The world has seen a great technological revolution in the last 100 years, but very few changes have been made with motors, and how motor starting works. Several industries in the world use motors to power up their machines, making it an in-demand and indispensable necessity in industrial application.
The induction motor, the most common type of motor utilized in building and industry processes, have relatively stayed the same in terms of function and operation. As such, induction motor starting is widely discussed and studied by careers relevant to the industrial field.
Induction motors work by generating rotation power through electrical conversion. This is with interacting magnetic fields. The back electromagnetic force (EMF), which is coupled with magnetic field build-up at the time of motor start, injects transient events that happen in the electrical system.
Such transitory conditions affect all equipment connected to the system and its electrical supply. Motor starting is carefully studied and inspected in industrial applications to limit such transient influence and to correctly accelerate the mechanical load of the motor.
Motor Starting Elements
Motor starting involves significant elements that need to be understood before one appreciates the process of working an induction motor.
Motor Starting Time
Motor starting time refers to when the electric supply is hooked to the time the motor accelerates to its full speed.
The motor starting time length depends on both the mechanical and electrical load the system carries. It can range from just less than a second to half a minute or longer.
During the motor start-up, a particularly high amount of current is needed. However, this could spell trouble for the electrical system supplying the motor and other equipment attached to it.
The transients refer to the duration of time it takes for the motor to run up to its designed speed after motor starting. This is dependent on the characteristic of load, both mechanical and electrical.
Motor Starting: How It Happens
As the motor starts, the current that is drawn initially is higher than the designated full load of the motor at running speed. Both the EMF and magnetic fields increase, which makes the mechanical load accelerate.
The current at motor starting can be anywhere from 5-8 times the motor’s full load.
Any electrical system is designed to carry and sustain a steady state during the running period. However, motor starting will cause electrical cables to carry a higher current compared to the steady-state. Greater voltage drops might also take place during the starting period, which may affect the mechanical load acceleration. Voltage drops may also affect other attached equipment, which is the reason for concurrent start failures.
Methods of Motor Starting
Motors are used in a lot of industries worldwide and are sometimes the root of issues faced by engineers on a daily basis. As an answer to these problems, various techniques and methods have been developed to make motors more capable and prevent failures.
Direct-on-line (DOL) is a straightforward method that is done by connecting the motor to the supplier directly at a specific voltage. Not every system can use this method; the most common examples are in well-dimensioned and mechanically stiff shaft systems. It can also be used for pumps and other equipment that have a stable supply.
Direct-on-line is the most common method particularly since it is the cheapest and simplest. It also causes the tiniest rise in temperature out of all the techniques in motor starting.
The issue with DOL is that the current can be as high as eight times or more than its normal load.
Star-delta starting method is used in three-phase motors. It is applied to minimize the starting current. At the motor start-up, the supply is connected to the star end for the stator windings to commence. As soon as it achieves running position, the current supply is reattached to the delta windings.
The advantage of using star-delta is the reduced starting voltage. The current at start-up for this technique is only a third of the DOL method. This system is applied to high-inertia models wherein loads are initiated at the time full loading speed is achieved.
The setback to using star-delta is that around 33% reduction in starting torque occurs. The effective changeover is required from star to delta so that the speed is maintained. If this fails or happens at a low speed, the current surge rises as much as in DOL, which can be detrimental to the entire system.
The auto-transformer starting is perhaps the fanciest of these three methods since it uses an auto-transformer that is coupled with the induction motor at the start-up.
This technique utilizes dual voltage reductions brought about by transformers, which also minimizes the voltage (around 50-80% of the full voltage) using the secondary auto-transformer voltage. This system causes reduced torque and locked-rotor current. It also causes a concurrent increase in possible torque per ampere line.
The auto-transformer start may also cause a pulsing current due switching from secondary voltage to the main one.
A soft starter ensures smooth motor starting as its name suggests. These devices are reminiscent of a semiconductor.
A soft starter limits the initial voltage of the motor, which then causes lowered motor torque. This device increases the voltage gradually and results in lesser current peaks and high torque.
However, like frequency converters, this system may disrupt all the other processes.
A frequency converter starting continuously feeds the motors but can only be used during start-up only.
The advantage of this method is the low current start-up requirement because of controlled current and torque during full speed. They are also considerably more cost-effective than soft starters, making them preferable.
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