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Wheel slip on locos such as the WAG-5/WAG-7,WAM-4,WCAM-1/2/3, is detected by a relay designated 'QD' which is a current differential relay. It detects the difference in the current flow between two traction motors. If all the traction motors are running at uniform and equal speeds, the armature of the relay remains balanced. However, if any of the axles are slipping, the current to this motor is slightly reduced producing a current imbalance in relay QD which is then triggered. QD gives a repeat to a relay 'Q48' which in turn may activate some automatic wheel-slip reduction procedures as detailed below, depending on the configuration in the particular locomotive. Operation of relay Q48 also lights the LSP (Signal lamp to indicate Wheel-Slip) on the driver's desk.
WAG-5/7,WAM-4,WCAM-1/2/3, etc., have been provided with mainly three methods to minimise wheel slip:
Sanders to improve adhesion: Sanders can be operated automatically by relay Q48 or manually by pressing a foot switch 'PSA' below the driver's desk.
Auto-Regression of the Tap-Changer to reduce tractive effort: Q-48 also gives an impulse to relay Q-51 (Relay for Auto-Regression of Tap-Changer) to reduce the notches which in turn lowers the voltage to the traction motors thereby reducing the tractive effort.
Field Weakening (Weak Field Operation): The DC series motors have their fields wound in series with the armature winding. Normally,the current through the field and the armature is equal but if the current through the field is partially bypassed, the torque of the motor is reduced. Hence, by shunting the field, the tractive effort of the motor is reduced. During wheel slipping, weak field is usually introduced on the leading axles of both the bogies because these are usually the ones to slip first due to dynamic weight transfer which tends to reduce the weight on the leading axles and proportionately increases weight on the trailing axles. (The same field shunting device is used to increase the speed of the loco above 21 notches but in that case it's applied to all the traction motors.) In WCAM-1 locos, a special switch 'ZQWC' has been provided in front of the driver the introduce weak field manually.
The above methods of detection and prevention also suffer from various drawbacks.
Relay QD has to have a built-in bias. It has to compensate for slight differences in the wheel diameters which also tend to introduce wheel slipping and skidding and may cause spurious operation of the relay.
Another problem is that on many locos, sanders are only partially operational and can't be relied upon.
Auto-Regression of the tap-changer is a mixed blessing. Repeated attempts by the driver to start the load may fail due to lack of progression of the tap-changer. This may result in the load being failed. In practice, many locos have the wire providing the impulse to relay Q51 disconnected by the sheds to minimise such failures on the line. In desperation, some drivers also tend to temporarily wedge the relay Q51 to prevent its operation and thereby prevent auto-regression.
Field-weakening is also not too reliable a method because it may reduce the power too much and may prevent the loco from starting altogether.
Microprocessor controlled locos: Microcontrollers on the newer generation of locos use a tachometer generator (tachogenerator) on all the axles which gives a precise output voltage for each axle, all of which are fed into a comparator. If all the voltages tally, the motors are running at the same speed but if an axle is slipping, it speeds up which increases the voltage measured. When the comparator detects such a difference, it can take the requisite action such as applying brakes slightly on the on the individual wheels or reduce the frequency of power supply to that motor.
In sections where wheel-slipping is a persistent and chronic problem, MU'ed (multiple unit) locos often give better results. This is because although a single loco may be sufficient to start and haul the formation, during the starting phase, wheel slipping may cause undue problems. In the case of a MU'ed consist, although the gross tractive effort is enough to start the load, the individual axles by themselves do not generate sufficient torque to induce wheel-slipping.