KNOW MORE OF THE DMU/EMU/MEMU. HOW DO THEY WORK??

Electric multiple unit (EMU) or Diesel multiple unit (DMU) 

Electric multiple unit (EMU) or Diesel multiple unit (DMU) refers to coaches/cars used for mostly suburban areas. These trains have multiple self propelled coaches/bogies either electric motors or diesel for each car.  As opposed to the convention where the passengers are in coaches that are not self-propelling and a locomotive hauls the train, in these cars that carries passengers also has the motive power. EMU/DMU have TRACTION motors installed to them in the bogies itself which propels the cars.

Following article will explore mainly about the shelf propelling bogie i.e DMC (diesel motor car) of an EMU/DMU.

image soucre

an EMU at Rail Coach Factory

WHAT ARE MULTIPLE UNITS (MU) ?

MULTIPLE UNITS are self propelled train coaches/bogies which are coupled with same or similar type of coaches.Traction motor is installed on the bogies which itself  They are called MULTIPLE UNITS because they are capable of operating with other units also.Beside hauling with a heavy locomotive, a number of traction motors are mounted on bogies of the coaches in a particular manner.They are interdependent on each other, it operates only when the right type of coaches are coupled in a right way. They can be operated from single cabin but driving controls are provided on both ends.

             These are classified on the basis of their power source, mainly of two types:-

• DIESEL MULTIPLE UNIT 
• ELECTRICAL MULTIPLE UNIT

DIESEL MULTIPLE UNIT

DEMU's :-  Diesel Electric Multiple Unit

DHMU's :- Diesel Hydraulic Multiple Units

HPDMU :- High Power DMU

     

    All of these are powered by Cummins prime mover , DEMU has a prime mover of 750 HP and DHMU having two prime mover having 350 HP each. All of these are manufactured in mainly 3 coaches formation but most of the time they are coupled with other units to cater the demand of the commuter traffic. The control system are made suitable for multiple operation of one, two and three unit formation. Nowdays ,  different formation are also used as in High Power DMU(mention below)

     
  DEMU( Diesel Electric Multiple Unit) , it works on the same principle as the diesel locomotives. Diesel genetator of 750 HP produces electricity which is fed to the traction motors of the power car and also in the trailer car having traction motor. Usually traction motors are  provided at both ends of the rake. A new aerodynamic design of DEMU introduced in 2004 on the SCR. It consists of 8 car rakes (Two motor car  and six Trailer cars ).

         DHMU (Diesel Hydraulic Motor Unit) , In this hydraulic torque converter ( a fluid coupling) is used to transmit the motive power of diesel engine to the wheels via propeller shafts and differential. The formation comes in 3,6 and 9 coaches. 

         HPDMU (High Power DMU) ,introduced in 1999 by ICF and  it has a prime mover of 1400HP and can 40% more passenger than conventional DMU's. Formation of 4 cars ( one power car and three trailer car ).

DMU rake (source)

ELECTRICAL MULTIPLE UNIT

EMU are preffered for suburban section as it have high accerleration and phenomenal stopping distances allowing them to  achieve higher speed at a short time and stopping efficiently hence it ultimately offers high throughput. Mainly two different emu's are in use , these are

EMU rake (source) 

• 1.5 KV DC traction 
•  25 KV AC traction. 

But due to switching from DC to AC traction power , EMU's with AC-DC traction is introduced. ICF manufactured all the EMU's ( AC-DC EMU rakes). WR signed contract with ALSTOM  to change the DC 1.5 kw EMU to AC-DC  traction power. SIEMENS and BOMBARDIER are also supplying the EMU's to the Mumbai but all the bodies are of ICF. 

MEMU's known as Main Line EMU. It was first  introduced in 1993 by ICF so that it can be operated in more areas. Unlike the EMU's they have vestibule connection (gangway connection) and lavatories. MEMU motor car seats 76 and trailer coach 108 with a top speed of 105 kmph. They are equipped with  electro-pneumatic brakes.

MEMU rake (source)

FORMATION OF AN DMU/EMU

DMU :-  

Most commom formation has 3 cars. These are

• DRIVING POWER CAR(DPC) :- The coach having traction motor/transmission and also the prime mover. Driving represents the presence of pilot cabin. Due to the prime mover , it can accomodate less number of passengers as compared to TRAILER CAR.
• DRIVING TRAILER CAR(DTC):- The coach having traction motor/transmission and pilot cabin but not the prime mover. 
• TRAILER CAR(TC):- The coach having no traction motor.

Sometimes when the traffic is higher , formation of 6 coaches and 9 coaches used. Now even 12 coach formation is also commisioned. 

        In case of HPDMU the formation is of 8 and 12 coaches runs. But nowdays due to more traffic, IR decided to only upgrade the power car (high power) of old units so that more trailer cars  can be added. so the formation nowdays are not specific.

EMU :-

In all these formation , the basic formation consists of 3 cars which are coupled together. These are 

• DRIVING TRAILER CAR (C)
• MOTORCOACH OR POWER CAR (B)
• TRAILER COACH (A)

Different EMU formation is in use in different areas. In 90's 4 car emu were also commisioned, sometimes they were coupled with 3 car unit. In mumbai 9 car formation was also a standard for long time. But later on due to increase in number of passengers 15-16 car formation are used. 

MEMU has most commonly 12 car formation in which one power car for every three coaches. Multiple pantograph is used in a single rake, each power has its pantograph mounted on the roof of same car.

WHY TRACTION TRATION MOTORS?

A 12 cars train draws about 4,000 amps from the grid. two incoming & leaving trains at a station would approx. require about 10,000 to 15,000 amps of currents. due to such huge requirement of the current , small traction substations would be needed to install at intervals in a route. 

Therefore AC/DC traction motors were only solution due to its low current requirement and the substations also can be installed at a larger distances.  

AC/DC COMPARISON

• ac travels large
• less substation
• cheap
• ac/dc both traction motors are available
• dc requires rectifier unit to convert ac to dc for operation
• ac motors can run directly through ohe supply via invertors
• more details at icfbogie.com

DIESEL MOTOR CAR (DMC)

An icf frame based bogie

 GEARED AXLE OF DMC

AIR SUSPENSION

WHY AIR SUSPENSION?

Air suspension is used in this type of bogies just because of load variation. In conventional bogies helical spring are used these are designed for a specific load but in these types of coaches passengers load is more.It provides

• Better riding comfort
• Improved reliability and reduced maintenance
• capacity to sustain super dense crush load
  
  
  
  

COUPLERS USED IN EMU/DMU

SCHAKU COUPLER:

Originally designed by M/s Scharfenburgkupplung, GmbH, Germany are used on the EMU stock of  Indian Railways.

In Semi permanent couplers the coupling is done manually and the two couplers after being aligned are secured by means of an adjustable cup sleeve joint. During the process of manual coupling the air pipe connections between the two coaches are also established automatically. The pivoted joints between the coupler and the coaches is designed and constructed in such a way that even in case of maximum difference in level, the coaches are capable of negotiating all track curves and of passing through points of change of gradient occurring during service. This coupler fulfills all other operational requirements.

For more details refer to 

TYPES OF COUPLING SYSTEMS USED BY "INDIAN RAILWAYS"