Applying Stepper Motors:

Application Questions You Must Answer & Things to Watch Out For

This webinar will be available afterwards at

designworldonline.com & email

Q&A at the end of the presentation

Hashtag for this webinar: #DWwebinar

Before We Start

Moderator

Miles Budimir Design World

Presenter

Tim Burke ElectroCraft

Applying Steppers

Applying Steppers

Application Variables

6

Mechanical Load reflected to motor

Maximum system speed reflected to motor

Available Voltage

Available Current

Ambient temperature conditions

Position increment & accuracy

Time to execute move

Applying Steppers

Stepping Motor Catalog Data

7

Applying Steppers

Torque-Speed Curves

8

0

10

20

30

40

50

60

70

80

0 2000 4000 6000 8000 10000

To

rqu

e (

oz.-

in)

Speed (Full Steps/s)

OV 15:1 OV 20:1 OV 30:1 OV 40:1

“Typical” Limits for stepper: • 3000 RPM (10K steps/s) for Size 17

• 1500 RPM (5K steps/s) for Size 34

Overvoltage Ratio: Ratio of Drive Supply voltage to Motor Voltage • Desirable to have this at least 15

Torque Margin: Utilize 70 % of available torque at any speed

Torque-Speed, Various Overvoltage Ratios TPP23-90A30

Applying Steppers

Electrical Equation

of Motor Winding: Vs=i*R+L*di/dt+Kb*w*sinA,

A=Elec Angle, w=rotor speed

9

Constant Current Drives & Motor Current Rise

i=Vm/R

i=Vs/R (eg i=15Vm/R)

High

Step Rate Med

Step Rate

Low

Step Rate

Time (mS)

Cu

rren

t (A

mp

s)

Area between curves is

where the extra torque

comes from

Applying Steppers 10

Current Profiles at Speed

Low Speed, Half Stepping Medium Speed, Half Stepping

No Current Regulation, Half Stepping High Speed, Half Stepping

Half Step Phase Energization

Phase A Phase B

1 Off On, + Current

2 On, + Current On, + Current

3 On, + Current Off

4 On, + Current On, - Current

5 Off On, - Current

6 On, - Current On, - Current

7 On, - Current Off

8 On, - Current On, + Current

1 Off On, + Current

Phase Currents

Applying Steppers

Stepper Torque-Speed Curves

11

2 Regions • Current Regulation

• Voltage Drive, current does not reach set point

Scaling • Low speed torque 70% of holding torque

• Knee scaled by overvoltage ratio (volt-amps if changing winding)

• Maximum speed scaled by overvoltage ratio (volt-amps if changing winding)

0

0.2

0.4

0.6

0.8

1

1.2

0 5000 10000 15000

No

rmali

zed

To

rqu

e

Speed (Full Steps/s)

Stepper Torque-Speed

Knee

Current

Regulation Voltage

Drive

Applying Steppers

40

Scaling Stepper Torque-Speed Curves

12

Comparison of test data against scaled curves • 48V scaled from 24V straight line

approximation

Stepper Torque-Speed

24V Actual

24V Estimated 48V Actual

48V Predicted

40

35

30

25

20

15

10

5

0 0 5000 10000 15000

Speed (Steps/s)

Torq

ue

(o

z.-i

n)

Applying Steppers

Connection

13

0

50

100

150

200

250

300

350

400

450

0 2000 4000 6000 8000

To

rqu

e (

oz-i

n)

Speed (Full Steps/s)

Series Parallel 1/2 Cu

Fixed Drive Current

Series Parallel ½ Cu

Terminal

Resistance 2R R/2 R

Power

Loss 2I2R I2R/2 I2R

Torque 2N*I N*I N*I

Inductance L L/4 L/4

Elec. Time

Constant L/2R L/2R L/4R

Torque-Speed Various Connections

Diagrams

Phase A

Phase B

Phase A

Phase B

Phase A

Phase B

Applying Steppers

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.00 20.00 40.00 60.00 80.00 100.00

Tem

pe

ratu

re (

de

g C

)

Time (min)

Typically conservative rating • NEMA ICS16 spec describes procedure

• Motor Hanging in free air

• Motor held in stall condition with full current in both phases

ElectroCraft motors rated for 130 °C

14

Thermal Ratings

Temperature Rise Size 17 Stepper

Applying Steppers

0.00

20.00

40.00

60.00

80.00

100.00

120.00

0.00 20.00 40.00 60.00 80.00 100.00

Tem

pe

ratu

re (

de

g C

)

Time (min)

Data 2 Node 3 Node

Simple Model • 2 Node: Winding Temp & Ambient Temp

• Some manufacturers publish data

• Thermal Resistance

• Thermal Time Constant

More Complex Model • 3-Node Model

• Winding temp

• Case temp

• Ambient temp

15

Thermal Model

Temperature Rise Size 17 Stepper

Tw Ta

Tw Tc Ta

Cw Cc

Cw

Applying Steppers

Can overdrive motor as long as thermal limit not exceeded • Thermal model allows calculation of

allowable duty cycle

• Torque falls off after rated current

• Note: Torque-speed curves are continuous rated operation

16

Intermittent Duty and Overdriving Motor

0

10

20

30

40

50

60

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

To

rqu

e (

oz-i

n)

Current (amps)

Torque vs. Current

Rated Current

2x Rated

Applying Steppers

Mechanisms of Loss

17

I2R Loss (Joule Heating) • Dominant at Low Speed

Core Loss • Primarily in stator lamination

• Both hysteresis and eddy current

• Hysteresis is f(step rate)

• Eddy Current is f(step rate)2

0

1

2

3

4

5

6

7

8

9

10

0

5

10

15

20

25

30

35

40

0 2000 4000 6000 8000

Po

we

r L

os

s (

W)

To

rqu

e (

oz.-

in)

Speed (Steps/s)

Torque-Speed I*I*R Core Loss Total Loss

Stepping Motor Losses

Applying Steppers

Resolution: • 360o/(# steps/rev.)

Accuracy: • Typically 3-5%

• Non-cumulative

• Influenced by:

• Friction (Application)

• Stiffness of motor (Motor Design)

Hysteresis • Error found when

approaching same position from either direction

• Typically 3%

18

Angle Accuracy

Applying Steppers

Full Step, Half Step & Microstepping

19

Microstepping

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.5 1 1.5

Torque Vectors – Full, Half & Microstep (Position shown in Electrical Degrees)

Full Step

Microsteps

Half Step

Full Step

• Microstepping accomplished by regulating phase currents

• Tq=Kt*Isin(wt)*sin(Q)+Kt*Icos(wt)*cos(Q)

Microsteps

Applying Steppers

Microstepping

Be careful if used for positioning • Friction band leads to

high error, may result in motor not moving

• Harmonics in torque profile lead to additional errors (cogging or detent torque)

20

-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0.5 0 0.5 1 1.5 2 2.5To

rqu

e

Mechanical Degrees

Full Step

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

-0.3 -0.1 0.1 0.3

To

rqu

e

Mechanical Degrees

1/16th Microstepping

Step 1 Step 2 Step 1 Step 2

Applying Steppers

Steppers capable of very high acceleration rates (high Torque/inertia ratio)

Start-Stop Rate – Step rate at which motor will pull into synchronism • Speeds above start-stop

will require acceleration

• Function of total system inertia

• Proportional to sqrt(1/total inertia)

21

Stepper Start-Stop Rate and Acceleration

Stepper Move Profile 18 Step Move, Constant Acceleration

Start-Stop Rate

0

500

1000

1500

2000

2500

0 0.005 0.01 0.015

Ve

loci

ty (

Full

Ste

ps/

s)

Time (sec)

Start-Stop Rate

Applying Steppers

Stepping at motor’s natural frequency • wn=sqrt(Kt*I*A/J), Kt=Torque Constant,

I=operating current, A=# of pole pairs, J=inertia

• Typically found at 150 to 450 steps/sec

• Influenced by reflected inertia of load

Remedies • Add mechanical damping or friction

• Microstepping

22

Common Failure Mode

0

50

100

150

200

250

300

350

400

450

0 500 1000 1500 2000 2500 3000

To

rqu

e (

oz-i

n)

Speed (Full Steps/s)

Torque-Speed Low Frequency Resonance

Applying Steppers

Common Failure Mode

23

Mid-Frequency Resonance • Less severe, but occurs at much

higher step rates

• Traced to oscillation of BEMF (amplitude and frequency modulation) leading to unstable torque

Electronic damping found in many newer drives will quiet this behavior

0

5

10

15

20

25

30

35

40

0 2000 4000 6000 8000 10000 12000

To

rqu

e (

oz-i

n)

Speed (Full Steps/sw)

Torque-Speed Mid Frequency Resonance

Applying Steppers

Linear Actuators • Conversion of rotary to linear

done within motor envelope

• Requires external hardware to guide screw & support loads

• Features • Low cost linear motion

• High resolution

• High force

24

Stepping Motor Linear Actuators

Applying Steppers

Translating Screw • Screw must be prevented from

rotating. For “long” travel must allow clearance behind motor for screw

Translating Nut • Must provide guide or rail

for nut to prevent rotation.

25

Types of Actuator

Applying Steppers

Types of Screw

Lead Screw • ACME Thread Design

• Efficiency Range from 0.3 to 0.7

• Function of coefficient of friction

• Backlash range from .002” to .007”

Ball Screw • Efficiency Range from 0.8 to 0.95

• Lower friction leads to better accuracy

• Backlash from 0.000” to 0.010”

26

Applying Steppers 27

Force Speed Curves

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15 20 25

Fo

rce (

lb.)

Linear Speed (in/s)

A-Thread Tested

B-Thread Tested

C-Thread Tested

Size 17S - Force vs. Linear Speed 48V, 2A RMS

Formula:

Eff=L/(dm*p)*((L+p*m*sec(a))/(p*dm-m*L*sec(a)))

• L = Thread Lead

• dm = Pitch diameter

Lead of screw is another variable to tailor performance • A-thread: 0.0625” lead

(16 Threads/in.)

• B-thread: 0.125” lead (8 Threads/in.)

• C-thread: 0.25” lead (4 Threads/in.)

Efficiency of lead screws • A-thread: 42%

• B-thread: 58%

• C-thread: 68%

• m = Coef. of Friction

• a = Thread angle/2

Questions?

Design World Miles Budimir mbudimir@wtwhmedia.com Phone: 440.234.4531 Twitter: @DW_Motion

ElectroCraft Tim Burke tburke@electrocraft.com Phone: 603.516.1255

Thank You

This webinar will be available at designworldonline.com & email

Tweet with hashtag #DWwebinar

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Applying Steppers