Fairford synergy, User Manual

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Synergy Series Soft Start User Manual
MAN-SGY-017. Version 01. 27/06/2016
185
8.3 Concepts and principles of fixed-speed induction motor starting and control.
Since it’s invention one hundred years ago, the standard 3-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.
8.3.1 Introduction
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 newest forms of control - namely, electronic, microprocessor-controlled,
optimising soft-starters such as synergy
TM
.
Note: Since there is a wealth of detailed literature available in the technical press, it is not
proposed to dwell too heavily on the specifics of realising the electronic control system, but
rather, to offer an outline of its various capabilities.
8.3.2 The Induction Motor
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 fulfils 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 earlier, 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 alters, up or
down, as the machine accelerates, reaching a peak at about two thirds full speed, finally to become zero at
Table 8.2.1– Applications (continued)
Table 8.2.1– Applications
Table 8.2.1– Applications
Application
Breakaway
Torque
(%FLT)
Remarks
Rolling mill 30–50 –
Saw, band 10–35 –
Saw, circular 25–50 May be high inertia; Plug brake may be useful
Screen, vibrating 30–60 –
Transformers, voltage regulators Nil Change firing mode
Tumblers 30–100 Can be eccentric load, may need high torque
motor
8. Applications (continued)