basic motor drive theory 40min

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.


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• 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


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.


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


G7 How an AC Motor Works

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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


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


G7Torque

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PERCENT SYNCHRONOUS SPEED

PER

CEN

T F

UL

L L

OA

D (

TO

RQ

UE &

CU

RR

EN

T )


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


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.


G7

R1L0

R2M


G7

• R1

– Primary Resistance• Resistance of motor windings

How an AC Motor WorksR1

L0

R2M

G7

• L0

– Leakage Inductance


L0

R2M

G7

• R2

– Secondary Resistance• rotor resistance


L0

R2M

G7

• M– Excitation Inductance


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

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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

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• 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


0V

Volts per Hz Relationship

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0 20 40 60 80

Frequency (Hz)

Vol

ts

46V - 3Hz


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

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• L0

– Leakage Inductance• effects speed and torque regulation at speeds greater

than 1/2 base frequency


• Not Used for Auto Torque Boost

R1L0

R2M

ASD Overview

G7

• R2

– Secondary Resistance• is used to calculate slip frequency


• 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 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

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• Open or closed loop vector control produce the same end result – Improved torque performance and better

speed regulation

ASD Overview

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Huge Starting Torque

• An additional benefit of vector control’s complete control over motor flux is starting torque

ASD Overview

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• 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

basic motor drive theory 40min

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