Induction Motor Starting Conditions

[GDL90958]

People,

We am currently looking at purchasing some large induction motors for a client to drive compressor and pump units (3MW - 10.5MW on offshore application). The original spec is outlining that the motors should be able to start @ 80% terminal voltage @ 3.2 LRC as a worse case scenario. Some of the motor people are saying they cannot achieve this (some are) and our mechanical people are looking at the compressor load requirements to increase this tolerance.

My question is can we simply change the requirement from 80% to say 85%? or should we stick to the original 80%? My reason for confusion is the way i understand the situation is the allowable run up time (ART) is calculated @ 80% terminal voltage and the ratio of ART and the run up time must be greater than 2 to ensure the restarting capabilty and which inturn ensures 2 successive starts with motor at running temp. If the motors cannot maintain the start up conditions @ 80% terminal voltage does this imply that the 80% value ART/RT calculations are effected.

Also could anyone explain what the significance of using the value of 80% terminal voltage in the outset if it is not tied into the ART / RT somehow?

Your advice would be mich appreciated.

Regards

Gavin

 

[peterb]

Before you think about changing the minimum starting voltage requirement, I think that you would need to calculate the actual minimum voltage at the motor terminals for your specific system conditions. This has been dealt with at length elsewhere in this forum. The voltage drop is primarily a function of the supply source impedance, the motor cable impedance and the starting current of the motor.

The motor torque is proportional to the terminal voltage, so a lower voltage may not allow the motor to accelerate the load to rated speed within the safe start time. 80% is a typical number, above which satisfactory starting should be assured for a "typical" motor.

 

[GDL90958]

Thanks for that peterb - we are specifying that the maximum volt drop at the motor terminal can never be greater than 15% of the terminal voltage, hence trying to increase the 80% value to 85%. Remember this is a new installation and the cable sizing we have carried out is preliminary and we don't know who we will buy it off of in the end - and as you are are the impedance characteristics will change from one manufacturer to another. this is why i was trying to investigate if the 80% value had any impact on the ART/RT ratio as a direct characteristic of the motor as opposed to the power system.

 

[electricpete]

I don't understand what 3.2LRC means. Can you explain.

You stated: "ratio of ART and the run up time must be greater than 2 to ensure the restarting capabilty and which inturn ensures 2 successive starts with motor at running temp"

I'm not too familiar with this way of looking at things. A reasonable assumption might be no power dissipation during the entire two-start sequence. In which case we need the ratio of actual rotor/stator heating energy divided by allowable be > 1/2. It appears your thumbule stated above is assuming that energy is proportional to starting time. I don't believe that is the case. It seems to me it would be more straightforward to come out and specify directly that the motor should be able to withstand two successive starts from initially cool condition (with 85% terminal votl)... rather than relying on this ratio of actual to allowable starting time.

 

[GDL90958]

Peterb - sorry LRC - locked rotor current - therefore the starting current is 3.2 x the nominal current.

We are looking for 3 starts cold and 2 starts hot. i am looking at some literature originated by Shell to be utilised when selecting motors. In this document it states: the allowable run up time (ART) is the time in which an electric motor can complete one or more starting cycles at 80% terminal voltage, in which the temp rise from normal running temp of any part exposed to the ambient air is restricted to the lower of either the allowable temp in gas group eg T3 or the temp limitations of material. it goes on to state if ample torque remains 80% nominal the nominal can be calculated with reasonable accuracy to ART(100%Un) = ART (80%Un) x (80/100)squared seconds. The doc goes on to say the running up time is the time required for an electric motor coupled with it's driven equipment to reach full speed from standstill with 100% terminal voltage. it states to ensure succesful accelaration or reaccelaration, the torque speed curve for 80% (worst starting conditions) should be at leat 10% of the rated load torque above the torque/speed curvefor the load. Also the ratio of ART/RT gives the mearsure of the restarting ability of any particular motor and preferably should have a value of 2. this indicates that the motor can tolerate 2 succesive restarts.

Can it be said that the above arguments will apply if 80% is changed to 85% remembering that the maximum permissible volt drop - 15% at any time?

The torque/speed characteristics from some of the vendors that we have received - does not achieve the above philosophy @ 80% and are asking for some relaxation of this particular value (one would think to remain competative) - our mechanical chaps looking after the compressors say they can work with a value greater than 80%. What i am trying to establish is the value of 80% a nominal figure or a figure that is required to ensure the correct ART? or should we just specify we want 3/2 starts and let the vendor size it accordingly? I would imagine that if we stipulate that the motor must be capable of starting at 80% volts and maintain all other criteria the stator size must increase and hence the capital cost must go up!

the 3.2 starting current (direct on line) is a restriction as we are not utilising any soft start mechanism and we must keep the rating of the motors down as we are limited to the gas turbine output at a particular frame size (even more capital cost)- i hope that explains the rule of thumb.

thanks for you assistance

Gavin

 

[gordonl]

A 3/2 motor as you have described is not the NEMA standard, from NEMA MG1 for large induction machines:

"Squirrel cage induction motors shall be capable of making the following starts, provided the Wk^2 of the load, the load torque during acceleration, the applied voltage, and the method of starting are those for which the motor was designed:
1.) Two starts in succession, coasting to rest between starts, with the motor initially at ambient temp
2.) One start with the motor initially at a temperature not exceeding its rated load operating temperature."

From the NEMA standard they make no reference to ART and RT, it sounds like a rule of thumb developped by the company. (Taking a quick look at motor data it seems to hold true) We normally use the 80% factor, because that is what we see during the start with dedicated motor transformers. From NEMA, you specify the worst case conditions to the motor manufacturer (torque,inertia, volatge drop, number of starts) and they will design to it. If you only need 85%, than that is the required limit. If unspecifeid the NEMA limit seems to be only 10%.

NEMA also mentions frequency variation plus or minus as being a factor to consider as well, it has never been a concern for me, but on a dedicated generator you may want to throw that in as well.

I would suggest you make very certain of the calculated actual voltage drop at the terminals before ordering the motor. Cable impedance is a function of size and installation, not manufacturer, so you should be able to get fairly close. I've never been involved in a design like this before, but I would say the generator is a critical piece of this puzzle which has to be in stone before the motor is in stone, or vice versa.

Just trying to helpful, hope I don't offend.

 

[electricpete]

I would say that your thumbrule (ART/RT>2 implies 2 starts can be performed) will remain true whether you use 80% or 85%.

My reasoning is fairly simple. Both the heating energy during a start and the starting duration can be calculated as an integral function with respect to speed involving inertia and motor torque and load torque as function of speed. If the rule is true for any motor, that means it is true for any motor torque-speed curve. Increasing the voltage is equivalent to selecting a different torque speed curve. So if the rule holds for any motor, then it will hold for any voltage.

I still question the original rule a little bit because that integral function for starting time is different than the integral function for energy.... and they do not differ by a simple constant. I'll work on that and post it shortly. Therefore it still seems more logical to me to tell the motor manufacturer more directly what you need.... that the motor shall be able to go through 2/3 starts without damage and without exceeding your temperature limit. You might also ask them to provide the calculations to prove that your requirement has been met.

 

[GDL90958]

Thanx people - much appreciated - i'm sure i'll have some more puzzles for you at some point!

good site!

Gavin