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interpretting temperature rise and duty cycle for an intermittent run motor

interpretting temperature rise and duty cycle for an intermittent run motor

interpretting temperature rise and duty cycle for an intermittent run motor

(OP)
I'm not familiar with rating practices for intermittent duty motors. Maybe some of you are more familiar than me and may have some insights on this question.

MOTOR DESCRIPTION:
  • Nameplate on slide 1:
  • 1.5hp, ~1800rpm, intermittent duty, Class H insulation, 115C ambient, 40C rise. FLA=2.25A
  • No, i don't have the ambient and rise reversed, this is designed to operate in a very hot steamy environment.
  • It is also sealed to prevent steam from entering into the stator, hence no external fan because that would require casing penetration (there are not even fins).
  • It drives a 3 piston positive displacement oil pump (slide 5). The oil pump intermittently pumps oil from the low pressure side of the system back to the high pressure side of the system to "charge up" the nitrogen-blanketed accumulator.
  • Some of the accompanying paperwork states "50% duty cycle" along with the other nameplate information listed above. It doesn't explain anything about that duty cycle and I'm not familiar with how intermittent duty cycle ratings.
  • The same pump/motor has been installed for 40 years. It's one of 8 similar units that operate relatively problem free
QUESTION 1: Should I interpret that at rated load with 50% duty cycle (of sufficiently short pulse width, more later) that it doesn't exceed 40C rise? Or maybe that it averages 40C rise?

Obviously there is a limit for how long the pulse width can be for a 50% duty cycle. If you run it one year on and one year off it's not 50% duty cycle. To my thinking the pulse width short be short relative to the thermal time constant. Thermal time constant is R*C where R is thermal resistance and C is thermal capacity. R is pretty high because there is not good heat transfer from winding to ambient on this beast. C is also somewhat high because there's a lot of extra mass in that casing along with typical stator and rotor core etc. So if I were to run that thing to steady state temperature and then shut it off, I figure it would take at least 30 minutes for the winding temeprature rise to get down to 1/e of it's orignal value. That leads to QUESTION 2.

QUESTION 2 - if the actual duty cycle was 5 minutes (which by my reckoning is much less than the time constant), then it should obey the nameplate rating of 40C rise (either peak or average as per question 1), correct?

PS - Questions 1 and 2 assume the loading magnitude does not exceed what was assumed in the rating. I'll get back to that assumption later.

ok, you may have figured out there is a backstory. I'll give the backstory but the details are a little fuzzy so I'm not necessarily asking anyone to solve it, but you're welcome to give ideas.

BACKSTORY (Weekend event)
  • Low pressure alarm on the oil system.
  • Pump found running continuously. Motor was around 280F on the DE bracket area and 260F on the ODE cap. (That doesn't necessarily surprise me, it's not designed to operate continuously)
  • Found and corrected nitrogen leak which was causing excess cycling.
  • Various other things happened and the motor was allowed to cool. The cooling time is critical information imo but since this happened on the weekend I honestly don't have the numbers but others assure me that it was long enough and the motor outside was room temperature prior to start (which imo doesn't ensure the winding was rooom temeprature). So I'm acknowledging some uncertainty in this data but it is what it is.
  • The motor was started an run for 5 minutes. They didn't see the expected rise in pressure. The pump/motor sounded "quiet". During those 5 minutes it rose to above 200F on the casing.
  • On the basis of the above indications the deiciosn was made to replace the motor and the pump with a new one.
  • the new/replacement stator had damaged leads, so the original stator (and cup-shaped frame) was re-used and combined/reassembled to the new rotor/bearings and new end bracket and coupled to the new pump.
  • After replacement , the pressure charges up as expected. The motor no longer runs hot.
FOLLOUP INSPECTION ON MONDAY
  • All of that happened over the weekend when I was at home. Then on Monday I was asked to explain what happened. I did have the opportunity to inspect the leftover pieces shown on slide 2-5 which is a frankenstein combinaton of the new stator and the old/removed rotor, bearings, pump.
  • Both motor bearings seem very loose, but they rotate smoothly. The grease was darkened, but it wiped off easily from the shaft adjacent to the bearing and the shaft underneath is shiny rather than varnished (which indicates to me the grease discoloration was from long-term aging, not from short-term overheating).
  • No obvious problems with the rotor.
  • The pump rotates smoothly in both directions. It was noticed that one of the three piston valves makes a different noise than the others.
DISCUSSION:

The removed parts will be sent off for shop testing, but I am left wondering what was the cause of the malfunction that led to the decision to replace the pump / motor. The basis was that it didn't develop pressure, it was quiet, and the motor got hot quickly.

I do agree that the 200F after 5 minutes seems unexpected, if we make the following assumptions:
  • ASSUMPTION 1 - the motor was truly cool at the beginning of the 5 minute run
  • ASSUMPTION 2 the connected load was the same as assumed in the rating (we'll talk about that more below)
IF those assumptions hold, then something doesn't add up. In that circumstances our ACTUAL ambient temperature was less than 40C. We operated for less than the time constant of the motor so I don't expect the winding would reach the 50% duty cycle temperature rise, but if it did that would be 40C+40C=80C (160F) on the winding and even lower on the casing. (Do you agree?).

There was a finger of suspicion pointed at the motor. But I don't think it can possibly be the motor because: The old stator was reused and is working fine; The old rotor looks fine; The old beairngs rotate fine even though they are loose.

So that leaves me with either the pump or the assumptions. (I'm going to rule out voltage unbalance or reduced voltage... no problems were seen in terminations of the old motor and the new motor is running fine on the same power supply).

QUESTION 3 - is there a valve malfunction on a smoothly-rotating PD pump that can cause it to overload the motor? (there are three pistons in parallel...they all suck from the same large open area in the center and discharge to the same triangle-shaped chamber on the outside although at different times since they are driven by eccentric center hub which pushes them at different times during the rotation... 3 equally spaced valve clicks are heard per rotation but one sounds different than the others)

Let's revisit assumption 1. I don't rule out that the motor felt cool on the outside but was still hotter on the inside from previous running , so then the outside warmed quicker than it otherwise would have during those 5 minutes. But there was another datapoint that went along with that the the people present concluded the pressure rise was not normal during this period. So that suggests maybe there was something else going on.

Let's revisit assumption 2. It is an assumption that I made myself is that the PD pump (assuming it's operating properly and setting aside the valve malfunction postulated above) consumes less than or equal to the amount of power assumed in the motor rating. I guess that's an assumption I invented without any basis. And maybe I am way off base. So I'd like to ask if that is typical design practice or not. In the real world application of this pump/motor, the duty cycle is far less than 50%. Does that mean the designer may have paired the motor with a pump that draws higher power than would be assumed in the duty cycle ratings? (yes I realize there are far more direct ways to get the info about the pump rating... I haven't found it in our paperwork... I did ask for a current measurement when the pump runs along with some measurements of casing temperature after various runtimes, but haven't gotten that yet).

Another thing I was thinking about. The motor never tripped during any of this including the initial event when it presumably operated for a long period of time and we saw 280F on the casing. Would you expect this indicates the motor should have tripped on overload?

Also the guy who took the initial measurements 280/260 over the weekend reported the temperatures at locations that seem to be linked to the bearings. I'm left with the impression that the area between them was not as high but I'll have to talk to him about that. I am mildly suspicious of all the information reported from the weekend since I'm getting it 2nd hand and in some cases 3rd hand (told to me by people who didn't see it themselves).

attachment link

RE: interpretting temperature rise and duty cycle for an intermittent run motor

First guess:
Air lock in the hydraulic system.
Pump was running at no load; hence, quiet and little heat generated, pressure not building.

Intermittent duty without knowing the duty cycle is an unanswered question, but:

Quote (Canadian Electrical Code)

Intermittent duty — a requirement of service that demands operation for definitely specified alternate
intervals of
(a) load and no-load;
(b) load and rest; or
(c) load, no-load, and rest.

While not directly applicable, the concept of RMS loading presented in the Cowern Papers may be interesting.

Quote (Cowern Papers)

Cowern Papers

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: interpretting temperature rise and duty cycle for an intermittent run motor

(OP)
Good points, Bill

I didn't realize there are different definitions of duty cycle either turning off or simply unloading. In our actual application of course it turns off but I don't know anything about what was assumed in the rating.

Air/gas in the system seems plausible after having troubleshooted a nitrogen leak. It might explain why the pressure didn't go up. It wouldn't exactly explain why the motor got hot quicker than expected, but maybe there are multiple things going on (like it was never truly cooled from previous running)

RE: interpretting temperature rise and duty cycle for an intermittent run motor

Can you do a growler test on the rotor?
Is it possible that a valve stuck closed and the motor was stalled?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: interpretting temperature rise and duty cycle for an intermittent run motor

(OP)
If I can't see anything visually, I wouldn't normally expect rotor problem bad enough to interfere with operation of the motor. But you never know. I'll add a rotor test to the shop requirements when we send the motor off.

Yes, I can imagine if a valve port is stuck closed it can create a high load. I might think about suggesting a performance test when that pump is sent to the shop. I should mention that when we listened to the valves as we rotated the pump shaft with the casing open and no fluid... the pump casing remained in one position... I might go back and explore whether the orientation of the valves with respect to gravity may have played a role in the sound by moving the casing around a little bit.

RE: interpretting temperature rise and duty cycle for an intermittent run motor

Energized for a long time and not building pressure.
Quiet.
Extreme heat build-up?
There would probably be some 60 Hz humming that may be mistaken for a motor running.
On a non-standard, high temp motor, I would not depend on our usual "Rules of Thumb" for evaluating operating temps.
The motor was at room temp, I assume mechanical contact between the windings, core and housing.
The windings were probably hotter than the housing, but not much hotter when we are talking about a motor rated to run in such a high ambient.
I have always considered intermittent duty motors to be more robust than standard motors due to possible frequent starting.
If the pump suction chamber was dry, the pump may not have been able to develop enough suction to start pumping.

Anecdote alert:
I have a 100 Gallon auxiliary fuel storage tank on my truck.
It has a 12 Volt transfer pump to transfer fuel into my truck fuel tank,(And into my tractor from time to time)
The pump is a positive displacement gear type pump.
The maximum suction lift is about 36 inches, depending on the level of fuel in the tank.
Occasionally, the pump drys out and won't pump.
Then I lift the fill hose so that a few ounces of fuel run back into the pump and it will be able to start pumping.
With the low cylinder volume of a higher pressure piston style pump, the effect of a dry pump may be worse.
Could the pump have run dry as a result of the nitrogen leak?
If the pump was not pumping due to vapour lock, the temperature may have been normal.
It's hard to say what to expect from a special high temp motor.

Pete; Is the oil hot?
If the crew was able to make measurements on the motor, at room temperature, I wonder why it is rated for such a high ambient temperature?

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: interpretting temperature rise and duty cycle for an intermittent run motor

(OP)
The oil is not hot during normal operation (including over the weekend).

The motor is designed to operate in either normal or emergency conditions.
In normal conditions the ambient temperature is typical < 40C.
There is a particular postulated emergency condition where the motor may have to run with the ambient much higher near 115C. So it was designed and specified to withstand that.
Since it is designed for emergency operation, I expect that it has plenty of thermal margin for normal operation.

RE: interpretting temperature rise and duty cycle for an intermittent run motor

Possibly suggest to the shop to investigate the pump pistons and cylinders for signs of scarcity of oil.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: interpretting temperature rise and duty cycle for an intermittent run motor

(OP)

Quote (electricpete)

Let's revisit assumption 2. It is an assumption that I made myself is that the PD pump (assuming it's operating properly and setting aside the valve malfunction postulated above) consumes less than or equal to the amount of power assumed in the motor rating. I guess that's an assumption I invented without any basis. And maybe I am way off base. So I'd like to ask if that is typical design practice or not. In the real world application of this pump/motor, the duty cycle is far less than 50%. Does that mean the designer may have paired the motor with a pump that draws higher power than would be assumed in the duty cycle ratings? (yes I realize there are far more direct ways to get the info about the pump rating... I haven't found it in our paperwork... I did ask for a current measurement when the pump runs along with some measurements of casing temperature after various runtimes, but haven't gotten that yet).
Now I do have results from a monitored 5 minute run of the motor after everything was returned to normal.
  • The current drawn was 1.55A (less then nameplate of 2.25A).
  • At the end of the 5 minute run, the temperature reached 114F on the inboard bearing area and 112F on the center stator casing and 112F on the outboard bearing area

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