The correct selection of the AC drive (VFD) is critical for the normal operation of the mechanical equipment electrical control system. To choose the best suitable AC drive, we need to know the specified machinery type, the load torque characteristics, speed adjusting range, static speed accuracy, the starting torque, and the environment, then determine the control pattern and protective structures of the VFD.
There are three mechanical torque load types: constant torque load, constant power load, and fluid load.
Constant torque load
In such loads, the load torque TL is unrelated with speed n, TL is always keeping constant or approx constant at any speed, the load power is linearly increased along with the load's speed increased. Conveyor, agitator, extruder and mechanical equipment, etc. friction loads, and crane, hoist, elevator, etc. gravity loads, are all constant torque loads.
When the AC drive drags constant torque loads, the output torque should be large enough at low speeds and has strong overload capabilities. If the AC motor needs operation at a low speed steady for a long time, we should consider the cooling capacity of the standard cage asynchronous motor to avoid the motor temperature goes too high.
Constant power load
The feature of the constant power load is the torque TL is roughly inverse proportional to the rotational speed n, but the power remains constant approximately. The spindle of metal cutting machine tool spindle, mill, paper machine, film production line coiler, uncoiler, etc., all are the constant power loads.
The nature of the constant power of the load is in a certain speed range terms. When the speed is very low, TL can not increase indefinitely due to the limitation of the mechanical strength, then convert to constant torque nature at low speed. Constant power area and constant torque area have a big influence on the selection of the transmission scheme. When the motor at constant flux speed adjusting, the maximum allowable output torque constant, is constant torque speed control; While in weak magnetic speed adjusting, the maximum allowable output torque and speed are inversely proportional, is constant power speed control. If the motor's constant torque and constant power speed range are consistent with the load's constant torque and constant power speed range, that so-called "matching", the size of the AC drive and motor are minimal.
This type of load's torque is proportional to the square of the speed, power is proportional to the cube of the speed. All kinds of fans, pumps, and oil pumps are typical of fluid loads.
Fluid loads are controlled by AC drive to regulate airflow, water flow, which can save energy significantly. Due to the fluid load demand in high-speed increases too fast, is proportional to the cube of load's speed, so we should not make this kind of loads in over frequency operation.