Control and electrical protection of electric drives train
Control and safety elements around electric drives depend on the application itself. Through this blog post, we will go through some applications of electric motors and different requirements depending on the environment and the task itself. The electric drive trains themselves can have several functions depending on the state in which the system is operating. It can perform useful work , or it can maintain the environment conditions for other systems to perform work, or to start a system that protects people etc.
In systems that do not have a safety function, the goal is to use as little energy as possible, to inform dependents of the system about the state of operation and not to affect other electrical consumers.
Some of the essential characteristics of the electric drivetrain:
| Duty type of electric motor (according to HRN EN 60034-1) | continuous running S1, short-time S2, intermittent periodic S3-S8 |
| Motor type | DC, AC, special |
| Power cable type | fire-resistant, self-extinguishing, shilded, non-halogen |
| Drive type | AC drive (VFD), special drive, Soft Starter, star delta, direct online |
| Power supply type | diesel aggregate, grid from two transformer stations, grid |
| Control location | local, remote |
Example of car electric drivetrain
Electric car’s drivetrain consists of an electrical motor and drive that enables control of the speed and torque that is transmitted to the wheels of the car. The car’s electric drive train is expected to be capable of offering high torque at low speed for starting and acceleration, and high power at high speed for cruising. It is evident from the requirements themselves that the input and output parameters depend on the type of driving, which sets high requirements for the electric motor drive that can only be realized with a fully controllable drive, which puts simpler ways of controlling and powering the motor out of use for this application.
Example of ventilation in tunnels
Fans in tunnels have two functions, the primary of which is safety and has requirements for continuous operation in the event of an accident. While in case of secondary use for hygienic tunnel ventilation. These two functions require a different approach to motor protection. In the primary function, it is necessary to ensure the operation of the fan as long as possible, even at the cost of destroying the fan or motor. In the secondary function, the electric motor should be protected against failure in order to prevent unnecessary downtime for repairs and to reduce unnecessary costs. When we look at the side of the protective elements in the primary function, none should work, and in the secondary function, the minimum overcurrent and overload protection should work. The most modern way of starting an electric motor with a frequency converter offers the most benefits for the function in question, where in addition to possible savings due to the possibility of adjusting the operating point, it has a safety function of working until self-destruction (Fire mode). As a function of the drive’s operation until self-destruction, it manages to work longer than an electric motor powered without protection. In case of impossibility to install the frequency converter, it is necessary to make sure that in the other starting modes, the motor overload protection is switched off from the control during safety operation. In Croatia, the recognized standards for designing ventilation in tunnels are the Austrian RVS standards. Through the relevant norms that have been taken over into the Croatian legislative, the kind of power supply and management that needs to be ensured is defined. Aggregate power supply is not used as a backup, but mains power from two separate lines is required.
A similar fan power function is required for smoke extraction of stairwells in buildings. Since in the event of a fire, the smoke extraction system must work as long as possible to ensure safe evacuation and extinguishing of the fire. While under normal conditions it can occasionally be used to ventilate the staircase.
Example of heating pumps in hospitals
As heating and cooling in hospitals provides the necessary climatic conditions for life-important rooms for patients, it is necessary to ensure that the pumps that bring the heating medium to the end rooms provide continuous power supply. Through the norm “Low-voltage electrical installations — Part 7-710: Requirements for special installations or locations — Medical locations (HRN HD60364-7-710)” define electrical installations in the hospital. It is not a problem for the pumps themselves if they remain without voltage for a short time, since heating is a slow system. On the other hand, the control system of the pumps must not remain in error due to a short-term voltage drop, but must continue the operation of the pumps after the voltage comes from the UPS and aggregate power supply. In order to reduce the risk of a pump failure interrupting the heating, a spare (service) pump is designed. With modern pumps that are designed to work in pairs, a smart management system on the pumps themselves enables the division of working time on both pumps equally. This function also checks that both pumps are in operating state. It is important to know the pump readiness because when a pump that doesn’t work for a long time it could break down when it needs to start in case of failure of the first one.
Example of the screw conveyor in dedusting system
The screw conveyor in the dedusting system (air filtering in industrial plants) has a requirement to be as efficient as possible in order to consume as little energy as possible. Its electric motor needs to be protected with classic motor protection, namely overcurrent and overload protection. When controlling, its operating point can be achieved by selecting the correct reduction drive or by changing the speed of rotation using a frequency drive. Since in case of failure or overloading of the electric motor drive of the screw conveyor in the system in question, it is important in the control logic to stop the fans so that the filter in which the screw conveyor is located does not fill up with filtered particles. The frequency drive enables the reduction of energy consumption in this example, but in smaller systems due to the low power of the motor, it does not justify the investment with a long return on investment. This example of an electric motor drive is powered from the grid supply.
Whether local and remote control is applied mostly depends on the size of the system and the required interaction with other systems. In any case, the selection of the type of control must be made locally so that during service there are no unwanted injuries to a person in the machine working zone. In the case of more complex electric drives, it is necessary to ensure the possibility of maintaining the disconnected state using a code, so that only the person who turned off the system after service could restart the system.
When choosing a control system, it is necessary to take into account the dissipation of thermal energy that should be removed from the device. This creates a particular problem in closed systems such as tunnels or areas threatened by explosions. The biggest producers of thermal energy are power electronics circuits that are found in frequency drives and soft starters. Versions of soft starters with a bypass reduce heat dissipation because the power circuit works only at the moment of start-up.