AC&DC Drive Basics
AC&DC Drive Basics
AC&DC Drive Basics
MOTOR
OUTPUT
LINE INPUT
All AC Drives convert “fixed” voltage and frequency into “variable” voltage
and frequency, to run 3-phase induction motors.
Types of AC Drives
In today’s marketplace, there are 3 basic AC
Drive categories:
• Open loop “Volts / Hz” Drives V/Hz
SENSOR-
• Open loop “Sensorless Vector” Drives LESS
VECTOR
FLUX
• Closed loop “Flux Vector” Drives VECTOR
All are Pulse-Width-Modulated (PWM)
Some manufacturers offer 2-in-1 & 3-in-1 Drives,
combining these attributes.
Open loop “Volts / Hz” Drives
o
l 230
t
s
0 30 60 Hz
900 1800 RPM*
(Base) *( 4-pole motor)
• Motor voltage is varied linearly with frequency
• No compensation for motor & load dynamics
• Poor shock load response characteristics
Sensorless & Flux Vector Drives
o
l 230
t
s
0 30 60 Hz
900 1800 RPM*
(Base) *( 4-pole motor)
0 30 60 Hz
900 1800 RPM
All PWM inverters (V/Hz, Vector & Sensorless Vector) share similar power circuit
topologies.
AC is converted to DC, filtered, and inverted to variable frequency, variable
voltage AC.
PWM Power Circuit:
AC to DC Converter Section
AC to DC
DC Filter
Rectifier
AC DC +
Input Bus
Caps
-
Input Reactor
(option)
DC Reactor
The AC input is rectified and filtered into fixed-voltage DC
• Certain manufacturer’s units contain an integral DC reactor (choke)
as part of the DC filter.
• Adding an external AC input reactor will yield similar benefits.
• Both reduce harmonics, smooth and lower peak current.
Power Switches
The IGBT: (Insulated Gate Bipolar Transistor)
An IGBT is a hybrid between a MOSFET and a Bi-polar Darlington Transistor.
COLLECTOR
GATE
= SWITCH
EMITTER
Application Issues:
• A 1 microsecond state-change will generate a 1 MHz RF pulse.
• Dv/dt (rapid voltage changes) can stress motor insulation systems.
PWM Power Circuit:
DC to AC Inverter Section
DC to AC Vu-v
DC Filter
Inverter
AC
Output
+
IGBTs U
V
M
- W
Imotor
IGBT Firing
Signals
S
E
LO
CA REF
L
v 1.3
microprocessor
E
Q
F
L
R controller
JOG W
RE
D
V
RUN STOP
RESET
RESET
Flux Vector Control Elements
Input, Feedback and Control Signals
Encoder Feedback
Torque reference
Vector algorithm
M L
R
F
JOG W
RE
D
V
RUN STOP
RESET
RESET
AC VECTOR CONTROL LOOPS
AC Vector Drive
Encoder
Speed Loop Torque Loop
Speed Error Torque Ref.
Speed Reference
Speed Torque PWM
Regulator Regulator
Torque Reference
Firing
Freq. & Voltage
Reference
Actual Torque
Torque
Calculator
Frequency Feedback
Speed Feedback
Typical AC Induction Motor
Speed / Torque Curve
“Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor
Breakdown point: Maximum
225 torque motor can produce
before locking rotor
Starting Torque
100
SLIP
Typical AC Induction Motor
Current & Torque Curves
“Across-the-line” operation @ 60 Hz, NEMA ‘B’ motor
650 Starting (inrush) current
225
%T 175
%I
150 Linear range: 40-150% load
100 (operating range in which current is
proportional to torque)
Speed
AC Motor Speed / Torque Curve family
on Inverter Power
225 Motor base speed:
1750 RPM
150
MOTOR SLIP
VOLTS FREQUENCY V/Hz
SYNC RPM - FULL LOAD RPM
%SLIP = X 100 460 60 7.66
SYNC RPM
Cast aluminum
Laminations of
end rings
high-silicon
Electrically joins rotor
bars at both motor ends content steel
Low-eddy current loss
magnetic medium
Elements of an Induction Motor:
The Stator
Stator Core
Lamination stack
of notched steel
plates
Elements of an Induction Motor:
Stator Windings (4-pole)
Steel Laminations
Slots
wye or delta
connection types
Stator Windings
Elements of an Induction Motor:
The Stator (4-pole)
t
Rotating
magnetic field
SLIP = (w s - w r ) / w s
w stator
w rotor
torque.
• Stator speed is known by frequency
• Rotor speed is measured with an
encoder (Vector).
• Rotor speed can be approximated,
knowing motor and bus current
(Sensorless Vector algorithm)
Rotor Magnetic Field Dynamics:
SLIP creates TORQUE
Magnetic Flux When rotor speed is near stator speed (light load),
Lines few stator flux lines are cut . Rotor bar current and
slip frequency are low.
Magnetic Flux Magnetic Flux
Lines Lines
R1 XLR XR
V XM RLOAD = R / Slip*
2
Magnetizing *(R2 is rotor bar resistance)
Reactance
Stator Rotor
Air
Gap Torque
Stator Leakage Rotor
Resistance Reactance Reactance Current
R1 XLR
XR
Total Current
Magnetizing XM RLOAD
Current
Motor Current Vectors
Magnetizing
Current
Torque-Producing Current
Autotuning on Sensorless Vector Drives
FACT: Most motor electrical parameters are
difficult to obtain from the manufacturer.
ROTOR RESISTANCE
ROTOR REACTANCE
MAGNETIZING CURRENT
STATOR RESISTANCE
LEAKAGE REACTANCE
?????
Not typically found on motor nameplate
1. Enter nameplate motor parameters (base speed, full load amps, voltage,
frequency, power factor).
2. Run the ‘AUTOTUNE ‘ function. The controller will pulse the motor &
determine approximate motor electrical characteristics for SENSORLESS
VECTOR Operation.
3. The S-V algorithm can now compute torque- and magnetizing current
vectors for more precise motor control.
Facts about Induction Motors
• High resistance
• High reactance
• Low amps
• Low torque
SPEED
AC Induction Motors
Effecting Base Speed through Volts / Hz Design
Motors on inverters don’t have to be wound for “60 Hz”
• Optimal power delivery occurs if voltage peaks at base speed
• Lowest amps occur at peak voltage .
• Drive price / component cost is related to amps.
VOLTS
0 20 40 60 Hz
600 1200 1800 RPM (sync.)
Motor Operation above Base Speed
Motor base speed: 1750 RPM (4-pole)
225 60 Hz
curve
% 175
T 150 120 Hz
Base curve
100
Peak Inverter Torque
(150 -200% current)
50 100% current
operating line
1800 3600
60 120
Hz
AC V/Hz Drives
Pro’s & Con’s
Advantages Limitations
• Simple, “look-up table” control of • Low dynamic performance on
voltage and frequency sudden load changes
• Good speed regulation (1-3%) • Limited starting torque
• No motor speed feedback needed • Lacks torque reference capability
• Multi-motor capability • Overload limited to 150%
80% HP = K x (RPM)3
80% 100%
Speed
Variable Torque Applications:
Centrifugal Fan Energy Savings
100%
Power Consumption
100%
Flow
Variable Torque Applications:
Centrifugal Pumps & Fans
100%
Base
RPM
100%
Most drive controllers have a special
“variable torque” V/Hz profile selection that
further cuts down on magnetizing current at
Volts light loads. Since magnetizing current is
purely reactive, motor losses are reduced .
60
Hz
Regenerative Operation of AC Motors
Example: 1750 RPM motor on 60 Hz power
Current
LOAD TORQUE & CURRENT
Regen Breakdown
4-Quadrant Operation of AC Motors
on Inverter Power
Clockwise
TORQUE
REVERSE FORWARD
REGENERATING MOTORING
- RPM + RPM
REVERSE FORWARD
MOTORING REGENERATING
Counter-
Clockwise
TORQUE
Conditions for Regenerating
on an AC Motor
AC Motors regenerate when pulled faster than their
sync speed at the applied frequency.
Regeneration conditions:
• Overhauling loads
• Fast deceleration of high inertial loads
• Stopping on a timed-ramp
• Cyclic loads or eccentric shaft loading
* 1750 RPM base
PULL
speed at 60 Hz
ROTATION
WEIGHT
AC Drive Regeneration
AC DC +
Bus IGBTs M
Input _
Caps
• Current flows back into the DC bus, via the IGBT switching & back diodes.
• AC Drive front-end rectifier is unidirectional; energy cannot flow back into the AC
line.
• Some returned energy is dissipated in losses in the capacitors, switches, and
motor windings (10-15%).
• Excessive regeneration can cause problems, such as DC Bus Overvoltage.
Dynamic Braking on AC Drives
V DC Feedback
DBR
AC DC +
Input Bus
_ M
Caps
SIGNAL
DB is NOT ACTIVE when:
DB is ACTIVE when:
DYNAMIC • Decelerating a frictional load
• Motor has an overhauling load
BRAKING • Stopping in coast-to-rest mode
• Fast decel of high-inertial load
CONTROL • Drive is disabled or if power
• Stopping in ramp-to-rest mode
is removed
M
AC Drives on a Common DC Bus:
Theory of Operation
AC
DRIVE
+ -
REGEN
AC
DRIVE
NET
As individual drives
POWER MOTORING
regenerate, the
returned energy is re-
AC distributed to
Net power usage DRIVE motoring drives via
is minimal, due the common DC bus.
to the efficient REGEN
use of returned
energy. AC
DRIVE
MOTORING
AC Drives on a Common DC Bus:
Typical Connection Diagram
THERMAL- MAG
BREAKER
INPUT LINE
REACTOR
AC AC AC
DRIVE DRIVE DRIVE
SEMICONDUCTOR
FUSES
INTERLOCKED
DC CONTACTOR
Line Regenerative AC Drives
BI-DIRECTIONAL POWER FLOW
V DC Feedback
LINE M LOAD
PWM
microprocessor
controller
• Two sets of 6 - IGBT bridges
• Gating control for both sets
• Converter IGBTs modulate on when bus voltage is excessive.
• More complex regulator design
• More conducted noise to power line
Cost of drive is 1.8 times standard non-regen AC Drive
Multi-motor Applications
Motor amps must total less than
controller amp capability
• Each motor must have its own overload
• Drive must be in the “V/Hz” control mode AC DRIVE
(V/Hz mode)
• Motor speeds will be within slip-speed OVERLOAD CONTACTS
range, with respect to each other. 30 HP
38 Amps
• Interlock output contactors to drive run
logic, when used.
2 hp 3 hp 10 hp 2 hp 3 hp 5 hp
2.8 amps 3.9 amps 12 amps 2.8 amps 3.9 amps 7.2 amps
Total HP = 25
Total Amps = 32.6
Application of Contactor
Bypass on AC Drives MAIN CB
Provides back-up, across-the-line
operation of motor
• Single-speed operation on line only
(must have mechanical control in place)
INVERTER
• Motor overloads are mandatory. DISCONNECT
INVERTER BYPASS
MOTOR
OVERLOAD
TYPICAL 3-POSITION
SELECTOR SWITCH
AC Drives and Power Factor
REACTIVE
FLOW
AC
Input M
A1
Armature
A2
F1
Field
F2
A1
A2
S
Typical DC Motor Armature
Current & Torque Curves
200
NO LOAD
100 MOTORING
Armature current is
%T RPM directly proportional to
0
% IDC torque throughout the
loading range.
-100 REGENERATING
-200
DC Motor Torque & HP vs. Speed
Motor nameplate: 250 / 1000 RPM
75
3 : 1 FIELD WEAKENING
25
4 : 1 FIELD WEAKENING
SPEED (RPM)
Power Switches
The SCR: (Silicon Controlled Rectifier)
a.k.a. - “Thyristor”
ANODE CATHODE
-
TRIGGER
GATE +
AC
Input
F1
AC
Input
Tachometer
F2 Feedback
(closed-loop)
Field
Control
A2
SCR Firing Signals Motor voltage
feedback
Signals
Line current
feedback
Microprocessor
0.75
KW
HEALTH
AC MOTOR DRIVE
200 V
S
LO
CA REF
v 1.3
controller
Speed or Torque E
EQ
L
PROG
Reference M L
R
F
JOG
W
RE
D
V
RUN STOP
RESET
RESET
Operator
Interface
Elements of a DC Drive:
Regenerative type A1
AC R
F R F R F
Input
F1
Tachometer
Feedback
R R R F2
F F F (closed-loop)
Field A2
Control
SCR Firing Signals Signals Motor voltage
FWD/MOT REGEN/REV feedback
Line current
feedback
AC MOTOR DRIVE
Microprocessor
controller
0.75 200 V v 1.3
KW
Reference M L
R
F
JOG
W
RE
D
V
RUN STOP
RESET
RESET
Operator
Interface
Dynamic Braking on DC Drives
M
A1
M
Braking Power
F1
DBR
F2
A2
time
M
• Dynamic Braking Resistors are shunted across the motor armature in a STOP or E-
STOP mode.
• Motor counter-EMF (back voltage from motor, acting as generator) appears across
resistor grids.
• Voltage diminishes as resistors dissipate energy.
• Braking Power diminishes exponentially with motor slowdown: P = V2/R
Not failsafe: DB will not function if field supply is absent (i.e. - if power is lost)
DC Regenerative Drives vs.
DC Dynamic Braking
AC Vector Drive
Encoder
“TEST” Speed Loop Torque Loop
Speed Error Torque Ref.
Speed Feedback
KW = HP x .746
Three phase Power
VL-L x I x 1.732 Pout
KVA = % Efficiency = X 100
Pin
1000
KVA = KW / P.F.
Power losses in AC & DC controllers
(5 - 100 HP; excluding motor; full speed & load)
DC: AC to DC
98% EFFICIENCY
SCR losses = 1%
Fixed losses = 500 -1000W
CONTROL
AC: & FANS
AC to DC
Cap losses = .5%
DC to AC
96% EFFICIENCY
SCR / Diode losses IGBT losses = 1.5%
= 1%
20% 100%
SPEED
DC Drive Advantages over AC