movement needed for current induction in the rotor. Back EMF The movement of armature windings of a direct-current or universal motor through a magnetic field, induce a voltage in them. This voltage tends to oppose the motor supply voltage and so is called "back electromotive force (EMF)". The voltage is proportional to the running speed of the motor. The back EMF of the motor, plus the voltage drop across the winding internal resistance and brushes, must equal the voltage at the brushes. This provides the fundamental mechanism of speed regulation in a DC motor. If the mechanical load increases, the motor slows down; a lower back EMF results, and more current is drawn from the supply. This increased current provides the additional torque to balance the load. In AC machines, it is sometimes useful to consider a back EMF source within the machine; this is of particular concern for close speed regulation of induction motors on VFDs. Losses Motor losses are mainly due to resistive losses in windings, core losses and mechanical losses in bearings, and aerodynamic losses, particularly where cooling fans are present, also occur. Losses also occur in commutation, mechanical commutators spark; electronic commutators and also dissipate heat. Efficiency To calculate a motor's efficiency, the mechanical output power is divided by the electrical input power: , where is energy conversion efficiency, is electrical input power, and is mechanical output power: where is input voltage, is input current, is output torque, and is output angular velocity. It is possible to derive analytically the point of maximum efficiency. It is typically at less than 1/2 the stall torque. Various national regulatory authorities have enacted legislation to encourage the manufacture and use of higher-efficiency motors. Electric motors have efficiencies ranging from at least 15% for shaded pole motors, up to 98% for permanent magnet motors, with efficiency also dependent on load. Peak efficiency is usually at 75% of the rated load. So (as an example) a 10 HP motor is most efficient when driving a load that requires 7.5 HP. Efficiency also depends on motor size; larger motors tend to be more efficient. Some motors can not operate continually for more than a specified period of time (e.g. for more than an hour per run) Goodness factor Eric Laithwaite proposed a metric to determine the 'goodness' of an electric motor: Where: is the goodness factor (factors above 1 are likely to