VFD for roots blower
Roots blower belongs to the positive-displacement blower. Its maximum characteristic is its volume revolution characteristics. It can be approximately believed that the maximum pressure reachable by the blower is unrelated with the revolving speed, which means that the achieved maximum pressure under different revolving speed remains unchanged, with the flow rate and the revolving speed in direct proportion. Therefore, roots blower can be regarded to be the load with constant torque and the corresponding roots blower system belongs to the typical constant-pressure output system. Therefore, after constant-pressure control, the system operation is in the constant-torque and variable flow status.
II. Improvement analysis
The traditional control mean is to enable the induction motor to revolve in constant speed. Depending on the inlet and outlet valves to conduct wind volume adjustment, then the pressure can be controlled in a certain scope, thus achieving the flow control. This results in that the roots blower has an energy consumption of 60% on the air inducing and blowing. However, if the VFD is used to regulate the speed of the motor, when the system flow rate requires to be reduced, the revolving speed of the roots blower can be decelerated. So that the roots blower can operate at the low flow point under the regulated pressure. Its input power has an approximate linear relation with the flow, as indicated in the picture 2. Therefore, the energy-saving result of the roots blower after using VFD is mainly equivalent to the operating flow. The power consumption of the roots blower is in direct proportion to the flow.
When the roots blower has went through improvement by using VFD, the system can realize soft starting and soft stopping, so as to reduce the shock of the system to the power grid in start-up, thus reducing the system start-up times and ensuring a stable operation. What's more, because of the deceleration of the revolving speed in roots blower operation, it can reduce the mechanical wear and prolong the operating life of both the motor and the roots blower. If the system employs pressure closed-loop control improvement, the fully automatic control and unattended operation can be realized.
III. How to choose a VFD
Roots blower belongs to the load in constant torque. In selecting the variable frequency drive for the roots blower, should avoide the VFDs specialized for centrifugal fan, water pump and so on. The general ATO VFD with constant torque load should be selected.
This type of VFD possesses decades of malfunction automatic protective function, such as overflow, overload, overheat, over-frequency and so on. The frequency can not only work under the automatic(regulate output frequency through pressure signal) and manual (set the working frequency artificially) statuses, but also possesses the mutual switch-over function between the auto-decompression start and VFD start. When one of the working modes is in repairing or having problems, it can be switched to the other working mode artificially, thus having no influence to the normal manufacturing. The intrinsic protection of the VFD is also complete, with smart and reliable protection to the phase loss, locked rotor, over flow, under voltage and so on.
IV. Parameter debugging of ATO VFD
|Function code||Name||Set value||Remark|
|P0.17||Acceleration time||120||Set according to the field condition|
|P7.00||Control selection of the closed-loop operation||1|
|P7.02||Selection of the feedback passage||6|
|P7.05||Digital setting of the preset amount||0.052||Set according to the practical condition|
|P7.09||Limit value of the upper limit pressure||0.100|
|P7.26||Constant-pressure water supply mode||1|
|P7.27||Range of the long-distance pressure gauge||0.100|