Fuses blowing in the wind... 2

[itsmoked]

I have a 2HP, 3Ph, 460VAC, 60Hz hydraulic pump motor.

On initial start up it was turned ON and run for ~2 minutes.
Turned OFF. Turned back ON within a minute. Turned OFF and turned back ON within 10 seconds. All three fuses blew.

Here's the fuses. (4A version)

http://www.bussmann.com/pdf/1001.pdf

Here's the motor:

[green]I think it was the repeated quick starts funneled thru time delay fuses. Do you agree? Got other ideas? I have to go deal with this Tues.[/green]


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[John2025]

ItSmoked,
I noticed that the OL has ground fault sensing. I don't know how sensitive it is, but maybe a leaky motor?
John

 

[itsmoked]

Thanks John.

The motor is brand new. The connections were just rechecked.
Now it seems it's not tripping.. Course they've never run it more than 15 minutes since. We shall see.

I can not find any specs on the imbalance aspects of that SS-overload relay. VERY ANNOYING!

Shame on SquareD to only mention the function in a data sheet not quantify it.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
That's because it isn't really an imbalance level sensing feature. As I said, it's most likely a biasing of the thermal model based on the effects of negative sequence currents that are a result of the imbalance. So any imbalance specifications they give you would be misleading. For instance, if your motor were only pulling 25% of the FLA, it could stand a 10% imbalance for a lot longer than another motor running at 90% FLA with only a 5% imbalance. So the thermal model biasing method is not adjustable, but it does cause fewer nuisance trips. Were it a true imbalance level protection scheme it would trip on a fixed percentage, regardless of the loading.

 

[itsmoked]

Now I get it.

I was wondering if at a higher loading the resulting imbalance would be less, percentage wise, and hence might not trip even though the actual load is higher?

Is that how that unit works, thermal monitoring of some internal elements?!?!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Yep, probably just CTs around some internal bus bars. Some people use Hall Effect transducers, but they are more $$ so you find them in more expensive versions where there is some added benefit, i.e. being able to use them behind VFDs where CT saturation might become a problem.

So they will take the CT secondaries (usually 1A) into A/D chips, integrate the current values with a time stamp from a run time clock chip and pump it all into a little mP or DSP that is running a canned algorithm tailored to calculate I²t. Then it compares that to a typical motor model stored on an EPROM and merged into the FLA setting you enter on the dial. Better ones have a real time clock chip so that it can calculate off time and thus maintain better cool down profiles, but these still work better than any bimetal OLRs.

 

[itsmoked]

Thanks Jeff. I should bite the bullet and hack one open sometime if I'm going to continue to use these.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com