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Role of Adhesion in Rail Traction

By on April 14, 2021

Synopsis: Adhesion is central to any discussion on traction- electric or otherwise. Therefore, it has remained a key issue in the development of railways.  In the context of high tractive-effort and power requirements from locomotives and limitation on axle loads in IR, need for achieving high adhesion has been underlined.

The article surveys important developments in this field during the history of railway. Factors affecting adhesion have been identified and their impact explained. Important conclusions regarding ways of improving adhesion have been drawn.

Finally, ‘slip-slide control system’ used in modern locomotives is described and need to incorporate this in conventional electric locomotives highlighted. The article emphasises need for more research in this field in Indian context as failures in form of wheel slip and wheel slide have not yet been eliminated.


Railway is a capital-intensive industry. If a railway has to remain profitable, it must achieve high productivity of its fixed assets in which huge capital is locked. This can be achieved by a) increasing the loading per train consistent with permissible linear loading density of the track and the loop length; and, b) by increasing the line capacity and utilizing it. These considerations finally translate to high tractive force and power from the motive power. As far as motive-power is concerned, consideration of economic fleet demands that more and more power is packed into each locomotive. Since the benefits are directly related to the power while costs (both initial and recurring) to the number of locomotives. An important component of recurring cost – energy spent on hauling of the locomotive itself – is directly proportional to its weight.  Therefore, the trend world-over is to go for more and more powerful locomotives consistent with the permitted axle load i. e. a high power to weight ratio.

The locomotive produces a force, which acts through the draw-bar pulls the train attached to it. The force required for pulling a train increases with the weight of the trailing stock, speed, degree of curvature & steepness of gradient (Up) of the track. While a locomotive produces draw-bar pull, its wheels exert a force against the rails at their point of contact with rails. Thus the mechanism of a locomotive’s working, both in traction and braking, is accomplished through adhesion between wheel and rail. There is a limit beyond which the tractive or braking force cannot be increased. This limit is imposed by adhesion.

If ‘T’ is the maximum tractive effort (TE) that can be applied without causing the wheel to slip, then

T = µ. W

Where ‘W’ is the effective weight carried by the wheel (axle load) and ‘Mechanical coupling of the wheelsets.’ µ the ‘coefficient of adhesion’ (CoA). Similarly, during braking, the coefficient of rolling friction (or adhesion) is the ratio of maximum braking effort (BE) that can be applied on the rolling wheel without causing it to slide.

It follows that maximum tractive effort that can be applied by a locomotive can be raised by increasing the number of axles and weight per axle i.e. the weight of the locomotive. There are, however, limitations on each of these. The first is decided by curve negotiation and lateral stability considerations, the second is discussed hereunder.


Track structure and eventually the underlying soil limit maximum permissible weight on an axle. In Australia and South Africa axle load of 25 tons is permitted. In the US axle load of even 30 tons is allowed on some railroads. However, the situation is not so in India. A substantial section of track in India runs on black cotton soil with low bearing capacity. Besides this, soil conditions vary widely across different regions of the country. Varying climatic conditions through different seasons add to the problem. While wide variations in temperature in North and West cause problems for maintenance of rail joints; heavy rainfall in East and Southeast leads to high monsoon runoff. Drainage of water presents formidable difficulties. Long stretches of track run on high embankments, which become weak during monsoons. And there are bridges, which have become very old. This coupled with the altered character of the flow of water through the rivers compounds the difficulties. This has serious implications for track maintenance.

All this leads to restrictions on axle loads and train speeds on all rolling stock. Presently allowed values for locomotives are as under:


It is necessary to appreciate that production of the tractive power is done by the equipment of the locomotive. But its utilization is facilitated by adhesion as explained earlier. Therefore, the key to increasing tractive effort and the power of locomotives lies in better utilization of adhesion. Steam/diesel/gas locomotives carry their power pack onboard, which constitutes a large part of the locomotive’s weight. Indeed, the overall power of the engine is limited by the capacity of the power pack. By contrast, an electric locomotive does not suffer from such limitations. It is, thus, essential to exploit maximum possible adhesion so as to pack more power into the electric locomotive and utilize it.

In this connection in a 1971 article (3), Fisher observed as follows

Railway electrification studies would not show an attractive result if the lower adhesion of diesel locomotives is assumed to prevail in all-electric operation. An electric design by copying existing diesel locomotives would show substantially lower return on investment in comparison with an advanced design of locomotive achieving a working adhesion of around 30%”

Fisher has also worked out the return on investment as a function of adhesion. Assuming the return to be unity corresponding to a value of m=0.18, he shows that return increases to 1.65 at m=0.25. Much has changed since then. Far higher values of adhesion are achieved now in both electric as well as diesel-electric locomotives.

Adhesion is central to all forms of traction including road transport. George Stephenson is said to have satisfied himself of the adequacy of available adhesion between wheel and rail before building his steam engine. Except for the adhesion-less drive based on linear induction motors, its importance for electric traction can be understood in the light of Fisher’s remark quoted above.


In the case of braking, failure of adhesion is far more serious for safety reasons. Hence much lower value of CoA (m) is adopted to reduce the chances of failure to a minimum. In conventional braking, every vehicle is equipped with brakes; thus, weight on all axles comes into play. The case of dynamic braking in locomotives is however different. In this case, only the weight of the locomotive comes into play stretching the ratio between braking effort and weight. Hence careful management of adhesion is even more important.

 Similarly, in urban transport vehicles like electrical multiple units (EMU), the production of tractive as well as braking effort is distributed among a large number of axles. However, the value of CoA (m) has to be kept low as failure is absolutely unacceptable. Varying load on motorcoaches like all payload vehicles poses problems to which interesting solutions have been found.


Several theories have been advanced to explain the underlying physical phenomena. ‘Interlocking theory’ which attributes the adhesive force to ‘meshing of protuberances on the contacting surfaces under pressure’ has found general acceptability. A lot of research work has been done by several European railways to understand the factors affecting adhesion with a view to finding practical ways of improving CoA(m) as well as establishing reasonable values of the same for the design of locomotives. The values obtained through measurements were found to have widely scattered values. In fact, to describe such nature of adhesion, it is the practice now expressed in terms of a value (x) with a certain level of confidence (Y%) in the stated condition of rail surface.

Though a number of materials have been identified as capable of improving adhesion if inserted between wheel and rail. But some such as powdered copper or silicones are too expensive for general use. The action of sand, though erratic, is promising. On dry rails with sand values of CoA (m) of about 0.45 and on wet rails about 0.26 have been recorded with sanding. However, the sand should be as coarse as possible and free from impurities such as calcite and mica and should be freshly deposited in front of each driving wheel.

Some of the important factors affecting adhesion will be discussed now.

Factors affecting Adhesion

Contact Area related factors

  1. Wheel Rail materials
  2. Hertzian stresses
  3. Creep

The coefficient of adhesion (µ) of virgin steel on steel is known to be between 0.6-0.7; but in practice, it is much lower. Hertz first studied the rail-wheel interaction. Stresses on the rail depending on the ratio of load carried by the wheel and its radius. Studies by General Motors (EMD) have also shown that adhesion becomes poorer with higher stresses on the rail. These studies are covered under the subject of ‘Tribology’. A better understanding of the phenomenon of creep and its utilization has opened new avenues for improving adhesion. We will discuss this at some length later.

Track related factors

  1. Rail profile irregularities
  2. Curvature of track
  3. Rail surface condition

A well-maintained track having a curvature of fewer than 3 degrees is certainly helpful. Indeed, the CoA (m) is much better on straight track than in yards. The surface condition of rails is perhaps the most important factor. Unfortunately, the rail surface is never clean. Adhesion is adversely affected by the presence of contaminants such as oxides, moisture, and oil. Oil layer of density as low as 10-6 g/cm2 (even residue) can have an appreciable deteriorating effect.

In 1878, Captain Galton carried out series of tests for braking. He concluded that for speeds up to 97 km/h the value of CoA(m) on dry rails was more than 0.2 and in some cases 0.25 or more. On wet and greasy rails, it fell as low as 0.15. Further tests have confirmed the same trend.v

The values of CoA(m) obtained at different speeds in another well-known 1944 study by E.W. Curtius and A. Kniffer, both of German Federal Railways, are shown in Fig-3. Their study concluded two separate bands of values of adhesion for dry and wet rails. Also, the limiting value of CoA(m ) decreased with an increase in speed. The proposed formulae indicating this relationship.
µ v= 7.5/(v+44) +0.16

This formula is still used for estimating the value of CoA(m) at any given speed in relation to its value at the start. Reduction in the value of CoA (µ) is believed to be due to the impact of vibrations on axle loads.

Chemical treatment of rails has been experimented with for a long. Some studies claim considerable improvement in adhesion by application of chemicals such as 1% solution of ethyl acetate and ethyl caprylate.

In India, certain special problems have defied easy solutions and continue to hamper the railway operation.  Poor adhesion resulting from the action of biological substances produced by crushing of fallen leaves on the track in autumn season (particularly, in Chakradharpur division); or the presence of particles of salt, molasses and oil on the track remain formidable challenges for everyday operation. Several cases of wheel slip do occur on cold and frosty winter mornings. Cleaning of rails with the help of brushes mounted on the locomotive has been used with some success.

Vehicle-related factors

  1. Mechanical
  2. Loco weight and axle weight distribution
  3. Weight transfer
  4. Speed
  5. Wheel size variation


  1. Method of torque control
  2. Traction motor characteristics
  3. Power circuit configuration
  4. Slip-slide control

The effect of axle load distribution needs no explanation. We have discussed the effect of speed already. Indeed, similar results were observed by SNCF, who came out with the following formulae to calculate the value of CoA (m) at any speed (v) as under. If µs is the static value then at a µv is given by:

µv= µs. (8+0.1. v)/ (8+0.2. v)

The strongest influence on adhesion undoubtedly comes from weight transfer. We will deal with it in some detail.

Weight Transfer

We are all familiar with the case of weight transfer, which happens when brakes are applied to a car suddenly. The rear wheels get offloaded and the front ones get overloaded as can be visualized from the position of springs on the front and rear axles.  Similarly, under the effect of tractive force, considerable weight shift can take place from front axles to rear ones.

In the case of a locomotive, however, the situation is quite complex. Weight transfer in a locomotive comprises three components, each of which depends on a number of features of the locomotive. The features of bogie and body that have a bearing on the weight transfer are summarised in table-2. It is assumed that the transmission of TE/BE is through center-pivot. However, European designs of locomotives generally employ an inclined traction bar for the purpose. This feature helps in reducing the weight transfer as the point of communicating TE/BE between bogie and body is brought down to almost the rail level. Also, traction motor (TM) mounting has been assumed to be ‘axle hung nose suspended’, which is the most common arrangement.

Resultant weight transfer on each axle has to be determined after taking into account each of the above factors appropriately. It involves tedious calculation.

Some designs of bogie (See annexure-II) make the weight transfer very large. The tri-mount bogie used on WDM-2, WAM-4 and WAG-5 is one such example. Some other designs reduce it substantially such as that of WAG-6C, WAG-7, WAG-9 and WDG-4. The trick lies in making the bogie reaction and TM nose reaction cancel each other. A comparison between features of WAM-4 and WAG-7 bogies is given below.

  1. Traction motors are axle hung and nose suspended in both bogies. But WAG-7 bogie has unidirectional noses against two forward /one reverse combination (and vice-versa on the second bogie) of WAM-4.
  2. Primary suspension of WAG-7 consists of sets of equalizers hung directly on end-axles boxes and supported on the middle axle-box through a link and compensating beam arrangement. This special mechanism re-distributes the loads equally on all three axles. Unlike this, WAM-4 has two sets of equalizer beams – one each between either end axle and middle axle – not connected to each other. Hence differential loading of front and rear springs can cause unequal loading of axles.
  3. WAG-7 is provided with a low center-pivot that does not carry any load. Secondary suspension comprising of stiff steel rubber pads is used. WAM-4 has a center-pivot at a higher level and a tri-mount arrangement.

Before taking this discussion further, let us understand the impact of weight shift on adhesion. We already know that the maximum TE that can be applied by a wheel is dependent on the load carried by that wheel at that particular instant of time. If the weight reduces, so does this limit. Further, since the controls of the locomotive are common for all traction motors (This statement is becoming inapplicable with the recent advancements in the technology of IGBT control and individual control of motor). TE produced by each motor should be within this limit. In other words, we can define ‘adhesive efficiency’ as a ratio of remaining weight on the lightest axle and the average axle weight. So, the effective coefficient of adhesion gets limited further by adhesive efficiency.

µeff = µ. η

Based on the results of weight transfer calculations (10), adhesive efficiency values have been worked out for various designs of locomotives as shown in Fig-4. The extent of the impact of weight transfer can be visualised from the values of efficiency. During starting, the performance of a locomotive can be affected severely.

 Torque Control

It is obvious that the smoother the control of TE the better it is. DC locomotives make use of resistance control. WCG-2 improves upon this by using finer ‘vernier control’. Conventional AC locomotives use tap-changer having 32 or more taps and also field weakening in steps for this purpose. A recent modification by RDSO, changes field weakening from 3 steps to 4 steps so as to soften resulting jump in TE for WAP-4 locomotives.

Thus far the discussion was primarily focused on static friction. The remaining factors are concerned with working as close as possible to the prevailing limit of static friction.

Kinematic Adhesion

When TE is applied to a rolling wheel its rotation is slightly increased beyond that corresponding to its longitudinal movement. This happens due to the elastic behavior of wheel and rail resulting in a relative movement known as “creep”. This additional movement increases with TE. Beyond a certain limit, this movement is increased greatly. The later phenomenon was named “Pseudo creep” by SNCF who first observed it. This is also called slip-stick. Any further increase in relative speed represents genuine slipping. Once slipping has started the value of limiting friction and, therefore, the TE that can be transmitted falls rapidly.

Consider if for some reason a wheel encounters a patch of the track with poor adhesion as shown in Fig-6. In such a situation, if the traction motor reacts to reduce the torque sufficiently rapidly so as to reach within limits of reduced adhesion, the slip will be arrested. This is termed as ‘dynamic stifling’ of slip. Else slip will continue to increase. Hence more steeply the TE Vs slip-speed curve declines, more the adhesion stability of the locomotive

The torque-speed curve of a TM at fixed voltage can be represented asτ=k. Ia; where a can vary from 1(shunt)-2(series). Then dt/dv = -a/(a-1). (t/v). Several motors and combinations have been compared in the next section.

Characteristics of traction motors in various circuit configurations

A detailed analysis by Moser (5) yields the results shown in Annexure I.

It can be noted from the analysis that the effect of adding resistors in series and connecting two or more motors in series is to make the motor characteristics shift towards right and response to wheel slip sluggish. This explains the low coefficient of adhesion in DC traction. Similarly, a bogie with high weight transfer, TMs in series and a mechanical governor explain lower CoA(m)  for WDM2. During tests starting adhesion at first slip without sanding was found between 0.152 and 0.26 (average 0.22), adhesion varied from 0.264 at 16 km/h to 0.17 at 27 km/h. Incidentally, the same bogie was adopted for WAM-4.  Fisher’s comment quoted in the beginning may be recalled.  Later developments addressed this issue effectively.

 Why is the DC series motor so popular for traction duty? The answer to this question lies in another equally important requirement of traction duty-namely load sharing. The wheel sizes do vary significantly, as also the characteristics of any two units of TM cannot be exactly identical. Yet load shared by various motors should be nearly the same.

Mechanical Coupling

A study by Swedish Railways compared the characteristics of a Bo-Bo locomotive with individual drive with coupling rod drive 1-c-1 locomotive were compared. As shown in Fig-7, about 15% higher values of CoA(m) were obtained with coupled rod drive. The characteristic of the coupled wheel is also shown in Annexure I.

Such drive offers better adhesion due to two reasons: first, weight transfer from one axle to another does not affect the total adhesive weight of the group of axles that are coupled; and second, mechanical coupling of wheels makes it impossible for a single wheel to slip. This property was utilized in the design of steam locomotives which otherwise give very poor adhesion. Rod drive is a legacy of the steam era. Indeed, the first electric locomotive (EF/1 or WCG-1) had connecting rods between wheels. This 125ton locomotive was able to exert 29 tons of TE.

Mono-motor Bogies

Later, famous French Engineer Nonvion utilized this concept to great advantage in his mono-motor bogies. Essentially, the wheels of a bogie are coupled through gearing. WAG-1, WAG-2 and WAG-4 locomotive have such provisions (See Annexure-II). Though difficult to maintain, these locomotives give exceptionally good adhesion performance.

It is worthwhile to take note of the fact that 3 phase induction motors connected in parallel fed by a common frequency supply give a similar effect as mechanical coupling without the problems of maintaining rods and gearing. This can be called as ‘electrical coupling’.


Consider no-slip prior to the instant t=0 when a slip begins and angular velocity ϖt exceeding ϖo develops. The power developed by TM will no longer be fully used for traction at the wheel rim. Instead, it will be divided into three components: useful traction power, power lost in friction between wheel-rail due to relative speed, and power causing angular acceleration of wheel-axle-TM armature assembly.

Slip-Slide control Systems

Those who have watched steam locomotive drivers starting their locomotives recall the alacrity with which the driver would bring down the regulator handle on noticing wheel slip. This is necessary to keep the situation from aggravating. Conventional electric locomotives use a ‘detect and control’ system for taking care of the wheel slip that may persist in spite of the stifling action of the traction motor. In most Indian electric locomotives, for instance, a relay detects a slip if the difference in currents taken by a selected pair of traction motors exceeds a threshold. It then causes the operating notch of the tap changer to regress, thus reducing the TE to zero. Obviously, such a control system serves only a minimal purpose.

The use of microprocessors in the late 1980s made a superior control possible thus permitting the locomotives to utilise the available adhesion better by working at a controlled level of creep. In 3 phase AC drive with VVVF control, the entire control of locomotive characteristics is achieved by controlling the firing of thyristors /IGBTs in the inverters. The slip-slide control system forms a part of the control system driving the power inverters. Favorable characteristics of 3 phase induction motors along with creep control systems that permit controlled wheel slippage (wheel surface speed may exceed the rail speed) make maximum use of available adhesion.

Though actual implementation may differ in different designs of locomotives, the basic philosophy is discussed here. The conditions of wheel slip/slide base on speed differential or acceleration/deceleration crossing a threshold are tabulated below.

The implementations of the above concepts with necessary refinements can be tedious. The first issue is how to determine the reference values of speed and acceleration discussed above. A commonly used method is to group the traction motors into two groups i.e.1,2,3 and 4,5,6 (Co-Co locomotives). Then compare the speeds of all motors (duly corrected for wheel diameters) and if the maximum speed difference exceeds a predetermined level during traction then the fastest moving wheel is considered as ‘slipping’. Depending upon the extent of slipping, sanding, or reduction in TE or both to the group having the slipping motor is applied. Anti-slip brakes may also be applied to concerned wheels. Similarly, in the case of breaking the speeds of all motors are compared and the slowest one is identified as sliding if the speed differential is more than a precept value. Both in WAG-9/WAP-5 (ADtranz make, electric) and WDG-4 (GM made, diesel-electric) locomotives groups of three traction motors are controlled together. Obviously, the system controlling each motor independently will be more effective.

Determination of these limits for various conditions is a matter of experience. The manufactures utilise their experience to program the logic. A crucial issue is the determination of the vehicle speed. Some designers of locomotives are satisfied with using the RPM sensors only and deriving the reference speeds through calculations. Others provide Doppler radar for measuring ground speed accurately. They claim that this method facilitates improvement in adhesion by making full use of creep. The goodness of a Slip-slide control system is also determined by how fast the TE or BE is restored after it has been reduced on the occurrence of a slip/slide. Because the faster this process, the more the TE/BE usefully applied. While simpler systems use pre-set limits of speed or acceleration differential to judge the stoppage of wheel Slip-slide, others calculate it concurrently fine-tuning the limits to prevalent situations with the help of computers onboard. Also, the rate of increase of tractive/ braking effort is computed by intelligent systems online depending upon the prevailing situation.

Determination of these limits for various conditions is a matter of experience. The manufactures utilise their experience to programme the logic. A crucial issue is the determination of the vehicle’s speed. Some designers of locomotives are satisfied with using the RPM sensors only and deriving the reference speeds through calculations. Others provide Doppler radar for measuring ground speed accurately. They claim that this method facilitates improvement in adhesion by making full use of creep. The goodness of a Slip-slide control system is also determined by how fast the TE or BE is restored after it has been reduced on the occurrence of a slip/slide. Because the faster this process, the more the TE/BE usefully applied. While simpler systems use pre-set limits of speed or acceleration differential to judge the stoppage of wheel Slip-slide, others calculate it concurrently fine-tuning the limits to prevalent situations with the help of computers onboard. Also, the rate of increase of tractive/ braking effort is computed by intelligent systems on line depending upon the prevailing situation.

The above strip chart compares the performance of two slip-slide controls one advanced other ordinary. Basically, the advanced system is able to operate at a higher level of adhesion taking full advantage of creep. It does not reduce torque if the speed variation is a limit calculated on-line. A less advanced system works on the basis of a predetermined limit that is lower. Secondly, unlike the previous system, its reaction to an increase in speed is violent and it recovers relatively slowly.

The values of CoA (µ)(starting & running at 30 km/h) measured by RDSO for various locomotives during performance evaluation are tabulated below.


The technology of vehicles on guided wheels rests on the rolling of steel wheels on steel rails. Adhesion, therefore, is central to railway traction. Hence a deep understanding of the phenomenon and the impact of various factors on it is a must for those connected to the railways or even roadways. Research during the last half-century has, indeed, increased the confidence of motive power designers and operators tremendously. Tribology of wheel-rail system continues to be a subject of active research abroad with a view to facilitating higher train speeds and eliminating the occurrence of wheel flats and extending the life of rails. These concerns are no relevant to us. Given the importance of the subject, the relative lack of published literature in India is a matter of concern. The subject deserves the attention of academics and practicing engineers since the failure of adhesion and consequent damages continue to cause concern.

The evolution of the design of electric locomotives in India from WAM-4 in the late 60s to WAG-7 in the late 1980s represents an increase in power to weight ratio from 33 hp/t to 41 hp/t. Surely this has been possible with measures taken to improve adhesion primarily improved bogie design and parallel connection of traction motors. To perfect the design, incorporation of a slip-slide control system is necessary. RDSO is in process of developing one microprocessor-based system, which will modulate the traction power by injecting additional field current into slipping motors. An efficient and reliable Slip-slide control system will enhance the performance of the locomotive.

Similarly, WAP-4 has been evolved within the tap-changer-DC series motor framework. Recently dynamic braking has also been provided. Management of adhesion at high speeds is a must for ensuring reliable performance. This need becomes all the more critical with dynamic braking.


  1. Andrews, H.I. Chapter III and IV in Text Book titled “Railway Traction” Elsevier Science Publishers. 
  1. Dorairaj, Dr.K.R.“Twenty-five years of AC Locomotives in India” Paper presented at IREE/ASME Joint Railroad Conference, New York April 1985.
  1. Fisher G T “50 KV through the rockies” Railway Gazette International, Oct 1971 as quoted in reference 2. 
  1. IRIEEN “Weight transfer and its compensation in locomotives” article in IRIEEN Journal Vol. X, July-Sep 1999. 
  1. Moser R.“A comparative study of the various types of electric traction motors in their specific fields of application” Paper in Brown Boveri Review, Dec. 1978.
  1. RDSO, Lucknow“Mechanical Engineering Reports” for various electric and Diesel electric locomotives e.g. M-162,  M-164, M-`98 and M-214 MT-111, M-531, MT-260, 
  1. RTRIQuarterly Review Sept’1998 Vol-39. 

8.  Tyagrajan, L An article titled ” Adhesion and Wheel Slip”. 

  1. Westinghouse Brakes Limited   “Railway Brakes” 
  1. Weight transfer calculations for different designs of locomotives.

WDM2 – ALCO’s Standard calculation code.

WAM-4/WAG-5 By suitably adopting the above.

WAG-6C _Hitachi’s document no T-6366 and further calculations by shri Himanshu Sharma and Shri Yogesh Kumar, IRSEE, 1999 batch probationers.

WAG-7- Calculations done by Sri V. Santhanam, then DSE,RDSO.

WAP-4- Calculations done by Sri G.K. Saini and Shri Raghwendra Kumar,IRSEE-1999 probationers.

WDG-4- Adopted from GMs calculation code.

Annexure -1

(Adopted from Moser (ref.-5)

Slope DT /D of various motor characteristics (including voltage drops but without regulation)

A = Normal operating range of a traction motor.

B = Principle of a regulated curve for F = Constant (traction vehicles with tractive effort regulation.)

Right-hand side: Behaviour on starting.

Left-hand side: Behaviour of the traction motors at a speed of 0.25 x Vmax

  1. = D.C., two motors in series, with starting resistor.
  2. = D.C., two motors in parallel, with starting resistor.
  3. = D.C., two motors in parallel, without starting resistor or chopper – controlled.
  4. =Pulsating – current motor with series excitation including transformer and possible smoothing reactor.
  5. = Pulsating – current motor with mixed excitation, series portion 50% (including voltage drops in transformers, rectifier and smoothing reactor )
  6. = 162/3 Hz single – phase AC motor including transformer.
  7. = 162/3 Hz single – phase a.c. motor with cross-connection for the motor fields including transformer.
  8. = Three-phase asynchronous motor with fixed stator frequency.
  9. = Mechanical coupling of the wheelsets.


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