Stepper Drives - Motion Control Tips

Three ways to reduce heat when operating a step motor

June 29, 2018 By Lisa Eitel Leave a Comment

Reduce motor heat by reducing idle and running current — or switch to closed-loop control.

Step motors are widely used motion-control devices because of their simple setup and accurate positioning with an open-loop configuration. But problems associated with heat generation can arise. That’s because the drive that powers an open-loop step motor doesn’t use feedback to control the amount of current provided to that motor. Instead, the drive provides full current at all times … regardless of torque demand on the motor. So the motor consumes power at excessive rates …. wasting energy and generating avoidable amounts of heat.

In some applications, operators run open-loop step motors with as much current as possible to generate the most torque possible. That ensures the motor will reach its position in the application. But as current increases, so does motor temperature. If motor temperature rises too high, windings short and the motor ceases to function. So design engineers (and motor end users) must find ways to maximize motor torque while avoiding motor overheating.

One industry approach is to set the drive to give the motor its rated current. Doing that will make the motor hot but not too hot. It’s normal for step motor temperatures to reach 70°, 80°, or even 90°C. While these temperatures make it too hot to touch the motor, the motor itself is unharmed.

But how else can design engineers (and motor end users) reduce motor heat? Here we’ll explore three more ways:

Reduce idle (holding) current
Reduce running current
Switch to closed-loop control

First — Reduce the idle (holding) current for the motor

Many motion control applications need the most torque when the motor accelerates and decelerates to meet its mark. But when the motor idles, or holds position, it requires much less torque. These instances are a good time to reduce motor current.

Most modern stepper drives do this automatically. For example, the drive might reduce idle current to 50% of the running current. More sophisticated stepper drives let design engineers program the idle current to any value between 0 and 100%. If a step motor spends even a modest amount of time sitting still, reducing idle current has a significant impact on motor heat.

Second — Reduce running current

Many design engineers err on the side of caution when selecting a step motor — often choosing a motor with more than enough torque (rather than just enough) for the application at hand. In these scenarios, it may be possible to reduce the motor’s running current with some trial-and-error testing. Basically, this requires reducing running current by a small amount and monitoring the motion output. If this trial run proves successful, reduce running current again and retest motor performance.

Continue making adjustments until the motor mispositions or stalls. Then slightly increase the current so the motor resumes accurate positioning. In some cases — especially when a step motor is oversized for an application — the engineer can reduce running current enough to measurably reduce the temperature of the motor.

Third — Switch to a closed-loop step motor system

The most dramatic reduction in heat generation occurs when replacing an open-loop step motor system with a closed-loop step motor system. Step motors are operable within a servo control loop when outfitted with a high-resolution encoder and powered by a suitable closed-loop drive. This relatively new system configuration necessitates that the engineer replaces the application’s motor and drive. But the magnitude of temperature reduction is usually worth it.

To review, a closed-loop step motor system uses feedback loops to accurately control step motor current, velocity, and position. The current loop ensures that the current provided to the motor by the drive is only the amount needed to meet torque demand. When the motor isn’t producing torque (or producing anything less than maximum torque) current to the motor automatically drops accordingly. This closed-loop control scheme significantly improves motor temperature … reducing it by as much as 50% or more in some applications.

Additional benefits of closed-loop steppers are quicker acceleration and higher throughput (up to 50% more torque than rated holding torque) as well as quieter operation (even to 10 dB quieter) because there’s less current in the motor windings. Plus without stalls, system accuracy is higher.

Closed-loop step motors incorporate a high-resolution encoder and are driven by an integrated, closed-loop drive that automatically reduces current to the motor when the load isn’t demanding torque.

Test results confirm that closed-loop step motors draw current only when needed, so they run cooler and with less power consumption than open-loop options. Consider actual tests that run both the open-loop and closed loop variations to get the same motion profile:

Acceleration = 100 rev/sec² and deceleration = 100 rev/sec²
Distance = 5 rev and speed = 10 rev/sec
Dwell time = 0.1 sec

Both carry a set load inertia and rotor inertia and operate from a 48-Vdc power supply. Results in the graph below show that the average power consumption of an open-loop stepper system delivering such output uses 43.8 W.

In contrast, a comparable closed-loop stepper system consumes only 14.2 W to deliver the same routine. That reduced power consumption translates into less heat generation and lower energy bills.

Results from tests conducted by Applied Motion Products confirm that closed-loop stepper systems consume less current than comparable open-loop stepper systems — and stay cooler too.

Applied Motion Products | applied-motion.com

Eric Rice graduated with an Electrical Engineering degree from the University of Illinois Urbana-Champaign.

He joined Applied Motion Products as an applications engineer in 1997 and specializes in step motors, servo motors, drives, and controls.

New series of stepper drives for ac-powered applications from Applied Motion Products

May 8, 2018 By Miles Budimir Leave a Comment

Applied Motion Products expands its family of STR Stepper Drives with two new ac-powered units for use in industrial applications powered by ac supply voltages. Offering the benefits of low noise, smooth motion, and high torque over wide speed ranges, the STRAC Stepper Drives bring high-performance motion control to basic step and direction applications. Units are ideal for larger motors and applications requiring maximum power.

Like their dc-powered counterparts, STRAC Stepper Drives feature advanced microstepping performance and current control with anti-resonance that improves motor smoothness and torque over a wide speed range. Units operate on ac supply voltages of 90 to 240 Vac and wire directly to 120 or 220 Vac line voltages to eliminate dc power supply requirements. The drives internally convert ac line voltages to high dc bus voltages to drive large step motors at maximum power.

Available in two models, the STRAC2 provides up to 2.2 A/phase while the STRAC8 provides up to 8.0 A/phase. Every stepper drive in the STR Series operates in step and direction or pulse/pulse control mode. Selecting between the two modes is simple by moving a jumper located under the drive cover or via dip switch on the front panel. Each drive microsteps to 25,000 steps/rev with a 1.8° step motor (1/125 step) even when command pulses are low resolution – thanks to a proprietary step smoothing filter. Users can set up drive parameters – including motor selection, current, and step resolution – using dip or rotary switches. No software is required.

The STRAC Stepper Drives are UL Recognized, CE certified, and RoHS compliant. Applied Motion Products offers a full complement of NEMA 23 and 34 frame step motors optimized for use with these new ac-powered drives.

For more information on the STRAC Stepper Drives, visit www.applied-motion.com.

Stepper drives: What’s the difference between an L/R drive and a chopper drive?

April 19, 2018 By Danielle Collins Leave a Comment

There are two fundamental types of stepper drives: L/R drives and chopper drives.

L/R stepper drives are also referred to as “constant voltage” drives, because they supply constant voltage to the motor windings. The current produced in the windings depends on the motor’s time constant, which is the relationship between its inductance and resistance (L/R). The time constant causes current to increase slowly with each voltage pulse. So for very high pulse rates (i.e. high motor speeds), full current, and thus full rated torque, may not be reached. This limits the use of L/R drives to primarily low-speed applications.

Torque vs. pulse rate for a typical unipolar L/R drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Current in the windings can be increased by using higher supply voltage, but motor temperature rise then becomes a problem. When the applied voltage is high, the “off” time (when no power is applied) must be long enough to prevent the motor from overheating. Another option is to add resistors in series to improve the time constant (L/2R or L/4R, for example) and increase the voltage. But resistors waste power through heat generation, so efficiency is reduced.

Resistance (R) affects the maximum current in the windings, according to Ohm’s law, I = V/R.

Inductance (L) affects the rate of change of current in the windings, according to the relationship, dI/dt = V/L.

Because of the torque and speed limitations of L/R stepper drives, an increasing number of stepper motor applications now use chopper drives. These drives are also referred to as “constant current” drives because they supply constant current to the motor windings, by “chopping” the output voltage (i.e. turning the output voltage on-and-off very rapidly).

With each motor step, a very high voltage (typically eight times the motor’s nominal voltage) is supplied to the motor windings. This high voltage gives a very short current rise time, according to the relationship for an inductor (dI/dt = V/L), and it causes higher current to be produced, according to Ohm’s law (I = V/R).

Torque vs. pulse rate for a typical bipolar chopper drive with a 15 degree step angle.
Image credit: AMETEK Inc.

Voltage chopping also modulates the width of the output pulses (pulse width modulation) and is typically done at a frequency of 20 kHz or higher — above the audible range. The impedance in the windings varies with the motor speed, so it plays a role in determining the voltage on-time.

At slow motor speeds — and low impedance — the chopper drive provides a short on-time for the voltage, producing a small pulse width. Alternatively, at high speeds —and high winding impedance — the chopper drive provides a long on-time for the voltage, producing a large pulse width and allowing time for the current to rise sufficiently to produce the rated torque.

Relationship between voltage and current in a (chopper) drive.
Image credit: Oriental Motor USA Corp.

A current-sensing resistor placed in series with each winding regulates the current and ensures the shortest time possible for current to build up and decline. As the current increases, voltage develops across the resistor. This voltage is monitored by a comparator, and at a predetermined reference voltage, the output voltage from the drive is turned off (chopped) until the next pulse occurs. This allows current to build and decline as the voltage is switched on and off.

By controlling the duty cycle of the drive through chopping, the average voltage and current are kept equal to the motor’s nominal voltage and current ratings. The result is precise torque control, and more importantly, higher torque production at high motor speeds.

Microstepping drives are common versions of chopper drives.

Feature image credit: AMETEK Inc.

Stepper drive is internationally certified for use in hazardous locations

March 6, 2018 By Paul Heney Leave a Comment

Applied Motion has announced the first and only step motor drive internationally certified for use in hazardous locations. The STAC6-Q-H Stepper Drive is ATEX and IECEx certified for Class I, Zone 2 locations, and UL certified for Class 1, Div. 2, Group C & D locations; Class 1, Zone 2 locations. The HazLoc Step Motor Drive is approved for use in hazardous industrial applications throughout the USA, Canada, Europe and the Middle East.

Addressing the need for step motors in hazardous locations in the oil, gas, paint, and printing industries, the STAC6-Q-H Stepper Drive operates in environments where flammable gases, vapors or liquids may be present in the air in quantities to produce explosive or ignitable mixtures. The STAC6-Q-H is currently in operation driving a step motor to control the braking and speed of the drill string of on oil rig.

Powered by single-phase 120 VAC that generates a nominal 160 VDC internal bus, the STAC6-Q-H provides superior performance when paired with high-quality NEMA 23, 24, 34 or 42 frame step motors. An advanced anti-resonance algorithm ensures optimal torque over a wide speed range, while microstepping to 51,200 steps/rev provides for smooth, high-resolution positioning.

An encoder feedback option is available for enhanced system accuracy using Applied Motion’s proprietary Stall Detection and Stall Prevention features, which overcome limitations of open loop step motor systems. The drive is capable of running stored programs created with Applied Motion’s Q programming language, and has sufficient input/output signals to connect to multiple external devices or sensors.

For more information on Applied Motion’s STAC6-Q-H Stepper Drive for Hazardous Locations contact the company for technical support at 1-800-525-1609.

Applied Motion Products
www.applied-motion.com

Absolute type AZ Series stepper motor driver with EtherCAT from Oriental Motor

November 22, 2017 By Lisa Eitel Leave a Comment

Oriental Motor now sells a DC-input AZ Series Multi-Axis Drivers with built-in EtherCAT communications. These are based on closed-loop absolute-type AZ Series driver.

The AZ Series Multi-Axis Drivers with EtherCAT is a space saving, DC input, multi-axes controller and driver designed for applications demanding 2 or more axis of control and integrating EtherCAT communications from an upper lever controller.
These new drivers come in 2 axes, 3 axes or 4 axes configuration. It is compatible with all of the DC input AZ Series family of products, including DC input stepper motors, linear and rotary actuators. The advantages of consolidating up to 4 drivers into 1 unit includes space saving benefits, reduced cost and simplified wiring.

The AZ Series Multi-Axis Drivers supports EtherCAT communication and is compatible with the CiA402 drive profile.
Learn more about the AZ Series Multi-Axis Driver with EtherCAT at the deep link on orientalmotor.com.

Since its founding in Japan in 1885, Oriental Motor has been a world leader in motion systems. For over a century we have concentrated on technological advancement and product design improvement—an emphasis evident in the sophisticated devices we market today. Oriental Motor’s sales and service network is international, with offices throughout North America, Europe and Asia. Domestically, ORIENTAL MOTOR U.S.A. CORP. was established in 1978.

Controller-drivers steer stepper-motor-driven sample-transfer machine

November 9, 2017 By Lisa Eitel Leave a Comment

Gatan Inc. makes specimen-preparation equipment and recently built an alpha prototype of a new machine. The iPrep prototype mounts to a scanning electron microscope (SEM) and shuttles samples back and forth between a processing chamber and SEM receiving dock. Within the processing chamber, two broad-beam ion guns polish samples.

The iPrep’s automated transfer mechanism allows use of SEM-related imaging techniques for 3D reconstructions of sample areas with exact control over slice thickness.

This is Gatan’s alpha prototype of an iPrep machine that next to a scanning electron microscope. It is a high-vacuum-compatible receiving dock — one that can withstand E-6 Torr — with a kinematic mechanism that lets samples get mounted with accuracy to better than 1 µm every time. In this application, the workflow also consists of dozens of very specific operations … all needing reliable handling thousands of times. The iPrep from Gatan balances several parameters to deliver on these design objectives.

The iPrep machine employs an AllMotion controller to command a transfer arm’s end effector to accurately open and close … and move a sample carrier to the polishing chamber and SEM. Another AllMotion EZHR23EN48V controller is part of a kinematic mounting system on the receiving dock inside the SEM. Stepper motors drive the end effector and receiving dock.

AllMotion EZHR23EN48V controller+drivers have selectable step resolution from half stepping to to 1/256 steps … and can deliver to 20 million microsteps per second.

The EZHR23EN48Vs are controller-drivers measuring 2.25 by 2.25-in. and bolt to the backs of the stepper motors they control. The controllers use a single four-wire bus with two power wires and two for communication. Simple commands are made via any serial terminal program or from the EZStepper Windows application.

Because every operation the iPrep performs occurs in a vacuum chamber, it’s impossible to hear or see when things go wrong. So Gatan relies entirely on sensor data (leveraging EZHR23EN48V wiring for OptoSwitch inputs) to identify problems in the chamber. If this does occur, workflow is halted for maintenance.

The main specifications for the system lie in the repeatability of placement in the SEM. 3D reconstruction only works with very low shift and rotation between placements. “Our tolerance for shift between images in the SEM (after transferring) is on the order of several nanometers,” said Thijs Hosman, project manager and research scientist for Gatan. Plus incorporating shift detection algorithms can compensate for some of the shift between images … but the mechanical placement accuracy still must be on the order of 1 µm. The tolerance for rotation is about 0.05° between images. The distance between the preparation chamber and the SEM is on the order of 1 meter.

New AC-powered STR stepper drives from Applied Motion Products

September 22, 2017 By Lisa Eitel Leave a Comment

Applied Motion Products, Inc. now sells an expanded line of STR Stepper Drives with the release of new drives for use in AC powered applications. STR stepper drives are designed for smooth motion and high torque over a wide speed range, bringing high performance motion control to basic step and direction applications. Two new AC powered STRAC drives expand the offering to cover larger motors and applications where maximum power is required.

Operating on AC supply voltages of 90 to 240 AC, these new STRAC drives can be wired directly to 120 or 220 VAC line voltages, eliminating the need for a DC power supply. AC line voltages are internally converted by the drives to high DC bus voltages that enable larger step motors to be driven with maximum power. These drives are designed for demanding applications where high torque and high speed are necessary.

The STRAC2 provides up to 2.2 A per phase while the STRAC8 provides up to 8.0 A/phase. All STR drives are great for OEM applications needing basic step and direction control of a 2-phase step motor. STR drives feature advanced microstepping performance and sophisticated current control with anti-resonance. Anti-resonance electronically damps motor and system resonances, which improves smoothness of motion and torque over a wide speed range.

“Step and direction drives make up more than 80% of the stepper drive market because customers want drives that are easy to configure and that can be commanded via simple, digital control interfaces. The addition of the cost effective, AC powered STRAC drives to the already strong STR offering means a more complete series of step and direction drives for the majority of users,” said Dennis Joyce of Applied Motion Products.

Every STR step motor drive operates in step and direction or pulse/pulse control mode. Selecting between these two modes is done by moving a jumper located under the cover of the drive. Each STR drive can microstep up to 20,000 steps/rev with a 1.8° step motor (1/100 step) and can even microstep the step motor when the command pulses are low resolution. This feature is called Microstep Emulation. All drive setup is done via dip or rotary switches on the side of the drive, including motor selection, running current, idle current, and step resolution.

Because of the high internal bus voltage of STRAC drives, step motors capable of withstanding high voltage should be used. Applied Motion Products offers a full complement of NEMA 23 and 34 frame step motors optimized for use with these new AC powered drives. Like the STRAC drives, many of these motors are UL recognized, CE certified, and RoHS compliant. To review, the feature of the stepper drives include:

Advanced functions for step and direction applications
Support for both step and direction and pulse/pulse control
Supply voltages from 90 to 240 volts AC
Up to 2.2 A/phase (STRAC2) or 8.0 A/phase (STRAC8) output current, peak of sine
Easy setup via dip switches and rotary switches … no software required
Air cooled enclosures — so no fans or moving parts
UL Recognized, CE certified, and RoHS compliant

For more information, visit Applied Motion Products’ deep link to the STR product page.

How do switched reluctance motors differ from stepper motors?

September 1, 2017 By Danielle Collins Leave a Comment

Switched reluctance motors operate by switching currents in the stator windings in response to changes in the magnetic circuit formed by the rotor and stator. The stator of a switched reluctance motor contains windings, similar to a brushless DC motor, but the rotor is simply made of steel that is shaped into salient poles, with no windings or magnets. To avoid a situation where all the poles of the rotor and the stator line up simultaneously (and no torque is produced), switched reluctance motors have fewer poles on the rotor than on the stator.

A typical 8/6, four-phase switched reluctance motor. Notice that when rotor poles 3 and 6 are aligned with stator poles B and B’, all other rotor poles are out of alignment with the stator poles.
Image credit: scienceprog.com

When the rotor and stator poles are out of alignment, the magnetic circuit between them has a high reluctance. As the stator pole pairs are energized, the rotor turns to align with the energized stator poles, which minimizes the reluctance of the magnetic circuit. This tendency of the rotor to move to a point of minimum reluctance produces what is referred to as reluctance torque.

Energizing of the stator poles must be precisely timed to ensure that it occurs as the rotor pole is approaching alignment with the energized stator pole. Unlike stepper motors, which can, and for most purposes do, operate in open-loop mode, switched reluctance motors require position feedback from an encoder or Hall effect sensors, to control commutation of the stator currents based on the precise rotor position.

This article provides an in-depth look at drive options for switched reluctance motors, and this article looks at how drive technologies could make switched reluctance motors more common in the future.

Switched reluctance motor with 6 stator poles and 4 rotor poles. Notice that the stator poles protrude toward the rotor, and the rotor poles protrude toward the stator. This is known as a doubly salient pole design.
Image credit: chargedevs.com

Switched reluctance motors have fewer poles and a larger stepping angle than stepper motors. While stepper motors are typically chosen for positioning applications, where step integrity and high resolution are important, switched reluctance motors are used in applications where power density is a primary concern.

Because switched reluctance motors have rotors with no magnets or windings, they have lower inertia and can therefore achieve higher accelerations and speeds than motors with permanent magnet rotors, such as stepper motors. The lack of magnets on the rotor provides other benefits as well – including the ability to withstand higher temperatures (less cooling required) and simple, lower-cost construction than permanent magnet motors.

Another difference between switched reluctance motors and stepper motors lies in the stator construction. In a switched reluctance motor, there is no overlap of coils between successive phases – in other words, the phases are independent of one another. This means that if one or more phases fail, the motor will still be operable, although with reduced torque output.

The fact that both the stator and rotor have salient poles (referred to as a doubly salient design) means switched reluctance motors produce more audible noise than stepper motors. The primary source of noise is distortion of the stator due to radial forces that occur when the stator pole pairs are energized. The energized pole pairs are attracted to one another, causing radial forces strong enough to distort the stator.

Salient poles are simply magnetic poles that essentially “stick out” from the diameter of the rotor (or stator), towards the stator (or rotor).

Torque ripple is also a common issue with switched reluctance motors. While both switched reluctant and stepper motors exhibit torque ripple, this effect is minimized in stepper motors with a higher number of phases (5 phases versus 2, for example). In a switched reluctance motor, torque ripple can be reduced by using a higher number of poles in both the rotor and the stator, but this decreases the motor’s average torque output, requiring a tradeoff between torque production and acceptable torque ripple.

IDEC releases line of stepper motion control products

July 10, 2017 By Paul Heney Leave a Comment

IDEC Corp. introduces a line of motion control stepper products in conjunction with Advanced Micro Controls Inc. (AMCI), an established developer of motion control systems. This AMCI by IDEC product line includes controllers, stepper motors, an integrated controller/drive, an integrated drive/stepper motor, and an integrated controller/drive/stepper motor. When combined with an IDEC FC6A PLC and its embedded motion control macro instructions, these products allow users to quickly and simply implement single- and multi-axis motion control in a wide range of industries and applications.

Many industrial applications require motion control, but users are often frustrated by the complexity and high cost of implementation, particularly with servo systems. IDEC and AMCI have partnered to address this issue by offering a simple yet highly capable stepper motion control system. Macro instructions embedded in the PLC WindLDR programming software can be configured with drag and drop commands to perform control of up to 12 axes.

The integrated controller/drive and drive/stepper motor units save space and money, and simplify installation, by combining multiple components into a single unit. The integrated controller/drive/stepper motor units offer the ultimate in space saving, cost effectiveness and system simplification.

All AMCI by IDEC products provide users with AMCI’s expertise in motion control combined with IDEC’s service, support and extensive distribution network. Motion control macro instructions embedded in IDEC’s FC6A PLC reduce design and programming time, allowing quick implementation in a wide variety of applications. Because all the components and the software modules are provided by IDEC and AMCI in close partnership, users are assured of correct operation right out of the box, with no need to perform integration among the components.

As compared to more complex servo motion control systems, steppers are a simpler technology, and require no tuning or adjustment. They also have excellent torque at low speeds, don’t require position feedback, and have very low maintenance requirements. These characteristics make them a great fit in a variety of industries including medical/pharmaceutical, printing, packaging, material handling and semiconductor.

Linear applications within these and other industries include conveyers, rail positioning, pushing applications/part eject, X-Y tables and filling stations. Rotary applications within these and other industries include indexing tables, material feed, cut-to-length, labelers and winding.

AMCI by IDEC’s iANF2E (2-axis) controllers can accept encoder feedback to provide stall detection and move verification. Each controller includes six discrete inputs for move conditioning and other functions, and four discrete outputs to indicate status and provide diagnostics. Each controller has a full complement of motion profiles, which can be selected by an IDEC MicroSmart FC6A PLC connected to the controller via an Ethernet-based Modbus TCP communications link. Up to six controllers can be connected to the PLC to control up to 12 axes of motion.

AMCI by IDEC’s iSM2340 NEMA 23 stepper motor comes in standard frame sizes with output torque of 130 or 240 oz-in.

AMCI by IDEC’s iANG1E integrated controller/drive unit can control one to six axes of motion, and it combines all the features of AMCI’s 1-axis controller and a 4.0A stepper drive into a single unit.

AMCI by IDEC’s iSMD23 integrated drive/stepper motor unit provides up to 240 oz-in of torque, and can be paired with the iANF2E to provide a complete solution. The unit can also be controlled by the PLC.

AMCI by IDEC’s iSMD23E integrated controller/drive/stepper motor unit provides all-in-one motion control with all the features of AMCI’s 1-axis controller, drive and stepper motor. No heat sinks are required due to its thermally efficient design. The integrated unit can operate over a wide range of input power, from 24-48Vdc.

IDEC
www.idec.com

Why is the electrical time constant important for stepper motors?

April 26, 2017 By Danielle Collins Leave a Comment

Motor windings have a property referred to as inductance, which is a resistance to any change in the electrical current through the winding. The result of inductance is the production of back EMF to oppose the change in current.

Where:

E_b = back EMF (volts)

L = inductance (H, or Ohm-s)

dI/dt = rate of change of current

Note that the magnitude of dI/dt is expressed as frequency times current (ω x I). The value of frequency is given by:

Where:

ω = frequency (rad/s)

S = motor speed (rpm)

N = number of magnetic cycles per shaft rotation

The windings also have a resistance to the current flowing in them. This resistance is proportional to the square of the number of turns in the winding. According to Ohm’s Law, voltage equals current times resistance: V = I x R. Ohm’s law expressed in terms of resistance shows:

Where:

R = resistance (ohms)

V = voltage (volts)

I = current (amps)

While resistance and inductance are similar properties, resistance determines the maximum current in the winding, whereas inductance determines the maximum rate of change of current in the winding.

The electrical time constant is the amount of time it takes the current in the winding to reach 63 percent of its rated value. The time constant found by dividing inductance by resistance.

Where:

τ_e = electrical time constant (s)

The time constant, τ, is the time it takes the current in the winding to reach 63 percent of its maximum rated value.
Image credit: NJR Co., Ltd.

Q: Why does the electrical time constant represent 63 percent of the maximum rated current (and not, say 50 percent or 85 percent)?

A: Because the time constant of an increasing system is the time that it takes the system’s step response (the resulting output when the input changes from zero to one very quickly) to reach 1 – 1/e of its final value. The value of 1 – 1/e is 0.632, or approximately 63 percent.

A lower time constant is usually desirable, because it means that current flows more quickly into the windings. This allows the motor to reach its rated torque before current is switched to the next phase. When the motor speed is high (high stepping frequency), there’s not enough time for the winding to receive sufficient current to produce the rated torque.

There are two ways to achieve high speed from a stepper motor: increase the rate of current flow, or decrease the time constant by keeping the inductance low. The first option, increasing the rate of current flow, requires increasing the voltage to the motor. While stepper motors can be run at higher than rated voltage, the higher voltage also means higher maximum current, which could cause the motor to overheat or the rotor to be demagnetized. So in this solution, some form of current limiting must be employed.

The second solution, using a motor with low induction (and therefore, a lower time constant), is a matter of motor selection. However, low induction means higher current, and a stepper drive with a higher current rating—and therefore, higher cost—will be required.