Electric Drives and Controls | 02.10.2012 | Ralph Reutter | DC-IA/SFS31 | Grundlagen...

Electric Drives and Controls | 02.10.2012 | Ralph Reutter | DC-IA/SFS31 | Grundlagen Frequenzumrichter mit ASM | © Bosch Rexroth AG 2012. Alle Rechte vorbehalten, auch bzgl. jeder Verfügung, Verwertung, Reproduktion, Bearbeitung, Weitergabe sowie für den Fall von Schutzrechtsanmeldungen.

Frequency converter with induction motor

Datum: 02.10.2012Ort: Lohr am MainErstellt durch: R. ReutterVersion: V01_en

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Basics Frequency converterServo – Frequency ConverterRectifier – Mains ChokePre-chargingInverterMotor FilterMains FilterBraking Resistor

Basics Inductance MotorConstructionStator Rotating FieldFunction

Basics motor controlV/fField orientated controlSensor less field orientated control

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Basics frequency converters

Difference between frequency converter an servo controller?

ServoHigh peak powerSM and ASMAlways with encoderMostly position mode

Frequency converterHigh continuous powerASM (inductance motor)With or without encoderMostly velocity controlled

The construction is identical Both are V-converters

Output of variable frequency and voltage

Differences only in application / possibilities of the Firmware

In the Rexroth language „open Loop“ means „without encoder“

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Mains Filter

Mains Choke

Rectifier Inverter

DC-linkMotor Filter

Motor

DC-link CapacityBraking Resistor

Construction of a Frequency Converter

Pre-charging

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DC-linkvoltage

u1

u2

u3

5

Rectifier

DC-link voltage



u1u2 u3

u12 u13 u23 u21 u31 u32

Without capacity smoothing

With capacity smoothingDC-link

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Mains-Side Phase Current without mains choke

Phase Voltage

Phase Current

DClink

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Mains-Side Phase Current with Mains Choke

Phase Voltage

Phase Current

DC-link

The choke extends the time ofcurrent flow• Equal current-time area• Reduction of the amplitude• Decrease of harmonics

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1 2 3 4 5 6 7 8 9 10 11 12 13

Without Mains choke

With Mains choke

Current

Order of Harmonics

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Frequency Spectrum



Pre-charging / Softstart

Control

Charging current is limitated bya resistor e.g. Braking Resistor

Controlled voltage by athyristor (phase shift control)

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U

V

W

UDC-link

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IGBT‘s are used as semiconductor switches

Inverter



U

V

W

UDC-link

ii

UDC

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U

V

W

UDC-link

i

i

UDC

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The converter output voltage is adjusted by the puls/break ratio

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Puls Width Modulation



Effect of the Puls Frequency

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Effect of the Puls Frequency

4kHz 12kHz

Current

Voltage

• Meassured with standard norm motor and HCS02• 20 Hz motor frequency• no load running

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Switching loss

U_DC

i

u

t

t

Switching losses

Conducting-state losses

u

i

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Risks for the motor windings

∆t

∆U

Rate of rise of voltageMade by the fast switching timeof the IGBT‘s.HxS : 5 kV/µsFv : k.A.

OvervoltageDetermined by inteferences of the voltage pulses

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Motorfilter / Linedumping

inverter

MFilter inductance

Leakage capacity

Lowpass – filtering high frequency parts

Adjusting of the wave impedance of the cable for a better load matching to avoid inteferinces

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Voltage Pulse

∆U

∆t

Without Motorfilter With Motorfilter

∆t

∆U

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Mains Filter / EMC Filter

Rectifier

InverterDC-links Motor

Mains

Mains Filter

Leakage current causedby the pulse frequency

i_lea i_lea

Should be avoided

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Brakingresistor

A braking resistor converts regenerative kinetic energy into thermal energy. The DC-link voltage is short circuited over the braking resistor by the braking transistor(Chopper).Switching on threshold between 780 and 820V.The difference between U_on and U_out is 30….40V.

U_on

U_out

DC_link

speed

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DC-link

Mostly inside the device

Mostly outside



Basics Frequency converterServo – Frequency ConverterRectifier – Mains ChokePrechargingInverterMotor FilterMains FilterBraking Resistor


Basics Motor controlV/fField orientated controlSensor less field orientated control

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Basics induction motor

Inductance motor - Construction

Connection Box

Bearing

Shaft

Fan

Stator-Winding

Rotor

Case

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IlBF **

Principle of a motor.On a current carrying wire which is inside a magnetic field, a force is be appealed

Principle of a generatorA voltage is been induced when the wire is been moved.

vlBU **

I

B

F

v

U

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Number of pole oairs

p = 1 p = 2

One U, V and W winding. Two U, V and W windings

50Hz rated frequency

Synchronous velocity of 3000 U/min

50Hz rated frequency

Synchronous velocity of 1500 U/min

Stator

Rotor

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U

V

W

U

V

W

U

V

W

U

V

W

i1i2 i3

t1 t2 t3 t4

t1 t2 t3 t4

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The asynchonous motor can be considered like a transformer

The stator current I1 generates a alternating magnetic field Φ in the air gap

Because the phase shifting of 120° a rotating field is generated.

The flux induces a voltage in the rotor. A current I2 flows through the short

circuit rotor. These short cut current effects a force

F.

Important: The speed of the rotor must be unequal (asynchonous) to the speed of the stator field.

Asynchronous motor

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• The torque Mi at the motor- shaft is proportional to the current in the short circuit rotor and proportional to the magnetic flux Φ of the rotor field.

• The speed nd depends on the frequency in the stator windings

d2i I~M

p

fn 1

d

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Rsσs σR

RRXh

Rs= Stator resistorσs= Stator leakage inductanceRR= Rotor resistorσR=Rotor leakage inductanceXh=Main inductanceU

IgesImag Itorque

Rating plate:6 values are always declared• Voltage• Speed• Frequency• Power• Current• Cos(phi)

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A constant flux in the air gap has to be generated.

magnIL

Lf

U

Lj

UImagn

2

When the ohmic part is neglect the magnetising current can be described by this formula.

To get a constant flux the following condition must be met.

.constf

UImagn

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Basics Frequency converterServo – Frequency ConverterRectifier – Mains ChokePrechargingInverterMotor FilterMains FilterBraking Resistor


Basics motor controlV/fField orientated controlSensor less field orientated control

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Basics motor control

Motor characteristic of an ASM with 50 Hz rated frequency

50Hz (rotating field frequency)

Slipfrequency

Slip= Difference between the real frequency of the rotor and the rotating field frequency

Rated point

Breakdown torque

Big slip

Small slip

Big torque

Small torque

Depending on the load the operation pointis moving on the charecteristic

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Inductance motor with frequency converter

f [Hz]

Normal operating area Field weaking area

M

Breakdown torque

Rated torque

2

)( *

n

nMM N

breakdownnbreakdown

n

nMM N

NnN *)(

The torque is reduced in the field weaking area

The characteristic is moved by the variable converter frequency

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V

f

VN

fN

Norm. operating area Field weaking area

V/f-control

It‘s an open loop voltage control

For every frequency (demand speed) the converter gives a specific voltage out.

The relation between voltage and frequency is constant to get a constant flux in the mashine.

Optimisations modify the form of the curve.

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Field orientated control- FOC

V

id1

iq1is1

phi1

Magnetising current

Active current

Summery current

The motor current is devided in two parts.Iq = Torque generating currentId = Magnetising current

Both currents can be independently of each other impressed into the maschine

iq2 is2

phi2

=id2

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Sensor less field orientated control - FXC , SVC

Current

Voltage

Field frequency

Motor control

Accurate motor parameters

Operation point of the mashine

Calculating of actual speed and slip

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Comparison of the three control modes

V/f FOC Sensorless FOC

Complete torque at standstill

no yes no

Speed accuracy - - +++ +

Knowledge of motorparameters

low much a lot of

Torquelimitter no yes yes

More motors at one converter

yes no no

Positioning no yes no

Electric Drives and Controls | 02.10.2012 | Ralph Reutter | DC-IA/SFS31 | Grundlagen...

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Transcript of Electric Drives and Controls | 02.10.2012 | Ralph Reutter | DC-IA/SFS31 | Grundlagen...