ME 407: Mechanical Engineering Design

Assistant Prof. Melik Dölen

Department of Mechanical Engineering

Middle East Technical University

Electric Motors/Drivers& Their Selection

ME 407 2

Outline – Electric Motors* • Classification of Electric

Motors– Stepper Motors

– DC Motors

– Brushless DC Motors

– Induction Motors

• Fundamentals of Motor Drives– DC Motors

• Motor/Driver Selection Procedure– Load Analysis

– Performance Curves• Applications

• Summary

[*] W. Voss, A Comprehensible Guide to Servo Motor Sizing, Copperhill Tech.Corp. 2007.

ME 407 3

Electrical Motors

• In most industrial applications, electrical motors are extensively used as actuators.

• Four motor systems are common alternatives in machine tool designs:– Stepper motors: Simple applications (e.g. desktop manufacturing

tools)– DC motors: Earlier CNC machine tools and specialized machine

tools– Brushless DC motors: Principle axis drives for contemporary CNC

machine tools– AC (Induction) motors: High-power spindle drives.

ME 407 4

Stepper Motors

• Stepper Motors– Permanent Magnet

• Relies on rotor magnets– Variable Reluctance

• Relies on rotor saliency– Hybrid Motors

• Relies on both rotor saliency and magnets

• Each pulse moves rotor by a discrete angle (i.e. “step angle”).

• Counting pulses tells how far motor has turned without actually measuring (no feedback!).

ME 407 5

Advantages / Disadvantages

Low costSimple and ruggedVery reliableMaintenance freeNo sensors neededWidely accepted in

industry

Resonance effects are dominant

Rough performance at low speed

Open-loop operation Consume power even

at no load

ME 407 6

(Simplified) Full-Step Operation

• Rotor of a PM stepper motor consists of a permanent magnet: – Stator has a number of

windings.

• Just as the rotor aligns with one of the stator poles, the second phase is energized.

• The two phases alternate on and off to create motion.

• There are four steps.

N

S

Current

S

N

Coil A

Coil B

Coil C

Coil D

S N

Coil A

Coil B

Coil C

Coil D

Current

NS

S

N

Coil A

Coil B

Coil C

Coil D

Current

N

S

SN

Coil A

Coil B

Coil C

Coil D

Current

N S

1 2

34

ME 407 7

(Simplified) Half-Step Operation

N

S

Current

S

N

Coil A

Coil B

Coil C

Coil D

S N

Coil A

Coil B

Coil C

Coil D

Current

NS

Coil A

Coil B

Coil C

Coil D

N SSN

Coil A

Coil B

Coil C

Coil D

Current

NS

1 2

78

Current

S

N

Coil A

Coil B

Coil C

Coil D

S N

Coil A

Coil B

Coil C

Coil D

Current

NS

S

N

Coil A

Coil B

Coil C

Coil D

Current

N

S

SN

Coil A

Coil B

Coil C

Coil D

Current

NS

3 4

56

S

N

S N

NS

Current

S

N

Current

S

N

Current

SN

S

N

Current

Current

ME 407 8

Half-Step Operation (Cont’d)

• Commutation sequence has eight steps instead of four.

• The main difference is that the second phase is turned on before the first one is turned off.

• Sometimes, both phases are energized at the same time.

• During the half-steps, the rotor is held in between the two full-step positions.

• A half-step motor has twice the resolution of a full-step motor. – Very popular due to this reason.

ME 407 9

Actual Stepper Motor*

• The stator of a real motor constitutes more coils (typically 8).

• These individual coils are interconnected to form only two windings:– one connects coils A, C, E,

and G:• A and C have S-polarity• E and G have N-polarity

– one connects coils B, D, F, and H:

• B and D have S-polarity• F and H have N-polarity

A

E

B

C

DF

G

H

N

SNN

SS

[*] Courtesy of Microchip.

ME 407 10

PM Stepper-Motor Animations*Full-step: Half-step:

[*] Courtesy of Motorola, Inc.

ME 407 11

Conventional DC Motor• The stator of a DC motor is composed of

two or more permanent magnet pole pieces.

• The rotor is composed of windings which are connected to a mechanical commutator. In this case the rotor has three pole pairs.

• The opposite polarities of the energized winding and the stator magnet attract and the rotor will rotate until it is aligned with the stator.

• Just as the rotor reaches alignment, the brushes move across the commutator contacts and energize the next winding.

• A spark shows when the brushes switch to the next winding.

Courtesy of Motorola, Inc.

ME 407 12

Brushless DC Motor

• A brushless DC motor (BLDC) has a rotor with permanent magnets and a stator with windings.

• It is essentially a DC motor turned inside out. The brushes and commutator have been eliminated and the windings are connected to the control electronics.

• The control electronics replace the function of the commutator and energize the proper winding.

• he energized stator winding leads the rotor magnet, and switches just as the rotor aligns with the stator.

• BLDC motors are potentially cleaner, faster, more efficient, less noisy and more reliable.

ME 407 13

AC (Induction) Motor

• Motor is essentially driven like an AC synchronous motor by applying sinusoidal current to motor windings.

• The drive needs to generate 3 currents that are in the correct spatial relationship to each other at every rotor position.

• High-resolution optical encoder is needed to control the commutation accurately.

• Very smooth low speed rotation.• Negligible torque ripple.

Servo-Motor Drivers• Most servo-motor drivers incorporate

motion controllers that allow the user to control– Torque (phase currents)– Speed– Position

• Once the user selects the control mode, the motor drivers must be connected to multi-axis controller unit (industrial PC, motion control card etc.):– Wiring configuration – Setting motor parameters via

• Manually (Control Panel / Memory Stick)

• Software assistance

ME 407 14

Torque Control Mode

• In this mode, the motor driver accurately regulates the motor phase currents in respect to the rotor’s position (namely, rotor magnetic flux linkage vector).

• Servo-motor acts like an ideal torque modulator to yield the electro-magnetic torque being demanded by the (position) control system.

• Torque command is issued through an analog input (usually a bipolar voltage).

• In precision motion control applications, this mode is frequently preffered.

ME 407 15

Velocity Control Mode

• Motor driver regulates the rotor’s angular velocity. – Relies on built-in incremental position encoder to measure

velocity.– Generally, a digital PI controller is employed to control the

velocity.• Velocity controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired”

controller to the driver.

• Velocity command is usually issued through an analog input.– Use of control data buses (such as CAN, SERCOS, Profibus,

RS-485, etc.) to send digital commands out to the driver is also common in industry.

ME 407 16

Position Control Mode• Motor driver regulates the rotor’s angular position.

– Motor driver again employs built-in incremental position encoder to measure position.

– Generally, a digital PID controller is utilized to control the position.

• Position controller feeds torque commands to the current/torque regulator. • User must upload the relevant gains and parameters of the “hardwired”

controller to the driver.

• For convenience, command is usually issued through two digital inputs (i.e. direction and pulse).– The servo-motor behaves like a position controlled stepper

motor.

• Advanced drivers support data buses (such as CAN, SERCOS, Profibus, RS-485, etc.) to send/receive digital information.

ME 407 17

ME 407 18

Operating Modes of DC Motor

M

+

_

ia

Va

Forward Motor

Tm

m

M

+

_

ia

Va

Forward Generator

M

+

_

ia

Va

Reverse Motor

M

+

_

ia

Va

Reverse Generator • In motor mode, the machine drives the “load” and needs energy from the supply.

• In generator mode, the “load-side” drives the machine and it generates power.

ME 407 19

“Forward Motor” Control

M+

VDC

Electronically controlledpower switch

+

_

Va

Va

VDC

t

Va

Tp

Td

• Electronically-controlled (unidirectional) switch is turned on/off rapidly.– Pulse width modulation

• Desired (average) voltage at the terminals of DC motor is obtained via controlling switching times:

La

+

S1

D1

Ra

ea

+

_

VDCBack

E.M.F.

DC Motor

ia dVTT

VV DCp

dDCa

where Tp is PWM period(constant) and Td/Tp = d is called duty cycle.

ME 407 20

Forward Motor Control (Cont’d)

• When S1 is turned off, ia flowing through the motor cannot be cut off immediately.– It must flow somewhere!

• The “clamp” diode allows current flow in Mode 2: – La drives a decaying

current.

• If D1 isn’t in place, a very large voltage will build up across S1 and blow it up.

La

D1 :off

Ra

ea

+

_

VDC

ia

S1 :on

Mode 1:

La

D1 :on

Ra

ea

+

_

VDC

ia

S1 :off

Mode 2:

ME 407 21

Four-Quadrant Motor Control

S1

S2

D1

D2

D3

D4

S3

S4

VDC

+

M

Half-Bridge Half-Bridge

• “H” bridge is used to operate the motor in four quadrants.

• Driver is composed of two half-bridges.

• Switches in a half-bridge cannot turned at the same time.– causes short-circuit.– If one of the switches is

turned, the other must be off.

ME 407 22

Forward Motor

• To go forward,– S3 is fully turned on;– PWM and ~PWM (inverted PWM) signals are applied to S2 and S1

respectively.

• Unidirectional switch S1 can carry current only in the indicated direction.

S1

S2

D1

D2

D3

D4

S3

S4

M

ia

VDC

Mode 1:

S1

S2

D1

D2

D3

D4

S3

S4

M

ia

VDC

Mode 2:

ME 407 23

Reverse Motor

• To go backward,– S1 is fully turned on;

– PWM and ~PWM signals are applied to S4 and S3 respectively.

S1

S2

D1

D2

D3

D4

S3

S4

Mia

VDC

Mode 1:

S1

S2

D1

D2

D3

D4

S3

S4

M

ia

VDC

Mode 2:

ME 407 24

Indirect Control System

CommandGenerator

Tor

que

com

man

d

Axis Control System

MotionController

Servomotor

Angular position feedback

Pos

itio

nco

mm

and

MotorDrive

Position sensor

Part

Ball Screw Shaft Nut

Table

Power

Courtesy of Heidenhain Corp.

ME 407 25

Direct Control System

CommandGenerator

Tor

que

com

man

d

Axis Control System

MotionController

Servomotor

Angular position feedback

Pos

itio

nco

mm

and

MotorDrive

Position sensor

Part

Ball Screw Shaft Nut

Table

Power

Linear Scale

Direct position feedback

Courtesy of Heidenhain Corp.

Generic Servo-Control System

CommandGenerator

Torquecommand(voltage)

Programmable Controller

Pos

itio

nco

mm

and

Drive + Motor

Mech.SystemO

utpu

tIn

terf

ace

Sen

sor

Inte

rfac

e

PositionSensor

Disturbance

ControlAlgorithm

e(k) m(k) x(t)

x(k)

x*(k) +

_

m(t) m(t)

Motortorque

Load's position

ME 407 26

Servo-Control (Cont’d)

ME 407 27

Factors to Consider

• The drive requirements must be defined before proceeding to motor selection:– How fast and at which torques does the load

move?– How long do the individual load phases last?– What accelerations take place?– How great is the mass-moment of inertia?

ME 407 28

Factors (Cont’d)

• Often the motor is indirectly coupled to the load shaft, this means that there is a mechanical transformation of the motor output power using belts, gears, screws and the like.

• The drive parameters, therefore, are to be reflected onto the motor shaft.

ME 407 29

Motor Selection

• Decide the motor technology to use (DC brush, DC brushless, stepper, etc.)

• Select a motor/drive combination

• Does motor support the required maximum velocity? If no, select next motor/drive.

• Use rotor inertia to calculate system (motor plus mechanical components) acceleration (peak) and RMS torque

ME 407 30

Motor Selection (Cont’d)• Does motor’s rated torque support the

system’s RMS torque? If no, select next motor/drive.

• Does motor’s intermittent torque support the system’s peak torque? If no, select next motor/drive.

• Does the motor’s performance curve (torque over speed) support the torque and speed requirements? If no, select next motor/drive.

ME 407 31

Selection Procedure*

ME 407 32[*] Courtesy of Omron, Corp.

Load Analysis

• Calculate inertia of all moving components– Determine inertia reflected to motor

• Determine velocity, acceleration at motor shaft– Calculate acceleration torque at motor shaft

• Determine non-inertial forces such as gravity, friction, pre-load forces, etc.

• Calculate constant torque at motor shaft• Calculate total acceleration and RMS

(continuous over duty cycle) torque at motor shaft

ME 407 33

Load Calculation

ME 407 34

sgn( )LL L L CL L D L

dJ b T T T

dt

sgn( )M DM M M CM M M

d TJ b T T

dt N

ML N

sgn( )M Le e M Ce M M

d TJ b T T

dt N

2ˆ Le M

JJ J

N

2ˆ Le M

bb b

N ˆ CL

Ce CM

TT T

N

Gearbox(N:1)

Servomotor Load

TMTL

TD

M L

Load Side:

Motor Side:

Gearing Ratio:

When combined:

where

Other Load Types*

ME 407 35[*] Courtesy of Omron, Corp.

Common Motion Profiles*

ME 407 36[*] Courtesy of Omron, Corp.

Performance Curves* (Brushless DC)

• There are two operating regions:1. Continuous Duty Region: Motor can deliver the torque continuously without

overheating. • Steady-state regime

2. Limited (Intermittent) Duty Region: Large torque can be developed with decreased overall efficiency.

• Transient regime (during acceleration/deceleration)

ME 407 37

[*] Courtesy of Pasific Scientific, Inc.

Performance Curve* (Brushed DC)

ME 407 38[*] Courtesy of Baldor, Inc.

Selection Criteria

• The motor’s rated speed must be equal to or exceed the application’s maximum speed.

• The motor’s intermittent (max) torque must be equal or exceed the load’s maximum (intermittent) torque.

• The motor’s (continuous) rated torque must be equal to or exceed the load’s RMS torque.

• The ratio of load inertia to motor inertia should be equal to or less than 6:1.

ME 407 39

Selection Criteria (Cont’d)

ME 407 40

,, ,( )L ss

M e M ss Ce C M ss

TT b T T

N

At Steady-State: (0 < M,ss < R)

,max,max ,max ,max( )L

M e M e M Ce P M

TT J b T T

N

Worst Case: (0 < M,max < R)

2, ,max

0

1( ) ( )

dutyt

M rms M C Mduty

T T t dt Tt

Note that duration to stay in the intermittent duty zone varies from oneservo-motor to another (0.05 s to 30 s)

In Duty Cycle: (0 < M,max < R)

RMS Torque

• The RMS torque (“Root Mean Squared”) represents the average torque over the entire duty cycle.

ME 407 41

Inertia Matching

• For optimum power transfer, the rule of thumb is that the motor’s (mass) moment of inertia should match to that of the load. – Ratio of 1:1 between load and motor inertia

would be the ideal scenario.– Rather, impractical – some mismatch is

allowed.

ME 407 42

Inertia Mismatch

• If the ratio of load over rotor inertia exceeds a certain range (for servo motors 6:1) consider the use of a gearbox or increase the transmission ratio of the existing gearbox. Servo motors should not be operated over a ratio of 10:1.

• Bosch Rexroth, for instance, recommends the following for inertia mismatch:

– 2:1 for quick positioning– 5:1 for moderate positioning– 10:1 for quick velocity changes

ME 407 43

Some Applications

ME 407 44

Proper Application:TM,rms < TC()TM,max < TP()

Failure:TM,rms > TC()TM,max > TP()

Applications (Cont’d)

ME 407 45

Low Speed Operation:< R

TM,rms < TC()

High Speed Operation: R

TM,rms < TC()

Typical Data Sheet*

ME 407 46[*] Courtesy of Pasific Scientific, Inc.

Data Sheet (Cont’d)

ME 407 47

Data Sheet (Cont’d)

ME 407 48

Summary

ME 407 49