The invention relates to a method for the reliable control of a braking torque of a drive system with a synchronous machine. The invention further relates to a drive system and a vehicle, in particular a rail vehicle with a drive system of this type.
Drive systems which have a power converter and a motor, also designated an electric machine, can make a desired torque available very precisely at the shaft of the motor. This concerns both torques for accelerating the motor and braking torques for braking the motor.
This matter will be considered below in detail using the example of rail vehicles. Nowadays, rail vehicles are mostly electrically powered. A motor provides that the vehicle can accelerate and also brake. During braking by the motor, electrical energy is fed back and is then conducted into the supplying network or is dissipated via braking resistors. In addition to the electrical brake, the vehicles also possess a mechanical brake, also designated a friction brake, with brake disks or wheel brakes. This second mechanical brake device is therefore needed above all to be able to brake the vehicle on a failure of the electrical brake reliably. The present-day drive systems function so reliably that the mechanical brake is still practically only needed for holding braking at standstill but nevertheless must be configured for the entire rotary speed range.
Regulations require a reliable brake for the vehicle, which brings the vehicle reliably to a standstill in every state of the vehicle. Nowadays, the mechanical brake which is provided for this purpose has a clearly evident construction, from which the operation is readily evident. However, it is subject to wear, so that it requires regular servicing.
With the present-day regulation of the power converter, within the nominal values of the machine and the current converter any desired torques, and therefore also braking torques, can be set. However, the regulation of the power converter is very complex due to its many input variables and internal calculation steps. For this reason, error-free regulation for all the possible vehicle and operating states is extremely difficult to achieve. Thus, the power converter with its present-day regulation is suitable only as an operating brake, which is complimented with a mechanical brake configured for the vehicle and its characteristic values, to ensure that the safety requirements are met.
From DE 101 60 612 A1, a traction drive with a traction power converter and a permanent field synchronous machine is known. With a synchronous machine, also known as a synchronous motor, a braking torque can be simply created in that the terminals of the synchronous machine are connected to resistors. The document describes the changeover to a resistance group in order to brake the vehicle reliably. The braking torque adapts via the parameters of the machine and the resistance value, as well as the rotary speed.
From US 2012/0043817 A1, a drive system for a use with an isolated alternating current network is known. This drive system has a three-phase electrical machine which can be operated as a motor or a generator.