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Locked Rotor Withstand Time (LRWT)

04/12/2011 8:33 AM

AA) Could an expert/expert define the meaning of "Locked Rotor Withstand Time (LRWT)" and how many seconds can be accepted and in which Standard? (NEMA or IEC, etc.)

BB) Is there any Standard that gives the definition and allowable 'LRWT' for Induction Moror having DOL Starting, 125HP, 150 FLA, 440 V/3Ph /50Hz and to which LRWT limits can be selected?

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#1

Re: Locked Rotor Withstand Time (LRWT)

04/12/2011 8:43 AM

→ British Standard 7671.

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#2

Re: Locked Rotor Withstand Time (LRWT)

04/12/2011 11:12 AM

You may also need to consult with specific manufacturers.

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#3

Re: Locked Rotor Withstand Time (LRWT)

04/12/2011 11:58 AM

Why did you not like the answers you got on your earlier thread? http://cr4.globalspec.com/thread/67777/Motor-With-High-Starting-Current-And-Long-Starting-Time

Normally, CR4 people don't like the same question being asked again. However.....

i think like this : The compressor motor in my fridge has a certain starting time, say 10 sec. The LRWT of that is only 5 sec say. Why? When it runs, the refrigerant circulating thru the motor cools it, so the starting inrush can last longer. If it stalls, no refrigerant circulating, so motor can burnout. An example of starting current and time being longer than LRWT. Kapish ?

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#5
In reply to #3

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 3:26 AM

Sorry to get in the way of your post. However, I believe the original poster of this request was not referring to an induction motor. A three phase 440V 50Hz, 125Hp motor is in no way related to a single phase induction motor as your link suggests. Three phase motors do not require an induction coil or a start up circuit to cycle operation. So, here is my question to the original post. What are you expecting this motor to do? It appears to be load dependent and I am not too sure what the maximum starting load is. There are three NEMA thermal trip class options for start up related to circuit breakers and, to a lower extent, system wiring if this is all you need, class A,B,C as relates to circuit beakers. If the load exceeds these limits you will likely need a stepper relay system or you may want to try a slip clutch or auto trans system. Good Luck!

Raga

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#7
In reply to #5

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 5:00 AM

My friend, if you had bothered to click on that link, it would have taken you to Candlez own post, which reads as follows:

How do I size coordinated circuit breaker, starterand power cable for motor having a high starting curent and long starting time such as the following motor:

A Squirrel Cage Induction Motor D.O.L starting, having 440 Volts, 90 KWs, FLC 150 Amp, LRC/FLC = 855%, Allowable Run Up Time (ART) at 80% Nominal Voltage is 44 Seconds and the locked Rotor Withstand Time (LRWT) at 80% Nominal Voltage is as follows: Warm 17 Sec. and Cold 50 sec. and considering the actual asymmetrical inrush current could be, according to NEMA manufacturers, as much as two times the LRC.?

Firstly, could you verify that the motor can operate normally in view of the above ART and LRWT values stated above?

Secondly, Can we use cable size, motor DOL starter and circuit breaker the same as if this motor has a starting performance as per NEMA MG1 letter 'G'? If not, why?

Detailed answer is appreciated ..

So, i am unable to make out what you are trying to say...single phase, three phase, induction coil...???

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#20
In reply to #3

Re: Locked Rotor Withstand Time (LRWT)

05/30/2011 1:27 AM

Hi

i am working in steel plant rolling mill here lot of roller table geared motors required to install which are always operating at higher temperature. all should run with the unoform speed. so i wondering about the stall time of motors ...?

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#21
In reply to #20

Re: Locked Rotor Withstand Time (LRWT)

05/30/2011 7:52 AM

What kind of motors are these ? Slip-ring motors? Are they controlled by solid-state starters ?

In any case, you need to get this data from the manufacturer of the motors. No code or standard will define your situation exactly.

For example, i have seen a complete turnkey rolling mill system set up in Bokaro by Siemens. They have done all the calculations and guarantee the coherent functioning...

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#4

Re: Locked Rotor Withstand Time (LRWT)

04/12/2011 4:43 PM

Locked Rotor Withstand Time is the time the motor can withstand without any damage when it draws the current equal to the locked rotor current. It is also called Motor safe stall point. Per NEMA MG 1, section 12.49, "Polyphase motors having outputs not exceeding 500 horsepower and rated voltage not exceeding 1kV shall be capable of withstanding locked-rotor current for not less than 12 seconds when the motor is initially at normal operating temperatures".

It is an important data for selecting the trip class of the motor overload relay. 12 seconds is the minimum time specified in NEMA, but most of the cases it is more than this. The motor data sheet should have this value. Generally there are two values specified for this parameter, one is cold condition and another is hot condition.

- MS

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#6
In reply to #4

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 4:25 AM

It is a good reply. The only additional thing I would like to add is ambient temperature consideration that has a bearing on with stand time.

Usually during starting, the starting current is same as locked rotor current till the motor picks upto its rated speed. The mechanical load torque on the motor during starting has a bearing on the motor starting time.

The safe time upto which the motor can have this locked rotor or starting current without causing any detrimental effects on any of its own components is the LRWT.

This characteristics is usually expressed as inverse characteristic.

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#15
In reply to #6

Re: Locked Rotor Withstand Time (LRWT)

04/17/2011 3:35 PM

What do you think ( RT) and (ART) ie. Run up Time and Allowable Run up Time mean, and how they are coordinated with LRWT ?

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#12
In reply to #4

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 8:22 AM

I add a score to the expert Mr.msamad. To support / strengthen the discussion I would like to add the following few lines.

The safe stall time or Locked rotor withstand time of an electric machine is often compared with the time required to accelerate the motor and the load from standstill to rated speed. Most users of electric machines feel that the safe stall time must be longer than the acceleration time in order to safely protect the unit against thermal damage. It is also felt that if the stall time with respect to the acceleration time is longer, the more frequently a large motor may be started. These ideas embody several of the many myths surrounding these two machine parameters since, in reality, greater thermal damage often results more from the frequency of starting than from the acceleration time being greater than the stall time.

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#17
In reply to #12

Re: Locked Rotor Withstand Time (LRWT)

04/17/2011 3:52 PM

Would you explain Run Up and Allowable Run up Time (RT) & (ART) in the same way you explained (LRWT) and how all can be coordinated with LRWT ?

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#14
In reply to #4

Re: Locked Rotor Withstand Time (LRWT)

04/17/2011 3:26 PM

To msamad

Would you explain Run Up and Allowable Run up Time (RT) & (ART) in the same way you explained (LRWT) and how all can be coordinated? Give detail if you wish.

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#18
In reply to #14

Re: Locked Rotor Withstand Time (LRWT)

04/17/2011 8:26 PM

Maximum Allowed Run up Time (ART) is the maximum time the motor can take to accelerate the load to full speed for Direct on Line (DOL) start. The ART should be less than the LRWT. In the relay coordination of the motor protection, the ART is shown in the motor starting curve and the LRWR is shown in the motor damage curve. The starting curve must remain left side to the motor starting curve and the protection device TCC curve must fit in between the motor damage curve and the motor starting curve.

If you are interested to know more detail about the device coordination, I would suggest you to take a look into this document. It is very good document for device coordination. See page 23 to 30 for Motor protection coordination.

http://epowerengineering.com/guides-part-5.pdf

- MS

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#8

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 7:22 AM

I'm curious why you want to know what LRWT is? If you have a 3PH motor that's taking more than a second to start you have a problem! There has to be a story behind this question.

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#9
In reply to #8

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 7:33 AM

Small diameter loads typically come up to speed quickly, but large diameter or other high-inertia loads (3500-rpm screw compressors, loaded conveyors, ball mills, big squirrel-cage fans, etc.) may take several seconds (or even tens of seconds) to reach full speed. WR2 (or GD2) really does matter.

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#10
In reply to #9

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 7:36 AM

I have an experience with a 21 MW Oxygen plant motor in a steel plant. Since it caused low voltage problem during starting, it was started with less than rated voltage. It took nearly 30 seconds to start as per the simulation exercises we did.

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#11
In reply to #9

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 8:17 AM

That's true but he stated it was DOL. He never made mention of any type of speed control. I have to wonder if he has a starting/breaker issue or was he just reading a spec sheet and wanted a definition?

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#13
In reply to #11

Re: Locked Rotor Withstand Time (LRWT)

04/13/2011 8:25 AM

True, we don't yet know the nature of the OP's load being started, but even DOL can take quite a few seconds on certain types of loads.

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#19
In reply to #13

Re: Locked Rotor Withstand Time (LRWT)

04/25/2011 9:53 AM

for MOTOR ,"greater thermal damage often results more from the frequency of starting than from the acceleration time being greater than the stall time"

please any gurus can tell me about this one by experience or by theoritically? thank you verymuch

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#16
In reply to #8

Re: Locked Rotor Withstand Time (LRWT)

04/17/2011 3:44 PM

Once we define the motor LRWT then we can simply know how to coordinate it with motor RT, ART, etc.

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#22

Re: Locked Rotor Withstand Time (LRWT)

01/27/2022 7:45 AM

Logic suggests the <...Locked Rotor Withstand Time (LRWT)...> is the time that a motor can withstand a locked rotor without damage, because if this time were exceeded it would be in the "not able to withstand" area.

It will vary slightly from motor to motor, it will vary significantly with ambient temperature, and a conservative value may well be published in the manufacturers' data sheets on specific motor types. If not, then it is surely worth a few phone calls to find out.

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#23

Re: Locked Rotor Withstand Time (LRWT)

01/28/2022 11:30 PM

Usually, the manufacturer catalogue presents the cold starting time and maximum stalling time hot [as per ABB catalogue]

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#24

Re: Locked Rotor Withstand Time (LRWT)

01/29/2022 11:42 PM

It seems to me, for low voltage induction motor, the policy it is not to check LRTW. See, for instance, IEC 60034-12.

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