Motor overload did not trip

[goomba]

First I will describe the setup. A 100 kva generator is operating a small ice plant. The voltage is 208 V/3 phase/60 Hz, Wye. The loads are as follows: 15 hp refrigeration compressor, 4 hp condenser fan, 1.5 hp pump, air conditioner (draws 24 amps) (all previously listed are 3 phase, 208V), (3) 1 hp single phase motors (augers, blower).

On 2 occasions, 2 different motors were locked up due to an ice blockage - one single phase motor operating an auger and one 3 phase motor operating a cutter assembly. The motor overloads did not trip (fuses in the case of the single phase motor and overload relay in the case of the 3 phase motor). Instead, the motors just started heating up in a locked condition. Luckily, I was able to find that the motors were stalled and I shut off power before the motors burned up. In the process of troubleshooting why the motor was not running, I checked the voltage. It was "pulled down" to 165 volts.
My theory on this is that there is not enough power in the supply to maintain the voltage to a sufficient level to allow the motor overload to trip.
Can anyone comment on this? If my theory is correct, I would look at sizing the overloads more conservatively and, in the future, specifying motors that have internal thermal protection. I assume the 100 kva generator is sized sufficiently.
Thank you!

 

[Bung]

First off, a fuse is not an overload device. The voltage being pulled down is entirely feasible, depending on cable lengths, and generator and motor impedances. What you need is an undervoltage and / or voltage unbalance tripping device. The fuse is only there for short circuit purposes, and may even not work even in that case if the the rating is too high and the short circuit capacity of the generator is too low. Also make sure the overload relay is a true overload relay, and not a short circuit tripping device (such as an MCB). A measurement of the current in the locked rotor case (using a clamp-on ammeter could tell you something about the actual currents)


Bung
Life is non-linear...

 

[electricpete]

I don't work with small motors much. Just throwing out some thoughts.

"My theory on this is that there is not enough power in the supply to maintain the voltage to a sufficient level to allow the motor overload to trip."

That doesn't sound right to me. I heard you have 208 system nominal voltage. I assume motor nameplate voltage is somewhere in the same neighborhood for the present connection. (correct me if I'm wrong... if it is a higher voltage motor that may make your theory more feasible). Your voltage droooped to 160v or approx 75% of nominal. In locked rotor condition at full voltage you should draw ballpark 500% of FLA. Due to reduced voltage that is reduced by 75% to the neighborhood of 300-400% of LRA. An overload should trip the motor normally in the neighborhood 125% after a period of time.

Maybe the motor has a higher nameplate voltage?
Maybe there is some additional protection perhaps internal protection and the motor was in fact already tripped when you looked at it? Note the motor won't cool off very fast when it's stopped (no cooling from rotor fan).






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[goomba]

You raise a good point. Except that it happened on 2 occasions with 2 different motors and I'm sure the motor protectors did not trip. The motors got so hot they started to smoke. Several minutes passed while I was running around with the volt meter. (maybe it just seemed that way!) They are rated for 208-230 volts. I wonder of a motor will pull a direct percentage of its locked rotor amperage under a reduced voltage condition?

 

[jraef]

OL relays should have nothing to do with voltage, only current. They work strictly (or should) on the thermal effect of current flow through the sensing element, regardless of voltage. Describe your overload relay and heater elements or settings. It sounds as though something is sized or set wrong, or the device is not actually an Overload relay.

Most 1 phase motors are already internally protected. it should say so right on the nameplate.

"Venditori de oleum-vipera non vigere excordis populi"

 

[bklauba]

Electric Pete's conclusions look right. You are lucky that the motors did not burn up, but they might have suffered some aging of the insulation quality, such as degradation/embrittling of the varnish on the windings. Certainly further treatment like what you describe will just push them further "to the edge".

Why did the motors sieze? It doesn't sound like they were underpowered (excessive voltage drop), but rather were mechanically overloaded. Are you sure that the devices these motors are powering are in good working order? Don't overlook degraded/poor installation, i.e., loose mounting, misalignment, out-of-balance, etc.

It does not sound like the old O/L relay did its job; it might be oversized, cal'd wrong, or ready to be rebuilt or replaced.

Once you are able to verify that they are not in need of service, consider upgrading/improving/replacing your overloads, including something for the single phase motors, with solid-state O/L's. These are quite accurate and repeatable, and age very gracefully, and are surprisingly inexpensive. ( 60 - 70 USD ) Compare that to the time and cost of replacing motors. There are numerous makers, Sprecher & Shuh being one (S&S also makes the ones marketed by Allen-Bradley and Cutler-Hammer).

 

[goomba]

That sounds like good advice. By the way, I looked on the motors for TP or Thermally Protected and could not find it.

Can anyone comment on the generator size for this load?

 

[jraef]

Sorry, I missed the generator issue somehow. Although it appears that 100kVA would be sufficient for this load (assuming this machine is all that it is connected to), there is theoretically a way for you to have seen the conditions you describe. I have seen it before, albeit on much larger equipment.

As described earlier, in a stall condition those motors will draw LRA of their nameplate rating, typically 500-600% FLA. However if your source is limited, as in a portable generator, you may not have that much power available. As a result, you may have been drawing significantly less than LRA. If, say for instance, the generator were only allowing those motors to have 300% of FLA, the OL relay may take up to 40 seconds to trip if it is a NEMA standard Class 10 tripping curve, and up to 90 seconds if it is a Class 20! If it were only allowing 200% FLA, those tripping times would be 110 and 250 seconds respectively. 300% current for 90 seconds or 200% current for 4 minutes is going to make the stator very hot to the touch, but is technically still within acceptable limits per NEMA.

What usually happens in this kind of situation is that the extreme generator loading causes a severe voltage drop or the prime mover to stall first. If you have a prime mover that is grossly oversized for that generator however, you can get into this situation. The prime mover barely notices the overloaded generator, so the generator impedance acts to limit the available current to the load. Most generators have UV tripping of some sort, so you normally don't see it for that reason as well. It is obvious that you generator does not.

As others mentioned, you would be well served to add UV protection to this system.

"Venditori de oleum-vipera non vigere excordis populi"

 

[goomba]

I will check - I believe there may be an undervoltage circuit breaker on the generator. I believe that the undervoltage condition occured more localized to the branch circuit feeding the motor.

 

[Walden]

Is the cutter motor fitted to a step-down gear box? If so then you will have difficulties in sizing the motor thermal overload relay to the load. A mechanical torque switch is often used in ice making machines to overcome this problem. The cutter motor needs to be electrically interlocked with the auger motor so that if the auger stops then the cutter stops. Otherwise there will be a build up of ice which will stall the cutter blade.

 

[goomba]

Bravo. Power was lost to the auger first and the cutter assembly backed up with ice. Very good point. Thanks