Locked Rotor Withstand Time (LRWT)

→ British Standard 7671.

You may also need to consult with specific manufacturers.

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...???

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...

When a motor <...stall[s]..>, the motor overload protective device should operate, thereby disconnecting it from the electrical supply. The operating characteristics of that device should be adjusted to match the motor during Commissioning. There should be a record of these settings that was created during that time, and that record will eliminate the <...wondering...>.

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.

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

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 ?

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.

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

<...Run Up...Time...> is a function of the inertia of the mechanical load connected to the motor. s motor manufacturers cannot possibly know the equipment to which motors are connected, the chances of finding that data from the manufacturer's data sheet is about zero. It is far better measured than calculated.

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.

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.

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.

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?

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.

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

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

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.

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

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