0 Members and 1 Guest are viewing this topic.
quote:Originally posted by guestThank You. Well, all i want to know is how an electric motor (that uses batteries to opperate) can have varying speeds. How is this "torque" applied? How do the speeds of SUCH motors change (some fast, some slow, yet the same size)and what are those small orange circle things attached to some of them for?
quote:The DC motor was the mainstay of electric traction drives on both electric and diesel-electric for many years. It consists of two parts, a rotating armature and a fixed field. The fixed field consists of tightly wound coils of wire fitted inside the motor case. The armature is another set of coils wound round a central shaft. It is connected to the field through "brushes" which are spring loaded contacts pressing against an extension of the armature called the commutator. The commutator collects all the terminations of the armature coils and distributes them in a circular pattern to allow the correct sequence of current flow. The motor works because, simply put, when a current is passed through the motor circuit, there is a reaction between the current in the field and the current in the armature which causes the armature to turn. The armature and the field are connected in series and the whole motor is referred to as "series wound".A series wound DC motor has a low resistance field and armature circuit. Because of this, when voltage is applied to it, the current is high. (Ohms Law: current = voltage/resistance). The advantage of high current is that the magnetic fields inside the motor are strong, producing high torque (turning force), so it is ideal for starting a train. The disadvantage is that the current flowing into the motor has to be limited somehow, otherwise the supply could be overloaded and/or the motor and its cabling could be damaged. At best, the torque would exceed the adhesion and the driving wheels would slip. Traditionally, resistors were used to limit the initial current.DC Resistance ControlAs the DC motor starts to turn, the interaction of the magnetic fields inside it causes it to generate a voltage internally. This "back voltage" opposes the applied voltage and the current that flows is governed by the difference between the two. So, as the motor speeds up, the internally generated voltage rises, the effective voltage falls, less current is forced through the motor and thus the torque falls. The motor naturally stops accelerating when the drag of the train matches the torque produced by the motors. To continue accelerating the train, resistors are switched out in steps, each step increasing the effective voltage and thus the current and torque for a little bit longer until the motor catches up. This can be heard and felt in older DC trains as a series of clunks under the floor, each accompanied by a jerk of acceleration as the torque suddenly increases in response to the new surge of current. When no resistor is left in the circuit, the full line voltage is applied directly to the motor. The train's speed remains constant at the point where the torque of the motor, governed by the effective voltage, equals the drag - sometimes referred to as balancing speed. If the train starts to climb a grade, the speed reduces because drag is greater than torque. But the reduction in speed causes the back voltage to decline and thus the effective voltage rises - until the current forced through the motor produces enough torque to match the new drag.On an electric train, the driver originally had to control the cutting out of resistance manually but, by the beginning of the First World War in 1914, automatic acceleration was being used in the UK on multiple-unit trains. This was achieved by an accelerating relay (often called a "notching relay") in the motor circuit (see next diagram below) which monitored the fall of current as each step of resistance was cut out. All the driver had to do was select low, medium or full speed (called "shunt", "series" and "parallel" from the way the motors were connected in the resistance circuit) and the equipment would do the rest.
quote:DC motorsOne of the first electromagnetic rotary motors was invented by Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors.The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected a spinning dynamo to a second similar unit, driving it as a motor.The classic DC motor has a rotating armature in the form of an electromagnet with two poles. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the classical motor going in the proper direction. (See the diagrams below.)DC motor speed generally depends on a combination of the voltage and current flowing in the motor coils and the motor load or braking torque. The speed of the motor is proportional to the voltage, and the torque is proportional to the current. The speed is typically controlled by altering the voltage or current flow by using taps in the motor windings or by having a variable voltage supply. The speed can also be controlled by using an electronic circuit that switches the supply voltage on and off very rapidly. As the "on" to "off" time is varied to alter the average applied voltage, the speed of the motor varies. As the DC motor can develop quite high torque at low speed it is often used in traction applications such as locomotives.However, there are a number of limitations in the classic design, mainly due to the need for brushes to press against the commutator. This creates friction, and the higher the speed the harder the brushes have to press to maintain good contact. Not only does this friction make the motor spark and create noise, but it also creates an upper limit on the speed. The imperfect electric contact also causes electrical noise. The brushes eventually wear out and require replacement. These problems are eliminated in the brushless motor. In this motor, the mechanical "rotating switch" or commutator/brushgear assembly is replaced by an external electronic switch synchronised to the motor's position. Brushless motors are available in two basic styles. In one type the permanent magnets are part of the rotor in the core of the motor, and the electromagnets ie coils are arranged around the inside of an outer shell. In the other type, the electromagnets or coils are in the core of the motor, and the permanent magnets are housed inside the outer shell. In both types the coils are held stationary to facilitate electrical connection, and the permanent magnet assembly rotates. Brushless motors are typically 85-90% efficient whereas DC motors with brushgear are typically 10% less efficient.Wound field DC motorThe permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance. Further reductions in field current are possible to gain even higher speed but correspondingly lower torque. This technique is ideal for electric traction (see Traction motor) and many similar applications where its use can eliminate the requirement for a mechanically variable transmission. Generally speaking the rotational speed of a DC motor is proportional to the voltage applied to it, speed control can be achieved by variable battery tappings, resistors or electronic controls. The direction of a wound field DC motor can be changed by reversing either the field or armature connections but not both, this is commonly done with a special set contactors (direction contactors).