IEC Motors

[Lc85]

Our clients technical specification says that IEC Motors must be capable of running satisfactorily with 75% of rated voltage (400V in this case)and frequencies within the range 48.5 to 51 Hz for 5 minutes. Our motor supplier says this is not possible. Has anyone seen this requirement before?

The motors are 1 off 110kW and 2 off 7.5 Kw

Thanks

 

[waross]

You may have to oversize the motors to meet that spec.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I think waross is right.
IEC 60034-1 standard “Rotating electrical machines –Part 1: Rating and performance”
7.3 Voltage and frequency variations during operation states:
“A machine shall be capable of performing its primary function within zone B, but may exhibit
greater deviations from its performance at rated voltage and frequency than in zone A.
Temperature rises may be higher than at rated voltage and frequency and most likely will be
higher than those in zone A. Extended operation at the perimeter of zone B is not
recommended.”
However according to Fig. 12 the minimum voltage recommended for continuous operation is 95% [extreme limit for zone B].
Duty type S2 – Short-time duty- if indicated by Manufacturer Example: S2 5 min. is referring only as permissible overload but voltage and frequency have to stay in the above limits.
Of course, at the start, for a few SECONDS, the supply voltage may drop up to 75% if the motor torque will always be more than load torque, any time.

 

[mikekilroy]

guess your motor supplier should know what they are supplying you. unfortunately often if they are a distributor they don't, so asking here is a good thing!

but you did not give enough info for a reasonable answer here yet. need more info to help provide a solution if possible...

so some more details please.

first, if 400vac 3ph is 75% f this iec motors rated voltage, that means it is rated 400/.75=533vac. wierd. is my math wrong?

second, you dont tell what this motors rated frequency is.

for instance, if really this motor is actually rated 480v@60hz, then it will be perfectly happy running 400v@50 all day long.

we need more info to answer.

third, what is the load?

for instance if the 110kw motors load during this 5 minutes is only 5kw, it will be fine also. details man. list more.

http://www.KilroyWasHere<dot>com

 

[ScottyUK]

Mike,

400V is the standard European line-line voltage.

The client's spec sounds like a requirement for power plant auxiliaries or something similar. It should be achievable but will require a bigger motor and bigger control gear. Unless these motors are on critical duty I wouldn't think the extra expense is worth it. Still, if the client is prepared to pay...

 

[mikekilroy]

This is why more info is needed.

I read 75% of rated voltage (400V in this case)=533v
rather than
75% of rated voltage (400V in this case)=300v

since these are "1 off" special motors, I am still not sure which it is.

But since they are "1 off," then the motor mfgr should be easily able to tell OP what the breakdown torque is as well as what the current draw will be no load and full load for both motors.

Adding the actual load on each motor to the mix of data given, the motor heating & actual current draw under that 75% of rated voltage condition can be calculated. Once calculated, it can be determined if it will survive 5 minutes and then how much time is required back at regular voltage to recover (cool back down).

I suspect the bigger issue may not be the motor but rather the supply capability as ScottyUK pointed out; whether vfd or other. Depending on what this supply is, the motor may draw anywhere upto close to its starting torque level (600% nameplate current) if the breakdown torque is not above about 250% (for instance, if the BD torque is 250% then it becomes 140% at 75% voltage, and if load is only 50% at that time, the current draw will not be anywhere near max, but if the "1 off" design has 180% BD torque, the motor if not vfd will draw its starting (DOL) torque possibly near 600% nameplate current if load is 100%)- all pretty calculable if the data is given. If motor is on a vfd it might be configured to handle this situation nicely too.

http://www.KilroyWasHere<dot>com

 

[waross]

Here is an example motor for the 7.5 kW motor:
7.5 kW = 10 Hp.
Next standard size = 15 HP
Select a 15 HP motor with a voltage rating of 208-230/460 Volts.
On 50 Hz this motor will develop 15 x(5/6) = 12 HP > Adequate for a 7.5 kW load.
Though not listed this motor will be suitable for 416V to 460V @ 60 Hz.
The voltage rating at 50 Hz will be 347V to 383V. But these motors are generally applied on 480 Volt circuits.
480V @ 60Hz >> 400 Volts @ 50 Hz.
The 383 volts is still a little high but we have a cushion. 90% of rated voltage will be 312 Volts. But we have a 12 HP motor with a service factor of 1.15 for an allowable short time HP of 13.8 HP and only a 10 HP load.
I am sure that this motor will be safe at 300 Volts for 5 minute intervals.

http://www.baldor.com/products/deta...08-230/460&winding=07WGW866&rating=40CMB-CONT

Specs.

http://www.baldor.com/products/spec...08-230/460&winding=07WGW866&rating=40CMB-CONT

You should look at the section of the Cowern Papers dealing with RMS HP loading of motors. There is a lot of information that is applicable. The paper also deals with torque at under voltage.
Excerpt;
The total amount of time for which RMS loading can be adequately calculated would depend somewhat
on the size of the motor but, in general, it would be safe to utilize this method for duty cycles that total
less than 5 minutes from start to finish (of one complete cycle). If the total time is beyond 5 minutes, then
the application should be referred to the motor manufacturer for more detailed analysis.
At 5 minutes at reduced voltage you are right on the edge.
See the paper here:

http://www.baldor.com/pdf/manuals/PR2525.pdf

You may have to get past sales to application engineering to size the 110 kW motor but I am sure that there will be an off the shelf solution.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ozmosis]

IEC 60038 states a standard voltage of (now) 230/400v +/- 10% as the 'nominal' voltage covering the LV range (690vac exception). I would be asking why someone wants an IEC Motor outside this range and, if the are going to reduce by 25% or increase by 75%, what else is on the circuit.

 

[ScottyUK]

Oz,

It's a fairly typical 'wish list' item on power plant specs. The last thing needed by a plant operator trying to cope with a major disturbance is for auxiliary drives to start tripping out. It's the kind of thing which could turn a bad day for a plant into a very bad day for the grid operator as stations start tripping off the system.

FWIW I'm not sure how often the 'wish list' is fulfilled these days - the older CEGB stations of yesteryear were better specified and better built than modern stations, but must have attracted a massive cost premium to achieve it.

 

[jraef]

I agree with ScottyUK, I have seen that type of spec in the PowerGen industry

lc85, if I am reading your info incorrectly,you are an equipment supplier or contractor that has been given a SPEC calling for motor performance that is outside of typical norms, meaning they have 400V 50Hz at the site, but they want the motor to perform at as low as 300V and 48.5Hz for up to 5 minutes, is that right?

If so, the rest of the question is, at what kind of performance level? At 75% of rated voltage, your running torque from that motor will be at 75% as well, so figure that your shaft power decreases accordingly and therefore the machine it s connected to must either have a reduced loading at that time, or you must over size the motor by a commensurate ratio (1/.75 or 133%). This then means that if running at expected full load and normal supply, the motor is going to present a worse power factor and may need correction, but they will have to tell you if that is necessary.

The secondary issue is that if it is expected to perform at that reduced voltage and START during that time, the acceleration torque (Starting Torque) available from that motor will be reduced by the SQUARE of the voltage drop, so 56% of normal. That could lead to stalling. So if these are crane motors for example, that could be a huge problem.

Bottom line you will need more clarification on what they expect from the machine performance during those low line conditions. As I have seen it in the PowerGen specs I came across, they had an acknowledgement that performance was expected to change, they just don't want anything tripping or burning up. We generally over sized the motors to 133% minimum and since it was always on centrifugal pumps, starting torque was never an issue.

"Will work for (the memory of) salami"

 

[Lc85]

Regarding my original question a few months ago, we also have some motors running off inverter drives. surely the inverter drive would not be able to take 75% voltage? although the clients spec only says motors so could we get round it that way? Also we have redundancy on the main drive ie we have 2 off motors each connected to an inverter model altivar 71? So if one goes down the other will take over.

Thanks

 

[waross]

We are confused by your terminology. One off generally means a special design and build of one special motor. One of may mean one motor at that rating, or it may mean something like one of possibly two or more.
eg: Three of four may mean that there are four motors or generators, any three of which can handle the workload with one unit in reserve.
Is 400 Volts 75% of rated voltage or the actual rated voltage?

Also we have redundancy on the main drive ie we have 2 off motors each connected to an inverter model altivar 71? So if one goes down the other will take over.

Two inverters or two motors on one inverter?
If both motors are on one inverter there is a good chance that one motor going down will take the VFD with it. But I will defer to jraef on this point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Bill,

Over here in GB "2 off" is a way of noting a quantity of two motors, especially in a bill of quantities. It's also written "2 qty." from time to time.

Not sure if that helps much - Lc85 still needs to clarify this a bit more.

 

[waross]

Thanks Scotty. I suspected that but at least one responder seems to have taken that to mean a special build.
The bottom line may be oversized and then derated motors.

Bill
--------------------
"Why not the best?"
Jimmy Carter