<P> Since the series - wound DC motor develops its highest torque at low speed, it is often used in traction applications such as electric locomotives, and trams . Another application is starter motors for petrol and small diesel engines . Series motors must never be used in applications where the drive can fail (such as belt drives). As the motor accelerates, the armature (and hence field) current reduces . The reduction in field causes the motor to speed up until it destroys itself . This can also be a problem with railway motors in the event of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far higher than they would do under normal circumstances . This cannot only cause problems for the motors themselves and the gears, but due to the differential speed between the rails and the wheels it can also cause serious damage to the rails and wheel treads as they heat and cool rapidly . Field weakening is used in some electronic controls to increase the top speed of an electric vehicle . The simplest form uses a contactor and field - weakening resistor; the electronic control monitors the motor current and switches the field weakening resistor into circuit when the motor current reduces below a preset value (this will be when the motor is at its full design speed). Once the resistor is in circuit, the motor will increase speed above its normal speed at its rated voltage . When motor current increases, the control will disconnect the resistor and low speed torque is made available . </P> <P> A Ward Leonard control is usually used for controlling a shunt or compound wound DC motor, and developed as a method of providing a speed - controlled motor from an AC supply, though it is not without its advantages in DC schemes . The AC supply is used to drive an AC motor, usually an induction motor that drives a DC generator or dynamo . The DC output from the armature is directly connected to the armature of the DC motor (sometimes but not always of identical construction). The shunt field windings of both DC machines are independently excited through variable resistors . Extremely good speed control from standstill to full speed, and consistent torque, can be obtained by varying the generator and / or motor field current . This method of control was the de facto method from its development until it was superseded by solid state thyristor systems . It found service in almost any environment where good speed control was required, from passenger lifts through to large mine pit head winding gear and even industrial process machinery and electric cranes . Its principal disadvantage was that three machines were required to implement a scheme (five in very large installations, as the DC machines were often duplicated and controlled by a tandem variable resistor). In many applications, the motor - generator set was often left permanently running, to avoid the delays that would otherwise be caused by starting it up as required . Although electronic (thyristor) controllers have replaced most small to medium Ward - Leonard systems, some very large ones (thousands of horsepower) remain in service . The field currents are much lower than the armature currents, allowing a moderate sized thyristor unit to control a much larger motor than it could control directly . For example, in one installation, a 300 amp thyristor unit controls the field of the generator . The generator output current is in excess of 15,000 amperes, which would be prohibitively expensive (and inefficient) to control directly with thyristors . </P> <P> A DC motor's speed and torque characteristics vary according to three different magnetization sources, separately excited field, self - excited field or permanent - field, which are used selectively to control the motor over the mechanical load's range . Self - excited field motors can be series, shunt, or compound wound connected to the armature . </P> <Ul> <Li> E, induced or counter EMF (V) </Li> <Li> I, armature current (A) </Li> <Li> k, counter EMF equation constant </Li> <Li> k, speed equation constant </Li> <Li> k, torque equation constant </Li> <Li> n, armature frequency (rpm) </Li> <Li> R, motor resistance (Ω) </Li> <Li> T, motor torque (Nm) </Li> <Li> V, motor input voltage (V) </Li> <Li> Φ, machine's total flux (Wb) </Li> </Ul>

What factors determine the amount of torque on the armature of a rotating dc electric motor