when not held at any other position. On the first step of the arm, full line voltage is applied across the shunt field. Since the field rheostat is normally set to minimum resistance, the speed of the motor will not be excessive; additionally, the motor will develop a large starting torque. The starter also connects an electromagnet in series with the shunt field. It will hold the arm in position when the arm makes contact with the magnet. Meanwhile, that voltage is applied to the shunt field, and the starting resistance limits the current to the armature. As the motor picks up speed counter-emf is built up; the arm is moved slowly to short. Four-point starter The four-point starter eliminates the drawback of the three-point starter. In addition to the same three points that were in use with the three-point starter, the other side of the line, L1, is the fourth point brought to the starter when the arm is moved from the "Off" position. The coil of the holding magnet is connected across the line. The holding magnet and starting resistors function identical as in the three-point starter. The possibility of accidentally opening the field circuit is quite remote. The four-point starter provides the no-voltage protection to the motor. If the power fails, the motor is disconnected from the line. Parameters and stats estimation Several studies propose either non-intelligent estimators which depend on the model, such as the extended Kalman filter (EKF) and Luenberger's observer, or intelligent estimators such as cascade-forward neural network (CFNN) and quasi-Newton BFGS backpropagation . See also Alternating current Brushless DC electric motor References Bibliography External links How Electric Motors Work (retrieved from Web Archive on 2014/31/01) DC motors Electric motors