UNIVERSITY OF SAN CARLOSEE/ECE DepartmentFABIAN, JAKE FLOYD G. 7-22-15BSME-4 Wed 4:30 7:30 PMENERGY CONVERSION LABORATORYExperiment No.3THE DC SHUNT MOTOR:STARTING AND CONTROLOBJECTIVES:1. To learn the correct procedure of starting a dc a shunt motor.2. To study a commonly used method of controlling the speed of a shunt motor.3. To study how to control the shunt motors direction of rotation.THEORY:A shunt DC motor connects the armature and field windings in parallel or shunt with a common D.C. power source. This type of motor has good speed regulation even as the load varies, but does not have the starting torque of a series DC motor. It is typically used for industrial, adjustable speed applications, such as machine tools, winding/unwinding machines and tensioners. In addition, the shunt wound dc motor falls under the category of self excited dc motors, where the field windings are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive of. And for this reason both the armature winding and the field winding are exposed to the same supply voltage, though there are separate branches for the flow of armature current and the field current as shown in the figure of dc shunt motor below.

Fig 1. Shunt motorThe armature current of the dc shunt motor is given by the equation,Ia = armature current (A)Va = armature voltage (V) = flux per hole (Mx)N = speed of rotation of the armature (rpm)Ra = armature resistance (resistance of the armature winding, The armature speed can be computed by just using simple algebra on the 1st equation.Atachometer(revolution-counter,tach,rev-counter,RPM gauge) is an instrument measuring the rotation speed of a shaftor disk, as in a motor or other machine.The device usually displays therevolutions per minute(RPM) on a calibrated analogue dial, but digital displays are increasingly common. The word comes fromGreek(tachos"speed") andmetron("measure").Just that there is one distinguishable feature in its designing which can be explained by taking into consideration, the torque generated by the motor. To produce a high torque,i) The armature winding must be exposed to an amount of current thats much higher than the field windings current, as the torque is proportional to the armature current.ii) The field winding must be wound with many turns to increase the flux linkage, as flux linkage between the field and armature winding is also proportional to the torque.Keeping these two above mentioned criterion in mind a dc shunt motor has been designed in a way, that the field winding possess much higher number of turns to increase net flux linkage and are lesser in diameter of conductor to increase resistance(reduce current flow) compared to the armature winding of the dc motor. And this is how a shunt wound dc motor is visibly distinguishable in static condition from the dc series motor (having thicker field coils) of the self excited type motors category.MATERIALS:DC motorArmature rheostatTachometerDC ammeter (0-6 A)Connecting wiresPROCEDURE:The following resistances were measured and these are the recorded result:a) Armature resistance, Ra = 36 b) Shunt field resistance, , Ra = 512 Starting a Shunt Motor

Fig 2.Shunt motor connections for the experimentAfter following the circuit diagram of the shunt motor, the maximum value of the current measured by the ammeter was recorded on Table 1. The armature rheostat resistance was set to maximum. The armature rheostat resistance was decreased by approximately 10% of the maximum value. This was done by turning the knob at the armature rheostat clockwise with each graduation corresponding to 10% of the maximum value.Table IArmature rheostat settingArmature current at starting (A)

1st (max)1

2nd2.0

3rd2.8

Discussion: It is observed in table 1 that as the armature rheostat resistance is decreased by 10 %, the reading constant current and the maximum current increases as well. The movement of the ammeter was also observed. And there was a sudden jolt or increase in the reading and then it decreases again back to its constant value. This means that there was a surge of current that occurred which is what that maximum current reading means.Speed Control Using the Armature RheostatTo start this part of the experiment, the armature rheostat resistance was set again to maximum then the source was switched on. The armature voltage Va, armature current Ia, and the armature speed N were measured when the speed of the motor has stabilized. These data were recorded on Table II below. Then, while the motor was running, the armature rheostat resistance was decreased approximately by 10% of max. value. The data were gathered until the 3rd deduction of 10% of max. value.Table IIArmature rheostat settingVa (V) Ia (A)N (rpm)

1st (max)32.60.3490

2nd45.60.4730

3rd60.60.5990

Discussion: It is obvious on the data gathered that as we decrease the armature rheostat resistance, we are increasing power (P = VI), thus increasing the speed of the dc motor as well.Control of the Direction of RotationThe speed of the motor was decreased to its nonzero minimum. By determining the polarity and using the passive sign convention, the direction of the field current was known. The direction of the armature current was determined as well. Then, the direction of rotation of the armature was also noted. This will be referred to as the reference directions .The next step was to know what will be the corresponding change on the rotation of the DC motor if the direction of the field current or armature current or perhaps both were reversed. The data was recorded on Table III below.Table IIIRefRev IfRev IaRef If & Ia

Field current directionF1 to F2 / F2 to F1F2 to F1F1 to F2F2 to F1

Armature current directionA1 to A2 / A2 to A1A1 to A2A2 to A1A2 to A1

Direction of rotationCW / CCWCCWCCWCW

Discussion: It is evident on the given data that when the field current direction was reversed, the direction of rotation reverses as well. Also, its obvious that when the armature current direction was reversed, the direction of rotation reverses as well. And if both current directions were reversed, the direction of rotation does not change from CW to CCW.

CONCLUSION:The experimenters were able to start a DC shunt motor. And then, the commonly used method of controlling the speed of a shunt motor was known. Also, the method of controlling the shunt motors direction of rotation was known as well. The shunt wound dc motor falls under the category of self excited dc motors, where the field windings are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive of. And for this reason both the armature winding and the field winding are exposed to the same supply voltage, though there are separate branches for the flow of armature current.