How to choose the suitable traction motor?
Three-phase motors have been state-of-the-art in traction applications since the evolution of power electronics. The key benefits of their use are the high power density, high efficiency and minimal maintenance. The asynchronous motor is the most widely used motor type today. Alternatively, permanent magnet synchronous motors have become an increasingly popular alternative solution because of their high efficiency in special operating conditions and environments.
Basic aspects for traction motor selection
Space is commonly limited in traction applications, so available space becomes an important consideration during vehicle propulsion design. The required size of any electric motor is mainly defined by the required torque and less so by power.
Common operating scenarios
Torque and power values in motor datasheets are commonly offered without considering thermal limits. Time limitations have to be assessed separately in order to compare different motors and to prevent overheating. One exception are operating points marked with ‘S1’ (usually also the nominal or rated point). S1 operation means that this point can be driven continuously if the specified cooling is ensured.
On the other hand, we design our motors according to the operational demands that your vehicle actually will be operating within. You send us operational simulation data and we calculate the energy consumption and the motor temperatures. According to this data, we can exactly tell whether the selected motor is suitable for the routes you want to travel.
Torque and power density requirements are very high in modern traction motors, therefore a proper cooling concept is crucial to avoid overheating during operation. Liquid cooling or forced-air ventilation using an external blower unit are most common. Motors with self-ventilated air cooling use a fan directly attached to the shaft, so no external cooling system is required.
Voltage and current
At the concept stage of a new propulsion design, the general voltage level is of minor importance as the motor can be adapted by changing the electrical winding accordingly. However, knowing the motor voltage is helpful for current calculation which will have a considerable impact on the converter design.
Principle design of an ASM
Asynchronous machines (ASM)
Asynchronous machines with a short-circuit rotor winding (‘squirrel-cage’) offer a rugged design. A converter and a cooling device are needed for operation as well as speed and temperature sensors. The traction converter can adapt the magnetic field inside the motor in order to ensure optimum efficiency under all operating conditions. Two (or even more) motors can be connected directly in parallel to one converter without additional equipment. Optimum efficiency can be achieved by the best possible coordination with the operating mode of the inverters.
Principle design of an PMSM
Permanent magnet synchronous machines (PMSM)
Permanent magnets create a magnetic field in the motor’s core of a PMSM. The highest efficiency values are achievable at low speed or utilization of full torque capability. Efficiency at higher speeds and partial load are reduced due to drag losses, caused by the permanent magnetic field. Motor control requires the knowledge of the actual rotor position. Each PMSM of a vehicle requires a separate converter. This is obviously no limitation for vehicles using only one (central) motor.
How to choose the suitable traction alternator?
In the end, it depends on your overall concept and your requirements. Learn more about the different functionalities of our permanent magnet synchronous generators and asynchronous generators in this chapter.
PMSM – The functional principle in a nutshell
TSA-made permanent magnet synchronous generators (PMSM) are equipped with high-energy permanent magnets inside the rotor. The machines are totally enclosed and liquid-cooled. The PMSM is excited by the permanent magnets only. So there is no separate electric excitation device in the PMSM, nor are there any connections for controlling the excitation.
When turning the rotor with terminals open, an electric voltage will always appear at the terminals due to the permanent magnets. If the PM generator is rotating and a conductive connection is present, electric current will always flow when the machine is rotating, both in normal operation and in the event of a short circuit.
The electric voltage depends on the speed of the machine. Higher rpm equals higher voltage. The voltage is also dependent on the load – the higher the load, the lower the voltage. Finally, the voltage depends on the actual temperature of the magnets. While the voltage is higher for cold magnets, it decreases when the machine becomes hot. The amount of this effect depends on the magnet material and should be considered during the design phase.
The PM synchronous generator is normally used together with a passive rectifier (B6 diode bridge), but operation with an active rectifier converter is also possible. In most applications, the passive B6 diode rectifier is used – basically only 6 power diodes are needed to convert the 3 phase AC voltage of the generator into DC voltage. The simple design of the power electronics is one of the major advantages of the PM generator over the asynchronous generator. However, the output voltage of the machine can only be changed by changing the speed of the machine. To take account of the voltage variation caused by the load or by the magnet temperature, a wider range for the allowed DC operating voltage is favourable.
Principle design of an PMSM
ASM – The functional principle in a nutshell
TSA-made asynchronous generators are equipped with squirrel cages made of copper bars and rings. There are neither permanent magnets nor slip rings. When turning the rotor with terminals open, no electric voltage will appear at the terminals.
For the generator to produce electrical power, it is necessary to create a magnetic field inside. For the asynchronous machine however, it is essential that the stator winding is connected to a 3-phase AC voltage supply.
For traction applications where no public grid is available, this 3-phase supply is provided from a DC source by an electronic variable voltage variable frequency converter (VVVF). Once this 3-phase supply has been connected to the asynchronous machine, the traction alternator can deliver electrical power to the converter. To simplify it: The converter provides the grid, the asynchronous machine forces the current to flow to the converter. It is not possible to simply connect the asynchronous machine to a passive rectifier (B6 diode bridge).
Principle design of an ASM