drive control


Thread Starter


i wanna know if it is possible to bypass any of the current controller or speed controller in a drive. This is specially for the time when there is problem in say tacho and the process is to be run for some time somehow. Is there any provision for it in drives.
I do not know whether one can bypass speed or current ckts into drive but latest series drive came with "smart bypass" where they are having inbuilt bypass contacter which bypasses entire drive ckt and keeps motor running. After u clear up the fault with drive u can resume to drive ckt. but ensure that drive does hv 'fly to catch' feature. if u want details plz mail me.

[email protected]
I've never seen a drive mfg allow direct access to anything deeper than the current loop. Any deeper and you would be open-loop controlling the
current command or raw PWM/scr firing etc. which could be dangerous!

Davis Gentry

Actually, there are drives out there (kollmorgen, MTS, and others) which accept direct PWM control, and our controllers do that in many applications to provide the tightest control with the lowest latency, but it certainly does not sound like that is what pp is talking about here. If he is using drives with tach (or with any other feedback directly attached) then it is highly unlikely that he can do much without the feedback, as the drive will have no way of keeping
track of what part of the phase cycle the motor is in at any given time. If he is lucky it will just stop moving...

Davis Gentry
Delta Tau Data Systems

Bob Peterson

Most DC drives have some capability to run openloop in speed mode. The output voltage is proportional to motor speed so it won't run away from you. The tach/encoder is just to dial it in closer, but even w/o the tach/encoder just open loop its typically w/i 5%.

VFDs may or may not be able to run w/o feedback. True vector mode requires motor position feedback. If all you care about is speed control, the VFD can run open loop and be very close as long as your load is OK.

Donald Pittendrigh

Hi All

It is not uncommon to do speed control from an external controller, I have dealt with several position control applications where the speed
controller was available in the external position controller so to avoid problems associated with loop tuning we set the drive ramp times to "0"
effectively bypassing the speed controller. To bypass the drives current controller externally is probably not a good idea unless the external
control loop is fast and accurate enough, I wouldn't recommend it and would be quite interested to know why you would need to do this. The tacho problem would usually only affect the speed controller not the current controller and bypassing the current controller for such a reason would be better done with a star delta or softstarter and external contactors.

Donald Pittendrigh
The Siemens 6SE70 drives can do this. For example the tach loss reaction P806 can switch the drive from closed loop vector to open loop on a tach loss. Check these drives out - they can do anything.


actually if there is some proble in tacho then the drive trips immediately as its not generating or generating less feed back. Now if in process plant it is required to run the motor for sometime then it becomes difficult. thats why i was asking for this procedure.