Electric Machines Drives

General Overview

Author : Marc Anthony Mannah

Cursus : ECAM Engineering
UE : Control Engineering S6

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 Details on the Electric Drives Course
 Electric Drives course details :
• Session 1 (2h): Introduction & Overview
• Session 2 (2h): DC Drives
• Session 3 (2h): AC Drives: Basics & Scalar Control
• Session 4 (2h): Field Oriented Control

• Tutorial 1 (2h): Power Electronics & Electric Machines

• Tutorial 2 (2h): DC Control & Closed Loop Control of DC motors
• Tutorial 3 (2h): Voltage & Frequency Control of AC motors

1 practical class (4h):

• Control of Induction motors: Scalar Control and Vector Control

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 Table of Contents

 Concept of Electric Drives

 Power Electronics Overview
 Electric Machines Overview
 Extras…

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 Concept of Electric Drives - Basics

𝑽𝑽 𝝎𝝎

PWM 𝝎𝝎𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎
𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪𝑪
𝑽𝑽𝒎𝒎𝒎𝒎𝒎𝒎𝒎𝒎

Objectives
• Controlling the dynamics of the machine: Speed, Position, Voltage, Current, Torque…
• Providing Electronic Commutation
• Motors Starting and Protection Circuitry
• Adjusting the input (Source) to meet the Output (Load) requirements

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 Concept of Electric Drives – Basic Methodology
I use power
I want to electronic to obtain
change variable voltage or
the motor excitation
speed… I use control and feedback
to accurately change the
values and control the PE
I can change its
components, in compliance
armature voltage
with the specs
or its excitation
current

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 Concept of Electric Drives – More into it

Electric Electrical Mechanical Load

Power Source P = V.I P = τ.ω P = τL.ωL
DC, 1P, 3P Electric Drive Motor Gearbox .
Power Electronics DC, IM, SM (Mechanical) Load
VF, VV, DC Vs, Is, … Drive .
Vr, Ir,
HMI, Internal τ,
α, f…
Loops… ω,
PWM θ
. Controller 4-20 mA Sensor
0-5V Physical Quantity
Reference PLC, 0-10V
Transducer
. Microcontroller … T, V, I, θ, ω, τ, Q…
Control System:
P, PI, PID, Scalar,
FOC…

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 Power Electronics Overview

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 Power electronics Overview

Diodes Rectifiers Single & Three Phase

Controlled Rectifiers Full Wave

Buck Converters Motoring & Regenerative

Boost Converters 4-Quandrant

Full Inverters Control Strategies

B2B Converters Switch Configurations

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 Power electronics Overview – Single Phase AC-DC Drives

SP FW Converter

Change Va by changing the delay (firing) angle α of the converter

Inductor Lm is a “smoothing” Inductor to prevent discontinuous current
A converter is also applied in the field circuit to control the field current by varying
the delay angle αf

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 Power electronics Overview – Single Phase AC-DC Drives
Single-Phase FW Converter Drives

For maximum Field current:

2Vm
α F = 0 : VF =
π

DC
2Vm
VA = cos α A
π
2Vm
VF = cos α F
π AC
Applications limited to 15 kW

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 Power electronics Overview – Three Phase AC-DC Drives

3φ
Supply AC/DC Va

Two-quadrant drive, by field reversal

Applications up to 1500 kW.

3 3Vm
Va = cos α a 0 ≤ αa ≤ π
π
3 3Vm
Vf = cos α f 0 ≤αf ≤π
π

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 Power electronics Overview – DC-DC Drives

 Mainly used in embedded circuits

 If VA or VF need to be reversed: 4 quadrants required
 4 possible modes:
• Power Control (Acceleration control)
• Regenerative Brake Control
• Rheostatic Brake Control
• Combined Regenerative and Rheostatic Brake Control

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 Power electronics Overview – DC-DC Drives
Power Control (Acceleration control)
Ripple-free Armature Current
DC

DC

Highly Inductive Load

J
Converter-fed DC Drive for a − Mechanical time consta nt : Tm =
Separately-Excited Motor B
L
− Electrical time consta nt : TE = A
RA

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 Power electronics Overview – DC-DC Drives

• The average armature voltage is

Va = kVs k duty cycle : 0 〈 k 〈 1

• The power supplied to the motor is

Po = Va I a = kVs I a

• The average value of the input current is

I s = kI a

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 Power electronics Overview – DC-DC Drives
Regenerative Brake Control
Variable voltage into constant voltage

DC

DC

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 Power electronics Overview – DC-DC Drives

• Motor turned by kinetic energy of the vehicle +

• Armature current flows as shown

Vav
• Turn the transistor on
• Armature current rises
-
• Turn the transistor off
• Diode turns on, current flows into the supply
Average voltage:
Vav = (1 − k )Vs
Regenerated power:
Pg = I aVs (1 − k )
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 Power electronics Overview – DC-DC Drives

Voltage generated by the motor acting as a generator is

Eg = K v I f ω
E g = Vav + Ra I a = (1 − k )Vs + Ra I a

E g = K vωmin I f = Ra I a K vωmax I f − Ra I a = Vs
RI Vs Ra I a
ωmin = a a ωmax = +
Kv I f Minimum Maximum Kv I f Kv I f
Braking Braking
ω ≥ ωmin ω ≤ ωmax
Speed Speed

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 Review on Electric Machines

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 Electric Machines Overview

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 Electric Machines Overview - Separately Excited DC Motor

dia
va = Ra ia + La + ea
dt
di f
v f = Rf if + Lf
Field circuit
dt

ea = K vω i f
Armature circuit

Td = K t iF i A
Pconv ea ia K vωia i f
Td = = = = K v ia i f = K t ia i f
ω ω ω
dω
Td = J + Bω + TL
dt
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 Electric Machines Overview - Separately Excited DC Motor

d
At steady state: =0 At rated load, IA = cst, Td≡If
dt
Va = Ra I a + Ea = Ra I a + K vωI f
Td = K t I a I f
ω ∝ Va
Va − Ra I a
So, ω = 1
Kv I f ω∝
If
VA Control ωrated IF Control

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 Electric Machines Overview – Induction Motors
Principle: Interaction of 2 magnetic fields: Tind=kBRxBS

The rotor windings Since rotor windings

The stator windings 3
are consequently in are shorted out,
phase voltages
a variable flux and then a current iR is
create a rotating field
thus a voltage eR is created leading to a
(at speed ω𝑆𝑆ync)
induced field BR

Vab, Vbc, Vca  Bs(t) Bs(t)  er(t) er(t)  ir(t)  Br(t)

ω𝑚𝑚 cannot be equal A torque is induced

to ω𝑆𝑆, otherwise and the rotor turns
torque becomes at ωm. The rotor will
zero. The machine accelerate until ω𝑚𝑚
slips below the becomes close but
synchronous speed less than ω𝑆𝑆

Hence the name : Asynchronous Motor

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 Electric Machines Overview – IM Model

120 f
ns =
P
n − nm ω s − ωm
s= s = ≡ slip
ns ωs

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 Electric Machines Overview – IM Equations

Input power : Pin = 3VL I L cos θ

Stator copper losses : Psu = 3I s2 Rs
3Vm2 Vs2
Core losses : Pc = ≈3
Rm Rm
Thus, the interest of
Rotor copper losses : Pru = 3I 'r2 R 'r operation at low s at a speed
R 'r close to ωs
Air − gap power : Pg = Pin − Psu − Pc = 3I 'r2
s
Developped power (converted power ) : Pd = Pg − Pru = 3I r2 R 'r (1 − s ) = Pg (1 − s )
Rotational losses (mechanical ) : PROT = Pfriction + Pwindage + Pstray
Output power : Pout = Pd − PROT
Pd Pg (1 − s ) Pg
Developped torque (induced torque) = Td = = =
ωm ω s (1 − s ) ωs

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 Electric Machines Overview – Terminal Characteristic

The value of Xm is normally large

& Rm is much larger, thus the
circuit can be simplified as follows:

Tmm
Vs
: I r' = 1
 Rr' 
2
 2
(
 Rs +  + X s + X r 
s 
) ' 2
sm s=0
 
1<s<2 s=1
2 '
' Rr s<0
Pg 3I r s 3Rr' Vs2
Td = = =
ωs ωs  Rr' 
2

(
sω s  Rs +  + X s + X r'
s 
)
2

 

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 Electric Machines Overview – Terminal Characteristic
 The starting torque is found by replacing s=1
 The maximum torque Tm is found by replacing s=sm where sm is found by calculating
the derivative dTd/ds=0

Slip for maximum torque :

Rr'
sm = ±
[R
2
s + (Xs + X ) ' 2
r ]
1
2

+ for Motor , − for Generator

Maximum developed torque :
3Vs2
Motor : Tmm =
[
2ω s Rs + R + X s + X 2
s ( )]
' 2
r
: pullout or breakdown torque

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 Thank you for your attention

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 Extras….

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 What we need to know ? Understand the behaviour
(electrical model, speed,
Machines Control rotor position in AC…)

Control Electrical Machines

Power Control of Control of AC Synchronous Asynchronous

motors (AM, DC Motor
Electronics DC Motors Motor Motor
SM)

Different configurations, Specs & Parameters Frequency variation,

transfer functions, identification, 4-quadrant PWM techniques, Scalar
sequencing, control operation, control control, Field Oriented
strategies… adjustments according to Control, Direct torque
the type of converter, control…
multi loops…

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 Control Overview - Generalities
Voltage control
 DC-DC converter
 Switched ON-OFF on high frequency (>10 KHz)
 Based on MOSFET, IGBT, GTO
 DC output is controlled by the duty cycle
 AC control
 By varying the pulse width, the amplitude of the sine wave can change - PWM
 By varying the pulse magnitude - PAM
 By varying the pulse repetition frequency - PFM
 DC outputs from choppers and PWM circuits are full of harmonics that may be eliminated due
to the inductive nature of the DC machine
 Since current flows only when switches are ON, these techniques are relatively lossless
 For continuous control, the inverter can be incorporated in feedback loops
 As speed changes, the control maintains the voltage constant

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 Control Overview - Generalities
Speed Control
 Speed changes with load variations
• Open loop: Manual
• Closed loop: Automatic

 DC machines
• If load changes, speed changes, and the machine has to deliver a different torque: current changes
• Speed is controlled by voltage control (outer feedback)
• Torque is controlled by current control (inner feedback)
• Voltage may deliver more current but without overriding current control, which must be controlled by the current loop

 AC machines
• Speed control depends on the supply frequency and voltage
• Variable frequency drives allow control for speed by varying these two parameters
• In case of Induction motor, V/F method is used with V/F constant to control the flux

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 Control Overview - Generalities
Torque and current control
 Use only current loop and omit speed loop
 Current is important at starting, transient and load changes
 Use methods such as (mainly important for Induction motors):

• Direct torque control DTC

• Vector controllers VC

Iref
ωref +
Speed + Current
M
Control Control
_ _ I
ωm

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 Control Overview - Compensators
If the error is too large or the transient response is too fast, the system may be
unstable and a compensation (controller in the feedback) is required

Though this may change the overall transfer function of the system, it will allow to
rectify one of the following situations:

1- Stable system, desired transient response but large steady-state error

2- Stable system but large transient response
3- Stable system but large transient response and large steady-state error
4- Unstable system

Basically, the
Compensator is in the
feedback (F4)
For simplicity purposes,
the compensator can be
instead inserted in the
main line (F1)

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 Control Overview - Compensators
1- Proportional controller (P): How far from set point? Response proportional with
offset
2- Integral controller (I):How long from set point? Adds time component to response
3- Derivative controller (D): How fast towards set point? Adds compensating element
for speed of response, e.g. It slows down the response and prevents overshooting

Vc Vc k i Vc
= kp = = s kd
Eω Eω s Eω
Paramete Settling Steady
Rise Time Overshoot Design Requirements
r Time State Error
Small  Steady-state error of the
Kp Decrease Decrease change Decrease
(Indefinite) motor should be less than 1%
Ki Decrease Increase Increase Eliminate
 Settling time of 2 seconds
Small
Decrease Decrease
Kd change No effect  Overshoot less than 5%
(Indefinite)

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 Control Overview - PWM
Case of chopper control
High frequency carrier

Vref
“Saw tooth”
triangular carrier

u(t)

With this
0 kT T comparison, we
could generate the
- Output magnitude is defined by the input magnitude control pulses of
- Output frequency defined by the HF carrier frequency the chopper.
- The reference & the carrier are continuously compared

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 Control Overview - PWM
Case of three-phase inverter control

We compare the triangle signal to the three sine waves. When the sine is over the triangle, we control K1 and
when it is under, we control K1’ and so on for the other sine waves.

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 Power Semiconductor Components
 Power Diode  Thyristor

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 Power Semiconductor Components
 Power Mosfet  IGBT

Ultra Fast IGBT Module

1200V/300A/20kHz

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 Power electronics Overview – DC-AC Drives

- Input: Battery, Fuel cell, Solar cell, DC link…

- Output: 120V/60Hz, 220V/50Hz,…
- Switches: IGBT, MOSFET, GTO…

- VFI: Voltage Fed Inverter: The input DC voltage is constant independent from the load
- CFI: Current Fed Inverter: The input DC current is constant independent from the load
- Variable DC link Inverter: The input DC voltage is controllable
- Resonant Pulse Inverter: The output is forced to pass to ‘0’ creating an LC resonant
circuit
- Multilevel inverter: Further switching combination and switching devices can be used
in order to minimize output harmonics

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 Power electronics Overview – SP DC-AC Drives

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 Power electronics Overview – 3P DC-AC Drives

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 Power electronics Overview – General AC Drives
Diode Rectifier Based Converter
DC power

Variable frequency and AC power at different

variable magnitude AC frequency and voltage
power level

PMSG
WRSG
uncontrolled controlled

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 Power electronics Overview – General AC Drives
Back 2 Back Converter
Control the active
Reduces the input current and reactive power
harmonics and harmonic flow to the grid
losses Bidirectional power flow

PMSG
SCIG

keeps the DC-link voltage constant,

Improves the output power quality by
reducing total harmonic distortion (THD).

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 Electric Machines Overview - DC Motors Comparison
Separated Series Shunt

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 Electric Machines Overview – Terminal Characteristic

Three regions of operation: Motor, Generator and Braking regions

1. Motor : As the slip s↑, torque ↑, while the air-gap flux is constant. Torque
reacts at maximum at s=sm, then starts to decrease due to the reduction
of air-gap flux.
0 ≤ s ≤1 0 ≤ nm ≤ ns

2. Generator: Slip s<0, and the rotor reluctance is negative: Power is fed
back from the shaft into the rotor circuit as the torque becomes negative.
s0 nm  ns
3. Braking region (Plugging): Developed torque opposes the speed and acts
as a braking torque. Also, s>0, I’r>> but Td<<: Energy due to excessive
current dissipates through the machine, produces heating: Not
recommended 1≤ s ≤ 2 n 0
m

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 Insights through Examples

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 First Example (Waste Sorting Plant)
I am the boss of a waste sorting plant, I want to redo
my entire installation :
 I want 3 units of conveyors at 3 different speeds, I want to be able to
easily change the speed of the chains.
 I want the plant to run continuously
 I want reduced maintenance and purchasing costs
 I don't need to have precise trash scrolling speed
 My factory is supplied with three-phase 400V / 50Hz by EDF.

Which solution are we going

to offer to this factory owner ?

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 First Example (Waste Sorting Plant)
 I want 3 units of convoyors at 3 different speeds, I want to be able to
easily change the speed of the chains.
• 3 motors controlled by 3 conversion systems for variable speed
 I want the plant to run continuously
 I want reduced maintenance and purchasing costs
• Brushless motors (PMSM or ASM), PMSM remains more expensive and
needs an auxiliary motor to start => Best solution : the cage ASM
 I don't need to have precise trash scrolling speed
• No need for precise speed, we can limit ourselves to a scalar control
because the factory owner does not care if the speed is shifted due to the
slip.
 My factory is supplied with three-phase 400V / 50Hz by EDF.
• I will be able to use frequency converters directly connected to the network

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 First Example (Waste Sorting Plant) Gear Box

 Schematic diagram :
80V/10Hz

Variator 1 ASM1 GB1 Conveyor 1

112V/14Hz
Power network ASM2
Variator 2 GB2 Conveyor 2
400V/50Hz

Three phase
240V/30Hz
poweer supply
Variator 3 ASM3 GB3 Conveyor 3

Speed variator

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 Second Example : Powering a lighthouse
I am the mayor of a seaside town, I want to supply the city's
lighthouse with a hydrokinetic turbine to rotate the lighthouse lamp
and make it light up.
• The rotation speed of the lamp is variable according to the messages to be
transmitted to the sailors and it must be precise.
• The brightness of the lamp is always the same.
• I don't want to have interruptions in the operation of the lighthouse
Which solution are we going
to offer to this mayor ?

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 Second Example : Powering a lighthouse
 First solution : with a DC motor
Battery (48V) with its Control of the
electronic charge chopper with a
Permanent Magnet
Synchronous Generator
management system PWM changing the
conduction duty
cycle k of the IGBTs

4 quadrants
PMSG Rectifier Fixed voltage DC output (48V)
chopper

Boost chopper Variable DC

48V-230V voltage to
adjust speed
Single
Lighthouse
phase
lamp with
inverter Fixed voltage DC output (230V)
its switch
230V/50Hz
DC Motor

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 Second Example : Powering a lighthouse
 Second solution : with an ASM

Battery (48V) with its

electronic charge Vector control of the
ASM to
Permanent Magnet
Synchronous Generator
management system compensate the
effect of the slip and
obtain a an
accurate speed.

PMSG Rectifier Fixed voltage DC output (48 V)

ASM

Boost chopper
48V-230V
Single Three phase
Lighthouse
phase inverter with
lamp with
inverter Fixed voltage DC output (230V) variable
its switch
230V/50Hz Voltage/Frequency

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