to the product of the armature current and the machine's total flux strength: where Speed equation Since and we have where Torque and speed characteristics Shunt wound motor With the shunt wound motor's high-resistance field winding connected in parallel with the armature, Vm, Rm and Ø are constant such that the no load to full load speed regulation is seldom more than 5%. Speed control is achieved three ways: Varying the field voltage Field weakening Variable resistance in the field circuit. Series wound motor The series motor responds to increased load by slowing down; the current increases and the torque rises in proportional to the square of the current since the same current flows in both the armature and the field windings. If the motor is stalled, the current is limited only by the total resistance of the windings and the torque can be very high, but there is a danger of the windings becoming overheated. Series wound motors were widely used as traction motors in rail transport of every kind, but are being phased out in favour of power inverter-fed AC induction motors. The counter EMF aids the armature resistance to limit the current through the armature. When power is first applied to a motor, the armature does not rotate, the counter EMF is zero and the only factor limiting the armature current is the armature resistance. As the prospective current through the armature is very large, the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter EMF. As the motor rotation builds up, the resistance is gradually cut out. The series wound DC motor's most notable characteristic is that its speed is almost entirely dependent on the torque required to drive the load. This suits large inertial loads as motor accelerates from maximum torque, torque reducing gradually as speed increases. As the series motor's speed can be dangerously high, series motors are often geared or direct-connected to the load. Permanent magnet motor A permanent magnet DC motor is characterized by a linear relationship between stall torque when the torque is maximum with the shaft at standstill and no-load speed with no applied shaft torque and maximum output speed. There is a quadratic power relationship between these two speed-axis points. Protection To extend a DC motor's service life, protective devices and motor controllers are