her publication on the subject of
Motor Management.
With these published fundamentals, of Motor Management, the user will have a growing
reference on the performance and operational data required for design and application.
Topics covered include: Protection of Motor and Drive, Selection and Operation of Switchgear, Communications.
The following manuals have already been published: Three-phase Induction Motors, discusses the structure, modes, selection, and sizing
of motors and Basics of Power Circuit Breakers, discusses the practical use of Motor Protective
breakers.
Electric motors can be found in every production process today.
The optimal use of the drives is becoming increasingly important in order to ensure
cost-effective operations. Motor Management from Rockwell Automation will help
you: to optimise the use of your systems, to reduce maintenance costs, to increase operational safety.
We hope these publications will help you find economical and efficient solutions for
your applications.
Copyright © 1997 by Rockwell Automation AG
All information given represents the current level of technology available and is not
legally binding.
i Motor Starting
Table of Contents
1
Traditional motor starting
1.1
1.1
Star-delta-starting
1.1
1.1.1
Normal star-delta-starting
1.1
1.1.2
Enhanced star-delta-starting
1.5
1.1.2.1 Combined star-delta-starting
1.5
1.1.2.2 Partially wound star-delta-starting
1.6
1.1.3
Uninterrupted star-delta-starting
1.6
1.2
Auto-transformer starting
1.8
1.3
Starting via chokes or resistors
1.9
1.3.1
Starting via chokes
1.9
1.3.2
Starting via resistors
1.10
1.4
Multi-stage motors
1.11
2
Soft starters
2.1
2.1
General
2.1
2.2
Power starting implementation
2.2
2.2.1
Motor torque reduction
2.3
2.2.2
Influencing the motor voltage
2.3
2.3
Starting types
2.4
2.3.1
Starting by means of a voltage ramp
2.4
2.3.2
Starting by means of a current limitation
2.5
2.3.3
Torque
2.5
2.4
Soft starter types
2.5
2.4.1
Single-phase full-wave controlled soft starter
2.6
2.4.2
Three-phase half-wave controlled soft starter
2.7
2.4.3
Three-phase full-wave controlled soft starter
2.8
2.5
Thermal load during start
2.8
2.6
Advantages of soft starters
2.9
2.7
Benefits to the customer
2.9
2.7.1
Mechanical
2.9
2.7.2
Electrical
2.10
ii 2.8
Possible Applications
2.10
2.9
Pump starting
2.11
2.9.1
Current and torque development for a star-delta-starters
2.11
2.9.2
Speed development for starts with a pump soft starter
2.12
2.9.3
Comparison of torque curves
2.12
2.9.4
Flow curve during start
2.13
2.9.5
Flow curve during stop
2.13
2.9.6
Requirements for a pump soft starter
2.14
2.9.7
Application areas
2.14
2.10
Options
2.14
3
Frequency Inverters
3.1
3.1
General
3.1
3.2
Structure
3.1
3.2.1
Mains rectifiers
3.2
3.2.1.1 Principle diagram of pulsating direct voltages
3.2
3.2.2
Intermediate circuit
3.3
3.2.3
Inverters
3.3
3.2.3.1 Principle diagram of pulse width modulation
3.4
3.3
Operational behaviour
3.4
3.3.1
Frequency-voltage-ratio
3.4
3.3.2
Voltage increase or boost
3.5
3.3.3
Slip compensation
3.6
3.3.4
Set value
3.6
3.3.5
Compensation
3.6
3.3.6
Motor protection
3.7
3.3.7
Change of rotation and braking
3.7
3.4
Advantages of frequency inverters
3.8
3.5
Radio frequency interference (RFI)
3.8
3.5.1
General
3.8
3.5.2
Standards
3.9
3.5.3
Measures
3.10
4
Comparing starting procedures
4.1
Motor Starting
iii
Starting Electric Motors
Due to their simplicity, robustness and cost-effectiveness, squirrel-cage motors are the
preferred choice of industry. During start-up, they develop currents of up to
approximately eight times the rated current and the high starting torque linked to this.
The high starting currents often lead to unwelcome voltage drops in the supply network
and the high starting torque put the mechanical elements under considerable strain.
Therefore, the electricity companies determine limiting values for the motor starting
currents in relation to the rated operational currents. The permissible values vary from
network to network and depend on its load-bearing capacity. With regard to mechanics,
methods are required which reduce starting torque.
Various starters and methods can be used to reduce currents and torque: Star-Delta-Starting Auto-transformer-Starting Starting via chokes or resistors Multi-stage starting Starting using electronic soft starters Starting using frequency inverters
In the following passages, the main starting methods used in practice are explained
further.
1
Traditional motor starting
1.1
Star-delta starting
A difference is made between: Normal Star-Delta Starters Enhanced Star-Delta Starters Star-Delta Starters with uninterrupted switchover (closed transition)
1.1.1
Normal star-delta starters
To enable the motor to start, the motor windings are configured in a star formation to the
supply voltage. The voltage applied to the individual motor windings is therefore
reduced by a factor of 1
_3 = 0.58 this connection amounts to approximately 30% of the
delta values. The starting current is reduced to one third of the direct starting current, i.e.
typically to 2...2.5 I
e
.
Due to the reduced starting torque, the star-delta-connection is suitable for drives with a
high inertia mass but a resistance torque which is low or only increases with increased
speed. It is preferably used for applications where the drive is only put under a load after
run-up, i.e. for presses, centrifuges, pumps, ventilators, etc.
Motor Starting
1.1 Motor Starting
1.2
M
L
7
6
5
4
3
2
1
0.25
0.5
0.75
1
3
2
1
M
M
N
I
I
e
I I
Y
I
A
M M
Y
n
n
s
L1
L3
L2
V
1
V
2
W
1
W
2
U
1
I
WU
U
2
U
e
I
WV
I
LD
L1
L3
L2
V
1
V
2
W
1
W
2
U
1
U
2
U
e
I
WU
I
LY
= 1
3 I
LD
Z
W
U
e
u3
Typical Current and Torque Curve for Star-Delta-Starters
Current ratios for star and delta connections.
Star Connection
Delta Connection
I
Motor current
I
e
Rated operating current of
motor
M
D
Torque for delta connection
M
E
Rated operating torque of
motor
I
LY
Supply current for star connection
I
LD
Supply current for delta connection
I
W
Winding current
U
e
Mains voltage between lines
Z
W
Winding impedance
n
Speed
n
s
Synchronous speed
M
L
Load torque
I
Y
Current in star connection
I
D
Current in delta connection
I
A
Current curve for star-delta start
I
LY
= I
WU
= U
e
_3 Z
W
I
L1D
= I
WU
+ I
WV
I
LD
= I
W
_3 = U
e
_3 = 3 I
LY
Z
W
I
LY
= 1 I
LD
3 After motor run-up, in most cases an automatic timing relay controls the switch-over
from star to delta. The run-up using star connection should last until the motor has
reached the approximate operational speed, so that after switching to delta, as little post-
acceleration as possible is required. Post-acceleration in delta connection will instigate
high currents as seen with direct-on-line starting. The duration of start in star connection
depends on the motor load. During delta connection, the full mains voltage is applied to
the motor windings.
To enable a switch-over from star to delta, the six ends of the motor winding are
connected onto terminals. The contactors of a star-delta starter switch over the windings
accordingly.
Starting in star, the main contactor connects the mains to winding endings U1, V1, W1.
The star contactor shorts winding endings U2, V2, W2. After successful run-up, the star
contactor switches itself off and the delta contactor connects terminals U1/V2, V1/W2,
W1/U2.
When changing from star to delta, attention has to be paid to the correct phase sequence,
i.e. the correct connection of the conductors to motor and starter. Incorrect phase
sequence can lead to very high current peaks during the cold switch-over pause, due to
the easy torque reduction following re-start. These peaks can damage the motor
windings and stress the controlgear unnecessarily. The rotation of the motor has to be
considered as well.
Motor Starting
1.3
Switch over from Star to Delta by means of Contactors Motor Starting
A sufficient time period has to be maintained between the star contactors de-
energisation and the energisation of the delta contactor, in order to safely extinguish the
star contactors disconnecting arc before the delta contactor is energised. During a
switch-over which is too fast, a short circuit may develop via the disconnecting arc. The
switch over time period, however, should be just long enough for an arc disconnection,
so that the speed decreases as little as possible. Special timing relays for a star-delta
switch over fulfil these requirements.
Motor Protection and Contactor Sizing
The overload relay is situated in the motor line. Therefore, the current to be adjusted is
lower than the motors rated current by a factor of 1
_3 = 0.58. Due to the third
harmonics currents circulating in the motor windings, a higher setting of the overload
relay may be required. This may only be carried out on the basis of utilising a
measuring device which records the correct r.m.s. value. Conductor cross-sectional areas
must be of a suitable size in order that they will be protected against temperature rises
resulting from overload conditions. Therefore, the conductor size selected must be in
accordance with the protective device(s) rating.
For motor protection by means of power circuit breakers with motor protection
characteristics, the power circuit breaker is switched into the network supply lines, as it
also carries out short circuit protection of starter and lines. In this case, the current is set
to the rated motor current. A correction of the set value because of the third harmonics is
irrelevant under these circumstances. The lines are to be thermally proportioned
depending on the power circuit breakers setting.
For normal star-delta starting, the controlgear must be sized in accordance with the
following currents: Main contac