Modern Drive Technology Helps Streetcars Roll Again

New power and automation technologies, such as smaller form factor inverters and hybrid drive systems, are behind a resurgence in mass transit manufacturing and deployment in the United States.

Outside of areas such as downtown San Francisco and New Orleans’ Garden District, streetcars are most commonly visible today only in black-and-white photos from a bygone era. But over the past several years, the nearly extinct U.S. streetcar has been quietly making a comeback in cities such as Portland, Tucson and Seattle. Atlanta will soon be added to that list.

The Metropolitan Atlanta Rapid Transit Authority (MARTA) has awarded Siemens Industry Inc. a $17.2 million contract to provide Atlanta with four new streetcars. Manufacturing will be performed at Siemens' railcar and locomotive plant in Sacramento, CA (a facility powered primarily by two megawatts of solar energy) with major components, such as the propulsion system and auxiliary power supply, being built at Siemens manufacturing facility in the metro-Atlanta suburb of Alpharetta. 

Street Power
Siemens’ Technology Drive facility in Alpharetta is the company’s center of competence for hybrid drives and rail propulsion systems in the United States. Traction systems comprise 70% of the business at the Technology Drive location, with the remaining 30% comprised of industrial drives.

The term “traction systems” refers to the inverters and motors that power mass transportation systems such as hybrid buses and rail applications. Siemens focuses mostly on IGBT (insulated gate bipolar transistor) power semiconductors with integrated PWM (pulse width modulation) control for its traction inverters. However, the Technology Drive facility still constructs a few GTO (gate turn off) thyristor drives for certain customers in the mining industries. The traction inverters manufactured by Siemens regenerate energy released during electrical braking to supply power back to the vehicle’s energy supply. 

Inverters for two types of light rail vehicles are manufactured at the Technology Drive location: the SD160 and S70. 

The SD160 light rail vehicle is used in Calgary, Denver, Salt Lake City and Edmonton. The T-shaped traction inverter cabinet, which also contains the auxiliary power supply, is affixed to the underside of the light rail vehicle. As a result of this positioning, the higher floor level of the vehicle means that a platform is needed to allow passengers to enter and exit. 

S70 is a smaller form factor light rail vehicle originally designed in the United States and is currently in operation in Houston, Charlotte, Portland, Norfolk/Hampton Roads, Salt Lake City and San Diego. This will be the system used in Atlanta as well. The auxiliary power supply of the S70 is housed in a separate cabinet. An important aspect of the S70 is that the inverter and auxiliary power supply can be mounted to the roof of the light rail vehicle, allowing for the design of a low-floor vehicle so that passengers can board directly from street level. The smaller size of the S70 also allows for creation of a shorter rail vehicle, which can reduce issues sometimes encountered during passenger stops where part of the vehicle extends into the intersection. 
Standard nominal voltage for the S70 is 750 V dc delivered via suspended catenary wire; low voltage power supply is 29.4 V dc. The 750v coming from the wire actually powers the propulsion inverters, which in-turn provides the power to the electric motors. The low voltage power supply also receives power from the line and, in turn, provides power for lower voltage equipment such as HVAC, lighting, and communications equipment. 

Similar to the S70 is Siemens’ ELFA hybrid drive. These drives, for which all components are manufactured by Siemens, consist of inverters and an overhead control that propel electric motors used on buses and trucks as well as seagoing vessels such as yachts. The power for this propulsion comes from energy storage in conjunction with either an internal combustion engine or fuel cell. Today there are more than 1500 ELFA hybrid drives in use worldwide.

ELFA hybrid drives provide a flexible design used mostly in a series hybrid setup where the internal combustion engine, combined with an ELFA generator, serves as the drive’s power source (this can be thought of as a “smart genset” and can be replaced by a fuel cell). In a parallel hybrid setup, the electric propulsion motor is coupled in parallel with the internal combustion engine so either can be used as needed. For the series setup, the electric propulsion motor is decoupled completely from the internal combustion engine, leaving the flexibility to locate the engine in the most convenient place or remove it completely and run on battery or fuel cell only.

Bringing Back the Streetcar
The first of the four new S70 streetcars for Atlanta is planned for delivery in September 2012 with service beginning in early 2013. These streetcars are part of the first phase of Atlanta's project to create a regional streetcar and light rail transit system. Initially, the streetcars will be used to bridge the gap between east and west downtown areas – a division created by the development of the I-75/I-85 connector that runs through the center of Atlanta. The connector was built in the late 40s and, not surprisingly, Atlanta street car use was discontinued in 1949.

The streetcars being built for the Atlanta project will be customized to meet Atlanta's operating environment. For example, the heating, ventilation and air conditioning will be modified to ensure compatibility with the Atlanta-specific environmental conditions, such as heat and humidity. The streetcar is built with an eye toward the potential use on future light rail systems and can enable multiple streetcars to be coupled together to operate as train sets at higher speeds, providing higher capacity and reduced trip time as the network grows. 

Although capable of reaching speeds of 66 mph, for streetcar use, the operational top speeds of the S70 has been reduced to 35 mph. 

Click HERE to read an in-depth description of Atlanta's new S70 Streetcar.


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