Large-scale electric drives are exposed to high levels of stress on start-up. Frequent starts without a sufficient interval for cooling can result in motors over-heating. Measuring the temperature of the actual rotors turning within the motor is complicated.
To enable reliable measurement results to still be obtained without resorting to estimated values, Siemens has developed a simulation known as the “virtual sensor” which is able to calculate the temperature inside the motor on the basis of mathematical models. This allows the cooling times required for electric motors to be significantly reduced, enhancing plant availability.
Motors which run hot not only pose a risk of damage at the drive itself, but can put the whole plant out of action. The possibility of overheating presents a real problem, particularly in large-scale electric motors whose rotors are exposed to extreme thermal loads on initial start-up. This makes it important for plant operators to be able to monitor the temperature situation on the inside. As measuring this is only possible with a great deal of difficulty if at all, generally the necessary cooling down times between two starts have always tended to be conservatively estimated. This involved using analytical and numerical design tools to determine the temperature distributions and associated limit value violations for worst-case scenarios.
To prevent critical violations, cooling times were then hard coded and stored in the control cabinet depending on the number of start attempts, meaning that adjustment to changing ambient and operating conditions was not possible. The “virtual sensor” simulator from Siemens allows more precise times to be stipulated. It calculates the temperature of the motor during its operation just as accurately as if it were measuring directly, by logging the current conditions and statuses and using this information to predict the moment at which the drive can be safely switched back on.
How the virtual sensor works is demonstrated using a table-top model. Two small electric motors are coupled by a shaft. One motor is rotation speed controlled, the other brakes the first one, creating a permanent load. While temperature sensors measure the outer temperature of the motor, data relating to operating time and load is logged.
These input parameters and a mathematical model of the motor supply the simulator with the basis for calculating the motor temperature. Depending on the type of motor, the temperature can reach a critical level at different speeds.
A noteworthy feature of the simulator is its speed: It is capable of measuring and supplying the data from inside the motor in real time. Siemens is benefiting from the work of its colleagues in the process industries and drives division, who use mathematical models as early as the motor engineering stage to log the geometry and materials required to determine the characteristics of the drives they design. With this simulator, the company is offering a development with potential for monitoring the temperature of electric motors.
Contact Jennifer Naidoo, Siemens, Tel 011 652-2795, firstname.lastname@example.org
Source: EE plublishers