3.12.5: Operation and Operation and Factors Affecting Factors Affecting Motor Output PoMotor Output Power Torquewer Torque The movement of a conductor in a magnetic field induces in it an emf, which we know from Lenz’s Law, will OPPOSEOPPOSE the motion producing it. That is to say, the induced voltage will oppose the supply voltage. This is called BACKBACK EMF.
EMF.
Back emf will never be as great as the supply input and the difference between them is always such that current can flow in the conductor and produce motion. The value of this current is dependent upon the value of the voltage across the conductor. This voltage, often referred to as the
EFFECTIVE VOLTAGE
EFFECTIVE VOLTAGE is equal to the difference between the applied voltage and the back emf. Therefore:
EFFECTIVE VOLTAGE = APPLIED EMF – BACK EMF
Example
Example
Back emf Back emf
Consider the diagram shown. A 24V supply is fed to a shunt motor with an armature resistance of 0.25 and a field resistance of 12. The motor takes a current of 6A under ‘no load’ . What is the back emf?
As the back emf is produced by the armature, the first thing to be calculated is the armature current.
a 0 25. a R 120 f R F
© Air Service Training (Engineering) Limited
Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Armature Current (a) = Total Current – Field Current (f ) A f 2 12 24 a6 2 4 A
Back emf (Eb) = Supply Voltage – Armature Voltage
) ( a a b V R E ) 25 . 0 4 ( 24 E b 23V
When the motor is ‘on load’ , the current it draws is 52A. What effect does this have on the back emf (Eb)?
The field current (f ) will remain the same (Ohm’s Law applied to a parallel circuit), so: A f 2 A a52 2 50 E b24(500.25)11.5V
It is noticeable how the back emf falls as the load is increased on the motor.
When the motor is ‘loaded’ it will tend to slow down, and as generated emf is directly proportional to the rate of change of flux linkage (Faraday’s Law), the value of the back emf will be reduced.
This will increase the Effective Voltage, and therefore the Armature Current and the motor speed will be restored. The back emf therefore determines the armature current and makes the dc motor a SELFSELF REGULATING
© Air Service Training (Engineering) Limited
EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
Motor Speed Control Motor Speed Control
Back emf determines the current in the armature, making the motor a self- regulating machine in which speed and armature current are automatically adjusted to the load.
At small values of load, the shaft torque exceeds the load torque causing the armature to accelerate and produce a larger back emf.
The increased back emf reduces the armature current, and therefore the shaft torque, until a state of balance is achieved and the speed is stabilised.
When the load torque is increased (with increasing load), it exceeds the shaft torque causing a fall in armature speed.
This results in a reduced back emf and an increased armature current.
This increase in armature current increases the shaft torque, restoring torque balance, and stabilises the speed again.
This variation of speed with armature current is known as the SPEEDSPEED CHARACTERISTIC
CHARACTERISTIC of the motor.
Although many motors run at a constant speed, it is sometimes necessary to be able to vary the speed to suit the application, and this can be carried out by varying the amount of current through the field windings.
When the amount of current flowing through the field is increased, the field strength increases, but the motor will slow down because a larger amount of back emf has been generated in the armature.
When the field current is decreased, the field strength decreases and the motor 'speed up' because the back emf in the armature has been reduced. A motor in which the speed can be controlled is called a VARIABLE-VARIABLE-
SPEED MOTOR
© Air Service Training (Engineering) Limited
Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Shunt Motor with Variable Speed Control Shunt Motor with Variable Speed Control
In the shunt motor shown, speed is controlled by a rheostat connected in series with the field windings. The speed of the motor will depend upon the amount of current flowing through the rheostat to the field windings.
To increase the speed the resistance in the rheostat is increased, which decreased the field current, resulting in a decrease in strength of the magnetic field and also the back emf in the armature.
This will momentarily increase the armature current and the torque causing the motor to automatically speed up until the back emf increases and the armature current decreases to its former value.
When this has happened, the motor will operate at a higher fixed speed than before. The opposite action takes place for a decrease in motor speed.
© Air Service Training (Engineering) Limited
EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
R R
Motors used for the operation of landing gear, flaps and other types of apparatus must be designed to work in either direction and are therefore called REVERSIBLE MOTORSREVERSIBLE MOTORS.
Reversible DC Motor Schematic Reversible DC Motor Schematic
The voltage polarity applied to the field and armature windings of any motor will determine that motor’s direction of rotation (clockwise or anticlockwise). To reverse the rotation of a dc motor containing an electromagnetic field, the polarity of the voltage applied to the field or the armature must be reversed. This will reverse the magnetic field of one of the two coils, hence reversing the motor’s direction.
Reversing a motor by this method is shown, where it can be seen that by moving the double-throw switch from the UPUP to the DOWNDOWN position the current through the field coil is reversed, thus reversing polarity, but stays the same through the armature. It would require quite a complex external circuit
© Air Service Training (Engineering) Limited
Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Split Field Reversible DC Motor Split Field Reversible DC Motor
The drawing shows a split field motor. It is operated by a single-pole double- throw (SPDT) switch which, when connected to either the CWCW (clockwise) or CCW
CCW (counter clockwise which is another way of saying anti-clockwise), positions will cause current to flow in the respective field winding. This makes it possible to change the direction of the motor at will by placing the switch in the desired position. The motor is reversed by changing the field polarity in relation to the armature polarity when the different field windings are energised.
Reversible dc motors are commonly controlled by single-pole double-throw switches, as with the split field type, but can also be controlled indirectly by the use of relays. The use of relays is dictated by the amount of current the motor draws while in operation. Any motor requiring more than 20-30 amps will operate more satisfactorily with a relay controlled circuit.
The separate field coils of a reversible motor are usually wound either in opposite directions on the same poles or on alternate poles. Since the field coils are in series with the armature, they must be wound with wire of a size large enough to carry the entire motor current.
© Air Service Training (Engineering) Limited
EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
The brushes in a reversible motor are usually held in box-type holders in line with the centre of the motor shaft. With this arrangement the brushes are perpendicular to a plane tangential to the commutator at the point of brush contact and the brushes will wear evenly regardless of the direction of motor rotation. On small motors the field and brush housing is sometimes made in one piece.
Brakes and Clutches Brakes and Clutches
Many motor-driven devices used in aircraft must be designed so that the operated mechanism will stop at a precise point. For example, when landing gear is being retracted or extended, it must stop instantly when the operation is complete.
If the driving motor is connected directly to the operating mechanism, a great amount of strain will be imposed upon the motor when it is forced to stop because of the momentum of the armature and other moving parts. In installations requiring an instantaneous stop, a clutch and brake mechanism is employed to prevent damage when the machine is stopped.
Clutch and Brake Assembly Clutch and Brake Assembly
Clutches of several types have been designed for the purpose of disengaging the motor from the load when the power is cut off. All such clutches are engaged by magnetic attraction when the power is switched on and disengaged by spring action. Two clutch faces are located within the clutch coil. One of the faces is mounted solidly on the armature shaft and the other is connected through a diaphragm spring to the drive mechanism.
When the clutch coil is energised, the two faces are magnetised with opposite polarities, hence they are drawn together firmly. The friction thus produced causes the driven mechanism to turn with the motor. When the power is cut off, the diaphragm spring separates the faces, thus disengaging the motor. Limit Switches and Protective Devices
Limit Switches and Protective Devices
Because of the limited distance of travel permitted in the driven mechanism, reversible actuating motors are usually limited in their amount of rotation in each direction.
It is essential, therefore, that the motor circuits be provided with switches which will cut off the power when the driven mechanism has reached the limit of its travel.
Switches of this type are called limit switches and are actuated by cams or levers linked or geared to the driven mechanism.
© Air Service Training (Engineering) Limited
Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
The adjustment of these switches is critical because severe damage may result if the motor continues to run after the limit of operation is reached.
Stripped gears and broken shafts are often the result of improperly adjusted limit switches.
If the driven mechanism is strong enough to withstand the torque imposed by the motor, the fuse or circuit breaker in the motor circuit will usually cut off the current to the motor.
Adjustment of the limit switches is accomplished by running the motor to the limit of travel and then adjusting the switch-actuating mechanism so that it has just opened the switch. The switches should be adjusted to open slightly
before the extreme limit is reached.
Reversible Motor Circuit with Thermal Protection Reversible Motor Circuit with Thermal Protection
Some actuating motors are provided with a thermal circuit breaker, or thermal protector, to protect the motor from overload and excessive heat. This device is mounted on the motor frame, and when heat reaches a predetermined limit, the circuit breaker will open and cut off the current to the motor. After the motor has cooled sufficiently, the circuit breaker will automatically close, thus permitting normal operation.
The diagram is a schematic diagram of a reversible motor circuit with a thermal protective device and a coil for operating the clutch and brake.
© Air Service Training (Engineering) Limited
EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
A circuit of the type shown would be used for operating cowl flaps, oil cooler shutter, air valves, and a variety of other devices.
Both the limit switches shown are normally closed. Since they open only when the motor has reached the limit of travel in one direction or the other, it is readily apparent that there will never be a time when both switches are open. Notice that the thermal circuit breaker and the clutch coil are both in the ground (negative) side of the circuit and are therefore in operation for either direction of travel.
© Air Service Training (Engineering) Limited
Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Notes: Notes:
© Air Service Training (Engineering) Limited
EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals