200hp motor starting across line every 8 minuets 3

[cuhead]

I'm working at a cold storage facility that has a compressor that is powered by a 200hp "us motor" desig B code F 440V 240 FLA 1200 RPM 65 year old motor. The controller is an across line size 5 Westinghouse starter (A200M5CXXZ1). The BMS system starts the motor evey 8 minuits and it runs for 4-5 minuets, satisfy pressure, then shuts off. The motor starter contacts consistantly fail and the line fuses blow. This problem has been going on since 1995. The owners of the facility are very fugal and save money wherever possible, hence no corrective action up to this point. What is causing the starter contact failer and what would be the most economical fix?

 

[davidbeach]

The most economical fix would be to run the motor continuously.

 

[itsmoked]

The contactor is failing through misuse, it's dying of premature aging.

You're not supposed to start a motor that big more than a couple of times an hour with at least 10 minutes between starts.

However, an old motor that's been doing this for almost 20 years must be ok with it - in this particular service.

When it dies it should be replaced with a much much smaller motor and compressor or pump that would run continuously to satisfy the same need.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

It is probably controlled by a pressure switch. Pressure switches with adjustable differential are available. Doubling the differential setting would increase the time between starts and give a correspondingly greater run time. This may double the life of the contacts.
A better fix, would be an unloader valve so that the motor may run continuously as David suggests.
There may already be an unloader valve that has failed and needs repair or replacement.

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

 

[jraef]

Actually motor starts-per-hour capability is a function of the mass/inertia it must accelerate, the load on it once accelerated, and the cooling time between starts. For all we know the load may be very light compared to the HP rating on the nameplate, which might explain the motor having lasted this long. That's what one would do if wanting to start a motor this often and not have it fail; up-size the motor.

But what is often forgotten in this process is that motor starters are designed for the BASIC starting duty of the size of motor they are selected to control, including a standard number of starts-per-hour as well, because the contacts must cool off in between starts too. So for example if the basic starts-per-hour rating of a 200HP motor is 1 cold start + 2 hot starts per hour with 15 minutes between starts, that will be what the starter was designed for as well. In addition, the hardest thing on contacts is not the starting, but the stopping of the motor; extinguishing the arc that forms as the contacts separate. That is where the damage happens, because the temperature of that arc is the same as the surface of the sun, the contact material is liquified on the surface. The more often it must deal with that arc, the less time it has to re-solidify in between, so more material is eventually vaporized each time.

Add to this the fact that Westinghouse has not made a motor starter in over 20 years. So in my experience here is what has taken place. The original starter lasted a long time, but the compressor did not cycle as often either. Then when the BMS was added and inappropriately programmed, it killed that old starter. But because it was by then obsolete, Cutler Hammer wanted a LOT of money for the replacement contacts because they were trying to convert people to their newer products. However your "frugal" owners found a source for cheaper replacement contacts from one of many aftermarket resources, most of which produce absolute crap, but they know that their primary market is exactly the type of cheapskates who would not know or care. So for years, they have been replacing bad contacts with bad contacts.

Be that as it may, the bottom line still remains that this is an inappropriate application. If your BMS is in total control of this compressor and nobody will deal with that duty cycle issue at that end, then the only viable option in my opinion will be a PROPERLY SIZED (because it too must cool off between starts) solid state starter, even if you do not bother with the soft starting aspect (although I certainly would). I would venture to say however that when the "frugal" owners see the cost they will elect to eskew that path as well, they will continue to throw good money after bad on replacing contacts at a very high long-term cost until the motor fails and maybe damages the compressor, forcing them to modernize the entire setup.

"Will work for (the memory of) salami"

 

[oldfieldguy]

Sounds like a good case for re-thinking the operation. I'd look at using the pressure switch to operate an loading valve instead of turning the motor on and off. That way, the motor will run continuously, saving wear and tear on the starter and motor.



old field guy

 

[electricpete]

Regarding motor (not contactor) starting limits), see also:
faq237-1285

The first link summarizes NEMA MG-10 guidance on the subject

http://www.joliettech.com/allowable_starts-intervals.htm

Column A - For 200 hp, 6-pole motor, they would suggest to limit starting to 4.5 starts per hour (column A).

Column B - column B gives another (possibly more restrictive) limit based on driven inertia in lb-ft^2

Column C gives some guidance about “rest or off time between starts”. I think this is intended to limit damage from short term (abuse where aluminum rotor may melt with repeated quick starts in short period of time regardless of long term pattern) as opposed to long-term chronic abuse limited by columns A and B. The words “rest or off time between starts” are an unfortunate choice of words imo, since the heat from the start itself is dissipated quicker while running (motor fan rotating) than while stationary (very little heat transfer). Assuming you intended to restart this motor in a few minutes and had flexibility in what fraction of that time is running and what fraction is secured, the worst thing you can do is start and immediately stop the motor… better to let it run most of the time between starts). For longer fixed starting intervals, the equation changes, starting heat may not be so much a factor at the time of next start. Most people easily grasp the cooling value of off-time, many people miss the cooling value of running-time for short-interval restarting.

I am not sure that upsizing the motor is a sure solution when we are concerned about effects of repetitive starting application on a motor (I'm not talking contactors). As you see in the chart, the starting limits are tighter for smaller motors. I acknowledge this does not take into account that the load inertia and torque will be smaller in relation to larger motor, but just wanted to caution against any blanket assumption that larger motor will cope with a repetitive starting application better than a smaller motor.


=====================================
(2B)+(2B)' ?

 

[electricpete]

Correction in bold

As you see in the chart, the starting limits are tighter for smaller motors.

should have been

As you see in the chart, the starting limits are tighter looser for smaller motors.

=====================================
(2B)+(2B)' ?

 

[DFordPE]

I haven't seen anything about using a Solid-State Reduced Voltage starter. My guess is that this system is so old that it was originally designed to run the motor constantly and let the compressor "load and unload."

The load inertia is only part of the equation because the motor sees inrush current every time it starts. Load only determines how long. Regardless, starting every 8 minutes is not recommended. Apparently, the motor is tougher than the starter.

VFDs can also be used to eliminate excessive starts and stops and potentially save some energy. However, it would require a better control system.

 

[squeeky]

How much history do you have? It would seem odd that someone designs this intentionally. The cycle time is too fast. It is time to climb out of the box. Is there anything you can do to slow the speed down like changing pulleys? I take it that this is a refrigeration compressor. Can you install some sort of an accumulator? Are you running at the correct temperature and is the differential correct? Open the differential. Check the on and off set points. If the 65 year old motor has been around a while and there is a new (less than 20 years) Building Management System which has been updated and the “Modern Engineer” not taken the existing design into consideration? Increase the air flow thereby increasing the demand on the system to draw more from it. Use part of the circuit to make ice to act as a storage medium and make ice in off peak and use it to supplement the cooling in high demand and save some money.

 

[jraef]

Refrigeration compressors are not as simplistic as air compressors as far as just putting in pressure switches etc., the entire system must be designed and operated properly. Having a BMS that bangs it on and off every 8 minutes is not right, but the solution is not typically in the EE realm, this is something that the ME responsible for the HVAC system must address as a system problem, then tell you what the electrical system changes need top be. That's why I said, if you, as the EE cannot get that taken care of, then in my opinion you, as the EE, can only deal with the EFFECTS of the problem. Using solid state controls will do that. The OVERALL problem will still exist, but the symptoms, as they relate to the electrical system, will be ameliorated.

"Will work for (the memory of) salami"

 

[waross]

From the age of the motor it is almost certainly a U frame motor. Compared to the replacement T frame motors the old U frames seemed almost a rugged as a hammer. The rewind shops would put extra insulation between the phases and we found rewinds to be even tougher than new motors. I spent some time in Western Canadian lumber mills many years ago. The mills had a practice of overloading and abusing their motors.
To survive a start every 8 minutes this HAS to be a U frame motor.

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

 

[DFordPE]

Wow, I didn't catch this originally, but 65 years old is pre-U-Frame. Not only that but some refrigeration compressors have specially built motors.

 

[racookpe1978]

"Frugal" owner, eh?

(Stupid and short-sighted more likely, but less us not get personal here.) 8<)

How much is this very high starting current, high runnning current, very short cycle, operation costing the owner is simple electric costs? What is the "cost" that is being used in electricity (bad publicity by not "being green" in today's market) by running the 200 HP motor this way?

Does that not give the EE a $$.$$ handle to demand answers from the HVAC/Building Management System/ME AND owner to start saving on the power bill?

Second half: The motor is inside the cooled area? Does not the excess motor current dump simple I^2R heat losses into the same AC system that essentially double the electric cost of this old system?