How long to idle 750 hp motor for cooling 4

[bacon4life]

How long should a 750 hp motor run at idle after unloading it to cool down the motor? This motor powers a water pump via a magnetic drive. The pump currently runs at idle for 30 minutes before shutting down. I was told the idle period was to cool down the pump, though I do not specifically know if the person was referring to the electric motor, the vertical lift water turbine, or the magnetic drive.

This long idle time appears to dramatically lower in overall power factor of the installation during months with low run times. Sometimes the installation has a power factor below 0.5, so the pump appears to be working for just a few minutes before idling for 30 minutes.

I remember thread mentioning that forcing a 2x start per hour motor to always run for at least 30 minutes might be much simpler control circuit than actually controlling the starts per hour. Before pursuing power factor correction, just want to check if there are any controls adjustments that should be made first.

 

[waross]

1. Power factor correction does not change the motor current, just the line current.
2. I was faced with a compressor on automatic start that was limited to 3 starts per hour.
That led to two issues.
a) The task of building a rolling counter to monitor starts per hour.
b) When air was needed by the plant, they could not and would not wait for the next allowable starting time.
Solution: Once started, the compressor ran for a minimum of 20 minutes. Air delivery during the run on time was controlled by an unloading valve.
With minimum 20 minute run times the three starts per hour could never be exceeded.
This seemed to be the simplest solution, and the simplest is often the most dependable with less points of failure.
If the installation is experiencing frequent low power factor I would address that issue, but as a separate issue, not particularly related to the run on time or starts per hour.
For power factor correction it may be better to look at the whole installation rather than at one motor.
Lightly loaded transformers will also hit the PF.

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

 

[Compositepro]

It appears that your real problem is that you have a severely oversized motor and pump for your application. Other solutions you should consider are:
>replace with one, two or, three smaller pumps to more closely match the load.
>larger storage tank to accommodate peak flow demands.

Is the low power factor actually costing you anything significant? When the motor is unloaded it will, of course, have a low power factor, but it will also be using little real power.

 

[bacon4life]

Bill-yes, that was the story I was thinking of. Luckily there seems to be enough auxiliary loads that the installation power factor is better than the penalty in months were the pumps are off the whole month.

Compositepro- Certainly eliminating the short cycling would help. The installation already has a several pumps, and mother nature set the size of the storage tank. When the pumps run fully loaded, the power factor penalty is fairly small. When the motor short cycles, the power factor penalty is pretty large.

 

[davidbeach]

Just out of curiosity, where are you pumping? Is this on the water is of the business, or is it some sort of pumped storage?

 

[LittleInch]

How exactly does a pump run "at idle"?? Open circuit will damage the pump, similarly dead head?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[edison123]

The magnetic coupling works on the eddy current principle to rotate the pump without directly contacting the motor shaft and the containment shell gets heated. The 30 minute idle spin is most likely for the eddy current shell to cool down. You can see a mag coupling simulation video here.

https://www.youtube.com/watch?v=fFXTioC1oJQ

A standard pf correction capacitor connected across the motor terminals, which is very common in industrial motors set up, can solve the low pf problem.


Muthu

http://www.edison.co.in

 

[LittleInch]

There's two types of mag coupling.

The one I'm familiar with is a permanent magnet type outside of the motor body.

The other type ("canned") essentially encapsulates the rotor within the pump itself.

Not clear which one we have here.

I'm having difficulty seeing that this thin metal shell will take long to cool down or indeed actually get that warm.

Or why a water pump needs a Mag drive coupling.

A 500 kW motor is a decent size so will have restrictions on starts, but you can usually do something by measuring winding temperature. No of starts is a fairly blunt tool.

Sounds quite like this post

https://www.eng-tips.com/viewthread.cfm?qid=470586

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LionelHutz]

I don't see why you need a cool down period. The motor shouldn't need any run time to cool. The magnetic drive shouldn't be producing any more heat once it's in the uncoupled state so nothing inside it should get any hotter than it was while working. The pump is stopped, so it doesn't need the motor running to cool. You need to dig in and find if there is any actual data which supports the cooling requirement.

The motor will have a specified number of starts per hour, but that is based on starting a load. With the motor on a magnetic drive it basically starts unloaded. So, you can get more starts per hour without thermally damaging the motor. Install a soft-starter if you're worried about the mechanical starting stresses.

Install a modern protection relay if you want to limit starts per hour and/or time between starts. A motor that size should have a decent protection relay (that is actually properly programmed) anyways.

 

[LittleInch]

It's the "idling" and "unloading" bit I don't understand.

Fixed speed electric motors and pumps don't "idle". a a large diesel engine can "idle"

Mag drives don't normally unload or disengage unless you're referring to a magnetic clutch??

Fill us in on the details and we can see what you can see, but if not we can't. It does sound like some confusion and an operating practice has developed over time, but now no one knows why.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

It's probably a magnetic coupling.
Diesel engines may have a "high idle".
That is; Running at speed but unloaded.
No matter how sophisticated your system is, monitoring winding temperatures, a high end protection relay, there is the possibility that the system will demand water and the protection system will refuse the demand.
With the simple cool down timer, the pump is always ready for a start if called upon.
There may be some discussion as to weather the 30 minutes cool down time may be shortened.

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

 

[LionelHutz]

The process makes no sense if you expect a 750hp pump to cycle on and off more than once or twice an hour.

 

[waross]

It is what it is.

during months with low run times.

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

 

[bacon4life]

Muthu-I'll take a look at the drives you mentioned to see about recommended cool down periods for the magnetic drives.

The cooldown requirement seems as strange to me as it does to LionelHutz and LittleInch. I work on the utility side of the meter, so I do not have access to very much detailed information about the installation. Before telling this customer that their installation makes no sense, I am trying to see if there are any reasonable explanations for the lengthy idle time.

This is an automated group of electric powered storm water pumps. The weather conditions impact pump run duration. I am not sure if the magnetic drives allow throttling of the pumps, are just for soft starting, or if they act as a clutch if/when debris clogs the pumps. I do not know exactly what the customer meant by running at idle, and whether they were referring to using the magnetic drive or some sort of valving arrangement.

 

[waross]

I expect that the drives are for soft starting.
Either all in or disconnected.
Running at idle will be running with the drive disconnected to cool the motor down.
Cool down time may be for the motor rather than the clutch.
OR
The manufacturer may have rated the motor/magnetic drive combination at two starts per hour.
With automatic start and with the flow depending on rain fall, it makes sense to keep the motor running but uncoupled for the cool down time.
The 30 minutes may start when the motor is started, not when the stop signal is given.
I have encountered similar situations with large air compressors.
The installation does not seem that bad.
A magnetic drive may be used for speed control in smaller sizes but not for 750 kW.
The losses in a magnetic drive on a 750 kW motor will be horrendous.
Consider the magnetic drive as a clutch.
Capacitors somewhere in the system will be good.

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

 

[LionelHutz]

I've seen magnetic drives that size used for speed control. The efficiency isn't that bad when you consider the variable torque characteristics of a centrifugal pump and that the pump speed stays at the upper end of the range.

Without more information, trying to come up with reasons is speculation. Maybe it is some application that has very large and short duration water demands.

I would bet that this cool down period isn't a cool down period, but for another reason. I don't see why any magnetic clutch or magnetic drive would require a cool down period. The motor certainly doesn't need to run idle to cool before being stopped.

The application needs a complete investigation, not just the blind installation of power factor capacitors because "that's how it is". When dealing with stuff like this I hate that answer because it's a lazy bullshit excuse for not putting in the time to figure out what is really going on. It's possible that leaving a 750hp motor running idle for most of 30 minutes every time a little water needs to be moved is the best solution, but you won't know until you dig into it and investigate the alternatives.

 

[LittleInch]

The only thing it sounds like is some sort of magnetic clutch arrangement similar to large air compressors where they commonly leave the motor running and the compressor "unloads". Now that could be disconnection of the load completely or that they let the air just drift blow thought the machine until you need more air. Reduces the number of starts as the air compressor start load can be one of the biggest on the plant.

Maybe a similar thing, but worth finding out.

If bacon4life (great handle btw) is on the utility side moaning about the terrible power factor then he needs to find out from the used what exactly is going on.

But the user really should have a better means of modulating the flow than just turning on a massive pump for a few minutes every hour.

Some different sized units or flow control would go a long way to solving this issue.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bacon4life]

The installation only runs during heavy rainfall events. I'll check to see if the customer can adjust the start/stop signals to give longer run times.

The power factor tariff is based on average utility cost to provide the service. For typical customers, the economic analysis for choosing to install PFC versus just paying the tariff rate for reactive power is pretty straight forward. For a unique customer like this with a highly unusual consumption pattern, I hope there is a more efficient alternative for all parties involved. In my region, rain is inversely correlated to peak loads. Thus the pumps only run at times it is easy for the utility to provide reactive power, and any PFC at this location would be offline at the system peak.

 

[Compositepro]

One reason to cool a large motor before allowing it to stop is to prevent thermal distortion and and unbalance. When stopped a hot motor will cool much faster on the bottom side than on the top side.

 

[waross]

When doing power factor corrections my first stop on site would be the accounting department.
"I need copies of the power bills for the last two years."
From the power bills I would glean the information needed to calculate the the needed correction.
In the absence of such information, I would suggest that correcting the PF of each motor will eliminate almost all PF penalties.

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