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Weight Transfer Compensation in Locomotives

By on April 13, 2021

The development of tractive effort by locomotive causes off-loading of some axles and over-loading of others-a phenomenon commonly known as weight transfer. Electric locomotives WCG-2, WAM-4, WAG-5, WAG-7& WCAM-3 have a provision for compensation of weight transfer by selective shunting of fields of traction motors. This paper reviews circuits of these locomotives to examine the efficacy of this scheme.  In some locomotives like WAG-7, the circuit is helpful (after recent modification); in others like WAM-4 WAG-5, and WCAM-3, it is of no help.

Introduction

The development of tractive effort (TE) by locomotives causes some wheels to offload while overloading others. In other words, weight transfer (or shift) takes place from some wheels of the locomotive to the rest of them. The magnitude and pattern of such weight transfer depend on the geometrical features of the locomotive – particularly its bogie. The end result is, obviously, problematic since the capability of a wheel to apply TE without slipping is directly proportional to the weight carried by that wheel. The wheels, which have become lighter, have lost some of their capacity to apply tractive effort. Hence the wheels on the lightest axle-wheel assembly (hereafter referred to as an axle) govern the total tractive effort that can be applied since all traction motors have a common control. In other words, for producing a certain value of TE the lightest axle will have to be worked to the highest coefficient of adhesion (m).

If it was possible to produce a different amount of TE from different axles such that over-loaded axles produced more and under-loaded ones less; the situation could become much better. It would then be possible to either increase the total TE for a limiting value of adhesion or be possible to produce a given amount of TE working the lighter axles to lower the adhesion limit. During starting such differentiation of TE is most desirable. With this objective in view, weight transfer compensation circuits are provided in locomotives such as WCG-2, WAM-4, WAG-5, WAG-7, WCAM-3 et al.

 Weight Transfer

 Before we take the discussion further, let us understand the weight transfer for the locomotives listed above. WCG-2, WAM-4 and WAG-5 locomotives share the same tri-mount Co-Co bogie. Likewise, locomotives WAG-7 and WCAM-3 share the same design of the bogie. Weight transfer calculations for WAG-5 and WAG-7 are given in Table-1. Actually, the dynamic load is directly proportional to the TE. The values in the table correspond to the starting TE (33t &44t respectively) for both locomotives.

It can be seen that both the magnitude and pattern of weight transfer are significantly different in the two cases. In WAG-5 axles 1,2 & 4 are offloaded while in WAG-7 axles 1,2&3 are off-loaded. WAM-4 has an almost identical pattern and quantum of weight transfer, as WAG-5 except its static weight is lower by 1.0 t per axle.  Similarly, WCG-2 has the same pattern of weight transfer but its starting TE is 35 t and static load 22t per axle. On the other hand, WCAM-3 will have an identical pattern of weight transfer like that of WAG-7, the quantum, however, may be lesser on account of lower TE. We will suitably adopt these values for respective locomotives.

Principle of Weight Transfer Compensation 

The torque developed by the traction motor (TM) is proportional to the product of field flux and armature current.  A part of the TMs field is diverted through a shunting resistor. Therefore torque produced by the motor and, consequently, TE at the corresponding wheels will be lower. In this way, fields of TMs on off-loaded axles are weakened while those of TMs on over-loaded axles are working to full strength. Thereby total TE of the locomotive is so distributed among axles that the ratio of TE to weight is more or less equal for all axles. This provided relief to offloaded axles that would otherwise have this ratio unduly strained -heightening the probability of slip. Another way of looking at the same phenomenon is that for a given maximum value of adhesion utilization, higher TE can be obtained from the locomotive.

Well, the above scheme appears workable. Indeed it is so in DC locomotives in which all TMs share the same armature current during starting due to series connection. But the situation changes in AC locomotives. Because of the fact that TMs are connected in parallel (in groups of two or singly), the current through the circuit with TMs having their fields shunted is more than that through the circuit with TMs not having such field shunting. This lessens (and may even nullify) the impact of selective field shunting described in the previous paragraph. With this background, we will now assess the exact impact in each case with the help of calculations with specific values for specific locomotives.

Circuit for Weight Transfer Compensation

 A spring-loaded switch named ‘ZQWC’ is provided on the driver’s desk. The driver is expected to use it by pressing it until the train starts rolling while starting the train on gradients. This switch operates a relay ‘QWC’, which in turn operates the shunting contractors to achieve shunting of fields of desired TMs depending upon the direction of motion. It may be noted that the driver is supposed to leave the switch the moment the locomotive has begun to roll. Therefore this circuit is relevant only before the moment in which back emf gets established.

 WCG-2

 We have mentioned DC locomotives earlier. Series connection at the start is done in this locomotive as well. As described earlier, axle -4 is the most critical one from point of view of adhesion utilization and wheel-slip. In this case, the fields of TMs 1&4 are shunted thus providing desired relief to wheels of axle-4. However, axle -2, which also is offloaded albeit to a lesser degree than axle-4, is not given any relief.

WAM-4

This is the first among a large family of conventional AC locomotives in Indian Railways. Perhaps due to a high degree of weight transfer, a scheme for compensation by electrical means was considered necessary for it. We will examine the circuitry of 2s-3p version, which is still in use. In this case operation of ‘ZQWC’ by the driver causes shunting contactors S141 when the locomotive is moving in the forward direction and S361 otherwise to pick up. As a result, fields of TMs on leading axles on both bogies are shunted. A simplified picture of the situation just before starting (when no back emf is established since no rotation has begun) is given below.

Using of values of current in various circuits estimated from the circuits given above and reading the values of  TEN for WAM-4 locomotive from the characteristic curves (TM TAO-659, gear ration 15:62, and half-worn wheels -1055 mm dia.) axle wise TE is given in Table-2. Weight transfer calculations have been adopted for a total TE of 33 t. Even though normally the rectifier over-current relay (QRSI) is set to 1500*2=3000 A,  calculations have been done for a setting of 3300 A (corresponding to 33 t) also to permit clear analysis.

A significant observation in the above table is that the tractive effort developed by the TMs with fields shunted and others are not different. This outcome is both not desirable and not in accordance with common perception. We all believe that shunting of TM fields must lead to reduced torque compared to the TMs whose fields are not shunted. However, actually, the torque remains the same due to increased armature current in these TMs vis-à-vis the other TMs. The effect of shunting of fields by about 11% (95-84) is nullified by an increase in armature current by about 4%. Therefore no advantage is gained by opting for this compensation circuit. Indeed this increase in current is harmful and avoidable.

 WAG-5

incidentally, the 2s-3p locomotives are being converted to 6p configuration. Therefore a detailed study has been done for WAG-5 locomotive which also has 6p configuration. The same bogie as that of WAM-4 has been adopted in these locomotives as well. And the scheme of weight transfer compensation was also extended to them with suitable changes to suit the 6p configuration.

Groups of three traction motors of each bogie are fed by one secondary winding of the transformer through a rectifier and smoothing reactor. The current distribution has to be worked out considering the above. As a matter of fact, the two circuits will carry different currents with the rear bogie TMs carrying slightly more (about 1%) current than those on the front bogie since TMs 3, 5&6 are shunted to exclusion of others. But the phenomenon within each of these groups is similar to WAM-4 whereby TM with shunted fields (field reduced by 95-72 =23% more) receive more (about 9%) armature current than those without field shunting. There is little difference in torque produced by TMs. Hence the intended advantage is not gained and the lighter axles have to be worked to higher adhesion. The armature current, however, undergoes an avoidable increase.

A thorough analysis of WAG-5HA locomotive calculating the current distribution, TE developed, and axle-wise adhesion utilization is given at annexure-1. To explore the issue further, different values of shunting resistance were tried for the two possible combinations of circuits. A conclusive summary is presented in Table-3.

It turns out clearly that axle-4 is worked to maximum adhesion in all the combinations. There is hardly anything to choose from as far as maximum adhesion utilization is concerned. Indeed as the value of shunting resistance increases (extent of shunting decreases), the maximum current reduces. This may not be very significant for HS-15250 TM but is important for TAO-659 TMs. Hence situation with no shunt is the best on these two grounds. A detailed perusal of annexure -1, however, suggests that adhesion utilization of axles 1 &2 is slightly better if TMs 1,2&4 are shunted with RS-3. Incidentally, the present circuit of  WAG-5 locomotive shunts TMs 3,5 &6 using RS-3.

WAG-7

 The weight transfer compensation circuit of this locomotive has been recently modified by RDSO following a suggestion from IRIEEN and S. Rly. The earlier scheme provided for shunting of TMs 3,5 & 6 for the purpose whereas the modified scheme provides for that of TMs 1,2, &3.

Adhesion Utilization while developing 44 t of tractive effort in WAG-7

 n a locomotive with the modified circuit, if the driver opts for weight compensation by switching on the ZQWC, the locomotive will develop the specified maximum TE of 44t by utilizing adhesion to the extent of 37.2%. If the loco is provided with an original circuit, the same TE will be developed by utilizing adhesion to the extent of 38.8%. Hence more chances of wheel-slip. This situation is worse than the one in which no compensation is provided. It may also be noted that the original circuit subjected the TM-3 armature to a maximum current of 1325 A for a short time whereas the current remains 1215 A only if no compensation circuit is used. The original circuit was performing poorly both in respect of adhesion utilization and maximum current.

WCAM-3

Another interesting case is that of WCAM-3 locomotive. These locomotives also have the same bogie as that of WAG-7. TE is, however, lesser. Hence the weight transfer will be correspondingly smaller but on the same pattern as that of WAG-7. In other words, the off-loading will occur on all wheels of the front bogie and not on the leading wheels of each bogie. But the field shunting for the purpose of weight transfer compensation is provided on TMs 1&4. Obviously, this will offer no advantage as far as slipping is concerned but reduce the overall TE slightly during DC operation. During AC operation there will be a mismatch between weight transfer and compensation pattern as the circuit is like that of WAM-4 and bogie like that of WAG-7.

Conclusion

 Weight transfer compensation should be provided to match the pattern and quantum of weight transfer and must be implementable in the given power circuit of the locomotive. It can be concluded from the analysis that the present scheme of weight transfer compensation has been essentially adopted from earlier DC locomotives. It does not work well on AC locomotives that have a different connection of TMs.

The circuits used on various locomotives of Indian Railways have been examined. In the case of WCG-2 also, it helps. In the case of WAG-7, it offers a clear advantage after a recent modification. Therefore all WAG-7 locomotives should be modified as fast as possible. But it does not offer any appreciable advantage in WAM-4, WAG-5 and WCAM-3 locomotives. No wonder that the circuit is isolated in several locomotives. It is, therefore, recommended that the circuit may be dispensed with in WAM-4, WAG-5, and WCAM-3 locomotives. It may not save anything by way of released material more than a relay, a switch, and some wiring. But like removing a nonfunctional appendix from a human body, it will surely save an odd painful failure.

 

References:

  1. Article titled ” Weight transfer and its compensation in locomotives” IRIEEN Journal Vol X, July-Sept. 2000.
  2. Circuit diagrams of locomotives WCG-2, WAM-4, WAG-5, WCAM-3 and WAG-7.
  3. Manuals of locomotives WCG-2, WAM-4, WAG-5, WCAM-3 and WAG-7.
  4. RDSO’s modifications
  5. WAM-4/3C dated 25/12/1978
  6. WAM-4/11K dated 26/12/1978
  7. WAM-4B/2 dated 20/11/1980
  8. ELRS/MS/0306-2001(Rev-0) dated 28/08/2001

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