30hp motors, start, stop repeat

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chris kennedy

chris kennedy

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Location
Miami Fla.
Occupation
60 yr old tool twisting electrician
I was at a facility today and witnessed material processing I had never seen. This is a C&D MRF (construction and demolition material recovery facility).
I am very familiar with these as I have been building and maintaining these for at least 10 years now.

Today they were processing only concrete and were starting and stopping 2 30hp vibratory finger screens at around 1 minute run time and 30 seconds rest time.
Normal C&D processing has these running continuously all day resting at lunch time only with the exception of stoppages for jams and such.

I'm concerned the heat generated at motor starting is not adequately being dissipated before the next start. DOL starting method.
Anyone else see this as a potential problem?

Attached is a crappy video taken today.
 
What are the motor temperatures after running that way for several hours?

How heavily loaded are they? It's possible (likely? hopeful?) that the original engineers considered the frequent starts & stop cycle, and factored that into the design by specifying larger motors.

What kind of overload-protective devices are in use? If the motors aren't getting adequate time to cool, thermal overload elements in the starters won't adequately cool off, either.
 
This is an older machine, job #2217, I finished a new install a month ago job #27??. The overloads are old school ELO's with dials and dip switches so I'm curios to see what they are set at. New installs use ELO's you can only adjust via remote data link from manufacture engineers or via blue tooth by me in the field.
The engineers designed this with a push button pause button for each finger screen. Old school, now pause and speed control are all on the HMI touch screen.

Monday I'll get with the facilities guys and the manufactures engineers and find out how often they do this and is the manufacture is aware of this practice.
Another issue I find with these facilities is they don't clean all the crap that settles on top of the motors smothering the cooling fins.
 
There is no start current issue, my concern for this particular process is that the motors aren't running long enough to cool down before they are stopped and started again.
 
I meant under normal operating conditions where these motors run from 6AM to 11AM then from 12PM to 4 or 5PM with the occasional pause for jams there is no issue with starting current.
 
If you DO need fast repeated starts, the solution is to over size the motor. So we don’t know that this has not already been factored in, ie from a torque and speed standpoint the task needed a 10Hp motor, but they upsized it to 30HP because of the duty cycle. That’s a question for the machine designer.

Now, if the motors burn out quickly, that would be the prime suspicion for sure. People tend to not think about overdoing things with machines, especially in that industry.
 
chris kennedy said:
There is no start current issue, my concern for this particular process is that the motors aren't running long enough to cool down before they are stopped and started again.
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let it run this way for a few hours and then go touch the motor. If you get 2nd degree burns you might have a problem. Third degree burns, a definite problem. No burns, or first degree burns, no problem.
 
petersonra said:
let it run this way for a few hours and then go touch the motor. If you get 2nd degree burns you might have a problem. Third degree burns, a definite problem. No burns, or first degree burns, no problem.
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And all these years I've been telling customers touching the motor won't indicate an overloaded condition. Now thay I have no customers, the truth comes out.🙂
 
chris kennedy said:
I meant under normal operating conditions where these motors run from 6AM to 11AM then from 12PM to 4 or 5PM with the occasional pause for jams there is no issue with starting current.
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You might be surprised what “normal operating conditions” are. For hazardous locations, see Section 500.8(B)(5). The concept is repeated in NEMA MG1 for other applications.

Typically for motors, it is the insulation rating that is the actual determining factor. Motors can be designed for multiple rapidly repeated starts. For application guidance it is best to consult with the motor manufacturer.

General purpose motors, even oversized, rarely can handle it because the starting current is not a function of the load until fairly “late” in the acceleration.

While ASD applications may also work, they generally aren’t as cost effective as a “task relevant” motor design.
 
When a motor is starting, the initial current flow is 4-7x the normal full load current. Motor conductor heating is proportional to the square of the current flow. So during starting the motor is generating heat much more rapidly then during normal operation.

However the motor isn't damaged by heating rate; it is damaged by insulation temperature. (Well, mostly. Changing temperature means things expanding and contracting which can mean components rubbing....) If the motor starts up quickly (very short duration for acceleration) and then runs with well below full load (so that heat production during run is below what the motor is rated for), then it will likely not have a problem with this rapid start/run/stop/rest operation.

You can measure this, if you have a tool which will take the RMS current where the averaging period is a complete start/run/stop/rest cycle. Heating is proportional to the square of current flow, and the motor is rated for a particular current flow, so if the RMS current flow averaged over a complete operating cycle is less than rated the motor should be fine. (Note: presuming a complete cycle is shorter than the thermal time constant of the motor. You can run a motor at 5x rated current for 4% of the time as long as the run period is short enough that you don't completely cook the motor before you get into the cooling period.)

-Jon
 
winnie said:
... You can run a motor at 5x rated current for 4% of the time as long as the run period is short enough ...
Click to expand...
True, but not quite complete.
There does need to be a favorable balance between total heat production and total heat rejection that keeps the motor's temperature down to an acceptable limit.
The rated continuous current is based on the motor's fan turning continuously. In start-stop operation, the fan is turning only part of the time and the total heat rejection is less.
 
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