200hp motor starting across line every 8 minuets
200hp motor starting across line every 8 minuets
(OP)
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?





RE: 200hp motor starting across line every 8 minuets
RE: 200hp motor starting across line every 8 minuets
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
RE: 200hp motor starting across line every 8 minuets
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
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"Why not the best?"
Jimmy Carter
RE: 200hp motor starting across line every 8 minuets
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"
RE: 200hp motor starting across line every 8 minuets
old field guy
RE: 200hp motor starting across line every 8 minuets
FAQ237-1285: Limits for repetitive induction motor DOL starting
The first link summarizes NEMA MG-10 guidance on the subject
http://www.joliettech.com/allowable_starts-interva...
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.
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(2B)+(2B)' ?
RE: 200hp motor starting across line every 8 minuets
should have been
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(2B)+(2B)' ?
RE: 200hp motor starting across line every 8 minuets
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.
RE: 200hp motor starting across line every 8 minuets
RE: 200hp motor starting across line every 8 minuets
"Will work for (the memory of) salami"
RE: 200hp motor starting across line every 8 minuets
To survive a start every 8 minutes this HAS to be a U frame motor.
Bill
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"Why not the best?"
Jimmy Carter
RE: 200hp motor starting across line every 8 minuets
RE: 200hp motor starting across line every 8 minuets
(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?