Chapter 6 DC Motors

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Chapter 6 DC Motors INTRODUCTION OF DC MOTOR Table 6-1. DC Motor Applications DC motor The construction of a DC motor is virtually the same as that of a DC generator and many machines may be operated as either, such as a starter-generator. Construction of DC motor • armature • the yoke (or casing) • the field coils Construction of DC motor • an armature mounted in bearings • a stationary magnetic field • commutator and brushes Direction of conductor tend to move armature  The armature is typically a soft iron drum mounted on the motor shaft, with the armature conductors set axially into the surface of the drum. Also mounted on the armature shaft are the commutator segments, to which the armature conductors are connected. The armature shaft is mounted in ball bearings at each end, the bearings being held in the ends of the motor casing. Field windings The field windings are attached to the inside of the yoke and form two poles fitting closely around the armature with a running clearance of about 2.5mm. Section through a DC Motor DC MOTOR PRINCIPLE OF OPERATION
 Back EMF The loop of an armature is moving through the stationary field as the armature rotates and this inevitably induces an EMF in the armature. This EMF produces a current flow that opposes the applied current from the battery and therefore reduces the total armature current flow. The induced voltage is known as back EMF. net EMF • The difference between the applied EMF and the back EMF is known as the net EMF, and it is this that determines the torque produced in the armature shaft. • In order to ensure that the net EMF is sufficient the resistance of the armature windings is kept as low as possible. Armature current The initial current flow through the armature, before it begins to rotate, is determined by the applied voltage and the armature resistance. If the resistance is low the current flow will be very high. As the motor gains speed the back EMF increases and reduces the current flow through the armature. Why there is a surge current when DC motor start? Limit the excess starting current To avoid excess starting current, some DC motors have a resistance built in to the armature windings, which automatically cuts out as motor speed increases. TYPES OF DC MOTORS TYPES OF DC MOTORS • series wound • shunt wound • compound wound Series Wound Motor Series Wound Motor • The field coils are connected in series with the armature. • At starting, when the current flow is very high. Consequently a characteristic of the series wound motor is high starting torque. This is useful in circumstances where the motor will be required to start against a high load and where the running load is also high. Examples Examples of instances where series wound motors are used are engine starter motors, flap operating motors and landing gear operating motors. Note • Series wound motors should never be allowed to operate without a mechanical load applied. • This is because they are liable to overspeed, possibly to destruction. Shunt Wound Motors Shunt Wound Motors • the field coils are connected in parallel with the armature windings. • The resistance of the field coils is deliberately set to limit the field current to that required for normal operation of the motor, and is much higher than the armature resistance. Shunt Wound Motors • On start up the current flow through the armature is high, because of its low resistance. • a characteristic of the shunt wound DC motor is low starting torque. • As the armature speed increases, increasing back EMF will cause the armature current to decrease. Shunt wound motors • Shunt wound motors are used when starting torque is low and increases with motor speed. • They are particularly useful where constant speed under varying load conditions is a requirement. • Typical applications in aircraft are fuel pumps and fans. Compound Wound Motors Compound Wound Motors It has two sets of field windings, one connected in series with the armature and the other in parallel. The low resistance series windings are shown in heavy lining, the higher resistance shunt windings in lighter lining. Compound Wound Motors The compound wound motor combines the characteristics of the series wound and the shunt wound motor. It is capable of high starting torque, but will not over-speed under light mechanical loading and will maintain a reasonably constant speed under varying conditions of load. Compound Wound Motors • The compound wound motor is suited to applications where loads may vary from zero to maximum and where starting loads may be high. • In aircraft they are often used to drive hydraulic pumps and used as a starter/ generator. Reversible DC Motors Reversible DC Motors Reverse the rotating direction could be achieved by means of a switching arrangement that reversed the polarity of the DC supply to either the field or the armature (but not both). This would reverse the magnetic attraction and repulsion and thus reverse the direction of rotation of the armature. split-field motor • In a split-field motor there are two sets of field windings, either wound in opposite directions on a common pole (or core) or on alternate poles around the inside of the motor casing. • used to operate flaps and landing gear, MOTOR SPEED CONTROL MOTOR SPEED CONTROL METHOD • armature control • field control. Armature Control Armature Control • This form of speed control is rarely used, as the high armature current requires a larger variable resistor to handle it. And it is inefficient. • it is just a theoretical method. Field Control Field Control Variable resistance would not directly affect the current through the armature. However, as the magnetic field strength has increased, so would the back EMF. The current in the armature would reduce and motor speed would also reduce. Tips To compare the magnetic field to a viscous fluid: The thicker it gets, the harder it is for the armature to turn. The thinner it gets, the easier it is for the armature to turn. AIRCRAFT DC MOTORS ACTUATORS • These are high-speed reversible series-wound motors whose output is normally converted into a driving torque via a step-down gearbox. • Motor actuators are self-contained units, which combine electrical and mechanical devices capable of exerting reversible linear thrust over a short distance or alternatively a reversible low-speed turning effort. Rotary Actuators Linear Actuators END OF CHAPTER 9

linairm 发表于 2011-10-22 20:28:42

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Chapter 6 DC Motors