AC drive inverter parameters setting (3)

7. Torque limit

It can be divided into drive torque limitation and brake torque limitation. It is based on the inverter output voltage and current values and calculates torque by CPU, to make the impact load recovery characteristics of the deceleration and constant speed running improved significantly. The torque limitation function can achieve automatic acceleration and deceleration control. Assuming the deceleration time is less than the load inertia time, it also can ensure the motor accelerate and decelerate automatically in accordance with the torque setting.
Drive torque feature provides a powerful starting torque, in steady-state operation, the torque function will control the motor slip, and limits the motor torque within the maximum setting value. When the load torque increases suddenly, even acceleration time is set too short, it will not cause the AC drive tripped. When the acceleration time is set too short, the motor torque also won’t exceed the maximum set value. The big drive torque is better for starting, set it to 80 – 100% is proper.

The smaller value of the braking torque, the bigger the braking force, it’s suitable for fast deceleration applications. If the value of the braking torque is being set too high, it will cause overvoltage alarm. If the braking torque is set to 0%, it can make the total amount of the regeneration on main capacitor close to 0, hence during the motor deceleration, it can decelerate to stop and won’t cause trip even if there is no braking resistor. But in some loads, such as the braking torque is set to 0%, it will cause idling temporary during deceleration to make the AC drive inverter start repeated, and current fluctuate heavily, in serious it will cause the AC inverter drive tripped, you should be attention.

8. Acc/dec mode selection

Also known as the acc/dec curve selection. Normally the AC drive has three curves: linear, nonlinear and S curve, usually most of them choose the linear curve; nonlinear curve is suitable for variable torque loads, like fans etc; S curve is suitable for constant torque loads, the acceleration and deceleration changes relatively slow. You can set it according to the load torque characteristics to select the right curve, but there are exceptions, I used to debug a boiler Fan variable frequency drive, select the acc/dec curve to non-linear curve, the ac inverter drive tripped as long as starting, it’s also no effect after I changed lots of parameters. Then I changed it to S-curve which worked fine. The reason is: the drawing fan rotate automatic before starting due to the flow of flue gas, and it’s reversed to become a reverse load, then select the S-curve, the frequency rises slower in starting, thereby avoid the frequency inverter from trip. Of course, this is the method used for the ac drive inverter that does not enable start DC braking function.

9. Torque vector control

Vector control is based on the theory: asynchronous motors and DC motors have the same torque mechanism. Vector control mode is decomposing the stator current into magnetic current and torque current and controlled separately, meanwhile, output the stator current to the motor after their synthesis. Therefore, we can obtain the same control performance with DC motor in principle. The motor can output maximum torque in variety operating conditions when adopts torque vector control function, especially the ac motor in the low-speed operating status.

Currently, the variable speed drive almost applies no feedback vector control, due to the ac drive enables slip compensation according to load current value and the phase, to make the motor has a hard mechanical characteristic, and is able to meet most systems requirements without the speed feedback circuit in the external of the inverter. This feature can be selected base on actual situation to enable it or not.
The relating function is slip compensation control, to compensate the velocity deviation caused by a load fluctuation, which can add the slip frequency corresponding to the load current. This feature is mainly used for position control.

10. Energy-saving control

Fans, pumps are all belong to decrease torque load, means the load torque decreases proportional to the square of rotation speed in the speed decline, the frequency inverter is designed with energy-saving control function dedicated V/F mode, this mode can improve the efficiency of the electric motor and the ac inverter drive, the output voltage can be decreased automatically according to the load current, to achieve energy saving purposes. You can enable or disable it according to the specific circumstances.

The 9, 10 parameters are very advanced technology, but some users simply can not enable them in equipment transform, means the ac drive trip frequently when they enable it, but normal after disable it. The reasons are:
(1) The original motor specification is far different with the frequency inverter requires.
(2) Do not know enough of the parameter function settings, such as energy-saving control function can only adopt V/F control mode, can not use vector control mode.
(3) Have enable the vector control mode, but have not do the work for the electric motor parameter manual setting and automatic running.