Two starts from cold... allowed for routine planned operations?

[electricpete]

The limitation of two starts from initial cold conditions from NEMA MG-1 is built into our operating procedures and all our operators know it by heart (*).

I see some occasions where our operations/maintenance staff does two starts from cold within a few minutes as part of routine planned evolutions.

I tend to think the purpose of the two starts-cold provision was to cope with an occasion when pump is needed to support operation and trips for some reason on initial start, this allows operators to restart once to support plant operation. I’m not sure it’s wise to do this any more than necessary.

I’m thinking about adding instructions that this should be limited to situations when required to support plant operation, but not for routine planned evolutions. For routine evolutions we should use a wait time of 30-minute running or 1-hour shutdown.

What do you think?
Has anyone else done this?
Are there any references, guides, published practices etc that might support this approach?

(* by the way, there is also the one restart hot limit... I tend to think this is less severe than two starts from cold when it comes to rotor stresses in large motors.)


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(2B)+(2B)' ?

 

[KllrWolf]

I only deal with fluid filled submerged motors, so our starting requirements may not be the same, but the concept is. Our requirements for customers is to allow at least 15 minutes from the motor stop until it is tried to restart again. The only exception is when the motor is energised briefly, or bumped, for a rotation check, and then for only 2-3 times before a full 15 minute cool down. This is because starting the motor generates a massive amount of heat over a very short period in the windings. When you try to start a motor that has just stopped down, you are adding the running temp and the extra heat from starting, which causes extra strain on the windings.

 

[GordS]

We don't place any such restrictions on our consecutive starts. My reasoning is typical stall times for our motors are in the range of 8-12 sec but typical start times are about 2 sec.

 

[LionelHutz]

The 2-starts cold rating always seems to be a vague rating. To be useful, it should be linked to a specific motor load. However, most manufacturers provide the number, yet I wonder how many could tell you how it applies to the installed system. I believe MG-1 has some maximum inertia ratings for lower HP motors. These inertia values would likely be considered the load where 2-starts cold applies.

Overall, the rating could be increased if the load was lower than what the motor was designed for. The start certainly doesn't heat the motor with a open shaft as much as it does when coupled to a fan or pump.

 

[rockman7892]

From everything I have heard and read my thoughts are in line with LionelHutz.

If the motor is driving a load which has a much smaller inertia than what it was designed for then it will be able to get the load up to speed much more quickly and thus be less taxing on the motor during starting.

I had a process fan motor once for wich the manufacturer data for the motor came with the 2cold/1hot start limitation. After starting this motor several times I noticed that the fan and motor would be up to speed in a coulple of seconds which was much less than the acceleration time given on the motor datasheet based on the load inertia. It turns out that the fan inertia was much less than was origonally expected and after proviging GE with the actual fan inertia value they came back to say that the number of starts could indded be increased (although I forget the exactt number)

From this experienced it became evident to me that a mtoor is allowed more starts if the inertia is smaller than what the motor is designed for and thus accelerates quickly.

The purposed of the overload curve in most electronic relays is the provide a thermal model of the motor. Ideally a motor that would be allowed 2 cold starts would take up about 50% thermal capacity on each start in the motor protection relay if set correctly. However in the case of the fan and other cases where the inertia is low the relay may only see about a 10-20% thermal capacity used during the start during fast acceleration. The relay can be set to allow starts based on avaliable thermal capacity so if only 10-20% is being used up during starting the relay would allow more than 2 cold starts.

 

[KllrWolf]

Well, I learned something. I did not relise that the typical motor had such a long start time. I am used to dealing with our motors attached to turbine pumps, which even on our 400+ HP have a normal start time of less than one second.