starting oversized motors

[JesperMP]

fmichael,
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.

awalinski,
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.

 

[DickDV]

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!

 

[paulc20]

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

 

[JesperMP]

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.

 

[PhilipW]

Jesper,

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.

 

[rsdoran]

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.
THIS DEPENDS ON LOCATION

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]

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!

 

[fmichael]

Reply to Jesper

Jesper:
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.
fmichael

 

[JesperMP]

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.

 

[DickDV]

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.