ELECTRICAL ENGG. DEPARTMENT SPEED CONTROL OF DC SERIES MOTOR PREPARED BY GROUP: F GUIEDED BY 140943109021 SHAIKH MOIN Z.

Speed Control of DC Series Motor The speed of a DC series motor can be controlled by using various method shown in fig. Speed control of DC series motor Flux control Rheostatic control Applied voltage control Field diverter method Armature diverter method Tapped field method Series parallel connection of field Fig 1.: various speed control techniques for a dc series motor

As shown in fig 1., there are four different method to carry out the flux control. We will discuss them one by one. We should keep a few facts about the dc series motor before discussing these methods. The current flowing through the armature and fie windings of a c series motor is same. The flux produced by any winding is given by. mmf Ø = reluctance = N * 1 S This expression tells us that we can change the flux produced by a winding by changing either the number of turns N or by changing the winding current. We will use these concept to explain the flux control techniques.

Flux Control for DC Series Motors: The flux control technique for controlling the speed of a dc series motor can be exercised in the following four ways: FIELD DIVERTER METHOD ARMATURE DIVERTR METHOD TAPPED FIELD METHOD SERIES PARALLEL CONNECTION OF FIELD The field diverter method is illustrated in fig.2 In this method a variable resistor R is connected across the field winding. This resistor is called as the filed diverter. we provide a parallel path for the filed current. By adjusting the value of R we can adjust the current flowing through the field winding. When the variable point of the rheostat is at point A. the field winding is shorted since R=0. all the current will be diverted away from the field winding. So Ø will be minimum & speed will be maximum. Thus point A corresponds to maximum speed.

Fig 2 (a) Field diverter method Whereas if we move the moving contact of rheostat towards point B in fig. 2(a), then the value of increase. Hence less current gets diverted through it. So more field current flows. This increase the flux and reduces the speed. thus as R increases, speed decreases

Point B in fig.2(a) corresponds to minimum speed point. Note that the minimum speed corresponds to the rated speed when no external rheostat is used. So using this method we can control the speed only in the region which is above the rated speed. The speed armature current characteristics is shown in fig 2.(b). CONCLUSION FROM fig 2.(b) From fig 2.(b) we can see that at a constant value of Ia = I’a , the speed goes on increasing as R decreases. For R = ∞ i.e. when R is absent the speed is equal to the rated speed N = N rated. As we reduce R, the speed increases above the rated speed. Thus with this method we can control the speed only above the rated speed. (b) Effect of R on n-Ia charateristics

ARMATURE DIVERTER METHOD In this method the variable resistor R (called as diverter) is connected across the armature winding as shown in fig 3. For the dc series motor T∞I2a. This method of speed control is used for the constant torque loads, and the speed control takes place as following: Fig 3: armature diverter method for DC series motor

As we reduce R by moving it upwards. SPEED CONTROL: As we reduce R by moving it upwards. The armature current Ia will decrease. But torque produce by motor T∞ØI2a. Therefore to produce required torque , motor draws more current from the source. (I increase) So field current increase. Hence Ø increases. Thus with reduction in R, speed decreases.

The set up for this method is as shown in fig. 4. TAPPED FIELD METHOD: The set up for this method is as shown in fig. 4. The change in flux is obtained by changing the number of turns of the field winding. A rotary switch is used to select the tapping. When the selector switch is in position 1, the entire field winding gets connected into the circuit. Hence the flux is equal to the rated flux and the motor runs at rated speed. As w move the selector switch to positions 2, 3 etc. the number of turns of field winding will reduce. This will reduce the mmf (ampere turns). Since Ø is proportional to mmf , the flux will also decrease. As flux decrease, the speed increase. Hence the motor speed increase as we move from tap 1 to 5. The maximum speed is obtained when the selector switch is at position 5. The method is useful for speed control above the rated speed. The speed variation in this method is not smooth instead it is in steps.

As flux decrease, the speed increase As flux decrease, the speed increase. Hence the motor speed increase as we move from tap 1 to 5. The maximum speed is obtained when the selector switch is at position 5. The method is useful for speed control above the rated speed. The speed variation in this method is not smooth instead it is in steps. Fig 4: Tapped field technique