DC Motor From Conventional DC Motor

DC Motor From Conventional DC Motor

The motor has two separate circuits. The smaller pair of terminals connect to the Weld windings, which surround each pole and are normally in series: in the steady state all the input power to the Weld windings is dissipated as heat – none of it is converted to mechanical output power. The main terminals convey the ‘power’ or ‘work’ current to the brushes which make sliding contact to the armature winding on the rotor. The supply to the Weld is separate from that for the armature, hence the description ‘separately excited’. As in any electrical machine it is possible to design a d.c. motor for any desired supply voltage, but for several reasons it is unusual to Wnd rated voltages lower than about 6 V or much higher than 700 V. The lower limit arises because the brushes (see below) give rise to an unavoidable volt-drop of perhaps 0.5–1 V, and it is clearly not good practice to let this ‘wasted’ voltage became a large fraction of the supply voltage. At the other end of the scale, it becomes prohibitively expensive to insulate the commutator segments to withstand higher voltages. The function and operation of the commutator is discussed later, but it is appropriate to mention here that brushes and commutators are troublesome at very high speeds. Small d.c. motors, say up to hundreds of watts output, can run at perhaps 12 000 rev/min, but the majority of medium and large motors are usually designed for speeds below 3000 rev/min. Increasingly, motors are being supplied with power-electronic drives, which draw power from the a.c. mains and convert it to d.c. for the motor. Since the mains voltages tend to be standardised (e.g. 110 V, 220–240 V, or 380–440 V, 50 or 60 Hz), motors are made with rated voltages which match the range of d.c. outputs from the converter (see Chapter 2). As mentioned above, it is quite normal for a motor of a given power, speed and size to be available in a range of diVerent voltages. In principle, all that has to be done is to alter the number of turns and the size of wire making up the coils in the machine. A 12 V, 4 A motor, for example, could easily be made to operate from 24 V instead, by winding its coils with twice as many turns of wire having only half the cross-sectional area of the original. The full speed would be the same at 24 V as the original was at 12 V, and the rated current would be 2 A, rather than 4 A. The input power and output power would be unchanged, and externally there would be no change in appearance, except that the terminals might be a bit smaller