Motor Inertia and Motor Starting Time
Understanding the importance of Torque-Speed curve and load inertia for motor selection
Moment of Inertia (Also referred as GD 2 or GD Square)
Moment of Inertia is a measure of an objects resistance to changes (to make it rotate from standstill and to accelerate). It depends on the bodys mass and axis of rotation. Higher the inertia, higher is the force required to change its rate of rotation. Moment of Inertia is equal to GD2 /4 and its unit is kgm2.
Applications like Pumps have low inertia and application like Fan and crushers are with high inertia
Torque-Speed Curve
The torque speed curve of a load is a plot showing its torque requirement on the y-axis versus the speed on x-axis. Its the torque requirement of the load from zero speed to full speed. Its application/load specific and varies with different load types. Loads like centrifugal pumps and fans follow quadratic speed torque curve, where torque varies square times the speed. Other loads like conveyors, crushers have constant torque requirement from zero speed to full speed. These are also called as constant torque loads or applications.
How these details are used for motor selection
The above details of Moment of inertia and load torque speed curve determine the starting time of the motor (with load) and this along with the starting current plays a major role in motor selection.
Method of starting and its relation to GD 2 and Torque speed curve:
We have different starting method requirement from the customer depending on factors like network availability and capex. Four major methods used for starting a LV motor are DOL, Star-Detla, Soft Starter and VFD. Each has its own advantages and disadvantages over other methods. GD 2 and Torque speed curve are critical in motor selection for first three methods, whereas not so critical for VFD driven motors.
Conclusion:
It is desirable to know the Load inertia and TS curve of application, more so for high inertia loads like fans or high torque demanding loads like conveyors and crushers. Both these factors have a major influence in motor selection, method of starting and designing motor protection scheme.
