One of the 700s blew up and they already replaced with 525 and 1400 that is communicating between it and the 1500. Is there no way to make this work even with having the extra 10' of slack?
Well, technically there IS, but not with the hardware you have now... What someone was doing (I'm guessing) was to somehow provide an educated guess at what was needed on the two drives to try to share the load, but it likely was not actually working. The fact that you ahve a slack loop is the indicator of that.
The 525 does have a 4-20mA output and torque can be selected. It will also use a 4-20mA input for a speed reference. Is this not the same as what torque following would be doing?
No, not the same. the output of the PF525 will tell you an estimate of what the torque is on the motor shaft, but it is not CONTROLLING that torque. The torque output of the drive will vary depending on what it needs to do to maintain SPEED. That is fine for the MAIN drive motor in your application, that is what you want. The problem is on the 2nd motor. It needs to
match the torque of the main motor, not the speed, otherwise it is not going to share the load. So it is the 2nd motor that needs to be a "torque follower" to the first motor, which means it needs to operate in
torque control mode. That is what the PF525 cannot do. The PF525 is a Vector Drive, but it is specifically a VELOCITY VECTOR drive; Rockwell states this over and over, but many people don't pay attention or don't understand what that means.
When you get into the world of Vector Control, it can generally be thought of as a more accurate way to maintain speed control, but there is more to it than that. You can have VELOCITY Vector Control and you can have TORQUE Vector Control, the difference being what takes priority in the response algorithm.
In Velocity Vector Control, it is the speed (velocity) that is considered the most important factor, torque is USED to maintain accurate speed and provide for a faster "step change response" to a change in the load,
AS IT RELATES TO SPEED. Since this is what MOST people use VFDs for, the basic Vector Control drives (like the PF520 and most other smaller/cheaper "component class" drives) come with a Velocity Vector Regulator loop in the microprocessor.
Torque Vector Control makes torque the prime point of control, even at the EXPENSE of speed. You want the torque to be the thing that the motor maintains as constant, regardless of what that means to speed. But BECAUSE it ALSO involves speed, the microprocessor in the VFD must then have a Velocity Vector
PLUS a Torque Vector Regulator loops in the drive, meaning the microprocessor has to do more math simultaneous processing. That then means a more powerful
mProcessor or adding a math co-processor inside of the VFD. More powerful or multiple processors means more power and heat, which means larger components and heat sinks etc. on the control boards. Smaller/cheaper drives like the PF520 series sacrifice that capability to be able to keep the size (and cost) down, compared to more capable drives like the PF700 (or now the PF750) class drives.
There is no free lunch...