Inquire Basket
  Home > Support > Theory of Stepper Motor

Hybrid Stepper Motor Catalogue
MEZ close loop stepper servo systems
AC Servo motors

New Contents

1.Stepper Motor
2.What is a Stepping system?
3.DC Brush Motor
4.Small Brushless DC Motor

Stepper Motor
    Motors convert electrical energy into mechanical energy.A Stepper Motor converts electrical pulses into specific rotational movements.The movement created by each pulse is precise and repeatable,which is why step- per motors are so effective for  positioning applications.
    Permanent Magnet Stepper Motors  incorporate a permanent magnet rotor,coil  windings  and magnetically conductive stators.Energizing a coil winding creates an electromagnetic field with a north and south pole .The stator carries the magnetic field.The magnetic field can be altered by sequentially energizing or "stepping" the stator coils which generates rotary motion.
    Figure 1 illustrates  a typical step sequence for a two phase motor.In  Step 1 phase A of a two phase stator is energized.This magnetically locks the rotor in the position shown,since unlike poles attract,W hen phase A is turned off and phase B is turned on,the rotor rotates 90°„ clockwise.In step 3,phase B is turned on but with the polarty reversed from Step 1,this causes another 90°„ rotation.In Step 4, phase A is turned off and phase B is turned on,with polarity reversed from Step2.Repeating this sequence causes the rotor to rotate clockwise in 90°„ steps.
    The stepping sequence illustrated in figure 1 is called "one phase on" stepping.A more common method of stepping is "two phase on" where both phases of the motor are always energized.However,only the polarty of phase is switched at a time,as shown in figure 2. With two phase on stepping the rotor aligns itself between the "average" north and "average" south magnetic poles.Since both phases are always on,this method gives 41.4% more torque than "one phase on" stepping.

    Half Stepping
    The motor can  also be "half stepped" by inserting  an off state between transitioning phases.This  cuts a stepper's full step angle in half.For example,a 90°„ Stepper Motor would move 45 on each half step,figure3.

However,half stepping typically results in a 15%-30% loss of  torque  depending  on  step  rate  when  compared to  the two  phase  on stepping  sequence.Since one  of  the  windings  is not energized during each alternating half step there is less electromagnetic force exerted on the rotor resulting in a net loss of torque.


DC Brush Motor
Principles of operation
    Reference to the chart reveals useful performance information valid for all fulling servomotors.
It shows speed n, current I, output power P and efficiency ¶« plotted against torque M for a given supply voltage U. Torque M is a function of the current I and the torque constant k (expressed in Nm/A). The motor develops its maximum torque Ms at stall (n=0), when the current is maximum and determined only by the supply voltage U and the rotor resistance R:
With increasing speed, an increasing back EMF E is induced in the armature which tends to reduce the current:

The value of E is the product of angular speed ¶ō (expressed in rad/s) and the torque constant (expressed in V/rad/s=Vs=Nm/A):
E = k¶ō
Thus, the supply voltage splits into two parts: RI, necessary to establish the current I in the armature, which generates the torque
M, and k¶ō to overcome the induced voltage, in order to generate the speed¶ō:
U = RI + k¶ō
No-load speed no is a function of the supply voltage and is reached when E becomes almost equal to U; no-load current Io is a function of friction torque:

Power output P is the product of angular speed¶ō and torque M (P = M.¶ō); for a given voltage it reaches its maximum Pmax at half the stall torque Ms, where efficiency is close to 50%. The maximum continuous output power is defined by an hyperbola delimiting the continuous and intermittent operation ranges.
Efficiency¶«is the mechanical to electrical power ratio (¶«= Pm / Pel). Maximum efficiency¶«max occurs at relatively high speed. Its value depends upon the ratio of stall torque and friction torque and thus is a function of the supply voltage:

The maximum continuous torque depends upon dissipated power (I2R), its maximum value is determined by:

Where Tmax is the maximum tolerated armature temperature, Tamb is the ambient temperature, Rmax is the rotor resistance at temperature Tmax and Rth is the total thermal resistance (rotor-body-ambient).
At a given torque M, increasing or decreasing the supply voltage will increase or decrease the speed. The speed-torque function varies proportionally to the supply voltage U.


Small Brushless DC Motor
Principles of operation
    The differences between a DC motor having a mechanical commutation system and a BLDC motor are mainly found in :
- the product concept
- the commutation of phase currents.
From the user's point of view, brushless DC motors follow the same equations as those with brushes: torque is proportional to current, speed depends on the voltage and the load torque.
The commutation of brushless motors
In the conventional DC motor commutation takes place mechanically through the commutator-and-brush system. In a BLDC motor, commutation is done by electronic means. In that case the instantaneous rotor position must be known in order to determine the phases to be energized.
The angular rotor position can be known by:
- using a position sensor (Hall sensor, optical encoder, resolver)
- electronically analyzing the back-EMF of a non-energised winding. This is called sensorless commutation.
Use of Hall sensors
In general, BLDC motor have three phase windings. The easiest way is to power two of them at a time, using Hall sensors to know the rotor position. A simple logic allows for optimal energizing of the phases as a function of rotor position, just like the commutator and brushes are doing in the conventional DC motor.
Use of an encoder or resolver
The rotor position may also be known by use of an encoder or resolver. Commutation may be done very simply, similar to the procedure with Hall sensors, or it may be more complex by modulating sinusoidal currents in the three phases. This is called vector control, and its advantage is to provide a torque ripple of theoretically zero, as well as a high resolution for precise positioning.
Use of Back-EMF analysis
A third option requiring no position sensor is the use of a particular electronic circuit. The motor has only three hook-up wires, the three phase windings are connected in either triangle or star. In the latter case, resistors must be used to generate a zero reference voltage. With this solution the motor includes no sensors or electronic components and it is therefore highly insensitive to hostile environments. For applications such as hand-held tools, where the cable is constantly moved, the fact of just three wires is another advantage.
The functioning of a sensorless motor is easy to understand. In all motors, the relation of back-EMF and torque versus rotor position is the same. Zero crossing of the voltage induced in the non-energised winding corresponds to the position of maximum torque generated by the two energized phases. This point of zero crossing therefore allows to determine the moment when the following commutation should take place depending on motor speed. This time interval is in fact equivalent to the time the motor takes to move from the position of the preceding commutation to the back-EMF zero crossing position. Electronic circuits designed for this commutation function allow for easy operation of sensorless motors.
Small Brushless DC Motor
As the back-EMF information is necessary to know the rotor position, sensorless commutation doesn't work with the motor at stall. The only way of starting is to pilot it at low speed like a stepper in open loop.
- for commutation, position sensors are necessary when operating in incremental mode
- sensorless commutation is recommended only for applications running at constant speed and load.
Operating principle of BLDC motors:
It follows the same equations as the DC motor using mechanical commutation except that parameters like iron losses and losses in the drive circuit are no longer negligible in applications where efficiency is of prime importance.
Iron losses
They depend on speed and, in the torque formula, may be introduced as viscous friction. The equation for useful motor torque becomes:

Losses in the electronics
The current and votage required by the motor and the drive circuit to operated at the desired speed and torque depend also on the drive circuit.
As an example, a driver bridge in bipolar technique will reduce the voltage available at the motor terminals by about 1.7V, and the total current must include the consumption of the circuitry.


What is a Stepping system?

Stepping systems have been used in industrial automation for many years to accurately position machine tools, x-y tables, feeders, etc. Unlike typical AC variable speed drives (used for velocity control), a stepping drive and Stepper Motor is used primarily for position control. A typical single axis Stepping system consists of a Stepper Motor controller/ indexer, a motor drive, a motor (with or without gearbox), and a power supply. A stepper is typically commanded by two digital inputs: a digital pulse train and a direction bit. A single pulse on the pulse input moves the motor one step increment in the direction (CW or CCW) set by the direction bit.

  • Controller/ Indexer
    The controller/ indexer is responsible for outputting the pulse and direction commands to the drive. PLCs distributed by MotionKing, as well as many other 3rd party products, can be used to control the SureStep line. The frequency of the pulse train controls the velocity of the motor, where the number of pulses determines the length of the move. The direction signal determines in which direction the motor will rotate.

  • Stepper Power Supply
    The power supply plays a dual role: it supplies the main power to the motors as well as the power to optically-isolated digital inputs. The motor power is typically provided by a linear non-regulated power supply. The SureStep power supply has a 32 VDC @ 4A (when fully loaded, 41 VDC unloaded) output to supply power to the motor via the drive. Also, the digital interface between the drive and the controller/ indexer should be powered by the isolated +5 VDC (500 mA max) regulated output. The motor power and the digital interface power are typically isolated since the motor power source is extremely "noisy" when loaded. Using the motor power for the interface might lead to false pulse commands thereby creating undesired system movement. One SureStep power supply can provide both motor power and interface power to a least 2 complete SureStep drive/ motor combinations.

  • Stepper Drive
    The drive translates the pulse and direction commands from the controller/indexer and converts them into actual motor movement. For each pulse from the controller, the drive will move the motor "one step" in the direction indicated by the direction command. The SureStep microstepping drive provides 4 different step resolutions for use in a wide range of applications. They range from 1/2 stepping (400 steps/rev) to 1/50 stepping (10,000 steps/rev) when used with a 1.8°„ (200 full steps / rev motor). The SureStep drive can be configured for use with all SureStep brand motors, as well as many other 3rd party 2-phase, bipolar Stepper Motors that require 0.4 to 3.5 Amps/phase to drive them. The drive also features a "test mode" that allow the drive and motor system to be tested without being connected to a controller/indexer. This feature enables an on-board indexer that moves the motor 1/2 revolution back and forth in half-step mode so the user can easily "move" the system while troubleshooting. An idle current cutback feature can be used to conserve power and reduce heat by cutting the power to the motor by 50% if no step pulses are received for 1 second. All the SureStep drive settings are done through a 9-position dip switch, which means that NO SOFTWARE or external resistors are required to configure the drive.

  • Stepper Motor
    The motor converts the power from the drive into rotational movement. Unlike AC motors, Stepper Motors have 100% current (idle-current cutback disabled) applied to them all the time regardless of load on the motor. The motor is moved in "steps" (one per command pulse) and will hold at its present position if no command pulses are received. The SureStep line of motors are built to provide high-torque for their frame size which ranges from NEMA 17 to 34. All SureStep motors have only 4 leads and are connectorized, which greatly reduces the chance of mistakes while wiring the motor to the drive. Many other motor brands typically have 6 or 8 leads, which can cause confusion while wiring the system.

NOTE: For further details or technical specifications, select the desired product category and select the "Products" tab and click on the desired "Item Code" link, or select the "Technical Info" tab.


  Copyright © 2006-2016 All Right Reserved | Sitemap

MotionKing Motor Industry Co.,Ltd.
Stepper Motor (China)
Tel: 0086-519-8988 6261
Fax: 0086-519-8515 1172
Live Chat by comm100