IM Speed Control Sep2023

8/8/2019 IM Speed Control Sep2023

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Induction Motor Drives

Speed Control

Induction Motor Drives

Advantages of Induction Motor

RuggednessLower Maintenance

Better Reliability

Lower Cost

Higher Efficiency Ability to operate in dirty and

explosive environment

The motor drives a load which has a certain

characteristics torque-speed requirement.

In general,

where, k may be an integer of a fraction.

In a feed drive,

In fans and pumps,

The motor can be connected to the load through a setof gears

The gears have teeth ratio and can be treated as torque

transformers

Load horsepower and torque

characteristics

Constant torque, horsepower varies as the speed

A pplications: General machinery hoists, conveyors,

printing press

Load horsepower and torque characteristics

Horsepower varies as square of the speed, torque

varies with speed

A pplications: Pumps, some mixers, some extruders

Load horsepower and torque

characteristics

High inertial loads

A pplications: Are typically associated with machines

using flywheel to supply most of the operating energy,

punch press

Load horsepower and torque characteristics

Shock loads

A pplications: Drives of crushers, separators, grinders,conveyors, and vehicular systems

Power converters and motors can be damaged if they

are not protected from the overload conditions

Methods of Speed Control of

Induction Motor

Pole Changing

Stator Voltage control Supply Frequency Control

Rotor Resistance Control

Slip Power Recovery Eddy-Current Coupling

Pole Changing

Sometimes induction machines have a special

stator winding capable of being externally

connected to form two different number of pole

numbers.

Since the synchronous speed of the induction

machine is given by

this would correspond to changing the synchronous

speed.

The two coils on A & C may be connected in series in twodifferent ways ² A2 may be connected to C1 or C2.

A1 with the other terminal at C then form the terminals of

the overall combination.

For a given direction of current flow at terminal A1, say into terminal A1, the

flux directions within the poles are shown in the figures.

In case (a), the flux lines are out of the pole A (seen from the rotor) for andinto pole C, thus establishing a two-pole structure.

In case (b) however, the flux lines are out of the poles in A & C. The flux lines

will be then have to complete the circuit by flowing into the pole structures on

the sides. Pole configurations produced by these connections is

a two-pole arrangement in fig. (a) and a four-pole arrangement in fig. (b).

Variable Terminal Voltage

The Torque Developed in IM is

Proportional to the Square of the Terminal

Voltage

The Rotor current is directly Proportional

to the Terminal Voltage

Thermal Over Loading decreases

A.C. Voltage Controller

AC Voltage

Controller

Fixed AC

Voltage

Source Variable AC Voltage

THREE PHASETHREE PHASE

AC VOLT AGE CONTEROLLER CIRCUITS AC VOLT AGE CONTEROLLER CIRCUITS

Speed-Torque

Characteristics for

Stator Voltage

Control Method

THREE PHASE AC VOLT AGE

ROLLER CIRCUIT

AC VOLTAGE CONTROLLERFour ± Quadrant Control

Forward Motoring and reverse plugging

operations

Low speed motoring and high speed braking Thyristor pairs A,B and C provide operation

in quadrants I and IV

Thyristor pairs A¶ , B and C¶ provide

operation in quadrants II and III

Zero current sensor [ dead time 5 to 10 ms]

W AVE FORM OF PHASE CURRENT FOR =60

SINGLE QUADRANT

CLOSED LOOP CONTROL

VARIABLE FREQUENCY

CONTROL The synchronous speed is directly

proportional to supply frequency.

The back emf induced

E= 4.44 N f NT kw

E E N f

N E ---------

N E ----------

[ if stator drop is neglected]

VARIABLE FREQUENCY

CONTROL

Constant Volts / Frequency Operation

Td= 3(V m/se)(p/2) {R r r /( R r+rLlr)2 }

Power Converters (contd¶)

2. Inverters ± Voltage and current source converters are

fed from a DC link. The DC link is generated with

either a controlled or uncontrolled rectifier.

Vc: controlled magnitude command

f c: frequency command

3. Cycloconverter

Power Converters (contd¶)

The controllers implement the control strategy

governing the load and motor characteristics

To match the load and motor, the input to the power

converter is controlled (manipulated) by the

controller

Controllers

Voltage Source Inverter

Low Internal Impedance

Any Short Circuit across its terminals

causes current to rise very fast The fault current can be cleared by Fast

acting Fuse links

erminal Voltage remains constantwith variations of load

Inverter Grade SCR

SIX ± STEP INVERTER

MODE OF SEQUENCE OF FIRING

OPERATION

I 1 5 6

II 1 2 6

III 1 2 3

IV 2 3 4

V 5 4 3

VI 5 4 6

Tripplen harmonics are absent

Classification of Converter Systems

PWM-VSI is now most commonly use

Draw Back of Six Step inverter

Derating of the motor

Torque pulsating

Over heat the machine at low speeds Commutation capability of thyristor inverter

reduced

PWM-VSI System

Diode rectifier for unidirectional power flow

PWM-VSI System

Options for recovered energy during

regenerative braking

PWM Inverters

Low harmonic content in the output

voltage

Lower derating of the motor and higher efficiency

No commutation problem

GENERATION OF CONTROL SIGNALS FOR

MODULATED W AVEFORM

SINUSOIDAL PULSE WIDTH MODULATION

OPEN-LOOP VARIABLE FREQUENCY

PWM INVERTER DRIVE

CLOSED-LOOP SPEED CONTROL WITH

PWM INVERTOR DRIVE

Advantages

1. The circuit for CSI, using only converter grade thyristor,

which should have reverse blocking capability, and also

able to withstand high voltage spikes during commutation,

is simple.

2. An output short circuit or simultaneous conduction in aninverter arm is controlled by the µcontrolled current source¶

used here, i.e., a current limited voltage source in series

with a large inductance.

he converter-inverter combined configuration hasinherent four-quadrant operation capability without any

extra power component.

Disadvantages

1. A minimum load at the output is required, and the

commutation capability is dependant upon load current.This limits the operating frequency, and also puts a

limitation on its use for UPS systems.

2. At light loads, and high frequency, these inverters have

sluggish performance and stability problems.

IM Speed Control Sep2023

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