104
Electric Motors and Drives
increase in speed is however obtained at the expense of available torque, which is proportional to Xux times current (see equation (3.1)). The current is limited to rated value, so if the Xux is halved, the speed will double but the maximum torque which can be developed is only half the rated value (point e in Figure 3.10). Note that at the point e both the armature voltage and the armature current again have their full rated values, so the power is at maximum, as it was at point b. The power is constant along the curve through b and e, and for this reason the shaded area to the right of the line bc is referred to as the ‘constant power’ region. Obviously, Weld weakening is only satisfactory for applications which do not demand full torque at high speeds, such as electric traction. The maximum allowable speed under weak Weld conditions must be limited (to avoid excessive sparking at the commutator), and is usually indicated on the motor rating plate. A marking of 1200/1750 rev/min, for example, would indicate a base speed of 1200 rev/min, and a maximum speed with Weld weakening of 1750 rev/min. The Weld weakening range varies widely depending on the motor design, but maximum speed rarely exceeds three or four times the base speed. To sum up, the speed is controlled as follows: . Below base speed, the Xux is maximum, and the speed is set by the armature voltage. Full torque is available at any speed. . Above base speed, the armature voltage is at (or close to) maximum and the Xux is reduced in order to raise the speed. The maximum torque available reduces in proportion to the Xux. To judge the suitability of a motor for a particular application we need to compare the torque–speed characteristic of the prospective load with the operating diagram for the motor: if the load torque calls for operation outside the shaded areas of Figure 3.10, a larger motor is clearly called for. Finally, we should note that according to equation (3.9), the no-load speed will become inWnite if the Xux is reduced to zero. This seems unlikely; after all, we have seen that the Weld is essential for the motor to operate, so it seems unreasonable to imagine that if we removed the Weld altogether, the speed would rise to inWnity. In fact, the explanation lies in the assumption that ‘no-load’ for a real motor means zero torque. If we could make a motor which had no friction torque whatsoever, the speed would indeed continue to rise as we reduced the Weld Xux towards zero. But as we reduced the Xux, the torque per ampere of armature current would become smaller and smaller, so in a real machine with friction, there will come a point where the torque being produced by the