Circuit Protection.... 2

[viperneo]

I have a problem with my cb tripping at a transfer. I have a 3 ph Fuse (30A Ferraz Class CC) that blows up when there's a upstream transfer from an ATS. The fuse is protecting a pump, and at normal condition (util power) the pump is able to operate as usual. But when power is out or a transfer is made to the generator, the fuse blows and I need to keep replacing it.

Can someone help me? Is it due to the type of fuse i'm using? Amp is not a problem b/c it's not blowing up under normal condition. I tried with a CB and it still does the same thing. Maybe it's a particular type of fuse/cb that I need to use. Any help?

Thx in advance.

 

[DRWeig]

Viper,

In my experience, if your transfer switch does not have a delay programmed in to allow residual voltage to dissipate from rotating loads (such as your pump), you'll almost always pop the fuse feeding a load with any inertia at all.

When normal power is lost, the pump motor immediately starts to slow down, but its residual magnetic field is keeping the voltage up at its terminals for a few cycles or more. When the transferred power hits the motor, it's out of phase with the residual and in most cases tries to snap the motor backward to synchronize it. Pop goes the fuse.

I always add a delay-in-neutral function to my transfer switches. If a particular pump is critical and can't be let down, it needs a UPS of some sort or a second mover on its shaft (industrial plants sometimes stick steam turbines on pump shafts to keep 'em up during power failures and transfers).

In sum, it may not be your fuse. Do you have a delay on transfer?

Good on ya,

Goober Dave

 

[itsmoked]

If your generator is putting out an undervoltage the motor can draw substantially more.

If the generator is undersized and is taking a long time to get the motor up to speed the fuse will have longer to heat during the restart by the generator. This can blow the fuses.

If the generator is running too slowly,(underfrequency) it may have your motor saturating and this could be blowing the fuse.

If the generator is running fast(overfrequency) the motor will be putting out more horsepower and hence will again draw more current and can blow the fuse.

Tell us more about the system.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[viperneo]

Keith,

Freq is not the problem b/c the pump had operated under the gen power, so I don't think voltage and size is a problem either.

It may just be what dave said, a delay issue. I would like to hear more ideas on what the problem may be.

Situation: ATS feed 15hp pump. Between ats and pump are 30A fuse (30A Ferraz Class CC) and FVNR starter.

 

[DRWeig]

Viper,

In addition to my comment above being a possibility, you also may simply have a too-small fuse (or breaker). I'm guessing here that your system voltage is 480, because you didn't say --

If the system voltage is 480 and your pump is 15 HP, a non-time-delay fuse may be sized up to 300 percent of full-load current, so 60A (for a 21 FLA motor) would be within code as long as the starter has running overload protection built in.

Look at NEC table 430.52 (1995) and the supporting text.

Holler back and let us know voltage and service factor of pump motor...

Goober Dave

 

[itsmoked]

If you can start the motor successfully using the mains OR the generator. But you have this problem occurring only during a transfer between the mains AND the generator. Then the problem is likely the lack of a time delay between running on the mains and switching to the generator. Exactly as DRWeig suggests. This is a common problem of switching between sources.

If you want to do a switch between the mains AND the generator while the generator is already running you will certainly have to use some form of synchronizing or you may soon be needing a new generator or motor.

Meanwhile you will need to somehow provide a second or two pause between transfers. If you cannot pause the transfer switch between sources you may need to: open the generator breaker -> transfer -> close generator breaker.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[oldfieldguy]

Some auto transfer switches have a synch before transfer function that compares the phasing from the source to the load, timing the close when any residual voltage on the load side is in phase with the source.

I had problems similar to those you're describing, and the fix was a simple as going into the transfer switch controller's program and turning that feature on.


old field guy

 

[viperneo]

Cool, thx. After DRWeig post, I suspect that to be the problem. I'll see if this is the case fri and hopefully it'll be solved.

Thanks guys and gals.

 

[Szalkus]

Many ATS mfgs. are out there. I'm aware of one who has offereings on Delayed/Programmed transition up to 600V, and 4000Amps. GE Zenith is one. If you are in US, refer to an ATS mfg. how meets UL1008 standard.

 

[waross]

Is your generator running all the time? Usually the time for a generator to start is more than adequate to allow any motors to spin down. The issues with regeneration usually manifest when a to rapid transfer is made from an existing source to another existing source.
itsmoked made some good points, but frequency is no longer an issue with modern sets. The newer Automatic Voltage Regulators respond to under frequency by lowering the voltage proportionally and the motors are quite happy. However, low voltage by itself may be an issue.
It is important to identify the cause of the problem. If this is a fast transfer issue, it must be resolved either by a synchronized transfer or by a delayed transfer. A larger fuse may result in motor damage.
If this is a low voltage problem it may be that the generator voltage is set too low. Use the same voltmeter to check both the utility voltage and the generator voltage. I have seen too many cheap, inaccurate voltmeters on Gen-sets.
There may be a voltage drop in the feeders from the generator to the transfer switch.
It may be that low voltage is a result of the AVR not reponding fast enough to the starting current of the motor. If you deem it not wise to increase the generator voltage, the go to larger fuses as a last resort. (Within code limits).
Keep us posted.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Blowing on transfer to generator or on transfer back from generator to utility? Big difference and lots of reasons why it would go out on retransfer if it isn't all set up just right. As said, if your generator has to start after loss of utility I can't see any reason for that transition to blow a fuse.