Download 1. Introduction

Satakunta Polytechnic
Electrical Power Engineering & Automation and Control
Laboratory work
SAMI GS FREQUENCY CONVERTER
1. Introduction
The cage induction motor is the electric motor most widely used in industry. It is popular because
of its simple and rugged construction and minimal need of maintenance. Its efficiency is good
and the weight low. It has internationally standardised dimensions and power ratings. Its
popularity and good characteristics have also made it cost-effective. Before frequency converters
were introduced into the market the use of the cage induction motor was limited because
 Its speed could not be controlled adequately;
 The motor loaded the supply network with a high starting current, i.e. 5 to 8 times the rated
current.
The introduction of frequency converters about 20 years ago began a revolution in speed control
of electrical drives. The static frequency converter provides an adjustable-speed drive whose
performance in terms of accuracy and reliability has never before been achieved. In the beginning
the initial capital costs where high but often justified by improved performance and lower
running costs. Today, the fourth generation of frequency converter challenges all other drives in
initial capital cost, performance and versatility.
In the short history of frequency converters, different principles of converters have been applied.
On the other hand more advanced power semiconductors have been developed and the most
recent, efficient, microprocessors have enabled increasingly sophisticated control.
The next figure shows the main circuit diagram of a typical, modern, frequency converter
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It has a three-phase diode rectifier that supplies the three-phase IGBT-inverter. The intermediate
dc-circuit has a dc-voltage Ud about
Ud = 1.35 U
where U is the phase to phase ac-voltage. The IGBT-inverter circuit controls both the frequency
and the voltage of the motor. The desired output frequency is produced by switching the IGBTswitches at the desired frequency and at a certain order. The sequence produces square wave acvoltages having the desired fundamental, a lot of harmonics and a voltage level near to the
original line voltage. However, at frequencies lower than the rated frequency, the voltage of the
motor should vary linear with the frequency. This property is achieved by pulse width
modulation (PWM), where the average value of the fundamental is lowered and at the same time
the output voltage approximates sinusoidal waveform.
2. Caution for Inspection and Operation
 After the power is switched off, the dc-bus capacitor remains charged at a high voltage for a
while. Wait five minutes for the capacitor to discharge.
 Do not touch electronic CMOS devices. The overvoltage due to static discharge may damage
components.
 Choose a 1000-rpm cage rotor motor to limit the maximum speed at reasonable values.
 At the end of the lab reset all parameters to the initial settings (1, p.32).
 Main circuit measurements using an oscilloscope, a spectral analyser or a voltage transformer
may cause damage to the inverter and to the measuring equipment. Before such the
measurements, consult your instructor.
3. Initial Tasks and Experiments
Find a personal computer with your favourite spreadsheet program and word processor. Copy
this instruction file, edit it, as you will, add your answers, comments, and results. In the end
finish and print your report.
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3.1 Rated Values
Check the rated values of the converter (1, p9)

Line current;

Output current;

Overload current and overload duty time;

Motor capacity;
Notice that for square type loads somewhat greater values are allowed.
Check the rating plate of the motor. The following nominal values are needed:

speed and slip;

output power;

current(s);

voltage(s);

number of poles;

cos.
The slip means the difference between the rotating magnetic field of the motor and the speed of
the rotor. The angular speed of the magnetic field depends on the line frequency and the number
of pole-pairs of the motor. For example, if the rated speed of a motor is 960 r/min, the nominal
slip of the motor is 1000 r/min-960 r/min=40 r/min or 4.19 radians/second.
The rating plate does not give the value for the rated torque. Calculate the rated torque. The value
will be required later in this exercise.
3.2 Block Diagram and Construction
Examine the block diagram of the converter (1, p10).
Open the device, study and find the following details

Construction of the converter;

Terminals for the ac-power source;
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
Terminals for the motor;

Terminals for the peripheral devices;

Ventilation.
Do not touch the CMOS-components.
4. Control Panel
Do not connect the motor cable before you have got familiar with the control panel.
4.1 First Exercise
Run the exercise (1, p24) and select the desired language (1, p32).
4.2 Start-up Parameters
 Select the language (1, p32).
 Select the application (1, p32). The application number 1 "FACTORY" is suitable in the
beginning.
 Restore the initial settings of the application "factory" (1, p32).
 Set the parameter "line voltage"
 Set the parameters of the motor according to the present motor (1, p33).
5. Operation Tests
5.1 Local Control
 Switch the main supply off. Disconnect the motor cable and run the local control test (1, p34).
 Switch the main supply off. Connect the motor cable, switch the main supply on and run the
test with the motor (1, p34).
5.2 Actual Values
Run the motor at the rated operating point. Use the control panel to monitor the actual values of
the following quantities:
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
Output current;

Output voltage;

Output frequency;

Dc-voltage;

output power;

Speed;

Temperature of the converter;

Torque.
Are the values reasonable?
6. Tasks, Experiments, and Measurements
6.1 Frequency and Voltage
Measure and plot the output voltage of the converter as a function of the frequency, when

The supply voltage has the rated value;

The supply voltage is about 10% low.
How will the converter adapt itself to the line voltage variations?
6.2 Current Limit and Torque
 Set the current limit according to the rated value of the motor. Run the motor at several
operation points both under and over 50 Hz and try to overload it. How are the current limit
and the torque related?
 Mount a flywheel to the shaft of the motor. Set very short, linear mode acceleration time (1,
p50). Set frequency reference to the value of 100 Hz and start the drive. Examine, how the
current limit will control the operation.
 Calculate the minimum acceleration time so, that when accelerated to 50 Hz, the motor
current will stay below the current limit.
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6.3 Starting (Release) Torque
 Lock the shaft of the torque measuring equipment. Measure the starting torque and frequency
at the rated stator current. At the beginning the parameters should have the initial settings.
 Since the magnetising current is not yet properly adjusted, the torque may deviate from the
rated value. Try IR-compensation (parameters 27.6 - 27.8 (1, p62) to reach such the situation
that the rated torque will be produced at the rated current and at the rated slip.
 Unlock the shaft and check that the current limit operates normally regardless of the past
settings.
6.4 Torque at Rated Current
Measure and plot the torque as a function of speed at the rated stator current. The speed range
should be from 0 to 2000 r/min. The IR-compensation should have the value found in the locked
shaft test.
6.5 Slip Compensation
Run the drive at a certain frequency. Set the current limit to the value of 1.5 In. Measure the drop
of the speed, when the load varies from no-load to the rated torque. Assume that a better speed
stability would be necessary. Examine, how the slip compensation will improve the behaviour.
What is the influence of the temperature of the motor to the speed stability?
6.6 Noise
The drive may be too noisy to be installed at certain places or noise may appear at certain
frequencies. Examine possibilities to reduce the noise by increasing the switching frequency of
the converter. Will there be undesirable consequences?
7. Braking
There are several different methods and arrangements to carry out braking in connection with
frequency converters. Some of those principles are presented in the next chapter.
7.1 Theory of Braking
The cage rotor motor has an inherent capability of braking. Braking occurs, when the speed of
the rotor exceeds the speed of the rotating magnetic field. There are two ways to get a motor to
act as a generator
 the load begins to drive the machine;
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 The supply frequency is suddenly decreased.
As well as the cage rotor motor, the PWM-inverter is capable of braking, i.e. converting acpower to dc-power.
The next circuit diagram shows an elementary frequency converter. When the motor is braking, it
supplies electric power through the inverter into the intermediate dc-circuit. Because the diode
rectifier is not capable of regenerative braking, the dc-voltage tends to increase.
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7.2 Braking by Losses
The first idea is that the system is not capable of braking. However, it may be used to decelerate
the motor and the revolving parts coupled into it. In the motor and in the inverter, power losses
are generated. The drive can be decelerated at such the rate that the mechanical braking power
will not exceed the sum of the losses. The braking ability is limited, but compared to coasting to
stop considerable shorter stopping time is achieved.
7.3 Dynamic Braking
If a braking resistor and a dc-chopper are added to the system, then excellent braking
characteristic is the result. Many small-size frequency converters have a built in braking chopper
and a resistor. The regenerated power will cause the intermediate dc-circuit voltage to rise. The
braking chopper discharges excessive power into the braking resistor. To restrict the temperature
rise of the equipment to a reasonable value, the duty time for such the braking system is often
limited.
As an option, braking resistors and choppers are available for most of the frequency converters.
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7.4 Dc- Brake
Dc-brake (dc injection brake) is an option for frequency converters that can be implemented
without extra cost. In normal operation, the motor has a rotating magnetic field. If dc-current is
fed into the stator winding and the rotor is rotating, then braking occurs. The braking energy is
dissipated into the rotor.
Test the operation of the dynamic DC-brake.
8. Advanced Experiments and Measurements
8.1 Remote Control
Modify the system so that instead of the local control (the control panel) the system obeys remote
control signals, as

frequency reference;

start/stop;

Direction.

Consider a system that should have two alternative remote control sources:

a control panel;

A PLC.
Both of the sources should give signals for start/stop, direction and frequency reference. The
control panel should furthermore have a switch "Automatic Control/Local Control" for the PLC
or the panel control. How would you implement the required system?
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8.2 Eliminating Troubles
 In some cases the drive should discover a sudden disappearance of the load. For example to
restrict the amount of the waste the drive should stop itself automatically. Find the solution
and test it!
 The drive supplies a mechanical system. The motor has four poles and the speed region is
from 500 r/min to 2200 r/min. Unfortunately the mechanical system has severe resonance at
540 - 690 r/min and at 1380-1560 r/min. SAMI GS has a built in solution. Test it!
8.3 Peak Torque Measurements
Modern frequency converters are capable of producing the fundamental frequency up to 500 Hz.
A 1000-r/min cage rotor motor would then have the speed about 5000 r/min. Unfortunately the
motor, having the rated frequency of 50 Hz, has very low torque performance at such the speed.
Run the frequency converter drive at the frequency of 150 Hz. Find the maximum output torque
that the drive is able to produce.
8.4 Time Domain Measurements
Monitor and record the format of the stator current and the phase to phase voltage at the
following operating points

the shaft is jammed and the motor is producing the rated torque at the rated current;

he frequency is kept unchanged, but the shaft runs freely and is unloaded;

the motor runs at the rated operating point;

the frequency is 50 Hz and the motor has no load;

The frequency is 100 Hz and the output power has the rated value.
8.5 Frequency Domain Measurements
Monitor and record the harmonic contents of the ac-supply current
 The drive runs unloaded;
 The motor runs at the rated operating point.
9. Study Questions
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What is the field weakening point?
Why is the operation at the frequencies over 100 Hz limited?
What is meant by slip compensation?
Why an ordinary thermal relay will not be capable of protecting a motor that is driven by a
frequency converter?
What may happen, if the frequency reference is decreased too fast?
SAMI GS is halted because of the loss of the load, but you don't know it yet. What to do?
The application is tested and the customer is satisfied. There are questions


If the supply voltage will be cut off, will the settings remain unchanged?
The operator will use the control panel to read actual values. What to do, so that the
parameters will not be changed accidentally.
What is meant by the abbreviation "IGBT"?
Sketch the symbol for an IGBT.
10. At the end restore the initial settings.
References
1. SAMI GS Frequency Converters ACS501 2.2 - 45 kW
Operation Manual, ABB Drives, 1992
2. SAMI GS Frequency Converters ACS501 2.2 - 45 kW
Application Macros, ABB Drives, 1992
3. J.M.D. Murphy & F.G. Turnbull
Power Electronic Control of AC Motors
Pergamon Press, 1989
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Terminology
acceleration time
actual value
at the rated operating point
at the rated stator current
Braking
cage induction motor
current limit
field weakening point
flywheel
frequency reference
influence of the temperature
inherent capability of braking
lower running costs
maintenance
nominal values
adjustable speed drive
performance
rating plate
Slip Compensation
The intermediate dc-circuit
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kiihdytysaika, 5
mitattu arvo, 4
nimellispisteessä, 4
nimellisvirralla, 6
jarrutus, 6
oikosulkumoottori, 1
virtaraja, 5
kentänheikennyspiste, 10
huimapyörä, 5
taajuuden ohjearvo, 5
lämpötilan vaikutus, 6
luontainen mahdollisuus jarrutukseen, 6
alhaisemmat käyttökustannukset, 1
huolto, 1
nimelliarvot, 3
nopeusohjattu käyttö, 1
suorituskyky, 1
arvokilpi, 3
jättämän kompensoiti, 6
tasajännitevälipiiri, 2