Fuse trip on motor startup

[ME69]

We are experiencing branch circuit fuse tip on 1 phase when a 100hp 460v NEMA-B motor starting a large fans in the x-line or VFD bypass mode. Motors start and run fine in VFD mode. Motor and wiring meg ok.
Fuses are 200 AMP Time delay. Their time curves indicate that they should hold the LRA (800) for 15 -20 seconds. Fuses often blow within a few seconds. Estimated acc time is about 8 seconds. We are in the process of recording starting amps and voltage, but do not have data yet. Normal running current and voltage balance is ok. There are 3 systems with 3 identical VFD,motors, fans switch gear etc. Each system has experienced this. Anyone with any similar experience?

 

[Barry1961]

Is the fan "free wheeling" in reverse when you try to start it? I have seen this problem before with multiple fans, in paticular if one fan is off while the others are running. That can cause the off fan to build up a good bit of speed in reverse.
Most of you large drives have a canned routine for this that stops the fan before starting. I forget if they use injection braking or reduced voltage to do it. I am sure Hitachi and ABB both have this as an option on some of their drives.

Barry1961

 

[rbulsara]

I agree with Baryy1961.

Tyr the following in that order.

1. Make sure fan is at standstill (not rotating in any direction) before starting across the line.

2. If there is any, close the damper (preferably outlet damper) before starting and open it once starting current drops down.

3. Above failing it is a simple case of having too high inrush due to high load inertia. You need to install larger fuses. Try 250A, 300A or even 400A fuse if need be. (regardless of what code says, if the load has high inertia you must accomodagte it or reduce it). In fact try with a large fuse like 400A, then bring it down in steps until you find the smallest size that does not blow.

 

[jraef]

I totally agree that windmilling is the most likely cause of your fuses blowing, but there are other possibilities.
1) Your bypass system is phased differently than the VFD, and if the fan is spinning in the CORRECT direction, you have the same problem as above when you energize it in Bypass. This is a very common installation error, because the phase rotation output of the VFD has absolutely no relation to the incoming pahse rotoation from the line. So if you only tested rotation using the VFD, you may be backwards in Bypass.
2) The transformer feeding this circuit is too small for X-Line starting. Using the VFD solves the problem, but when you try to start X-Line in bypass, the voltage drops and the current surges too high for the fuses.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

Check the motor shaft speed before the VFD by-pass is operated. If the VFD frequency is lower than 60 Hz the motor still goes into accelerating mode.The speed should be very close to the final full load speed and as Jraef said in the very same direction to avoid high current inrush while by-passing the VFD.