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G7How an AC Motor Works
G7 Objectives
• Provide overview of basic induction motor theory
• Identify key operational requirements of variable frequency drives
• Define sensorless vector and feedback vector control and the differences between them
G7 Target Audience
• Induction motor users or potential users
• Adjustable speed drive users or potential users
G7
• An AC Motor has two basic components– Stator– Rotor
• 3 phase AC Power to the stator = rotating magnetic field– Speed(rotating magnetic field) =120xHzpoles– Synchronous Speed
How an AC Motor Works
G7 How an AC Motor Works
G7
• The STATOR is the outer body of the motor which houses the driven (powered) windings on a laminated iron core
– Voltage rating is determined by• number of turns on the stator windings
• total circular mill of a cross section of the winding.
– HP is determined by • winding loss
• lamination loss
• windage loss
• the ability of the motor to dissipate the heat generated by these losses.
• Stator design determines the rated speed of the motor and most of the full load, full speed characteristics.
How an AC Motor Works
G7
• The ROTOR is comprised of a cylinder made of round laminations pressed onto a shaft and a number of short circuited windings.
– Windings are made of copper or aluminum bars• passed through rotor around the surface
• bars protrude beyond rotor laminations
• bars connected by shorting ring at each end
– Rotor windings exhibit inductance and resistance • Rotor inductance is dependant on the frequency of the current flowing in the rotor.
• The frequency of current flowing in the rotor is the difference in the rate of rotation between
– Stator magnetic field
– Rotor RPM
• ROTOR design determines the starting characteristics
How an AC Motor Works
G7
• The rotating magnetic field induces current into the rotor bars (same as a transformer)
• Current in the rotor bars creates a magnetic field
• The rotor’s magnetic field chases the stator’s rotating magnetic field.
How an AC Motor Works
G7
• At start the difference in speed between the rotating magnetic field and rotor is very large
• Both stator and rotor currents are very large
• As the motor accelerates, the difference is speed decreases
• Stator and rotor current start to decrease
How an AC Motor Works
G7 How an AC Motor Works
0
50
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350
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500
550
600
650
0 10 20 30 40 50 60 70 80 90 100
PERCENT SYNCHRONOUS SPEED
G7
• At full speed under no load the difference in speed is very small– A small amount of magnetic lines of flux cut
through the rotor bars– A small magnetic field is induced in the rotor
• This small magnetic field generates a small amount of torque, just enough to overcome friction and windage
How an AC Motor Works
G7
• When a load is applied to the motor shaft, it slows down
• The difference in speed between the magnetic field and the rotor increases
• More magnetic lines of flux cut through the rotor bars
• More torque is generated
How an AC Motor Works
G7Torque
0
50
100
150
200
250
0 10 20 30 40 50 60 70 80 90 100
PERCENT SYNCHRONOUS SPEED
PER
CEN
T F
UL
L L
OA
D (
TO
RQ
UE &
CU
RR
EN
T )
How an AC Motor Works
G7
• The more load applied, the more torque the motor produces until breakdown torque is reached
• If load continues to be applied, the motor stalls
How an AC Motor Works
G7
• The difference in speed between the rotating magnetic field and the rotor is what generates torque
• This difference in speed is referred to as slip
• Under proper conditions slip is directly proportional to the torque applied to the load.
How an AC Motor Works
G7
R1L0
R2M
How an AC Motor Works
G7
• R1
– Primary Resistance• Resistance of motor windings
How an AC Motor WorksR1
L0
R2M
G7
• L0
– Leakage Inductance
How an AC Motor WorksR1
L0
R2M
G7
• R2
– Secondary Resistance• rotor resistance
How an AC Motor WorksR1
L0
R2M
G7
• M– Excitation Inductance
How an AC Motor WorksR1
L0
R2M
G7 ASD Overview
• What is an ASD
• Conventional V/Hz PWM Drives
• Vector Control
• Closed Loop versus Open Loop
G7
• An AC Motor will happily produce rated torque at any frequency if the proper voltage is applied
• Inductive impedance is directly proportional to frequency:– Impedance () = 2 x x f x L
ASD Overview
G7
• Since inductive impedance decreases linearly with frequency, only 1/2 of the voltage is required to provide the same amount of current at 1/2 frequency (1/2 speed)
• I = V
ASD Overview
Volts per Hz Relationship
0
100
200
300
400
500
600
0 20 40 60 80
Frequency (Hz)
Vol
ts
G7
• Now that the very basic requirement of a drive is understood what do we do to provide this constant volts per hertz relationship
• A drive is relatively simple. It converts AC power to DC then reconstructs an AC output waveform
ASD Overview
G7 ASD Overview
G7
• The problem is that a motor’s needs aren’t quite as simple to provide 1/2 voltage for 1/2 speed and 1/10th voltage for 1/10th speed
• The simplified equivalent circuit has stator resistance that causes an internal voltage drop
ASD Overview
460V - 60Hz
Full current - 23V drop
437V
23V-3Hz
Full current - 23V drop
0V
Volts per Hz Relationship
0
100
200
300
400
500
600
0 20 40 60 80
Frequency (Hz)
Vol
ts
46V - 3Hz
Full current - 23V drop
23V
57V - 3Hz
150% current - 34V drop
23V
35V - 3Hz
50% current - 12V drop
23V
G7• Conventional PWM drives simply dealt with
this stator winding voltage drop with a programmable voltage boost
• Later drives offered automatic voltage boost but it was marginally effective at best
• The problem with voltage boost is that it has no intelligence to it
• ENTER VECTOR CONTROL
ASD Overview
G7
Flux Vector ControlVector Control Simplified
ASD Overview
G7
• There are only two things that a drive can vary to control a motor:
– Voltage
– Frequency
• So what does vector control do differently?
ASD Overview
G7
• First - let’s go backwards
• AC Drives originally gained popularity because a “standard” AC motor can be used
• Early drives had poor starting torque & poor speed regulation
• DC drives remained the electric ASD of choice for high performance applications
ASD Overview
G7
• The objective of vector control (or flux vector) was to improve speed regulation plus improve torque performance to at least that of DC drives
• To properly understand the basics of vector control let’s take a look at the motor’s equivalent circuit again.
ASD Overview
G7Simplified Equivalent Circuit
R1L0
R2M
ASD Overview
G7
• R1
– Primary Resistance• compensation for voltage drop on motor windings
• effects torque at low speeds
• Used in all Vector Control Modes
• Used in all Auto Torque Boost Modes
• Used in Torque and Positioning Modes of Operation
R1L0
R2M
ASD Overview
G7
• L0
– Leakage Inductance• effects speed and torque regulation at speeds greater
than 1/2 base frequency
• Used in all Vector Control Modes
• Not Used for Auto Torque Boost
R1L0
R2M
ASD Overview
G7
• R2
– Secondary Resistance• is used to calculate slip frequency
• Used in all Vector Control Modes
• Critical if regeneration exists at below 15Hz
• Critical if operation will extend above base frequency
R1L0
R2M
ASD Overview
G7
• M– Excitation Inductance
• Used for calculation of current used to excite the motor.
• Used in all Vector Control Modes
• Used in all Automatic Torque Boost Modes
• Used in Position and Torque Control Modes
R1L0
R2M
ASD Overview
G7
• Vector control uses a different philosophy which provides a mechanism for feedback– Vector control monitors:
• Output current magnitude
• Magnetizing or flux current
• In phase current
• Motor slip (torque)
ASD Overview
G7
• Since current and motor slip are monitored, the microprocessor can provide complete control over the motor– Flux current can be controlled by increasing or
decreasing voltage– Motor shaft speed can be controlled by either
varying slip or increasing output frequency– Torque can also be fully controlled by either
changing the amount of slip or flux
ASD Overview
Simplified Vector Diagram
Flux Current (If)
Vector Sum (Actual Current drawn by motor)
In Phase Current (Ia) Voltage Vector
Vector Control (Control of Magnetizing and In Phase Currents)
G7
• If flux current is kept constant, slip is directly proportional to torque
• If flux current is kept constant in phase current is directly proportional to torque
• This means that if flux is kept constant, slip is directly proportional to in phase current
• The drive can therefore accurately calculate motor slip hence actual shaft speed
ASD Overview
G7
• It can be demonstrated that a vector drive can provide very good speed regulation without having direct speed feedback from the motor shaft
• This technique is referred to as “Open Loop Vector” or ‘Sensor less Vector Control”
• Speed regulation can approach 0.1% for a properly set up open loop vector drive
ASD Overview
G7
• Open or closed loop vector control produce the same end result – Improved torque performance and better
speed regulation
ASD Overview
G7
Huge Starting Torque
• An additional benefit of vector control’s complete control over motor flux is starting torque
ASD Overview
G7
• Closed loop uses actual rotor speed and current for feedback– 0.01% speed regulation– Zero speed holding torque
• Faster response to step changes in load
• Additional components (rotary encoder) which may reduce system reliability
ASD Overview
G7
• When investigating drives, evaluate your needs and pick the most appropriate technology based on:– Performance requirements– Overall system reliability– Costs of sparing (motors and drive parts)
ASD Overview