Motor Question for DickV

I have a customer who has standby power via emg generator for his building. The emg generator is suppooosed to be sized to operate the hvac system, lighting, and one of the two elevators. The problem is that when the elevator starts the generator bogs down and the elevator pump motor cant get up to speed thus letting the resident smoke out of the motor due to a severe lack of CEMF etc..

Get this..The generator contractor says it’s a problem with the elevator because everything else works properly, and the KW rating on the generator matches all of the collective demand. ??? I tried to explain to the customer the technical aspects of locked rotor current, and starting current, frequency shifts etc.. etc.. I told him that we should run a test on the generator to watch the frequency on a scope, and current shift when the elevator pump motor is starting under full load conditions. ( I figured if the generator is sized right, it may be a problem with the governor on the generator not properly reacting to the load). So the customer told the generator contractor what I said. The generator contractor went to the site, started the generator, and pointed to the Hz meter that read 60 Hz, then to the tag that showed the proper KW rating for the pump motor FLA. (we were not invited to this little ceremony). He then told the customer that he should have us install a soft start on the elevator so that we can control the starting current ramp. I again tried to explain to the customer again that the generator must be sized to accommodate the collective starting currents of all inductive loads. However my powers of persuasion are not as strong as the generator contractors. (lowest bid gets the job), and the generator guy convinced the customer that we must install the soft start to resolve the issue.

I told the customer that I will not guarantee that the soft start will solve the problem. The customer responded by issuing me a P.O. for the soft start. ??? banghead

OK so I cant talk myself out of this job and must accept my fate to sell something to somebody that may have no redeeming value.

So now my problem is this: When I install the soft starter, since there is only one set of settings, the motor starting current ramp must be set to the lowest common denominator (the generator). I know that the customer will not be able to tolerate the long starting time when the elevator is on normal power.

To get around this problem I see one of two choices: 1) to install an AC drive that I can vary the ramp time based on the power supply (via analog signal from a plc, or digital drive), or 2) Install two soft starts, one to run the pump motor under normal power, and the other to run the pump under generator power. To me either choice is unacceptable. We are dealing with a 50 hp motor @ 240vac, so either one would be really expensive.

Do you know of a soft start that has two settings that would provide two different starting ramps on two different start/run inputs? Or am I all wet? What to do??

Thanks, Mike.

Just for a quick response:

Have you thougt of a bypass contactor for your softstarter?
I mean, under normal conditions, the elevator is run by the normal starting equipment, but when the generator is on, the bypass is deenergized.

Just a thougt

Only a hope....
Most soft starts can come with a by pass contactor, arrange that contactor to be energized if bulding is on mains power, if mains fail contactor drops out, switching in the soft start. That was you should get normal operation on mains and reduced on generator.


Further some AC motor drive , Emerson Unidrive SP hav an option to run on 48Vdc as emergency recovery system, for lift systems, charge a big battery.....lol

What voltage is the generator putting out?

Except for the fact that this is a real and serious problem, it would be absolutely laughable. A standby generator salesman comes in and sizes a standby generator system for average continuous load without any regard for peak load. If I did that in my line of work, I'd be without work pretty guickly.

No, the simple fact is that the gen set is sized wrong. Sadly, that won't change anything and its not an argument worth winning, it seems.

I would say the the other posters' suggestion of a softstart with bypass contactor is a good one. I rep Motortronics softstarts and I do believe that their microprocessor controlled unit can set two different ramps but, a bypass would be cheaper and simpler.

Regarding the setting up of a softstart, the capacity of the supply (in this case, the gen set) has little bearing on the limits you have available for adjustment. The starting current is ENTIRELY determined by the worst-case breakaway torque of the load, in this case it sounds like a hydraulic pump. It should be obvious that you can't set the starting current and torque less than breakaway or the motor will not start to move. And if it works out that breakaway current is still too high for the gen set, you will not have fixed the problem.

So, be careful here. The softstart is not a magical solution unless you are sure that load breakaway current is low enough for the gen set to hold its output up.

Oh, and regarding magical solutions! It sounds like the gen set guy is really into that kind of thing, first the undersized gen set and then, an "all you got to do is" solution with a softstart!

How do people like this stay employed? Whoops, there I am again. Baffled, baffled, baffled!

May be dumb question, but have you checked load of other items, the pump motor should be as name plate but lighting can grow in size and demand and you may not have as much left for lift as you expected.

Dick, and all

The proposed starter (that we usually use) is HPV100 from Magnetech (elevatordrives.com). I have thought of the bypass idea by way of possably installing two soft starters. The mechanical contactor supplied with the soft starter is a Fault contactor that breaks the lines oposite the SCRs in event of a fault, like a shorted SCR.

Note: I have taken to the practice of dropping the fault conactor when the motor is not running, rather than leaving it up all the time. This is due to the fact that we once had a severe fire due to a partually shorted motor, and the fault contactor only being disengaged when the starter had a fault. We sequense it like this: pick fault contactor - pick run - drop run - drop fault contactor.

Dicks response to my question is what I'm very afraid of. I'll have to heavly document my file in the event of litigation when this all dosnt work out. Or I can just out-right refuse the job.

Dick,
I would like to see some documentation on the two ramp starter, if you could provide it. I wont do anything until I know that I have exhausted all other options. (this may be a problem since the owner is changeing the building feeder in two weeks and will need the elevator to run off the generator for two days!)

I apperciate your response.

Thanks, Mike.

I have encountered exactly the same situation. In my case it was a 200 hp motor. A soft start did solve the problem, but I had a variable torque load and I could set the current low enough and the ramp time long enough to control kVA loading on the generator set. Your pump is probably a constant torque load, which will make starting current requirements worse.

In some cases I've had to put delays on various loads, so that if the generator is running only one piece of equipment at a time will start.

Someone is going to have to do some analyis. Since the generator supplier is obvioulsly in "CYA" mode (Cover Your A$$) and the owner is clueless, that means you. You will have to look at the kVA rating of the gen set, the starting current of your pump and other loads, and see if the caapacity is there.

(You might also ask the generator guy if he knows why transformers and generators carry kVA ratings and not just kW!)

Tom,

I would ask him your proposed question, but he's avoiding me like the plague.

Well DickDV, isn't it always the situation?
Some flaw in the design and the first respons is "just rewrite the software". Yes, that blackbox of a PLC does the magic

ejessen,

It seems that you and I are on the same page! Check out my post, on another thread, http://www.plctalk.net/qanda/showthread.php?postid=43544#post43544]HERE![/URL]

I'm glad I'm not the only one....

Disclaimer: Since I don't know the details of your hydraulic system, this may be totally unfeasible.

Could you add a 'load/unload' valve so that you don't have to start the motor against full pressure?

In the 'unload' position, the hydraulic fluid is trapped between the cylinder and a check valve, and the output of the pump is directed back to the tank. Once the pump is up to speed, you shift the valve to the 'load' position so that the pump output is directed to the cylinder.

It's possible that the cost of the additional valve and plumbing might be less than the cost of the soft starter.

elevmike,

Your thread leads to another classic
Been there, done that, bought the t-shirt sooooooo many times - and I guess it'll continue eternally (hopefully I'll only be around for the next 60 years or so ) - well, unless some programmer extends our life by some enginous code...

Back to the serious so we won't be

Soft Starts are a good way to start motors powered by generators. I have successfully used Soft Starts on ships and mobile equipment where the generators were unable to start the motors across the line. However, as DickDV pointed out, the generator must still be able to produce enough current to start the motor using the Soft Start, which is going to be more than the motor full load current. Assuming the hydraulic valve doesn't open until the motor is up to speed, the motor starting torque requirements will be low and you should be able to start the motor in a reasonable time with 150-175% of the motor full load current. Ask the generator salesman if the generator can produce this much current, along with current for all the other loads, for a short time. Let him decide if the generator is suitable.

If you go the Soft Start route, purchase a starter which can be set up with two different ramps. You select which ramp is used by closing or opening a contact. The normal power ramp can be very fast and the motor will start almost the same as across the line. The generator ramp can be set to increase the current gradually up to a maximum which will start the motor in a reasonable time. The secret to starting a motor connected to a generator is to gradually ramp up the current allowing the governor time to respond.

Soft Starts with bypass contactors generally start the motor with the Soft Start and energize the contactor after the motor is up to speed. When stopping, the contactor drops out first, then the Soft Start shuts off. The contactors are not rated to continuously start and stop the motor across the line. You may be able to get a Soft Start with a "Emergency Bypass" contactor which is full rated or you could use the present starter for normal starting. Either way, there will be some control logic required. The two ramp Soft Start will be much easier to implement.