Reduced Voltage Starter?

[tjmurf]

Is there a minimum motor HP size where reduced voltage, soft start or other controller should be used rather than across the line starter.

This is assuming 3 phase 460 volt motors.
Also assume the driven load does not require a reduced tork start.

Thanks!
TM

 

[oldfieldguy]

A lot depends on your power source. Down here on a heavily industrial area, most of the local petrochemical plants have the benefit of a very stout power grid at 69,000, 138,000 or 230,000 volt transmission levels, and almost ALL motors are full voltage start, from fractional horsepower to 15,000, 480 volts to 13,800.

Several miles up the interstate, a new facility is fed from a 13,800 volt overhead line, and its only fairly large motor, a 4160-volt induced draft fan at 600 HP, is on a soft-start, a situation required by their service agreement with the local electric utility company. Full-voltage starting would cause a lot of problems with other customers on that 13,800-volt service as voltage was drawn down by a full-voltage start.

Another consideration for soft-start may be the mechanical requirements of the connected device.

These are just a few of the factors that go into making the decision on full-voltage versus soft or reduced voltage starting.

old field guy

 

[itsmoked]

Nicely put oldfieldguy.

Depends also on the load as some would rather be started gently.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[rybrk]

it depends on starting torque of motor and line votage drop of the grid.instead of soft starter you can use VFD for better results and energy saving purpose.Based on the applications generally we will choose VFD/soft starter/line starter.

thanks

rybrk

 

[jraef]

Aside from all these other mitigating factors, from a quasi-regulatory standpoint each utility has its own rules usually based upon starting amps/HP at their various service voltages. The rules may also change by application and location within a utility's service area and in alsmost all cases you can always ask for a variance anyway.

For example, I am in Northern California served by PG&E. Their official cutoff is 25HP at 480V. What that means is that below 25HP you usually don't need to ask. Starting at 25HP, when you apply for service they will ask you how often you will be starting that load, then based on where you are on the grid they may or may not require RV starting. Generally speaking, at 50HP 480V they are going to get more insistant, but even then you can appeal; for instance if your load only starts once per day or even less. I saw a recent applicatios where they allowed 2 x 150HP motors to be sarted Across-the-Line because they ran continuously for months at a time. By the same token, I have also seen them require RV starting on a 15HP motor where the user was at the very end of a 3 phase transmission line served by a relatively small existing transformer. PG&E offered him the choice of usng RV starting or paying for new lines to feed a larger transformer, but obviously the soft starter was going to cost less.

When I lived in Seattle 9 years ago, the cutoff was 15HP from Seattle City Light, but 25HP from Puget Sound Energy, the adjacent utility and 50HP from those served directly by the Bonneville Power Administration. Bottom line, it varies by utility, then varies again within each one.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[oldfieldguy]

Another aspect is that with the modern equipment, the distinction between variable frequency drive, "soft-start" and reduced voltage start are blurring badly.

You older guys remember that a couple of decades ago, reduced voltage starting was complex and expensive, resitor, auto-tranformer, delta-star, etc. Variable spped drive was even more so, and a nightmare if you needed more than a few horsepower.

The new solid-state stuff makes these options easier to fall back on. In 1970, variable speed was essentially the domain of DC. In the eighties, I saw the first variable speed AC drives, and today I am installing 9000 horsepower variable frequency drives, and integral to the characteristic of variable frequency is the soft-start cahracteristic.

Life is good when it's working, but the old guy with a voltmeter coupd find and fix problems with a autotransformer reduced voltage starter. With the solid-state stuff, well, a good supplier with spares better be available...

old field guy

 

[dpc]

I've always found the fixed hp limit on full-voltage starters pretty amusing. In the case of the 15 hp limit, if the motor in question was 30 hp and a reduced voltage starter was used, the end result would be basically the same as starting a 15 hp motor across the line. So if the system can tolerate that, why is RV required at 15 hp?

Of course, it's simpler to use motor hp as the criteria rather than the actual voltage dip, but it doesn't really make a lot of sense.

 

[jraef]

Saves them from having to calculate voltage dip on every small installation. Case in point, a small store with a 15HP elevator motor. If it's a freight elevator that starts maybe 3 times per day, they don't worry. If it's a passenger elevator that starts 10 times per hour, they do. But the project is too small to warrant assigning an engineer to calculate all the variables involved in VD, so they just make them put an RV starter on it.

JRaef.com
Eng-Tips: Help for your job, not for your homework Read faq731-376