Let’s continue with our motor management topic, part 2. In my previous blog post, the electrical, thermal, and mechanical constraints necessary to consider for large motor starting have been presented in detail. We will focus now on the main motor application characteristics impacting the selection of motor starting methods.
So, what are the motor application characteristics to consider before selecting the starting method?
There are two main types: mechanical and operating characteristics. Application torque and inertia are the main mechanical characteristics.
The above figure shows various forms of load torque evolution with speed. By far, quadratic torque applications are the most frequent in number and suitable for all the starting methods — direct on line, progressive start, and variable speed drive. Applications with that kind of torque include pumps, fans, compressors, and practically more than 70% of applications. The other torque shapes are seen in electro-intensive segments for conveyors, extruders, positive displacement pumps, crushers, mixers, etc. These applications generally use direct on line or variable speed control and starting.
Another important mechanical characteristic is the application inertia. The inertia can be defined as a dynamic, acceleration-dependent torque acting as a damper for the speed variation. The inertia is mathematically proportional to the mass of the object, so larger applications will have higher inertia than smaller ones of the same type. For motor starting, the inertia increases the starting time and the thermal stress on the motor.
High inertia applications are typically fans for which the progressing starting solutions such as soft-starters or auto-transformers need to be carefully sized and set up to avoid overheating.
Operation-related characteristics are the frequency of starting, acceleration time or speed control, application type, and flow control needs. Frequently started motors will require some adaptation in the starting method to smooth the start-up and damp the mechanical and electrical constraints. When acceleration time or speed control are required, a variable speed drive will be selected. For pumps, the well-known water hammer effect on starting and stopping will impose the use of progressive starting. Requirements in flow control may also impact the starting mode. Variable speed drives can easily perform starting and stopping of the motor and in addition vary the flow, bringing energy savings.
In the next post in this series, I’ll compare the motor starting methods with examples.