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The purpose of this report is to investigate and compare the efficiencies of AC and DC motors. Investigating into these devices to find out how they work and see if there are any relationships between the power output and the speed of the motor with varying current and voltage input. Also identifying any links between the loaded torque applied to the motor and the effect it has on the power output and the input current of the motor.
The most important characteristics of a motor, are the output speed, torque, power and the efficiency. The output power is determined by the speed of the output shaft and the torque available from it. The electrical power input is determined by the voltage of the supply, the current taken from it and (for an ac motor) the power factor. The efficiency is the mechanical power output divided by the electrical power input.
Power electronic switching systems are widely used for controlling the speed of ac and dc motors. How do they work ? What range of speed can they give ? How efficient are they ? How do they affect the ac mains ?
Two motors will be investigated in this experienemnt. The AC motor used in this investigation is the 12 Leroy Somer 3 ~ AIS model. It is designed for connection to a three-phase supply of 380 Volts, 50 Hertz and to give a maximum output power of 300 Watts at a speed of 1440 rev/min. The DC motor was the Moteurs Leroy Somer, model - C85, 1500 min-1-180W. This model has separate connections for the armature (the rotating part) and the field (the stationary part). The armature is designed for connection to 270 Volts dc and the field for 240 Volts dc. The maximum power output is 180 Watts at a speed of 1500 rev/min. For the tests in this investigation it is convenient to supply the field at a voltage of 220 Volts which does not greatly affect the performance. When efficiency is calculated it must include the power taken by the field winding. This is constant at 220 Volts and 0.33 Amps, i.e. 73 Watts.
The conventional electric motor is a device which works on the basis of converting electrical energy into mechanical energy, the principals behind the electric motor are primarily based on Fleming's 'Left Hand Rule' and Faraday's 'Law of Induction'.
Flemings 'Left Hand Rule' is the concept which relates a conductor carrying an electric current in the presence of a magnetic field would produce a force. The amplification of parallel field lines from both magnetic fields, one from the magnet and the other produced by the current in a wire produces a catapult effect which in-tern provides a resultant force in a specific direction. Here is a graphical illustration which shows the Fleming's left hand rule:
From the diagram above it illustrates the effect of Flemings 'Left Hand Rule' quite coherently for instance, if there is a current that is perpendicular to the magnetic field then a resultant force in the upwards direction would be induced, this is in effect why the electric motor windings rotate when a current is passed through them in the presence of a magnetic field. Here is an illustration depicting the interaction of the magnetic fields between both the current baring conductor and the permanent or electromagnet this interaction produces a catapult field:
The interaction between two fields induces a turning effect.
A simple DC motor has a coil of wire that can rotate in a magnetic field. The current in the coil is supplied via two brushes that make moving contact with a split ring. The coil lies in a steady magnetic field. The forces exerted on the current-carrying wires create aÂ torqueÂ on the coil.
The force F on a wire of length L carrying a current (i) in a magnetic field B is i x L x B x sine of the angle between B and i. The direction of F comes from Flemings right hand rule:
The two forces shown here are equal and opposite, but they are displaced vertically, so they exert aÂ torque. (The forces on the other two sides of the coil act along the same line and so exert no torque.) The coil can also be considered as a magnetic dipole, or a little electromagnet, as indicated by the arrow SN. Note the effect of theÂ brushesÂ on theÂ split ring. When the plane of the rotating coil reaches horizontal, the brushes will break contact (not much is lost, because this is the point of zero torque anyway - the forces act inwards). The angular momentum of the coil carries it past this break point and the current then flows in the opposite direction, which reverses the magnetic dipole. So, after passing the break point, the rotor continues to turn anticlockwise and starts to align in the opposite direction.
The torque generated over a cycle varies with the vertical separation of the two forces. It therefore depends on the sine of the angle between the axis of the coil and field. However, because of the split ring, it is always in the same sense.
The power produced by a DC motor can be calculated by calculating the current travelling through the DC motor, the equation for this is as follows:
Where Current travelling through motor
WhereVoltage applied to motor
Where Counter emf
Where Resistance of armature
After gaining the value, the mechanical power rating can be calculated using the following equation:
Where Mechanical power out
Where Current travelling through motor
Where Counter emf
But power can also be calculated using the formula
Where P = Mechanical power out
Where = Angular frequency
Where T = Torque of Motor. Torque can be calculated using: ,
Where Constant that relates the number of stator turns, number of phases and configuration of magnetic field. B = Magnetic flux density and I = Induced rotor current.
An electric motor which is driven by alternating current. The motor used in this investigation is an induction motor which has its power supplied to the motor by means of electromagnetic induction.
With AC currents, we can reverse field directions without having to use brushes. This is good news, because we can avoid the arcing, the ozone production and the ohmic loss of energy that brushes can entail. Further, because brushes make contact between moving surfaces, they wear out.
Induction motors use shorted wire loops on a rotating armature and obtain their torqueÂ from currentsÂ inducedÂ in these loops by the changingÂ magnetic field produced in the stator (stationary) coils.
The current in the stator coil is in the direction shown and increasing. TheÂ induced voltageÂ in the coil shown drives current and results in a clockwise torque.
3.0 Apparatus and Methods
The Induction Motor (AC): The work station used for this investigation was number 4. The apparatus was set up as shown in Fig.1. Switch C was firstly turned on checking that the knob F was set to 0, the dynamometer control was set to "T Manu" and that the "Manu" knob was also at 0. Switch B and D were then turned on. The setting of knob F was slowly increased. The voltage, current, electrical input power and power factor were all noted as shown on the energy analyser. The voltage was increased to 380 Volts (+/-3V). On the dynamometer the output torque was checked that it was at zero and the power was at near zero (+/-3 Watts).
The "Manu" knob was then turned on the dynamometer to increase the load torque to 0.2 Newton meters. The output torque, power and speed on the dynamometer were noted. This was repeated at intervals of 0.2 Nm until the motor reached its rated power output of 300W.
The motor was then tested at higher loads at intervals of 0.4Nm. The output torque, power and speed on the dynamometer were recorded. This was repeated until the motor was overloaded and stopped. Once this has happened the load was reduced to zero and kept running so that its own fan could help it cool down. The switches B,C and D were turned of after about five minutes.
The DC Motor: The work station used for this investigation was number 6. The apparatus was set up as shown in Fig.2. Switch C was firstly switched on checking that the knob F was set to 0, that the dynamometer control was set to "T Manu" and that the "Manu" knob was also 0. Switch B and D were turned on, and the voltage supplied to the motor was shown on the panel-mounted voltmeter on the 0-500 Volt scale. Knob F was slowly increased until the voltage was about 30 Volts. The voltage was increased in steps of 30V until the motor was running at its rated voltage of 270 Volts. The voltage, motor speed and current were recorded.
Then the output torque, power and speed on the dynamometer were checked that the output torque should be zero and the power should be near zero (+/- 3 Watts). The "Manu" knob on the dynamometer was turned to increase the load torque to 0.1 Newton-meters. The voltage, current, power, output torque and speed on the dynamometer. The same procedure was repeated a intervals of 0.1 Nm until the motor reached its rated power output of 180 W.
The motor was then tested at higher load torques at intervals of 0.2 Nm until the motor current reached 2A.
Observations and Results
Induction Motor (Ac):
Electrical Input Power
After the motor reaches its rated power output of 300W:
Table 1: Results for DC motor with no load:
Motor Speed (Rev min-1)
Table 2: Results for DC motor with load:
Table 3: Results for DC motor after the motor reaches its rated current 2A:
Table 4: Calculation for DC motor:
Motor Speed (Rad-1)
Power In (W)
Power Out (W)
Example of calculations:
Power In = Voltage x Current
= 30 x 188 = 4.50 W
5.0 Analysis and Discussion of Results
From the results we can see that the fluorescent lamp is cheaper to run for 5000 hrs. It is the most economically friendly. By using the fluorescent lamp a saving of £37.10 can be made.
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Sherman, 1990, Viscous Flow, McGraw -Hill Series in Mechanical Engineering
White, F.M. 2003, Fluid Mechanics, 5th edition, McGraw-Hill
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