Optimum Selection of One Common Spare Motor Against Three

Well, what do you think?

If a motor is process critical for a high value product you carry a spare on stock. Expensive it may be but what are the down time costs?

You'd be better off going with one of each if you can't decide. You don't have any head room with any of these and at those high loads what is your current efficiency?

Hopefully, you have provided your vendors adequate information to allow them to make an informed decision.

Please explain further about the Part Load entries. (One interpretation would be that all the motors are oversized.)

Thanks to all participants.

ELECTRICAL SIDE:-

We have two options

a. Either to select 800 kW, 1492 RPM motor.

i. Its rated torque is 5120 N.m.

ii. No issue for motor-A

iii. Motor-B & Motor-C running torques are 4715 & 4520 N.m.

iv. In simple words it will be 531 kW, 992 rpm motor.

OR

b. Select 800 kW, 992 RPM motor.

i. Its rated torque will be 7694 RPM.

ii. No issue for motor-B & Motor -C

iii. When operated at 1492 RPM for Motor-A , its rated torque will be reduced from 7694 N.m to 5120 N.m. Running torque demand of Motor -A is 2700 N.m

I think one of us may have miscalculated the torque of the 2nd and 3rd motors. I assumed they were 6 pole based on the RPMs being nameplate, not as used, you likely assumed they were 4 pole and are being used at the lower speeds via the VFD.

In truth, the OP did clearly state it as the motor NAMEPLATE data, so motors 2 & 3 are 6 pole, making their rated torque HIGHER than the 4 pole motor in #1.

Instead of selecting 50 Hz motor, 60 Hz motor looks a more suitable choice.

Let Selected motor is 800 kW, 690 V, 60 Hz, 1194 RPM

i. 800 kW Motor Rated Torque: 6400 N.m @ 1194 RPM

Meet rated torque capacities of motor B ( 6732 N.m ) & C ( 5675 N.m )

ii. 800 kW Motor Rated Toque: 5120 N.m @ 1492 RPM

Meet rated torque capacities of motor A ( 5120 N.m)

FACTORS NEED FURTHER INVESTIGATION:

i. It seems convenient to operate 60 Hz motor from 40-75 Hz as compared to operate a 50 Hz motor from 40-75 Hz.

ii. One factor that need to be investigated is that what would the noise level for motor operation between 40-75 Hz operation in case of 800 kW 50 Hz as well as 800 Kw 60 Hz motor.

iii. Variable frequency drive de-rating in span of 40-75 Hz operation.

What is the probability your process will change enough within the lifespan of these motors that you might need all 800kW of your largest motor?

Or that your mill production might be increased with some easy modification, so that you might need 800kW at Mill B or 700kW at Mill C. In that case, an easy selection. The losses attributable to running an oversized motor will be very small, higher windage against lower I2R losses, don't look at efficiency numbers, calculate the actual kW losses for each motor and compare directly.

If you try over-speeding your 6 pole motors into constant HP range, you might squeak by, on the 4 pole application, easy calculation to see if it will work. Balance speed might be important to specify.

There is not enough information on what you can or cannot change for the motor envelope to evaluate the benefits of a single spare, you need to disclose your constraints. It is normally not as insurmountable as not having enough torque available.

All that one has to do is compare the value of the motor with the value of the down time while the replacement motor is ordered and delivered. As this is commercially-sensitive information, no recommendation can be found here.

You said: It is requested that keep focused on technical aspect instead of posting like contact OEM etc. We are already working on it with different vendors. Purpose of this post is to share ideas regarding optimum choice and major points to be considered in such type of selection.

How would you know if the information that an anonymous person replying here would be correct?

If you had sufficient engineering knowledge within your local group, you would not need to reach out to an online forum, unless you believe that you can somehow beat or circumvent the local suppliers.

In my experience, large-company suppliers are better. Call them.

1. In my view 715 kW, 690 V, 992 RPM is the best choice.

2. Delivery time of such motor may take 18-20 weeks therefore two cement lines cannot be stopped for such long time ( if one of above motor out of three fails ).

3. Every difficult situation is resolved in steps. I am really thankful to the all participants specially JRef for addressing the issue as needed.

4. Sorry to say but one of the most disappointing thing about CR4 is that in response, instead of sharing experience irrelevant stuff is shared which poster or reader already know. Engineers hesitate to ask or discuss technical issues since by default replies like contact to OEM etc are received within no time and no one is interested even to read these comments.

Don’t get worked up about it. As in all things, take what you need or find valuable, ignore the rest. In this age of “social media” people often seem to be compelled to react or respond to every post, be it positive or negative. Selective disinterest is the key to happiness now.