ME DC MOTOR CHARACTERISTICS by nikeborome

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									        ME 2143-1        MOTOR CHARACTERISTICS
                           (EA-03-06)


                            SEMESTER 4
                             2010/2011




      DEPARTMENT OF MECHANICAL ENGINEERING
         NATIONAL UNIVERSITY OF SINGAPORE



INSTRUCTIONS
 o The lab report must be submitted within one week of the completion of
   the lab work. It must be typewritten. The completed report should be
   placed in the bin kept in the Motor Characteristics lab.
 o Formal reports should be at most 11 pages long. They should contain:
   Title page, Objectives/Aims, Background/Summary of Experiment,
   Results and Observations, Discussions and Questions, and Conclusion.
 o Informal reports should be at most 8 pages long. They should contain:
   Heading, Objectives/Aims, Results and Observations, Brief discussion
   and short answers to questions (one to two sentence answers) and
   Conclusion.
 o You should attend the lab only on the day assigned to you. Free jumping
   to alternate days is not allowed.




                                   1
                      !!! WARNING !!!
     High voltages are present in this Experiment!
  DO NOT make any connections with the power on!
The power should be turned off after completing each set
                   of measurements.
Safety Precautions:
  1. Ensure appropriate attires: no slippers, sandals or open-toe footwear
     allowed.
  2. Long hair should be properly tied.
  3. Make sure hands are dry when conducting experiment. KEEP WATER
     BOTTLES AWAY FROM EXPERIMENT AREA.
  4. Make sure all power supplies are switched off before commencing with
     connections.
  5. Make circuit connections with test leads. Use only ONE hand when
     making connections to avoid closing circuit with your body.
  6. Signal tutor or technician to check and verify your wire connections are
     correct.
  7. Switch on power supply and proceed with data collection for experiment.
  8. After each set of readings, switch off power supply before making any
     changes to wire connections.
  9. When disconnecting test leads, remove the main power supply
     connections first, i.e. DC positive voltage output or AC voltage live
     output.




                                      2
1.0    Objectives
To be familiar with the wiring and basic characteristics of the following motors:
       i.      D.C. Series Motor
       ii.     D.C. Shunt Motor
       iii.    A.C. 3-Phase Squirrel Cage Induction Motor.

To examine the relationship between Torque, Speed, Voltage, and Current for various types
of motor connections in no-load and loaded configurations.

2.0    Apparatus
       For the DC Motor Experiments:
               LAB VOLT 8211 - D.C. Compound Motor
               LAB VOLT 8911 - Electrodynamometer
               1 A, 0-240VDC, DC Power Supply
               Machine Enclosure

       For the 3-Phase AC Motor Experiments:
               LAB VOLT 8221 - 3-phase AC Squirrel Cage Motor
               LAB VOLT 8911 - Electrodynamometer
               LAB VOLT 8821 - Integrated AC Power Supply Module
               Machine Enclosure

       Other Common Meters and Accessories:
              Test Leads
              Multimeters
              Tachometers

3.0    Introduction – DC Motors
A DC motor consists of a stator, a rotor, and other mechanical parts, such as the bearings,
shaft, and the housing. The stator contains the field windings (or permanent magnets) that
establish the magnetic field. The rotor (also called as Armature) is the rotating part inside
the stator. The rotor has its own windings.

A voltage E A is induced in the armature due to the motion of its conductors relative to the
magnetic field. This voltage is usually referred to as back emf and is given by

        E A  K m            (1)

       where:
                K is a machine constant that depends on the winding and structural details of
                the motor,
                 is the magnetic flux produced in the stator (Webers),
                and  m is the motor angular speed (rad/s)




                                               3
Another important relationship for the D.C. motors involves the terminal voltage, the back
EMF generated by the rotation of the armature, the resistance of the armature circuit and the
armature current. By Kirchoff's Voltage Law, we have

       VT  R A I A  E A      (2)

       where:
                VT is the terminal voltage applied to the motor terminals,
                RA is the resistance of the armature circuit in ohms which, in the case of a
                series motor, includes the resistance of the series field coils and
                IA is the current in the armature in amperes.
Combining the two equations gives:

        K m  VT  R A I A   (3)

Re-arranging this equation gives:

            VT  I A R A
       m                  (4)
               K
The torque developed by a D.C. motor can be calculated from the equation

       Tdev  KI A            (5)

       where:
                Tdev is the torque developed by the motor (Nm),
                IA is the current in the armature (A).

In a series motor, the field winding and the armature are in series connection. Therefore the
armature current IA is the same as field current IF. The flux  is directly proportional to the
field current when the motor is operating in the linear range. Thus for series motors, the flux
can be assumed to the proportional to the armature current. As the flux is dependent on
the armature current in a series motor, at low current on light loads, the speed of the series
motor can become excessive. For that reason a series motor should never be operated on light
loads on full voltage.

In a shunt motor, as the field windings and the armature is in parallel connection. The
armature current IA is different from the field current IF. While the armature current IA is
dependent on the load, the field current is independent of the load conditions. As a result, the
flux for a shunt motor can be considered to remain constant. You will learn in theory that the
field produced by the armature current in the armature conductors can affect the field strength
but the shunt field flux can be considered to remain unaffected by the load. As the flux for a
shunt connected motor is independent of the armature current the only effect the load can
have on the speed is to increase the effect of the voltage drop in the armature.

One of the outcomes of this experiment will be experimental proof of the curves showing the
relationship between speed, torque and current in D.C. motors and an understanding of these
curves and what they tell you about the motor. Comparing these curves for the various D.C.
motor configurations will help you select the correct motor for a given purpose.


                                               4
4.0 Experiment on DC Series Motor
WARNING!! High voltages are present in this Experiment! DO
NOT make any connections with the power on! The power must be
turned off after completing each set of measurements. Use only one
hand when making connections with the test leads.

4.1       Constant-Load Test
  a) Connect the DC Compound motor as shown in Figure 1. Please note the terminal
     numbers when making the connections. Set the control knob of the
     electrodynamometer at 10%.
  b) Turn on all relevant power supplies. Start the motor by adjusting the 0-240 V, 3 Amp
     power supply to 180 V. Wait for one minute. Then reduce the supply in steps of 20 V
     until 60 V.
  c) At each step record the speed and current. Enter the data in the no-load table.
  d) When data collection is completed, switch off all relevant power supplies.
  e) Plot a graph of the current against speed, and voltage against speed.




      +




                         Fig. 1   DC Motor Series Connection




                                             5
                 Table 1: DC Series Motor Constant-Load Test Results
                         Volts (V)      Speed (rpm)       Current (A)




4.2              Load Test
      a) Adjust the 3 Amp DC power supply to 180 volts to start the motor. The supply should
         be maintained at 180 VDC for this test. Wait for one minute.
      b) With the control knob of the electrodynamometer turned fully counter-clockwise
         (ccw), read the no-load data from the meters and enter it into the table.
      c) Adjust the control knob of the electrodynamometer to increase the load applied to the
         motor in steps of 0.1 Nm until a maximum of 1.0 Nm. The load applied can be read
         directly off the scale marker on the stator housing of the electrodynamometer. At each
         step record the speed and current. Enter the data into the Load Test Table. Continue to
         increase the load until the load reaches 1 Nm or the motor current reaches 1.1A.
      d) When completed data collection, turn the control knob of the electrodynamometer
         slowly in ccw direction until it is fully ccw. (Turning the knob too fast would cause
         the load to start rocking, which is undesirable.) Reduce the motor supply voltage to
         minimum (0V) and switch off all relevant power supplies.
      e) Plot a graph of torque against speed, and current against speed.



                       Table 2: DC Series Motor Load Test Results
               Voltage ( V )    Speed (R.P.M.)       Torque (Nm)      Current (Amp)




                                                 6
4.3        Questions
      a) Why must the DC series motor be always started under load and give examples in
         your answer?
      b) What is the relationship between Torque and Current for a DC series motor?
      c) Try your best to briefly explain the shape of the Torque versus Speed graph obtained
         in this experiment.

5.0        Experiment on DC Shunt Motor
WARNING!! High voltages are present in this Experiment! DO
NOT make any connections with the power on! The power must be
turned off after completing each set of measurements. Use only one
hand when making connections with the test leads.
5.1        No-Load Test
      a) Connect the DC Compound motor as shown in Figure 2. Please note the terminal
         numbers when making the connections. Set the control knob of the
         electrodynamometer at its full ccw position, i.e. minimum load.
      b) Turn on all relevant power supplies. Start the motor by adjusting the 0-240 V, 3 Amp
         power supply to 240 V. Wait for one minute. Adjust the variable resistor until the no-
         load motor speed is about 1500 rpm. Re-adjust the supply to 240 V. Then reduce the
         supply in steps of 20 V until 60 V.
      c) At each step record the speed, line current and field current. Enter the data in the no-
         load table.
      d) When completed data collection, switch off all relevant power supplies.
      e) Plot a graph of the field current against speed, and voltage against speed.




       +




                               Fig. 2 DC Shunt connection

                                                 7
                      Table 3: DC Shunt Motor No Load Test Results
          Volts (V)     Speed (rpm)     Field Current, IF (A)    Line Current, IL (A)




5.2      Load Test
      a) Adjust the 1 Amp DC power supply to 240 volts to start the motor. The supply should
         be maintained at 240 VDC for this test. Wait for one minute.
      b) With the control knob of the electrodynamometer turned fully ccw, read the minimum
         load data from the meters and enter it into the table.
      c) Adjust the control knob of the electrodynamometer to increase the load applied to the
         motor in steps of 0.1 Nm until a maximum of 1.0 Nm. The load applied can be read
         directly off the scale marker on the stator housing of the electrodynamometer. At each
         step record the speed, line current and field current. Enter the data into the Load Test
         Table. Continue to increase the load until the load reaches 1 Nm or the motor current
         reaches 1.1A.
      d) When completed data collection, turn the control knob of the electrodynamometer
         slowly in ccw direction until it is fully ccw. (Turning the knob too fast would cause
         the load to start rocking, which is undesirable.) Reduce the motor supply voltage to
         minimum (0V) and switch off all relevant power supplies.
      e) Plot a graph of torque against speed, and line current against speed.

5.3      Questions
      a) Explain how speed is regulated in a DC shunt motor and give examples.
      b) From no-load to full-load explain in your own words why there is little speed
         variation over this range.
      c) Try your best to briefly explain the shape of the Torque versus Speed graph obtained
         in this experiment.




                                                 8
                        Table 4: DC Shunt Motor Load Test Results
  Volts (V)       Speed (rpm)   Torque (Nm)   Field Current, If (A)   Line Current, It (A)




****************************************************************

6.0      Experiments on AC Three-Phase Induction Motor
6.1      Illustration of Induction Motor Operating Principle
AC induction motors are ideal for most industrial and commercial applications because of
their simple construction and low number of parts, which reduce maintenance cost.
Three-phase AC power is supplied to the windings of the stator of the induction motor. A
rotating sinusoidal magnetic field is produced. The speed of rotation of the stator magnetic
field is described as the synchronous speed ns and is given by

           120 f
      ns                (6)
              P
      where:
         ns is the synchronous speed (rpm)
         f is the frequency of the AC supply (Hz)
         P is the number of poles in the motor.
For example, in Singapore the AC supply has a frequency of 50 Hz. For most common AC
motor, which has 4 poles, the synchronous speed can be calculated as:

                 120  50
          ns              1500rpm
                    4
The rotor of the induction motors can be either wound rotor type or squirrel cage type.
Wound-rotor induction motors have a three-phase winding, similar to the stator winding, on
the rotor. Wound-rotor induction motors can be controlled to operate at different torques and
speeds. However, they are usually significantly more expensive than squirrel cage rotor
motors.



                                               9
Squirrel cage motors are the most common type of induction motor. In a cage rotor design,
there are solid conductors (usually cast using aluminum) in slots on the rotor. The ends of the
conductors are short-circuited at each end of the rotor using an "end-ring". There is no power
supplied to the rotor. This is a great advantage when compared with other motors.
AC motors should ideally be operating in the synchronous speed. When operating under load,
there would be a drop in operating speed n m . The speed of the stator field relative to the rotor
is ns  nm .

The slip s is defined to be the relative speed as a fraction of synchronous speed:

                ns  nm
           s                  (7)
                   ns

Slip s varies from 1 when the rotor is stationary to 0 when the rotor turns at synchronous
speed.
In this section of the experiments, we will study the characteristics of the Squirrel cage
motors by observing the change of the speed with respect to the change of the load and
plotting the torque and speed curve of the motor.


6.2    Experiment Procedures
WARNING!! High voltages are present in this Experiment! DO
NOT make any connections with the power on! The power must be
turned off after completing each set of measurements. Use only one
hand when making connections with the test leads.




                   4


  VARIABLE
      AC
                   5
   OUTPUT



                   6




                   Figure 3: 3-Phase AC Induction Motor Connection



                                               10
a) Connect the 3-phase AC induction motor as shown in Figure 3. Please note the
   terminal numbers when making the connections. Set the control knob of the
   electrodynamometer at its full ccw position, i.e. minimum load. (VERIFY)
b) Turn on all relevant power supplies. Start the motor by adjusting the variable AC
   output to 400 VAC. Wait for one minute. With the control knob of the electro-
   dynamometer turned fully ccw, read the no-load data from the meters and enter it into
   the table.
c) Adjust the control knob of the electrodynamometer to increase the load applied to the
   motor to 0.1 Nm. The load applied can be read directly off the scale marker on the
   stator housing of the electrodynamometer. At each step record the speed and line
   current. Enter the data into the Load Test Table. Continue to increase the load until
   the load reaches 1 Nm or the motor current reaches around 0.46A.
d) When completed data collection, turn the control knob of the electrodynamometer
   slowly in ccw direction until it is fully ccw. (Turning the knob too fast would cause
   the load to start rocking, which is undesirable.)
e) Reduce the motor supply to 0V and switch off all relevant power supplies.
f) Plot graphs of torque against speed and line current against speed for the motor.



                  Table 5: AC 3-P Motor 400VAC Results
    Volts (V)      Torque (Nm)       Speed (rpm)         Line Current (A)




                                          11

								
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