Motor or overcurrent relay problem? 3

[dbaec]

Problem
I have had two 2HP single phase dual voltage motors that trip the overcurrent device on startup. The first motor blew the starting capacitor.

History
These are on a hood exhaust fan on a restaurant. The exhaust fan was installed new 12/08. The fan gets turned on at opening runs about fifteen hours and turned off at closing. In two months the OCD tripped three times. The first two times I was called out to check it out. I reset it, checked startup and run currents, both were normal and all else seemed fine. The third time, under pressure to keep it running, the OCD was bypassed. The thinking was that the OCD was defective. Upon starting the motor the starting capacitor blew. A new motor of the same make and type was sent to replace it. One week later the OCD tripped again.

Technical
The motor is 2HP Dual voltage 115/208-230V.
Connected for 230V.
FLA is 12.7 at 230V.
S.F. is 1.15 at 230V.
Motor has two starting caps, both 295uH.
OCD is set for 125% FLA, ~15.8A.
OCD is three phase (ambient compensated bi-metallic)
(class 10 with single phase sensetivity).
Voltage to motor not running is 246, while running is
243.
Start up current is ~57A decreasing to a stable running
current of 12A in four seconds.

Findings
After double checking the wire connections, fan
direction, belt tension and alignment, I cycled the
motor on and off more than twenty times. During that
time the OCD tripped three times. Each time was within
about five seconds of startup. My ammeter would read
in the high 50s and drop to 12A in about four seconds
then the OCD would trip.

Questions
Is there something my ammeter is not telling me?
Even so, that class 10 OCD should handle ~95A for ten
seconds before tripping. (600%x15.8)
If I'm using the wrong or defective OCD what caused
cap in the first motor to blow?

Any suggestions would be appreciated.

 

[dbaec]

skiier

This is a cooking hood. The hood, fan, Ansul system and temp switch were all provided new 12/08 by the same company. I wired the temp switch and wall switch in parallel and supplied power to the hood fan. I'm not an expert on hood systems so if there's a better way to switch it I'd love to know.

anyone

What are the odds of two bad motors in three weeks. Same motor same hood, 2hp single ph 240V vertical mounted. I rewired the OLR last monday as jraef suggested and set it at 100% 12.7A. On Saturday, five days later it burned out again. This time it ran for an hour then shut down. The first time it burned out it just never started. There were burn and smoke marks coming out of the vent holes around the base (the side with the makeup chamber).

I've checked voltage and current draws several times all seem good. I don't have access to a logging meter or I'd use that.

My questions now are, is it possible that this motor should not be mounted vertcally, and what are the odds of two centrifugal starting switches (I'm assuming) in a row being bad in a vertically mounted motor?

 

[jraef]

The issues surrounding mounting position of a motor are usually around the bearings and thrust changes on them associated with mounting vertically if not designed for that. The end result around bearing failure is not usually spark inducing. The only caveat I can think of to that might be a failure of the centrifugal switch as you suggested. But is there a starting switch at all? It's common practice to use PSC motors in vent hoods and other high temperature applications such as that.

Can you post the motor data, i.e. mfr., part # etc?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[edison123]

May be a 3 phase motor (eliminating the troublesome villains of capacitors and centrigugal switches) would be the solution here.

 

[waross]

How is the condition of the starting switch after the motor fails? Is it covered with cooking grease? I am curious about the high temp switch. Some fire codes mandate that fans shut down in case of fire to avoid fanning a fire.

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

 

[dbaec]

Here's the motor nameplate and tag on the hood unit. I hope to get better pictures of motor and hood tomorrow.

 

[dbaec]

Sorry, here's the motor nameplate.

 

[jraef]

Those are capacitor start TEFC high efficiency motors. The standard product in their catalog is rated for 104F (40C) operation, so that's something to consider in this application, although they are special OEM versions so it's hard to tell what the OEM would have asked for as special features.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[dbaec]

I believe this is an ODP design. I also questioned the 40C ambient temp for this use. The rooftop temp here (Arizona) easily exceeds 104F, but I don't think that would cause a breakdown in only two or three weeks.

Some pics of motor and OLR setup. Currently a 1/2hp fan is being used in place of the other unit. In the OLR setup the two motor leads are tied directly to the 120V from the switch in the kitchen instead of to the OLR. The 240V for the motor had to be used also for the OLR coil, thats what the contactor on the right is for.

 

[dbaec]

motor

 

[dbaec]

burnout

 

[jraef]

dbaec,
Not to appear harsh, but that wiring is ALL WRONG!
The way it is wired, there is NO overload protection being provided. NONE. In addition, it appears as though you are running the motor power THROUGH the auxiliary contacts (97 and 98) of the OLR in stead of the Tele contactor coil.

I don't know what that Siemens contactor is supposed to be doing, but it is not doing anything useful there, in fact it appears to be not only redundant, but may also be partly the cause of your problems. It appears to be switching the tele contactor on, but also sending power through the OLR aux. contact directly to the motor. Without that contactor, the system would not have worked at all, but by adding it, all that was accomplished was to "fix" the error in a way that leaves the motor unprotected.

It also appears as though that motor power circuit is only sending 120V to that motor. How is it connected in the motor; 120V or 240V? If as you said you are connecting them for 240V, you are only sending 1/2 voltage to the motor! If that is the case, you would be overloading it immediately, but because your overload relay is not even being connected in the circuit, it is doing NOTHING and you are burning out that motor. I'm surprised they last as long as a week, in fact I'm suprised there has not been an open flame fire!

The entire setup needs to be scrapped, with the possible exception of the Tele contactor, since NOTHING is going through that except its own coil power. Then a qualified electrician who KNOWS how to connect a 1 phase motor power circuit nees to be brought in.

This is a fire waiting to happen, please cease running this equipment IMMEDIATELY.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[dbaec]

I appreciate your concern jreaf, your right about the setup in the picture not doing anything. I think I didn't make it clear that the contactor and OLR are disconnected and power is being passed straight through to a temporary 1/2hp unit. I assure you that it is wired correctly in normal operation.

Anyway the cause of the motor failures is the temp switch in the hood cycling on and off after busy days. So while the OLR is not seeing an overload condition, the motor doesn't have a chance to cool down between startups, eventually smoking the windings. We're going to use a timing device to ensure the motor stays running for several minutes anytime it's started.

Thanks for all the input.

 

[jraef]

You may also want to consider using a solid state OL relay, one with a few more sophisticated features such as Minimum Time Between Starts and Starts Per Hour limitation. These are relatively inexpensive now and would forgo the need for a timer that someone may not understand the need for at a later date. Some can also have their Phase Loss protection turned off for use on single phase motors, eliminating the need for that loop back wiring as well.

My recommendations for low cost units:
Symcom 777 Motor Saver (very popular), also brand labeled by Square-D and others
Motortronics RX Series



"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376