How to select AC Drives?

The AC drive should be of equal size to the electric motor or bigger. In some applications requiring high starting torque due to the load characteristics, the AC drives may need to be oversized to suit. You need to be aware of the load characteristics and size the drive accordingly. Even a fan application which can have low starting torque requirements, still requires full power when running at full speed. The only time that ac drive smaller than the motor would be acceptable would be if the electric motor was never going to run at or above base speed for a variable torque load, even then you have to be aware of the run up times and starting torque requirements.

AC Drives has to be selected by current and not power. That should be same but is not always. Immerged pumps for examples are using more current than standard ac motor for same mechanical power. In general current is more less linked to torque. Be aware than bellow 50% of nominal torque/current the AC drives selection can be different.

Some applications such as conveyor belts in mining applications can be kilometers long and require upwards of 30% of full voltage to start. A fan application may require high starting current to overcome inertia, but current can drop once it is moving and running below base speed. The characteristics of the load need to be taken into account when sizing AC drive application.

The definition of the right set depends on three steps one must properly calculate: 1. Kinematics, 2. Dynamics and 3. Inertia balance and power coupling. As it is an iterative process, it is not possible to get the right set until those three calculations have been resolved. For the first one we need to know what are the speed, acceleration and jerk cycles so first limits for gear and motor appear. Then we need to know what the load is and how it behaves. Is it an elevation? Is it a horizontal movement? Combination? Does the load vary? These parameters allow to calculate power and torque. Together with the values of kinematics you can define the gear+motor options, resources to a few. Finally one must check the inertia ratios between the load and the geared motor (taking itself into account to get the right balance, what makes the process iterative). Perhaps you have enough power and speed but the inertia ratios make the variable frequency drive unable to control the load and unable to transmit the power properly. Finally with all parameters fixed, you can even optimize the ser selection by analyzing the set terminally. This makes profit of the part of the cycle the motor is stopped or its load is low. This is why sometimes you can see lower power sets compared to nominal theoretical values (this is engineering). If you need to fit well an application (i.e. You are a machinery builder and need to optimize much to reduce costs), we can perform the consultancy for it.