Tuesday, 4 November 2014

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We know that the speed of shunt motor is given by:

where, Va is the voltage applied across the armature and φ is the flux per pole and is proportional to the field current If. As explained earlier, armature current Ia is decided by the mechanical load present on the shaft. Therefore, by varying Va and If we can vary n. For fixed supply voltage and the motor connected as shunt we can vary Va by controlling an external resistance connected in series with the armature. If of course can be varied by controlling external field resistance Rf connected with the field circuit. Thus for .shunt motor we have essentially two methods for controlling speed, namely by:

1. varying armature resistance.

2. varying field resistance.

Speed control by varying armature resistance 

The inherent armature resistance ra being small, speed n versus armature current Ia characteristic will be a straight line with a small negative slope  In the discussion to follow we shall not disturb the field current from its rated value. At no load (i.e., Ia = 0) speed is highest and Note that for shunt motor voltage applied to the field and armature circuit are same and equal to the supply voltage V. However, as the motor is loaded, Iara drop increases making speed a little less than the no load speed n0. For a well designed shunt motor this drop in speed is small and about 3 to 5% with respect to no load speed. This drop in speed from no load to full load condition expressed as a percentage of no load speed is called the inherent speed regulation of the motor.

It is for this reason, a d.c shunt motor is said to be practically a constant speed motor (with no external armature resistance connected) since speed drops by a small amount from no load to full load condition.
Since eT=kI φ, for constant φ operation, Te becomes simply proportional to Ia. Therefore, speed vs. torque characteristic is also similar to speed vs. armature current characteristic
The slope of the n vs Ia or n vs Te characteristic can be modified by deliberately connecting external resistance rext in the armature circuit. One can get a family of speed vs. armature curves for various values of rext. From these characteristic it can be explained how speed control is achieved. Let us assume that the load torque TL is constant and field current is also kept constant. Therefore, since steady state operation demands Te = TL, Te = akIφ too will remain constant; which means Ia will not change. Suppose rext = 0, then at rated load torque, operating point will be at C and motor speed will be n. If additional resistance rext1 is introduced in the armature circuit, new steady state operating speed will be n1 corresponding to the operating point D. In this way one can get a speed of n2 corresponding to the operating point E, when rext2 is introduced in the armature circuit. This same load torque is supplied at various speed. Variation of the speed is smooth and speed will decrease smoothly if rext is increased. Obviously, this method is suitable for controlling speed below the base speed and for supplying constant rated load torque which ensures rated armature current always. Although, this method provides smooth wide range speed control (from base speed down to zero speed), has a serious draw back since energy loss takes place in the external resistance rext reducing the efficiency of the motor

Speed control by varying field current

In this method field circuit resistance is varied to control the speed of a d.c shunt motor. Let us rewrite .the basic equation to understand the method
If we vary If, flux φ will change, hence speed will vary. To change If an external resistance is connected in series with the field windings. The field coil produces rated flux when no external resistance is connected and rated voltage is applied across field coil. It should be understood that we can only decrease flux from its rated value by adding external resistance. Thus the speed of the motor will rise as we decrease the field current and speed control above the base speed will be achieved. Speed versus armature current characteristic  for two flux values φ and 1φ. Since 1<φφ, the no load speed 'on for flux value 1φ is more than the no load speed no corresponding to φ. However, this method will not be suitable for constant load torque.
To make this point clear, let us assume that the load torque is constant at rated value So from the initial steady condition, we have 1=L ratedea ratedT=TkIφ. If load torque remains constant and flux is reduced to 1φ, new armature current in the steady state is obtained from 11aL ratekI=T φ. Therefore new armature current is


But the fraction,1 1>φφ; hence new armature current will be greater than the rated armature current and the motor will be overloaded. This method therefore, will be suitable for a load whose torque demand decreases with the rise in speed keeping the output power constant  Obviously this method is based on flux weakening of the main field. Therefore at higher speed main flux may become so weakened, that armature reaction effect will be more pronounced causing problem in commutation

Speed control by armature voltage variation

In this method of speed control, armature is supplied from a separate variable d.c  voltage source, while the field is separately excited with fixed rated voltage . Here the armature resistance and field current are not varied. Since the no load speed  N  0=Va/knφ,  the speed versus Ia characteristic will shift parallely for different values of Va. As flux remains constant, this method is suitable for constant torque loads. In a way armature voltage control method is similar to that of armature resistance control method except that the former one is much superior as no extra power loss takes place in the armature circuit. Armature voltage control method is adopted for controlling speed from base speed down to very small speed as one should not apply across the armature a voltage which is higher than the rated voltage. 
Ward Leonard method: combination of Va and If control 
In this scheme, both field and armature control are integrated Arrangement for field control is rather simple. One has to simply connect an appropriate rheostat in the field circuit for this purpose. However, in the pre power electronic era, obtaining a variable d.c supply was not easy and a separately excited d.c generator was used to supply the motor armature. Obviously to run this generator, a prime mover is required. A 3-phase induction motor is used as the prime mover which is supplied from a 3-phase supply. By controlling thefield current of the generator, the generated emf, hence Va can be varied. The potential divider connection uses two rheostats in parallel to facilitate reversal of generator field current 

First the induction motor is started with generator field current zero (by adjusting the jockey positions of the rheostats). Field supply of the motor is switched on with motor field rheostat set to zero. The applied voltage to the motor Va, can now be gradually increased to the rated value by slowly increasing the generator field current. In this scheme, no starter is required for the d.c motor as the applied voltage to the armature is gradually increased. To control the speed of the d.c motor below base speed by armature voltage, excitation of the d.c generator is varied, while to control the speed above base speed field current of the d.c motor is varied maintaining constant Va. Reversal of direction of rotation of the motor can be obtained by adjusting jockeys of the generator field rheostats. Although, wide range smooth speed control is achieved, the cost involved is rather high as we require one additional d.c generator and a 3-phase induction motor of simialr rating as that of the d.c motor whose  speed is intended to be controlled.  

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