Testing Motor Overloads

[CuriousElectron]

Hi Sparkies,

This may be a stupid question, but how would you test an overload device on 60 HP motor? Unless you substituted the existing load with a load that draws more Amps, I don't see how you would confirm your new OL picks up on overload. It seems to me that you'd set your overload device based on your calculations, and hope that it does its job.

Regards,
CE

 

[waross]

Back in the day we used an instrument called a Multi-Amp. This would output a calibrated current that was passed through the overloads. The multi-Amp also had a timer that could be wired to the O/L contacts to show the time to trip.
You don't test with a load at line voltage. An overload relay responds to current and so is tested with a calibrated current at a low voltage.

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

 

[controlsdude]

Also, a lot of overloads have a built in tester. Simple push it with a flat head screwdriver usually trips them.

 

[waross]

Manual tripping does not check calibration.

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

 

[jraef]

MultiAmp was (is?) a brand name of what’s called a “Prinary Current Injection Tester” and the description is accurate; controlled current at low voltage. Circuit breakers are tested the same way, although they also get tested for instantaneous trips, which OL relays don’t have.

https://us.megger.com/products/relay-and-protection-testing/primary-injection-test-systems

Why are you asking? Do you have a particular instance that is making you suspect that your OLs are not tripping correctly? Current injection testers are very expensive and require training because they can be dangerous, typically bought and used by people doing this a lot such as electrical testing companies. If yours is a one-off thing, I suggest calling one. If you are in North America, look for a local NETA (National Electrical Testing Assoc.) provider.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

We have a Phenix Technologies HC1 test set. Pretty simple unit to test relays or breakers that don't expect balanced 3-phase 120 degree separated currents. Just put the 3 poles of the relay or breaker in series and dial-up the test current. I don't recall it being super expensive, but still definitely thousands of dollars, not hundreds of dollars.

If you search for breaker test set you can find other similar test sets from other manufacturers.

 

[controlsdude]

I am kind of puzzled by why the need to check an overload that a manufacturer has deemed correct based on its internal testing of its product. I have never tested an overload with one of these testers in over 25 years in commissioning systems that had motor overloads part of systems.

Care to explain the reasoning for the testing?
Maybe this is a Planned maintenance issue?

 

[panter]

Electron,
if you do not have any of the power sources and the requirement to test the overcurrent relay, you can reduce relay setting and expect a switch off breaker at the rated current of the motor. Most over-load relays work with double current in range between 50 and 100 seconds. Pay attention because motor exclusion may cause some problems in process.

 

[CuriousElectron]

I forgot to mention that this is a solid-state overload device, so we'd be interested in testing the relay settings. It sounds like CB-832 would do the job.

Thanks!

 

[ScottyUK]

Many solid-state relays have imbalance detection and trip more quickly for an unbalanced set of currents. To test these you'll need a three-phase primary set, or you could contrive something fairly easily using a three-phase variac and a set of matching transformers. We built one using toroidal transformers with only the primary windings wound onto the core, the secondaries being our own leads with a handful of turns through the aperture. Most of the low-volume manufacturers will supply oddities like this. Big variacs turn up from time to time on the surplus market as universities retire stuff.