Dc Motor/Generaton Theory Flashcards
In a DC motor, the stator’s magnetic field, due to armature reaction, moves.
in the opposite direction of rotation of the armature.
A small air gap between magnetic poles results.
in the same field as a large air gap, for the same magnetising force.
Magnetic field (number of flux lines) is the same regardless of distance between poles. However, the field density (flux density) will increase with a smaller air gap.
The electromagnetic brake coil in an actuator would be energised.
only when the actuator is running.
In a shunt wound direct current motor with a constant voltage field supply, the torque developed by the motor is.
directly proportional to armature current.
Decreasing the field current in a shunt motor will.
increase speed and increase torque.
Decreasing the field current in a shunt motor will reduce the back emf in the armature so the armature current will increase giving a rise in torque and the RPM will increase as a consequence.
To calculate generator output you need to know the.
armature speed and field strength.
In a shunt motor, if you reverse both field current and the armature current, the motor will.
continue to run in the same direction.
Prove it with Fleming’s left hand rule.
A starter generator has a.
low resistance series field and a high resistance shunt field.
If the brake coil on an actuator motor goes open circuit, the actuator will.
stop.
The brake coil is wired in series with the actuator motor coil. It withdraws a spring loaded lock to release the actuator when current is applied.
The diagram shows a 200 V long shunt generator. What is the voltage dropped across the series resistor?
0.12 V.
Don’t let the field configuration confuse you. The 0.04 and 100 ohm resistors are part of the generator and can be ignored. Just consider the 0.06 ohm (the series resistor) and the load resistor. Calculate current (about 2A) then calculate voltage across 0.06 ohm resistor.
If a generator speed is halved and the field strength is doubled, the voltage output will be.
unchanged.
How can the direction of rotation of a DC electric motor be changed?
reverse the electrical connections to either the field or armature windings.
The only practical method of maintaining a constant voltage output from an aircraft generator under varying conditions of speed and load is to vary the.
strength of the magnetic field.
Electric motors are often classified according to the method of connecting the field coils and armature. Aircraft engine starter motors are generally of which type?
Series.
Series motors have the highest starting torque.
If a heavy mechanical load is removed from a series motor.
the speed will increase and the armature current will decrease.
Speed increases and increases the back emf which decreases the armature current.
If the correct supply were connected to a DC shunt motor which had lost its residual magnetism it would.
run normally.
Its a motor, not a generator!.
One purpose of a growler test is to determine the presence of:
a shorted armature.
The purpose of an end-travel microswitch in a linear actuator is to.
remain closed and opens at end travel only.
See Pallett - Aircraft Electrical Systems, pg 140.
Linear actuators used in aircraft are of.
split field series wound type.
See Pallett - Aircraft Electrical Systems, pg 141.
If a generator sparks, a possible reason is.
the brush springs are loose.
If the field strength of a generator is doubled, and the speed is doubled, the output EMF will.
quadruple.
NIL.
In a DC motor the pole pairs are.
embedded within the field coils.
A series wound DC motor, the field has.
few turns of thick wire.
Doubling the running time of an electrical machine would.
double the joules used.
Power (watts) is the rate of using energy (Joules). So doubling running time will not change the power. Current is the rate of charge flow - so more charge (coulombs) will be used but not at a faster or slower ‘rate’.
