HomeDefinitionVFD parameters setting principle

VFD parameters setting principle

Generally, there are dozens of function parameters in variable frequency drive (VFD), or even hundreds for option. In practical application, it is not necessary to set every parameter, most parameters can be keep factory default settings.

1. Acceleration/deceleration time
VFD acceleration time is the time required by output frequency from 0 Hz to the maximum frequency, deceleration time is from maximum frequency dropped to 0 Hz. Usually use the frequency setting signal rise and fall to determine the acc/dec time. It needs to restrict the rise rate of the frequency in acceleration during the electric motor start period in order to prevent over-current, and limit the frequency decrease rate in order to prevent over-voltage during deceleration period.

Acceleration time setting requirements: limits the speed up current below the VFD's over-current capacity, make the over current speed loss don't cause the VFD drives tripped; The deceleration time set point is to prevent smoothing circuit voltage excessive, make regeneration overvoltage don't cause the variable frequency drive tripped. Acc/dec time can be calculated according to the load, but to practical experience it's better to set longer acc/dec time at the debugging, to see if there are over current or over voltage alarms during the electric motor start/stop; Then shorten the acc/dec time gradually base on the principle with no alarm in operation, repeat several times, you can determine the best acceleration and deceleration time.

2. Torque boosting
Also known as the torque compensation, it's to compensate the torque decrease in low speed for the stator winding resistance of the AC motor, by increase the low-frequency range of V/F. It's enable the voltage during acceleration goes up automatically to compensate the starting torque when it's set to AUTO, to ensure the electric motor accelerates smoothly. Adopting manual compensation, you can get better curve through test according to the load characteristics, especially the load starting characteristics. For variable torque loads, the output voltage maybe too high in low speed by the inappropriate chooses, to cause a waste of electricity energy.

3. Electronic thermal overload protection
This function is set to protect the electric motor from overheating, it calculate the AC motor temperature rise by the VFD internal CPU base on the operation current and frequency, thus to enable overheating protection. This function applies to "one VFD drag one motor" only, it should be installed thermal relay on each motor when you want one ac drive drag several motors.
Electronic thermal protection setting value (%) = [electric motor rated current (A) / ac drive rated output current (A)] × 100%.

4. Frequency limitation
Means the VFD output frequency maximum and minimum value. Frequency limitation is to prevent the failure of impropriate operation or external frequency setting signal source from outputting frequency too high or too low, it's a protective function to prevent damage the devices. In the application you can set it according to the actual situation. This feature can also be used for speed limiting, like in some belt conveyors, due to not so many materials being transported, adopt the variable speed drive to reduce the wear and tear of machines and belt, by setting the variable frequency drive maximum frequency to a certain value, this allows the conveyor belt running at a fixed, low speed status.

5. Frequency bias
Sometimes it's called deviation frequency or frequency deviation setting. Its purpose is when the frequency is set by an external analog signal (voltage or current), use this function to adjust the output frequency value when the frequency setting signal in minimum value. Some VFDs deviation value is available from 0 to fmax when the frequency setting signal is 0%, some variable speed drives even can be set on the bias polarity. In debugging, when the frequency setting signal is 0%, the VFD output frequency is not 0 Hz but x Hz, then the deviation frequency can be set to negative x Hz to enable the variable speed drive output frequency 0 Hz.

6. Frequency setting signal gain
This feature is only effective when the frequency set by external analog signal. It is used to compensate the inconsistencies between the external setting signal voltage and the variable frequency drive internal voltage (+10v); at the same time it's convenient for voltage selection of the analog signal, in setting, when the analog input signal is maximum (10v, 5v or 20mA), obtained the output frequency percentage of V/F graphics and treat it as a parameter to set it; Take an example, external setting signal is 0 - 5v, VFD output frequency is 0 - 50Hz, then signal gain value is set to 200%.

7. Torque limit
It can be divided into drive torque limitation and brake torque limitation. It is based on the VFD 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 VFD 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 VFD start repeated, and current fluctuate heavily, in serious it will cause the VFDs tripped, you should be attention.

8. Acc/dec mode selection
Also known as the acc/dec curve selection. Normally the VFD 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 VFD, select the acc/dec curve to non-linear curve, the variable frequency 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 VFD 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 VFD 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 VFD. 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 and 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 VFD is designed with energy-saving control function dedicated V/F mode, this mode can improve the efficiency of the electric motor and the VFD 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 VFD trip frequently when they enable it, but normal after disable it. The reasons are:
(1) The original motor specification is far different with the VFD 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.

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