Starts per hour with MV motors

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philly

Senior Member
I am aware that there is really no specified starts/hour with medium voltage motors (4.16kV), and that most manufacturers use the NEMA MG-1 standard that says the given motor can have two consecutive cold starts and one hot start for a given load intertia.

Although its not specifically spelled out, I believe it is a standard approach to limit starts to 2 starts per hour, and we somewhat follow this philosophy when setting up our relays.

What if however the driven load has an inertia much less then specified on the datasheet, can you then achieve more consecutive starts, or more starts per hour? For instance what if your relay curve is set below the motor cold rotor thermal damage curve to protect the motor, and when starting you only use up 20% of the thermal capacity? At first glance this would tell me that I would be capable of ahieving much more than 2 consecutive starts (maybe 5) before I reach 100% thermal capacity. My question is should we allow more consecutive starts in this case or should we still limit to the 2 consecutive starts? This would kind of go hand in hand with the number of starts per hour as well.
 

mivey

Senior Member
You can start anytime the temperature has stabilized to normal running temperature. You can do two starts anytime the temperature is within 5C of ambient.

If the motor can cool back down to a stabilized normal temperature 10 times an hour, you can start 10 times. It is all about the internal temperature, not necessarily time. The time becomes a factor only because of how long it takes to cool down the innards.


Add: MG-1 has more info than just the 2-1 data. You could always ask the motor manufacturer for more detail on increasing the "default" number of starts.
 
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philly

Senior Member
If the motor can cool back down to a stabilized normal temperature 10 times an hour, you can start 10 times. It is all about the internal temperature, not necessarily time. The time becomes a factor only because of how long it takes to cool down the innards.

What are you referring to as stabilized normal temperature?
 
You can start anytime the temperature has stabilized to normal running temperature. You can do two starts anytime the temperature is within 5C of ambient.

If the motor can cool back down to a stabilized normal temperature 10 times an hour, you can start 10 times. It is all about the internal temperature, not necessarily time. The time becomes a factor only because of how long it takes to cool down the innards.


Add: MG-1 has more info than just the 2-1 data. You could always ask the motor manufacturer for more detail on increasing the "default" number of starts.
MG-1 is a minimum standard. Keep in mind though that there is no, or very limited, punitive recourse if a manufacturer says that their motors are constructed to MG-1, but in actuality they are not meeting the requirements of MG-1. Since there is no third party independent testing practiced, it remains a wild-west territory.

MG-1 is clear that the restart restriction refers to 100% inertia. The motor algorithm in most SS motor protection relays calculates the safe restart temperature and based on that may give you the safe restart time. When, in addition to the current, RTD's are available and connected, this number becomes less conservative. The relay usually initialized, that occurs at the first start where it profiles the start-up characteristics T/C and builds a mathematical model for the motor/load combination. Each manufacturer of relay has their motor algorithm. Obviously those who have access to motor manufacturing data would have a better chance to build a more realistic model. These would be Siemens, GE, ABB, etc. that builds both relays AND motors.

While Mivey is correct that the determination is temperature based, but the safe restart temperature varies with the inertia and specific motor configuration. Eg. the larger the motor mass is the lower the required ' 'safe' temperature will be. The same is true inversely with the driven mass. The ambient temperature will also have effect.

MG-1 gives no other guidance than to talk to the motor manufacturer. The other warning it states that: 'It should be recognized that the number of starts should be kept to a minimum since the life of the motor is affected by the number of starts.'

So talk to the manufacturer AND add a protective relay that will help you determine the safe restart.


 

mivey

Senior Member
So talk to the manufacturer AND add a protective relay that will help you determine the safe restart.
Do you find most of the manufacturers of these large motors to be very responsive? I wonder why they don't include this type of information to start with (no pun intended)?
 

rcwilson

Senior Member
Location
Redmond, WA
Good advice above.

The difference in running and stopped cool down times surprised me when I first saw it on a relay's motor winding temperature recording. A 2000 HP motor drove a pump through a hydraulic variable speed coupling so it could start and run essentially unloaded. When the motor started and ran unloaded for 10 minutes, the temperature was much less than after an aborted start when the motor hit full speed, tripped, coasted to a stop and sat for 10 minutes.

When it was running at light load, the cooling fan cooled the motor. The only cooling when stopped was by radiation and conduction so it took a lot longer to cool down.

The number of starts depends on what the motor is doing between the starts and the nature of the start.

These motors hit full speed so fast (< 2 .5 seconds) that it was possible to do 5-6 starts an hour after we verified that the cool down times and other values in the protection relay matched the actual motor characteristics. The motor vendor was involved.

One concern was rotor heating during acceleration. Rotor temperature is not measured by the RTD's. It has to be estimated by the relay software. Just depending on a winding RTD to tell if its OK to start may lead to an overheated rotor with melted bar joints.
 
Do you find most of the manufacturers of these large motors to be very responsive? I wonder why they don't include this type of information to start with (no pun intended)?

It is not that simple. They need your data in order to calcualte it and it is a case-by-case data. I guess they should be able to supply you with a curve, but on most applications this seem to be woring well. It is perhaps also why MG-1 does not give requirements at any other loads.
 

Besoeker

Senior Member
Location
UK
FWIW we usually specify two consecutive starts from cold and not more than four per hour. That said, we have a couple of 1250kW 11kV starters going through our works at the moment where I have made it not more than two per hour. Special circumstances and, as Laszlo says, you need to look at that for each case.
 
It is not that simple. They need your data in order to calcualte it and it is a case-by-case data. I guess they should be able to supply you with a curve, but on most applications this seem to be woring well. It is perhaps also why MG-1 does not give requirements at any other loads.

Off subject. I hate it that we don't have a spell-checker on this dang machine. I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.

I 'learned' typing in the air force with two fingers, hunt and peck. Now I advanced to four fingers, but still don't look at the screen when I am typing. When I went to school we did not have keyboarding. Does that sound ancient to some of ya?

Anyways, this is the only public apology I offer for my typing, but I am always regretful when some of that slips through. Don?t cut me slack though for bad English, nor should you expect the same from me.:mad:
 

mivey

Senior Member
Off subject. I hate it that we don't have a spell-checker on this dang machine. I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.

I 'learned' typing in the air force with two fingers, hunt and peck. Now I advanced to four fingers, but still don't look at the screen when I am typing. When I went to school we did not have keyboarding. Does that sound ancient to some of ya?

Anyways, this is the only public apology I offer for my typing, but I am always regretful when some of that slips through. Don?t cut me slack though for bad English, nor should you expect the same from me.:mad:
You can get a spell check that runs in IE if the IS department will let you download it (I can't imagine why they would think it was an issue). There may be a grammar checker as well but I have not looked.
 

Besoeker

Senior Member
Location
UK
I often just cut and paste it into MSWords, let it catch the errors and paste it pack. AT home I sue Firefox on Vista and it DOES spell-check right in the forum as I type.
Make sure you have plenty of money before you start that action........:grin:
Struck me as funny given the context. And it's also an example of something that MSWord wouldn't pick up. I've also had a few odd "corrections" offered by Word. In a quotation I was preparing (in an old version) it wanted to change "Borehole Pumpset" into "Brothel Dump Site". I was vaguely tempted to leave it but, regrettably, common sense kicked in.......

Like you, I also use Firefox. It's on my laptop which is mostly what I use everywhere. The spell check is handy. My one reservation is that I can't find an option for British English. Obviously US English is fine here but, if I used it in other places, I might have to apologize for spelling apologise incorrectly.
:cool:
 

Besoeker

Senior Member
Location
UK
Tools -> Options -> Content tab -> Languages section. There are options for us/uk/ie/nz/au/etc. Twelve dialects of English in all.
It is set to British English [en-gb] but it still doesn't like colour or apologise.
 

jdsmith

Senior Member
Location
Ohio
Do you find most of the manufacturers of these large motors to be very responsive? I wonder why they don't include this type of information to start with (no pun intended)?

If you can provide accurate load curves, they will provide you with start/hour information based on those curves. The problem is that the pump, fan, and compressor curves that we typically see are not all that accurate. When you give semi-accurate load curves to the motor manufacturer they can give your starts/hr, but it is only as accurate as the data it is based on.

...
One concern was rotor heating during acceleration. Rotor temperature is not measured by the RTD's. It has to be estimated by the relay software. Just depending on a winding RTD to tell if its OK to start may lead to an overheated rotor with melted bar joints.

It is possible to get rotor RTD's on synchronous motors. RTD's and instrumentation can be installed on the rotor with a wireless link to a stationary receiver. This only works on synchronous because the rotor instrumentation needs a power supply. A wireless rotor RTD package can be installed for $25k or so, which is a good investment on a motor worth a million dollars or more. I just priced 3000 HP 1200 RPM blower motors - roughly $200k each for induction and $800k each for 1.0 PF synchronous. For that kind of money I would install rotor RTD's and get a real picture of what the temperature is in there.
 
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