starting oversized motors

awalinski

Member
Join Date
Sep 2002
Posts
38
Is it at all possible to start a 60 hp (103Amp) motor across the line?
I tried to do that and was accused of not taking in consideration power interruption. Does anyone has any related experience?
I could not find anything in the NEC.
Thanks
 
Who is doing the accusing? We use across the line starting on 250hp motors. It sounds more like a utility company issue than a code issue. And remember, if you are flying the right flag there, it's the CEC, not NEC. ;)
 
I have never seen a 60HP 480vac or higher ....like an air compressor, that werent across the line. In most cases you try to make sure the motor has NO LOAD on startup ..air compressor good example, no back pressure.

Overall a 100 amp load is not that big of a surge and should not cause interruptions even if gets to 150 on startup, with no load shouldnt really overamp that much for that long.

I think of larger things like 150HP or larger when it comes to using wye-delta starting, now we are talking hitting the line with 300+ amps(maybe), thats a load.

Every situation is unique, there may be factors involved at your place we arent aware of...sounds to me like someone is trying to make someone look bad to the powers that be. If there may be an issue, with power or politics, then use a Wye-Delta starter, softstart, or AC Inverter. I would try it and see if there is an issue.
 
60 HP is no problem starting across the line. NEC T430.150 list 3ph 460vac motor as 77 amps and 154 amps at 230v. Where does the 103 amps come from? If you by chance are at a very small place with a limited power source like 200 amp 3 phase 230 volt service I could see their concerns. But this would be uncommon for an industrial facility.
 
Hang on a minute here, folks. Starting inrush current on most NEMA and IEC motors is six to eight times FLA when started across the line. That's current that has to be available from the AC supply for at least a couple of seconds minimum. On high inertia load like flywheels, centrifuges, and fans, this inrush can last ten's of seconds.

The question for awalinski is whether his supply system can supply that level of current for the time required. In most industrial facilities there would be no problem but, it bears checking. It's not the first time I've seen a motor starter close and a bunch of other smaller starters drop out and all the flourescent bulbs in the building go out and then relight.

If the system for whatever reason cannot bear this inrush condition, then soft-starting can help if the load is capable of being soft-started. Remember, the degree of soft-starting available is determined solely by the load.

Finally, that 60hp 103amp nameplate on the motor is suspicious. As pointed out above, it clearly is not a 230 or 460V rating. Is it possible that this is a 380V 50Hz motor? And, if so, how are you reconciling this rating to North American 60Hz power, normally 230, 460, or 575V?
 
I am glad to see I am seeing things that should be obvious...I have learned a rule of thumb for each 1HP @ 575vac the load will be approximately 1 amp, @480 1.25A. The 103 indicated a 380vac source of 3 phase power to me. I am glad to see others think same.

I stated there may be other factors...but if its an industrial or even commercial application and has 3 phase @ 380vac then a 100A load shouldn't cause that many problems.
BUT
Not all countries/locations have the same power/line capabilities.

AS I stated, there may be factors, political or pertaining to power usage that we are not aware of. I also have seen where starting a fair sized motor (25HP) has caused issues.
 
You can start a 60 hp motor across the line. I've seen it done with 300 hp 460 VAC motors. I've also seen 700 hp 4160 Volt motors with across the line starting. The real question is "should you?" The avove referenced motors would dim the lights when they were started, and stalled out standby generators.

As a general practive most utilities want motors above 40 or 50 hp started at reduced voltage to minimize inrush, and some electric utilities dictate this. It is good practice to use reduced voltage starters on motors above 40 hp, and the cost of reduced voltage solid state starters has dropped to the point that they are feasible at even lower power.
 
depends on the load

Starting a 60HP motor acroos the line with just a standard electromechanical starter as opposed to using a soft start is not a problem for a 60 HP motor as long as you have a load that is consistent and not varying wildly. For example fan is not a problem.
A crusher that would be a problem.
I have not seen a rule of 40HP limit and a properly sized fan does not have high inertia that would cause the tens of seconds of high
inrush current.
 
At 4kV the supply should have such a low impedance that it will likely cope with DOL starting of motors this size. But this WILL need to be confirmed with your network supplier.

Nor is it cheap to source low cost soft starters at this voltage, but they do exist. In recall Rockwell does have an MV soft start product that will do motors at this voltage just fine.

Another possibility I've also seen in use was a pair of large (8000HP, 6.6kV) refiner motors run up to speed (unloaded) with a much smaller DOL motor, and then the main contactor closed once synchronous speed has been confirmed.
 
You know all this talk about the line and i haven't even seen it. Could someone point me to the line so I might step over it and figure out what the heck it is? GUS-----Remember

Everyone that looks at this stuff doesn't know what "Across the line" means.
Take me for example.
 
"Across the line" just means connecting the motor directly to the power source via a contactor (contactor = big relay). Starting the motor from a standstill requires a lot more current than when it's running.

Using a soft-starter (not in the "cottony soft" sense) is one method of limiting the amount of current required to start a motor.

Speaking of contactors... Our local electrical supply house used to get a kick of of some of the purchase orders from my last company. The purchasing department would run the order through a spell checker, and always ended up ordering NEMA contractors... :p

beerchug

-Eric
 
Hello 1
If you want to minimize the starting "amps" and don't want by a soft starter you could use a STAR-DELTA connection/switch.(hope it went the right way?)
Start the motor STAR-connected and when it's running change to DELTA-connection.
We used it in the navy/an a ship for starting the firewater pump think the motor was about 300 kw.
 
Peak inrush on a across-the-line start motor will see an instantaneous peak value of 12-16 times FLA, and drop 6-10 times FLA after 1-2 cycles, before dropping to it's nominal running amps, unless under locked rotor condition. The new energy efficient motors may see the highest peak values during startup.
I would reccommend a soft start unit. Your compressor unit should have unload capabilities. With the softstart the initial voltage/current can be reduced to a minimal peaking level if setup correctly.
Your utility company can provide you with the information you are seeking. If you have an 600 amp service, you will see the starting effects more than you would on a stiff 2000-4000 amp service, However you are still paying for all those starts, especially if you have a higher demand rate. A compressor that starts once an hour may not be an issue, but one that cycles every few minutes may be. :cool:
 

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