WARSAW UNIVERSITY OF TECHNOLOGY
INSTITUTE OF ELECTRICAL MACHINES
ELECTRICAL MACHINES IN THE POWER ENGINEERING AND AUTOMATIC
Induction Generator test
Introduction
For the over-synchronous speed (n > nn, s < 0), induction machine enters generator state
of operation with changed direction of induced voltage Uir, changed direction of power flow
changed direction of active current, but unchanged direction of reactive (magnetizing) current
(Fig.1).
Fig.1. Torque-speed characteristic with generating operation area.
The last feature is the main disadvantage of induction generator. Another disadvantage is that
induction generator can’t individually force voltage frequency.
Fig.2. Operation of induction motor as induction generator.
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Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego
Using external drive with regulated rotational speed, it is possible to force synchronous
state of operation of induction motor by increasing motor rotational speed up to synchronous
speed (s=0). Drive system delivers power to cover all mechanical losses in induction motor
(bearings and ventilation), and balancing parasitic electromagnetic torques. On the other side
grid delivers active power to cover winding and core losses as well as reactive power
necessary to build up flux in the motor (Fig.2).
After increasing shaft speed over synchronous speed (n > ns) slip became negative (s < 0)
and internal motor power became negative machine starts to operate as a generator.
Ratings of the double speed Induction Motor to be tested:
Voltage
Power
Power factor
Current
Rotational speed
Ratings
UN
PN
SN
IN
nN
Test should be performed for upper rotational speed (supply voltage connected to lower
set of terminals).
To being able to operate induction generator autonomously there is necessity to deliver
reactive power, source of reactive power could be external block of capacitors (excitation
capacitors) connected to the motor phases. Excitation capacitors capacity is determined by
two border states of load (explained bellow).
- Minimum value of connected capacitor Cmin is determined by it’s ability to deliver
reactive magnetizing power to the motor in the no load state. This could be determined
experimentally using circuit connection presented in Fig.3.
Fig.3. Induction Generator test circuit connection.
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Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego
Induction machine operates as a motor. Set of variable capacitors should be tuned in such
a way to that all reactive power necessary to magnetize motor is delivered from the capacitors.
This state of operation is reached when indications of power meters working in Aron
connection are equal. Induction Motor accepts only active power from the grid (power factor
cosφ=1).
cosφ=1
Cmin [μF]
- Maximum value of connected capacitor Cmax is determined by maximum load state
of induction generator and reactive value of load current. After starting the autonomous
generating mode of operation of induction motor capacitors value should be increased until
the following operating condition is reached: I=1,1IN for U=1,1UN.
No load state of operation of Induction Generator
The aim of no load test is to determine the minimum value of rotor speed for which Induction
Generator is able to self-excite and maximum value of rotational speed for which induced
voltage doesn’t exceed 1,1UN and capacitor current Ic is lower than nominal current IN. The
following characteristics should be evaluated:
1- Dependence of induced voltage as a function of capacitance: Ui=f(C) for constant
rotor speed equal to synchronous speed: n=n1;
n=n1=
1
2
3
4
5
6
7
8
Capacitance [μF]
Cmin=
Cmax=
Induced voltage [V]
Ui=1,1UN=
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Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego
2- For chosen value of capacitors Cfix from the range <Cmin – Cmax> the dependence of
induced voltage as a function of rotational speed Ui=f(n), Ic=f(n) for rotational
speed range 0,7n1<n<1,1n1.
Cfix=
Rotational
speed [rpm]
0,7n1
1
2
3
4
5
6
7
8
Induced voltage
[V]
Capacitor current
[A]
1,1n1
External characteristics of Induction Generator
External characteristic is dependence of Induction Generator voltage as a function of
load current for constant value of excitation capacitors and rotational speed: U=f(I) for
C=const and n=const. Test should be performed only for active load (water resistor).
C=const=
n=const=
Load current [A]
Output voltage [V]
1
2
3
4
5
6
7
8
Exemplary external characteristic of induction generator are presented in Fig.4.
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Materiały dydaktyczne dystrybuowane bezpłatnie.
Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego
Fig.4. External characteristics of induction generator.
Literature:
Adam Biernat, Asynchronous Machines – part of the lecture: Electric Machines in the Power
Engineering and Automatics.
Instruction elaborated by Adam Biernat
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Materiały dydaktyczne dystrybuowane bezpłatnie.
Projekt współfinansowany ze środków Unii Europejskiej w ramach Europejskiego Funduszu Społecznego