VFD/PLC Based Synchronous Transfer Control

VFD/PLC Based Synchronous Transfer Control

 In any system where one VFD controls multiple motors, the problem becomes “How do we efficiently move pumping units from the 50Hz utility bus to the VFD output or variable frequency bus and vice-versa”?

VFD/PLC based sync-transfer controls have the following components:

• VFD per system

• PLC per system

• VFD contactor per motor

• Bypass contactor per motor

During synchronous transfer control system operation it is common to set the VFD output frequency to match the 50 Hz operation of the electrical utility. This allows paralleling of the utility 50 Hz bus and the VFD output bus by closing both the VFD and bypass contactors. Once the output frequency matches the utility’s operating frequency (with equal voltage magnitude and phase angle), the latter motor bypass and VFD output contactors may be simultaneously operated to quickly transfer the motor from the VFD output to the 50 Hz utility bus and vice-versa.

This process allows the VFD to sequentially control multiple motors using the closed circuit, sync-transfer operation. The latter is the basis of “VFD/PLC based sync-transfer control” where one VFD controls many motors. The VFD and associated PLC based control system is programmed to implement the synchronous transfer control process by the use of two basic commands:

UP Transfer - transfers the motor from the VFD to the Utility bus.

DOWN Transfer - transfers the motor from the Utility bus to the VFD.

UP Transfer Command Process [See figures 1 through 4]

 

The PLC based control system UP transfer command involves the following operations - assuming all motors are stopped on receipt of an operator command the PLC and VFD operate as follows:

1. PLC closes motor1 VFD contactor

2. VFD accelerates motor1 to the operator set point

3. PLC receives a start motor2 command

4. PLC sends the UP transfer request for motor1 to the VFD

5. VFD accelerates motor 1 to 50 Hz and confirms synchronization OK

6. PLC closes motor1 bypass contactor transferring motor1 to the 50 Hz bus

7. PLC opens motor1 VFD contactor and closes motor2 VFD contactor

8. VFD ramps its output and accelerates motor2 to the operator set point

Note: This process continues until all requested motors are operating.

DOWN Transfer Command Process

The PLC based control system DOWN transfer command involves the following operations - assuming at least two motors are operating on receipt of an operator command the PLC and VFD operate as follows:

1. PLC receives stop motor2 command from the operator

2. PLC requests VFD to decelerate motor2 to minimum speed

3. PLC opens VFD contactor

4. PLC sends DOWN transfer request for motor1 to VFD

5. VFD accelerates to 50 Hz and confirms synchronization OK

6. PLC closes motor1 VFD contactor and opens motor1 bypass contactor

7. VFD decelerates motor1 to the operator set point

Note: This process continues until only one motor is operating on the VFD.

Soft Start Advantage of Synchronous Transfer Control

A great benefit of the VFD is the ability to avoid across the line starts by ramping the unit motors from zero speed to the minimum speed over several seconds. The soft start advantage of VFDs allows large motors to be sequentially started while maintaining compliance with the local utility flicker limits and minimizing the mechanical strain on the electric motors.