View Full Version : starting oversized motors

September 22nd, 2003, 05:57 PM
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.

September 22nd, 2003, 06:19 PM
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. ;)

September 22nd, 2003, 06:25 PM
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.

September 22nd, 2003, 06:34 PM
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.

September 22nd, 2003, 06:53 PM
Is it at all possible to start a 60 hp (103Amp) motor across the line?
dear sir : awalinski
i have air compressor Overall a 100 amp load i used soft starter
and you can make delta starting
i hope help to you
thanx :p

September 22nd, 2003, 07:11 PM
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?

September 22nd, 2003, 09:35 PM
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.
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.

Tom Jenkins
September 23rd, 2003, 07:39 AM
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.

Jiri Toman
September 23rd, 2003, 12:11 PM
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.

September 23rd, 2003, 01:23 PM
We've just ordered two 1250HP and five 1750HP (4160V) motors that will be started across the line. I'll have to wait to see what happens when we start them. :D :D

September 23rd, 2003, 06:56 PM
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.

September 23rd, 2003, 10:43 PM
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.

Eric Nelson
September 24th, 2003, 01:40 AM
"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



September 24th, 2003, 01:59 AM
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.

September 24th, 2003, 05:06 AM
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:

September 24th, 2003, 06:43 AM
a "typical" AC-motor of the squirrel-cage type has a starting current approx 7 times that of FLA, not 12-16 times. There are exceptions, but as long as we are talking about commercial standard motors, then 7 times is a good rule of thumb.

what does the nameplate of the motor say about power, voltage, current, maybe in various combinations ?
And at what voltage are you intending to use it ?
As some of the others has pointed out, if the motor is 60hp and 103FLA at 380V,
then it will be derated to approx. 20hp and 34FLA at 230V.
But you will have to check with the nameplate.

34FLA will give a starting current of approx. 220A with direct start (now I too know what "across the line" means :)). Thats the value that your electrical provider should say OK or NOT OK to, provided that the above assumptions are correct.

You have a number of options to reduce the inrush current,
star-delta, softstarter, transformerstarter, VFD.
Today low cost softstarters are so cheap that they can replace star-delta starters. Low cost softstarters are barebones with the softstarting thyristors in only 2 phases. All the major manufacturers are selling them now, I use some from Siemens.

September 24th, 2003, 07:04 AM
Jesper mentions another very important consideration when starting larger motors, that of peak inrush currents generating peak demand surcharges from the utility.

In North America, many (probably most) utilities use peak demand metering which can easily cause an electronic softstarter to look like a bargain.

One caution here tho. Even with nothing loading the motor, about the best a softstart can do is reduce the inrush from 6-8 times FLA to around 2.5 times FLA. If this is the only large motor in your facility, this can still represent a significant short term shock to your AC supply.

While I have never been able to justify an inverter at these horsepower levels solely to eliminate inrush currents, the fact is that this same motor could be started faster and with currents never exceeding FLA with an inverter.

So, the bottom line is that there are several choices, each with advantages and disadvantages. Isn't just about everything like that!

September 24th, 2003, 07:27 AM
In regard to Max Demand Metering, here in the part of Auastralia I work in the supply authorities charge a Maximum Demand charge as well as an energy charge on some tariffs. This is not an instantaneous maximum but the highest 15 min average during the billing cycle. Some of the older meters were a thermal type, the newer are electonic.

This would probably not be greatly affected by a DOL start peak of a few seconds

September 24th, 2003, 07:35 AM
I once was involved in a project where the authorities would DOUBLE the rates for one years electrical supply (it was in Vienna - Austria), if there was even the shortest peak value above a certain limit. There were special detection equipment installed by the authorities.
So we used VFDs as softstarters to make damned sure that this couldnt happen.

By the way, I do believe that normal softstarters lets you limit the starting current to 100% FLA. Never had the need to go this low with a normal softstarter though.

September 24th, 2003, 07:39 PM

Two points:

1. The 12-16 times FLA mention by fmicheal is ONLY for a few mains cycles while the magnetic field of the motor establishes itself.

It then settles back to around the 6-7 times FLA you mention until the motor establishes rotation at normal slip speed.

Capturing the waveforms of the real instantaneous current drawn by a motor starting DOL on a storage oscilliscope is quite educational. The peaks involved are quite amazing.

2. Soft starters are very much a compromise technology. They ONLY exist because they are cheaper to make than Variable Speed Drives (invertors). Their single biggest drawback is that they only reduce the starting voltage and so inevitably they also reduce the starting torque.

Because the motor torque drops roughly by the square of the voltage reduction, by the time you have reduced the starting current in most motors to 300% FLA, there is barely enough motor torque to accelerate the motor to speed. Most times 350% is the lower limit. At only 100% most motors are unlikely to start in a reasonable time, or they will stall completely.

September 24th, 2003, 09:53 PM
Seems we now get to my statement about verifying NO LOAD on startup...?

We can debate this forever...simple fact is we did NOT get enough info.

A 60HP motor at 480 vac (maybe 415 for some countries) should not run at 103 amps on a DOL connection. YES the startup current may be higher but in many cases not a significant factore.

A 60HP motor at 380vac probably would.

ARE there other factors involved in WHY DOL shouldnt be used? THe poster has not responded so WHO KNOWS?

As I stated in my original reply, there are always factors to be considered, some are political and some are financial. Financial concerns cost associate with demand created by any electrical device. Politics can be internal to the company OR involvement with the electrical utility..there may be more for either.

Last but not least...ie THE MOST IMPORTANT FACTOR ...safety.

Maybe when y'all stop debating things that are fairly obvious the poster will respond with enough information to properly evaluate the situation.

Tom Jenkins
September 24th, 2003, 10:27 PM
A 60 hp motor @ 460 AC has a full load current draw of 77 Amps per NEC tables. Individual motors will vary, and nameplate data should be consulted.

A 60 hp motor @ 460 AC has a locked rotor current draw of 435 Amps per NEMA tables. Nameplate code letter and kVA per hp data shold be used to verify this. EPACT motors generally have higher locked rotor current.

I dont have hard data on the one or two cycle inrush current, but it is probably safe to ignore it in most cases.

Started across the line, all motors will draw locked rotor current for some period of time - the duration depends on the inertia and torque requirement of the load. A loaded conveyor or a hoist may need full torque during acceleration to overcome the load. In that case a high torque motor should be used, not a standard Design B motor.

Most motors will start most loads just fine at reduced voltage and torque, whether it is a solid state "soft start" or an autotransformer or other reduced voltage starter. A high starting torque load should be analyzed carefully!

Starting across the line causes high voltage drop in all but the largest distribution systems, and at high horsepower it should be looked at very carefully!

September 25th, 2003, 04:21 AM
All induction squirrel cage type motors have a peak instantaneous (note the words "peak instantaneous") value of 12-16 minimum times FLA. This is the value that establishes the magnetic field. After a few cycles, yes the motor will drop to a value of approximately 6 times FLA, barring a locked rotor condition. Energy efficient motors may exhibit a higher inrush value.

September 25th, 2003, 04:39 AM
I stand corrected.

Just a small thing: Even with a locked rotor you will still get only the 6-8 times value after the first few cycles of the magnetic field.

September 25th, 2003, 08:28 AM
In line with PhilipW's comments, while most softstarters will allow a reduction to 100% FLA, there is no point in going that low because below about 250% FLA the motor doesn't have enough torque to turn itself, much less any load that might be coupled to it.

Curious, isn't it! A softstarter which is dissynchronous requires 250%+ full load current to turn the motor while an inverter which is nearly synchronous, will start the same motor with around 33% FLA.

This gives a little insight into how far from ideal conditions 50 or 60Hz starting of a stationary motor really is.

And with respect to Ron Doran's comments, I agree about the lack of sufficient data but, as happens so often here, the thread has moved into a more general coverage of motor starting and related inrush currents. It's educational, I suppose, even tho off target. And, it seems to me, probably a good thing anyway. Shucks, I've learned something again on this thread even tho the original poster may or may not have gotten his answer.