The various methods of controlling two AC Motors in load share are:
Supply both motors from one Single Inverter Drive unit
This just needs the Inverter sizing to match the combined motor current and thermal overloads adding for individual motor protection. The Inverter must be set in VxF control. The load will share between the two motors naturally, if the loads are evenly distributed and the motor slip frequency is large enough. Small motors have a much larger slip frequency than large motors, so this method is best employed for small power applications and where speed control accuracy is not important.
Simple Inverter Drives connected to each motor
The Inverters must be set in VxF control and have exactly the same speed set-point, ramps and start instruction. The load will share between the two motors as long as the loads are evenly distributed and the motor slip frequency is large enough. Small motors have a much larger slip frequency than large motors, so this method is best employed for small power applications and where speed control accuracy is not important. If the drives have any form of ‘slip compensation’ you will need to turn this feature off.
Simple Inverter Drives connected to larger power motors
The Inverters can be set in VxF control or Sensorless Vector mode and will need to have exactly the same speed set-point, ramps and start instruction. The load will share between the two motors as long as the loads are evenly distributed and the Inverter Drives are set to reduce their speed set-points with increasing load. The speed ‘droop’ forced by the Inverters combined with the motor slip, will need to be large enough to balance the loads. If the ‘drop’ is not large enough, the drives may have a tendency to push and pull to trip one another on ‘Bus Over Voltage’ or shake the mechanical connection between them. In this case it will be necessary to either increase the ‘droop’ or use drives with multiple torque limits and to turn off the braking torque of both drives. This method is best employed for medium power applications and where speed control accuracy is not important.
‘Vector’ Inverter Drives connected to each motor for all motor powers and where higher accuracy of speed control is required
The Inverters must be of a type including programmable function blocks. The drives would be arranged have exactly the same speed set-point, ramps and start instruction, or as ‘Master’ and ‘Slave’ with a single ramp located in the ‘Master’ drive providing the speed set-point to both drives. The load or motor current signals are compared in a free ‘PID’ function block in the ‘Slave’ drive and set to trim the ‘Slave’ speed set-point to maintain the same load or motor Current for each drive. This method is suitable for reasonably smooth loads, as the PID function trimming the ‘Slave’ drive could be comparatively slow to respond.
‘System’ Inverter Drives connected to each motor for all motor powers and where higher accuracy of speed control is required
The Inverters must be of a type that has high-speed processors and internal separate speed and torque loops. The drives would be arranged as ‘Master’ and ‘Slave’ with a single speed loop located in the ‘Master’ drive providing the torque demand for both drives. The ‘Slave’ drive speed loop (PI) output would be disconnected internally from its own torque demand and the ‘Master’ torque demand signal brought across and connected to the ‘Slave’ torque demand or frequency control block. Fast processors and separate speed loops are present on Parker 690P, 890 and ABB ACS850 Inverter Drives. The loads and load changes for each motor will need to be reasonably evenly matched for this method of control.
Full Closed Loop Vector ‘System’ Inverter Drives connected to each motor for all motor powers and where the highest accuracy of speed control is required
The Inverters must be of a type that has encoder feedback and set in full closed loop flux vector control. The drives would be arranged to have exactly the same speed set-point, ramps and start instruction, or as ‘Master’ and ‘Slave’ with a single ramp located in the ‘Master’ drive providing the speed set-point to both drives. The load signals are compared in a free ‘PID’ function block in the ‘Slave’ drive and set to trim the ‘Slave’ speed set-point to maintain the same load for each drive. You may be able to use a simple proportional (+- Input) function block for this purpose and keep the drive response more closely matched for both drives. This method is suitable for less smooth loads where some speed difference can occur. The product entries on the site include Technical Manuals and in some cases free to download programming software for the more demanding applications.