Where are traction motors used in vehicles?
ABB has now launched a modular concept for traction motors. The new, optimized motor is based on standard modules and can therefore be adapted to customer specifications much more quickly than conventional motor designs.
A lot is demanded of traction motors for driving rail vehicles: They not only have to withstand difficult operating conditions such as pulsating modes of operation, strongly changing loads, different weather conditions, dust and dirt, but should also be energy-efficient and have a service life of 40 years.
This often leads to specifications with conflicting requirements that are difficult to meet. So it's no wonder that the manufacturers of locomotives, multi-part railcars, trams, underground trains and other rail vehicles usually develop their own concept for the optimal traction motor in each case.
ABB, a manufacturer of traction motors since 1909, saw this as an opportunity, but one where tradition had to be broken. "We saw that we were competitive in this market and that we could grow," says Peter Isberg, technical director for traction motors. "The challenge here was to set higher standards while creating a level of standardization and flexibility in traction motors that had never existed before."
Isberg and his team thought of developing an electric motor that should be as energy efficient as possible. “We wanted to build a compact motor that efficiently delivers both high power and high torque,” says Isberg.
The more power you get out of an electric motor, the lower the efficiency. The reason for this is the rise in temperature in the active components of the engine. One way out was to make the engine bigger, but that would have run counter to the goal of the project. Instead, the team focused on optimizing the electrical design. An important point here was the thermal behavior of the motor when it is supplied with the voltage and frequency required to control the motor power from a converter that converts alternating or direct current into square-wave pulses of different frequencies. The converter and motor had to be designed as a system based on the operational characteristics of a particular train application.
ABB has its own FEM software for electrical design which, in combination with software for modeling thermal networks, can simulate the expected motor temperature during operation with great accuracy.
A converter can also generate a voltage where the phases in the motor winding are interconnected (common mode voltage). This in turn leads to another technical problem: bearing currents (current flow through bearings that can cause damage).
"It is our job to inform the customer about the risk of bearing currents, to show various technical solutions and to present the alternative that is safe," comments project manager Henrik Carlsson. "In this case, our choice fell on hybrid bearings."
Hybrid bearings have steel races and ceramic rolling elements that act as insulators. This makes it practically impossible for a current to flow through the bearing under normal operating conditions. In addition, the relubrication intervals are significantly longer than with pure steel bearings.
“These two advantages were enough to convince us of this storage solution,” continues Carlsson. Although hybrid bearings are more expensive, the rail operator saves costs thanks to the higher reliability of the bearings and fewer operational downtimes. A traction motor is usually located in the bogie. In order to be able to remove the engine for maintenance purposes, the car body must usually first be separated from the chassis.
During our tour of the well-organized factory hall, Carlsson jokes with one of the drivers. The atmosphere is professional yet informal. A feeling that is reinforced by the size of the plant, which, given the production volume of over 3,000 engines per year, seems quite small.
We stop at a traction motor that is currently being assembled and take a closer look at the standardized modules: variably positionable junction boxes, sensor and cooling devices, brackets and, last but not least, a scalable design. With a basic concept, ABB can meet a large number of customer specifications. “We are eagerly waiting for the first trains with engines from the new platform to go into operation,” says Lars Fredrikson, who works in ABB's marketing and sales department.
The new engine shouldto be used for the first time in 2011 in Caracas, Venezuela. The customer is the Spanish company CAF / Trainelec, which will install 1,100 modular traction motors in subways for the Caracas Metro.
“CAF / Trainelec has chosen us as a new supplier for traction motors,” explains Fredrikson. “There are a number of engine manufacturers, which means that the competition is fierce. In this case, a reliable construction and short delivery times, which we were able to offer thanks to the modular design, were decisive. "
Longer on the rails with SKF hybrid bearings
When ABB decided to develop a modular concept for traction motors, the company turned to SKF for bearing solutions.
The two main objectives of the project were to minimize downtime by eliminating harmful currents through the bearings and to reduce maintenance costs through longer relubrication intervals. ABB also wanted to reduce the number of storage types in order to benefit from higher purchase volumes in the long term in terms of price.
The two companies opted for a flexible bearing concept and ended up choosing different SKF hybrid bearings, each best suited for each type of ABB traction motor.
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