Saturday, July 28, 2012

Starting Methods

This section outlines some of the available motor-starting methods that are used in the industry.

Direct-on Line Start
 
A direct-on line (DOL) start is the simplest, most common, and
least expensive method of starting squirrel-cage induction and synchronous motors. Direct-on line starting offers high-acceleration torque and reduced acceleration time.
However, the following criteria must be checked for large motors:
The power system must be stiff, and the voltage drop caused by direct-on line starting must not exceed flicker limits at all voltage levels.
With synchronous motors, high accelerating and oscillating (pulsating) torque may be a problem for the driven equipment during starting. A torsional analysis is required for such applications.
With squirrel-cage induction motors, high starting and breakdown torques may cause a shock for some types of driven equipment. 

Reactor Start 
 

 
A reactor is connected in the motor circuit, either in the line or at the neutral end, during start of the motor. The reactor is bypassed by switching a contactor or a circuit breaker when the motor has attained the rated speed. The starting current is reduced linearly, and the torque is reduced by the square of the voltage at the motor terminals. One advantage of the reactor starting is that the motor torque increases with the speed as the starting power factor improves. This increase in torque is an added boost for synchronous motors, where the motor torque drops off at the end of the acceleration.

The reactor ohmic value must be selected to reduce the motor-starting current drawn from the system to a value that will cause acceptable voltage flicker and at the same time provide sufficient voltage at the motor terminals to accelerate the load.

Autotransformer Start

This method is similar to the reactor start, except that an autotransformer is switched into the motor circuit during starting and bypassed at the end of the start. Two switching arrangements, open transition and closed transition, are used.

In the open-transition scheme, the switching sequence is: close 1 and 3, open 1 and 3, close 2. This scheme provides open-circuit transition from reduced voltage to full voltage and is not recommended.

In the closed-transition scheme known as “Kordorfer,” the switching sequence is: close 1 and 3, open 3, close 2, open 1. This switching arrangement introduces a portion of the autotransformer as a series reactor in the motor circuit during the transfer to full-motor voltage, thus reducing the transient current and torque.
The salient features are:
  • Usually provided with 8%, 65%, and 50% taps. This permits the adjustment of the motor terminal voltage.
  • Two additional contactors or circuit breakers and an autotransformer are required.
  • The starting current as seen in the network is reduced as the square of the transformer ratio.
  • Because the motor current is greater than that in the line with an autotransformer starter, the starter produces more torque per ampere of line current.                                                                                                 Capacitor Start
   

A capacitor is switched with the motor, which compensates part of
the VAR drawn by the starting motor. The bank size is usually selected to provide about 50% of the motor-starting VAR. The capacitor is switched off at about 95% of the motor rated speed, and the motor current drops to about full load. The salient features of this method are: 
Because C ≈ V2, a smaller bank size at a lower voltage can be used to match with the motor-starting duty.
The method improves the acceleration torque and reduces acceleration time, which makes it suitable for high-inertia or high-starting-torque loads.
A resonance check shall be made to ensure that the selected capacitor bank does not create a system problem.

Reactor-Capacitor Start

 

This method is a combination of reactor and capacitor starting, as
described earlier. This method can be used where the network is weak and reactive VAR is needed to improve motor performance during acceleration.

Captive Transformer Start

 
The motor is energized through a two-winding transformer. The
switching and protection are provided at the primary only. The transformer must be sized and built for motor-starting duty (impact loading). The salient features are:

Can be used for large motors where the switching is done at a higher voltage than the motor rated voltage, e.g., a 50,000 hp, 13.8 kV motor can be switched at 34.5 kV through a captive transformer.

Reduces short-circuit currents from the motor to the primary switchgear and from the system to the motor.
High-resistance grounding can be provided for the motor, thus reducing iron damage.

Variable-Voltage Start
This method is known as a “soft start” and is basically a reduced voltage starting method similar to reactor start. In this method, a rectifier-inverter using IGBT is applied to vary the voltage at the motor terminals to reduce the starting current. Because the frequency remains the same, the motor-starting torque is also reduced in proportion to the voltage squared. The soft-start controller is bypassed by switching a contactor or circuit breaker when the motor has attained the rated speed.
Because the output waveform from the controller is not a sine wave, there is some reduction in the motor torque compared with the reactor start. The controller will also inject current harmonics into the power system during the starting period. This method is not suitable for high-inertia and high-torque loads.
 

Variable-Frequency Start 

 
In this method, the ratio of voltage to frequency (volt/Hz) is maintained constant during the acceleration period or operating speed range. The voltage and frequency of the power supply to the motor is reduced to a low value to increase the ratio of the motor torque to the motor-starting current. At reduced frequency, the applied voltage and starting current are lower.
The motor is accelerated through a frequency converter, and upon reaching the system frequency, the motor is transferred to the network. The motor-starting torque can be shaped to suit the load characteristics. Two types of drives — load commuted inverter (LCI) and pulse-width modulation (PWM) — are used. LCI is a currentsource inverter and is common with synchronous motors. The PWM drive is a voltage- source inverter, using switching devices such as IGBT, IGCT (integrated gate commutated thyristor), or IEGT (injection enhanced gate transistor) to chop the DC into pulses. The salient features are:
  • SmoothSmooth acceleration and a negligible voltage drop in the network.
  • No transient torques.
  • High cost and requires a large space.
  • One starter can be used for more than one motor.
  • Harmonic generation, with the order depending upon the number of pulses.Filters may be required.
Part Winding Start

 

This method can be used for applications where a synchronous motor is started unloaded. The motor is wound with two sets of Wye-connected stator windings, with the neutral end of one winding connected through a vacuum contactor or a circuit breaker. Upon starting, the contactor at the neutral end is open, and only one winding is in operation. The contactor is closed just after the motor has reached the synchronous speed, and the motor excitation is turned on. With this arrangement, the motor-starting (inrush) current is on the order of 65% to 75% of the normal starting current.
This method offers a limited reduction in starting current, relatively low starting torque, and is not suitable for all ratings and speeds.

Source:Industrial Power System. Shoaib Khan.
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