Soft starting a 1,200 HP 4160 volt motor

Sure this is done all of the time. Best check with your motor manufacture first about this application. Then call ABB, I have seen more of their medium voltage stuff than anyother manufacture in this kind of application.

Seems pretty weird that your company would install equipment that they can not start. Or is this something your looking to buy?

Actually it is a client. It is a fan for a flash dryer and they are limited by what the utility supplies. They are looking to install this equipment. We normally don't deal with this aspect of the design. Thanks -S

| Actually it is a client. It is a fan for a flash dryer and they are limited | by what the utility supplies. They are looking to install this equipment. We | normally don't deal with this aspect of the design.

Is the supply 4160 delta, or 4160/2400 wye? Can you access each of the three (sets of) windings individually? Maybe one option you have it to do a 2400 volt start then switch to 4160. I've seen controllers for this kind of thing, though at low voltage. Medium voltage ones might exist.

Another option might be one or more smaller motors that can be connected on the same lines, but start OK with smaller power. Their rotational inertia could help start the larger motor.

Exactly how much power does the utility provide? Do you have any other nameplate data on the motor, like locked rotor amps?

manufacture

If they are considering it. Then look into a axial fan and controls. The motor starts with the fan blades closed so it is almost a "no load" start on the motor. Then as time progresses the controls can open the blades and load up the motor. Did this on a 480v 3phase motor for smoke removal. Starting amps was an issue with the generator.

Along the same lines, you can use an auto transformer for 4160. Similar in principle to Delta-Wye. It will still pull 2-3x nameplate during start.

The proper information that is required is the SCA(Available short circuit current) at the intended installation feed point. That would tell you exactly how much starting current you could have while avoiding more than X% voltage drop.

X%=SCA/Start>>

Try using a freq drive. That is if the motor will allow the freq change. Works great for chillers.

Works great for chillers.

A huge frequency drive on a high impedance system could give voltage distortion problems. And maybe even liability problems.

j

Works great for chillers.

This doesn't make a lot of sense to me. What type of motor is it? How do you know you can run the motor if you can't start it? Are you manually starting it? What does it mean "limited power from the utility?" Is there insufficient current but the no load voltage is ok? What does the voltage droop to when the motor tries to start? What happens, does it blow breakers? Are all of the phases connected properly? Since a fan doesn't put much of a load on a motor until it gains a fair amount of speed there may be more to this than meets the eye. Bob

Thanks for all the replies. We looked at VFDs, spinning it up with smaller motors and even uglier ideas. We finally told the client to talk to the utility. The problem is the supply to the plant is inadequate and the utility is complaining about the expense of the miles of wire and equipment to deliver the power.

The utility will most likely not be of much help. They tend to tell users to either cough up a ton of money to upgrade the feed to their new system, or they tell them to limit the starting kVA. Along those lines, there are 3 choices.

1) A VFD, but at 4160V you will be looking at $250,000 by the time it is all said and done. The advantage: a VFD can start the load at 100% FLA, and since you alrerady know the system can handle 100% FLA, you know in advance that the VFD will work. 2) Pony motor starting. This is the suggestion from above wherein you can use a small motor to get the fan moving, then switch over to the main motor after it is already spinning. Theoretically it could work, but when the 4160V motor is connected, there is still a massive inrush even if the fan is already moving, just smaller by a few percent. This is very expensive to experiment with only to find out it didn't work! They also tend to add mecahnical safety issues to the problem. 3) Reduced Voltage Starting. This is what was mentioned above as either Autotransformer (RVAT) starting or Solid State (RVSS) starting. In this HP size the RVAT will be slightly cheaper (if you ignore motor protection issues), but riskier and very big and heavy in comparison. Risky because again, you are not 100% sure it will work until you try it. RVSS is going to cost more up front, however most RVSS starters now come with protection systems comparable to Multilin Relays built-in, so if you add the cost of that to an RVAT, it comes out even. The best part is that RVSS manufacturers will often do a Transient Motor Starting analysis for you for free if you can provide all the motor, power system and load data to them. I used Motortronics on 3 projects last year and every one of them came out dead on to the TMS analysis they did prior to my purchasing them. So I knew in advance that it would work before I purchased them. On a 4th project, the utility restrictions were too severe so I had to use a VFD. Motortronics ran the TMS study for me and told me that any RVSS would not work, so essentially they lost the order. So at $25,000 each for the RVSS's and $250,000 for the VFD, I looked at it as though Motortronics saved me $225,000 each on 3 projects! The Motortronics is the one sold by ABB as well as mentioned above. The first ones I used 5 years ago came from ABB as part of a package deal, and I would still go that way again if the project called for it, but when I just wanted the starters alone, I went directly to the manufacturer.

If you want to get more opinions on this, try logging onto

and go to the Electric Motors and Controls forum to post a question. Great resource, free as long as you register, and they DO NOT sell your email address.

Reduced voltage starters that I have worked on will not limit the current much. Starting current values on our 4160 chillers are 3/4 of full load. 'course that could be the operators. I just look at the load data once a month.

No one ever said that medium voltage was inexpensive.

One other thing that you should be aware of, Unless the technology has gotten a lot better in the last couple of years. Medium voltage motor controls really do not like heat. I know of a water pumping plant in N. Phoenix that has a 10 ton air conditioner sitting on top of their medium voltage VFD's. In the summer time the room runs constantly at about 85-90 F. So if your application in in a warm place best check with the manufacture before going very far. Eaton/Cutler-Hammer used to produce low voltage VFD's and Softstarts that were rated at 40 C. When I called they thought I was crazed until I sent the temp information for my desert. We ended up ducting a/c directly into the bottom of the vfds to keep them running in the summer months.

Across the line starting current is normally given as 600% or so of full load current. Often soft starters limit current to 300% of full load current, or maybe 150% of full load. Dropping starting current to 75% of full load sounds very low. There are instances where this wouldn't even produce enough torque to get the motor running.

j

I was not speaking of a soft start. Reduced voltage stuff we have at 4160 v has an auto-transformer in the circuit until the start windings are used.

I learned about the starting torque problem a few years into my career while programming PLC controls for centrigual chillers. Fortunately, the motor controls weren't my problem. One site, a new central utility plant, had two 4500 HP and one 1750 HP chillers with RVAT starters. They started one big chiller with no problem and put a load on it. When they went to start the second one, the entire building lost power almost immediately after the start contactor closed. It turned out someone at the engineering firm had underestimated the starting demand, and the brand new substation had tripped on overcurrent. When asked about switching the autotransformer tap from 65% to 50%, the chiller manufacturer's engineers determined that the motor wouldn't have enough torque to start turning the motor, speed increaser (1:3.2 gearbox) and compresssor. The owner and engineer were forced to put in a larger service.

Closing the intake dampers on startup will also help. I watched the contractors go mad trying to start a 100 HP multistage centrigual aeration blower with the intake valve fully open and not trip the solid state starter. They were convinced that pulling a vacuum against a closed valve took more current than moving air.

Here's another thought - is it any less expensive to install and operate cogeneration at the plant than to pay the utility to install more new equipment with more capacity? It's more complex, and depends on the availability of fuel, usually natural gas, oil or excess plant steam. If a 1200 HP load pushes the existing supply to its limits, where will the power come from for any future growth?

Mike

I tend to refer to autotransformer starters as soft start too. I have not looked into medium voltage applications but at low voltages, the lowest taps I see on Reduced Voltage Auto Transformer starters are 50%, giving 25% of across-the-line starting current at the primary -- however across-the-line starting current is typically 600% of full load current, so RVAT starting current is still 150% of full load current. A starter that dropped starting current to 75% of full load current would be great for starting a motor without jolting the mechanical load. Or for starting a motor on generator power. I am surprised to see a number that low (75%), is all.

j