Automation Direct Stellar SR55 Series, Руководство пользователя

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Page B–8 Stellar
®
SR55 Series Soft Starter User Manual – 1st Ed, Rev F – 09/18/2019
Appendix B: Soft Starter Application Considerations
b.3 – c
oNcepTs ANd
p
RINcIples of
f
Ixed
-s
peed
I
NducTIoN
m
oToR
s
TARTING ANd
c
oNTRol
Since its invention one hundred years ago, the standard three-phase induction motor has
become one of the most familiar items of industrial equipment ever known. Due to its
simplicity of construction, low cost, reliability, and relatively high efficiency, it is likely to
remain the prime source of mechanical energy for the foreseeable future.
b.3.1 – i
nTroduCTion
Energy conversion, from the electrical supply to rotating mechanical energy, is a characteristic
of all motors. To regulate energy flow, most motor circuits require a mechanism to connect
and disconnect them from their electrical power source. Electro-mechanical switches, known
as “contactors,” are the standard means of achieving this control. Even today, more than one
hundred years after their introduction, contactor-based systems remain the most widely used
method of motor control.
Nevertheless, there is a definite trend towards more sophisticated electronic systems of
control being applied to fixed-speed motor drives. This section will discuss these forms of
control; namely electronic microprocessor-controlled optimizing soft starters such as the
SR55.
b.3.2 – T
he
i
nduCTion
m
oTor
In order to appreciate the benefits of using an electronic controller, it is important to have
some understanding of the characteristics and limitations of the induction motor and the
electro-mechanical systems currently used to control them.
The standard, fixed-speed induction motor fulfills two basic requirements:
• To accelerate itself and its load to full speed (or speeds with multi-speed motors)�
• To maintain the load at full speed efficiently and effectively over the full range of loadings�
Due to the constraints of materials and design, it can be difficult to achieve both objectives
effectively and economically in one machine.
So, how does a motor start in the first place? As mentioned previously, motors convert
electrical energy drawn from the power supply into a mechanical form, usually as a shaft
rotating at a speed fixed by the frequency of the supply. The power available from the shaft
is equal to the torque (moment) multiplied by the shaft speed (rpm). From an initial value
at standstill, the torque varies, up or down, as the machine accelerates until reaching a peak
at about two thirds of full speed, and then dropping to zero at synchronous speed. This
characteristic means that induction motors always run at slightly less than synchronous speed
in order to develop power (the ‘slip speed’), and hence the term asynchronous. Figure B.3.2.1
shows a graph is of an induction motor torque/speed curve, and illustrates this important
characteristic of asynchronous three-phase induction motors.