Tom Jenkins
Lifetime Supporting Member
I just got back from a start-up trip with GE VFDs supplied by others. This is on a multistage centrifugal blower application, of a type and size I have done successfully many times before. These were two 350 hp and two 200 hp VFDs, GE Model 6KGME4___DSL1001 "clean power" VFDs. The blowers are a fairly high inertia load.
I normally set the accelleration and deceleration on the VFDs for 15 to 30 seconds. This is to prevent over current trips on accelleration and over voltage trips on decelleration. In rare cases I may have to go to 40 seconds. I cannot recall ever needing more than 45 seconds. I also have the same time for both in all projects that I can recall.
On this job the GE starty-up technician was not as experienced as I might have hoped. (See - I can be diplomatic!) We started at 30 seconds for both accell and decell times. Accell was fine, but on decell we had overvoltage trips during a normal stop cycle until we got the decell time up to a minute. The GE start-up technician indicated he really thought we should be using braking resistors. We did not see any elevated temperatures on the VFD or have any temperature faults.
Finally, the questions:
1) I generally expect a VFD to have similar decelleration torque capability as accelleration torque capability. I expect to use braking resistors only if I have either a short decel time requiring more torque, or repeated braking requiring more heat dissipation. Is this assumption incorrect?
2) On A-B and C-H and Magnetek VFDs I haven't had any problems with decell. Is the GE drive generally less capable in this area than some of their competitors?
I normally set the accelleration and deceleration on the VFDs for 15 to 30 seconds. This is to prevent over current trips on accelleration and over voltage trips on decelleration. In rare cases I may have to go to 40 seconds. I cannot recall ever needing more than 45 seconds. I also have the same time for both in all projects that I can recall.
On this job the GE starty-up technician was not as experienced as I might have hoped. (See - I can be diplomatic!) We started at 30 seconds for both accell and decell times. Accell was fine, but on decell we had overvoltage trips during a normal stop cycle until we got the decell time up to a minute. The GE start-up technician indicated he really thought we should be using braking resistors. We did not see any elevated temperatures on the VFD or have any temperature faults.
Finally, the questions:
1) I generally expect a VFD to have similar decelleration torque capability as accelleration torque capability. I expect to use braking resistors only if I have either a short decel time requiring more torque, or repeated braking requiring more heat dissipation. Is this assumption incorrect?
2) On A-B and C-H and Magnetek VFDs I haven't had any problems with decell. Is the GE drive generally less capable in this area than some of their competitors?