• Electrical India
  • Jan 5, 2016

MAGNA START
New Generation Slip-Ring Motor Starter

Rheostatic starters utilise one or several resistors for obtaining stated motor torque characteristics and for limiting current. During starting period, it cuts out successive one or several resistors provided in the rotor circuit...

- V N Prajapati


 Slip-ring motors are widely used when high starting torque is required. Rheostatic and liquid immersed resistance starters are used for starting the slip ring motors. In this article, we have explained concept of eddy current effect of inducers for starting slip-ring induction motors.

  The major advantages of slip-ring induction motor are:

  • High starting torque compared to squirrel cage induction motors about 2.5 times full load torque.
  • Pull out torque can be achieved at even zero speed.
  • Squirrel cage induction motors take 6 to 7 times full load current but slip-ring induction motors take low starting current approx 2.5 to 3.5 times full load current.
  • Speed can be controlled easily.

Starters For Slip Ring Induction Motors

Rheostatic rotor starter

  Rheostatic starters utilise one or several resistors for obtaining stated motor torque characteristics and for limiting current. During starting period, it cuts out successive one or several resistors provided in the rotor circuit.

  The value of resistance is chosen on the basis of the mean torque required during starting. The rated data of the motor can be used to calculate the resistance that will give rated current and rated torque on starting. The resistance R2 is worked out with the formula
R2 = U2 / √3 x I2
Where U2 =Rotor Voltage at standstill
I2 = Rotor Current at rated power
This method has following major drawbacks:

  • Reduction in resistance is achieved by timers as the motor accelerates. Design parameters of the resistance do not take into account the varying load conditions.
  • Each starter has to be individually designed to suit the motor and once installed, its starting characteristics can rarely be altered to suit changing requirements of the customer.
  • By virtue of large number of contactors and other moving parts, resistance banks are bulky, complex and liable to malfunction.
  • These have a limited starting capability.

Magna Start System – New Generation Slip Ring Induction Motor Starter

  Magna Start System is a specifically designed system for starting slip-ring induction motor, which consists of control and protection system with magna start inducer for rotor circuit. The Magna Start inducer is physically much smaller than the equivalent resistance, requires only one shorting contactor because of which the size, and complexity & cost of the overall system gets considerably reduced. All the components are enclosed in a single dust & vermin proof enclosure.

  The schematic arrangement of Magna Start System is given in Figure 1. The major advantage of Magna Start System is that it provides smooth acceleration to full speed (the time – current characteristic is given in Figure 2), and thus reduces electrical stress.

Schematic Diagram of Magnastart System...

Time Current Characteristics of Starting...

Advantages Of Magna Start:

  • Size: Though physically smaller than conventional systems, the large thermal capacity of Magna Start permits the starting of high inertia loads without problems.
  • Simplicity: Magna Start requires only one shorting contactor, reducing overall system complexity, size and cost.
  • Maintenance: Magna Start has no moving parts and does not use liquids or chemicals. Magna Start will give many years of service without any maintenance or adjustment.
  • Reliability: The robust construction permits heavy usage under the most arduous conditions. Comprehensive thermal protection prevents any possibility of damage.
  • Flexibility: Each inducer model is designed to start a wide range of motors. The front panel (Fig.1) allows the user to select the exact starting torque and current he/she requires.
  • Smooth starting: Smooth acceleration to full speed reduces electrical stresses in the motor and mechanical wear. 
  • Performance: Magna Start is ideal for inching, reversing and plug braking without any additional controls in the rotor circuit.

Working Principle of Magna Start Inducer

  The Magna Start inducer consists of 3 coils wound on specially shaped steel cores. Eddy currents in these cores are reflected in the windings as impedance, which is dependent on rotor frequency. When power is applied to the motor, the frequency in the rotor is the same as that in the stator. As the motor accelerates, the rotor frequency and the inducer impedance decrease. This results in a smooth acceleration to full speed at which point the slip-rings are short circuited.

Front Panel of Magnastart inducer...

  The heavy mass of core material allows the inducer to absorb a considerable amount of heat. The advanced thermal insulation protects the coils that remain relatively cool and several consecutive starts may be obtained on drives with high inertia loads. 

  Each of the coils is tapped at 56% and 84%, all four connections brought out to studs on the front panel in a rectangular array. The front panel of Magna Start Inducer is shown in Figure 3. By using star and delta combinations as given in Table 1, up to 17 different starting impedances are available. Thus, the inducer can be set to provide the exact starting characteristics required on a wide range of impedance for the motors.

Features Of Magna Start

  • Naturally automatic step-less operation
  • Intrinsically load sensitive starting
  • Constant torque, constant current acceleration
  • Suitable for online plugging

Electrical Characteristic And Performance

  Magna Start design ensures that a high power factor (0.8 or more) is maintained in the rotor circuit in order to prevent excessive starting currents being drawn. Magna Start also provides a good linear characteristic of impedance against frequency (a representative graph of impedance v/s speed is shown in Figure 4) so that a constant current is drawn and the same torque is delivered during the run-up. This enables the fastest possible run-up time within a given current limit. There is a value of current where maximum starting torque is obtained. Any current lower or in excess of the above value will give lesser torque. Refer Figure 5.

Figure 4

Figure 5

Selection Of Magna Start Inducer

  The size of the inducer required is decided solely by motor HP rating and the required starting duty. Factors to be considered in determining the starting duty are:

  • Number of starts per hour
  • Starting time
  • Maximum permissible rotor starting current as a factor of full load rotor current

  The ratings of the standard Magna Start range have been calculated on the basis of 10-second starts, drawing a maximum of 3 times the full load rotor current. Technical Information of inducer is given in Table 2. The user can check the maximum rotor current and rotor voltage from the motor name plate. Other rating inducers can be developed as per requirement of customer.

Table - 1: Impedance Combination...

Conclusion

  • Magna start system provides load sensitive starting and smooth time current characteristic.
  • Magna start system has advantage of size, simplicity and no maintenance
  • Required impedance can be achieved by adjustment of tapping of the inducer.

V N Prajapati is AGM (D & D, QC), JSL Industries Ltd, Gujarat.

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