choice. It is recommended to apply a supply voltage at least U = 5 x R x I of the selected motor winding.

3. Verifi cation of the resolution
It is assumed that the application requires a resolution of 9° angular. The selected motor AM1524 has a step angle of 15° which means that the motor is not suitable directly. It can be operated either in half-step, which reduces the step angle to 7,5°, or in micro stepping. With micro stepping, the resolution can be increased even higher whereas the precision is reduced because the error angle without load of the motor (expressed in % of a full-step) remains the same independently from the number of micro-steps the motor is operated.  For that reason the most common solution for adapting the motor resolution to the application requirements is the use of a gearhead or a lead-screw where linear motion is required.

General application notes
In principle each stepper motor can be operated in three modes: full step (one or two phases on), half step or microstep. Holding torque is the same for each mode as long as dissipated power (I2R losses) is the same. The theory is best presented on a basic motor model with two phases and one pair of poles where mechanical and electrical angle are equal.
In full step mode (1 phase on) the phases are successively energised in the following way:  1. A+   2. B+   3. A–   4. B–.
Half step mode is obtained by alternating between 1-phase-on and 2-phases-on, resulting in 8 half steps per electrical cycle: 1. A+   2. A+B+   3. B+   4. A–B+   5. A–   6. A–B–   7. B–   8. A+B–.
If every half step should generate the same holding torque, the current per phase is multiplied by √2 each time only 1 phase is energised.

The two major advantages provided by microstep operation are lower running noise and higher resolution, both depending on the number of microsteps per full step which can in fact be any number but is limited by the system cost. As explained above, one electrical cycle or revolution of the field vector (4 full steps) requires the driver to provide a number of distinct current values proportional to the number of microsteps per full step.

For example, 8 microsteps require 8 different values which in phase A would drop from full current to zero following the cosine function from 0° to 90°, and in phase B would rise from zero to full following the sine function.

These values are stored and called up by the program controlling the chopper driver. The rotor target position is determined by the vector sum of the torques generated in phase A and B:
where M is the motor torque, k is the torque constant and Io the nominal phase current.

For the motor without load the position error is the same in full, half or microstep mode and depends on distortions of the sinusoidal motor torque function due to detent torque, saturation or construction details (hence on the actual rotor position), as well as on the accuracy of the phase current values.

4. Verification in the application
Any layout based on such considerations has to be verified in the final application under real conditions. Please make sure that all load parameters are taken into account during this test.

Stepper Motors

Two Phase with Disc Magnet

Features

The rotor consists of a thin magnetic disc. The low rotor
inertia allows for highly dynamic acceleration. The rotor
disc is precisely magnetized with 10 pole pairs which
helps the motor achieve a very high angular accuracy.
The stator consists of four coils, two per phase, which
are located on one side of the rotor disc and provide the
axial magnetic field.

Special executions with additional rotating back-iron
are available for exceptionally precise micro-stepping
performance.

Benefits

• Extremely low rotor inertia
• High power density
• Long operational lifetimes
• Wide operational temperature range
• Ideally suited for micro-stepping applications

Stepper Motor Two Phase

Features
PRECIstep® stepper motors are two phase multi-polar
motors with permanent magnets. The use of rare-earth
magnets provides an exceptionally high power to volume
ratio. Precise, open-loop, speed control can be achieved
with the application of full step, half step, or microstepping
electronics.

The rotor consists of an injection moulded plastic support
and magnets which are assembled in a 10 or 12 pole
confi guration depending on the motor type. The large
magnet volume helps to achieve a very high torque
density. The use of high power rare-earth magnets also
enhances the available temperature range of the motors
from extremely low temperatures up to 180 °C as a
special confi guration. The stator consists of two discrete
phase coils which are positioned on either side of the
rotor. The inner and outer stator assemblies provide the
necessary radial magnetic field.

Benefits

• Cost effective positioning drive without an encoder
• High power density
• Long operational lifetimes
• Wide operational temperature range
• Speed range up to 16 000 rpm using a current mode chopper driver
• Possibility of full step, half step and microstep operation