Motor Question for DickV

[elevmike]

Ok guys,

I've lost count of how many emg-gen I've hooked up to in the past. The most frequent problem is the frequency shift that takes place under starting current draw. Under normal power you may get low voltage, but the frequency holds, allowing the motor to get up to speed under no load. With emg power the starting load often boggs down the generator causing the frequency to drop, preventing the motor from getting up to speed, causing more current draw, causing the generator to bogg more ..etc.. until something tripps, or blows or whatever. Like I said, they are keeping me out of the loop, (no pun intended) as hard as I may try. I still hope aganst all reason that the generator may be sized properly, but it cant react to the sudden load for some reason. I'm not a standby power expert but I'm sure that can be fixed, as in the past.

It is not my responsability, or expertise to do a full energy audit of all loads connected to the generator. It is my contention that the generator should replicate normal power for all loads, including the elevator. The generator guy thinks differently, and has convienced the customer to do this. I am going to provide the owner with a document that will sheild me from liability in the event that the generator guy is wrong. He has agreed to it in princeaple, over the phone. However.. a new twist, he is going to ask the generator guy to assume responsabiliy for failure.

I'll bet that sometime tomarrow I'm going to get a call from the generator guy!

There will be a full blown document signing ceromoney before the project commences.

In the mean time I'm looking for a two ramp soft starter.

Allan, I found out the reason I have 5 of those AB starters. They trip out on very small frequency shifts. We had to replace them with contactors/heater overloads. I just cant win!banghead

I need something that will give me two ramps and 60hz. It looks like it may be a drive! $$$$ Theres a big edifference between drives and soft starters. A drive will have the capability to provide a constant or even ramping waveform dispite the incomming frequency. With the soft start it's GIGO.

APB,

EXACTLY!!! Thats my whole point. However if you read my posts I failed in my attempt to be witness to the "test". I'm not responsable for the entire building, I'm just the elevator guy.

Doug_P,

Good very good try, but that's not gonna happen, I'm sure your a very good programmer as your other posts indicate.

Rick,

I tried my best to explain the operation of the valve but I may have fell short. For more info (if your courious) check one of our suppliers HERE. http://www.eecovalves.com/

Eric,

You really do need to go back and explain to that guy about the concept of time, and how 24:59 dos'nt exist on our planet...not yet anyway. He'll have to wait a few million years for the rotation to slow a little.

Best regards to all, and thanks very much. This has been a great thread!

I'ts past 13 O'clock and I have to go to bed.

Mike.

 

[Rick Densing]

Mike- I asked because of the assumption that if the oil is not being used to lift the elevator, the pump motor is unloaded. With a fixed displacement pump dumping through a relief valve, you are not. From your description of the pump and from what I can glean from the site, this may be the case. The load horsepower setup is flow times pressure. Since the flow is constant and the pressure is constant, your horsepower load is constant, weather or not the elevator is moving.

You may already know this, but I say it for the benefit of the audience, also.

 

[elevmike]

Rick,

The control valve acts more like a diverter valve. When the coil is energized a small piston in the valve moves thru a cylinder and diverts the oil from a full bypass and forces it to the elevator jack. As this happens the motor goes under load, and thus the current rises. It is not a presssure relief valve; it is a full bypass.

When the pump starts, all the oil is pumped directly back into the tank thru a open no-load circuit. The actual delivered HP is about 50% or less.

The elevator does have a pressure relief valve to protect the system from damage due to over-pressure; however it dos not come into play in the context of this issue.

Everybody,

Please dont misunderstand me; I do appreciate all of your responses, questioins, and suggestions. If I seem short, or sarcastic at times, I really dont mean to be, as we are all on this forum to expand and share our knowledge.

Doug, As for your suggestion to install a smaller motor to start the larger main motor. I have seen this type of setup before in the case of turbines. My sister & brother-in-law work for the local power company. At the Conner Creek facility the have two huge gas turbine generators, used only for peak loads, that are started by smaller turbines that are started by electric motors.

In this case the load from the smaller motor (after start) would require a higher HP main motor, ultimately defeating the purpose of my efforts.

Thank you all.

Mike.

 

[Tom Jenkins]

"Your solution would seem to make sense, however in the case of a closed hydraulic system, continual bypassing would heat the hydraulic fluid beyond acceptable limits."

I'm inclined to agree with Rick on this one.

Heat generated in a hydraulic system:

BTU/hr = 1.48 x gpm x psi x (1 - efficiency)

If you have a dump valve that opens from pump discharge right back to the reservoir the pressure is nearly zero, and the heat generated is also nearly zero. A simple solenoid operated one way valve would do. I used to design hydraulic systems on aerial work platforms and that is how we handled it - with a propane engine for power you can't conveniently start and stop every few seconds.

I looked at the valve info, and it appears to include counter balance valve functions and a lot of other stuff as well. I realize there may be code restrictions or such on elevators that make my suggestion impractical, but heat generation shouldn't be a problem.

Of course, you are right that the real answer is to have the gen set properly sized!

 

[DickDV]

elevmike, APB has make some excellent points, some of them mirroring my comments early in this thread.

Primarily, this is the gen set people's problem. That you are trying to accommodate them ought not push you into anything that would compromise performance or safety. If it does, it's time to lay down the gauntlet and simply refuse.

I definitely would not get into any redesign of the system for accommodation. Adding a softstart is simple enough and may, that's MAY, help but I wouldn't go very far beyond that.

 

[Greg Gauper]

For positive displacement pumps:

Motor HP = (Pump Volume (in GPM) * Pressure (in PSI))/(Pump Effeciency * 1714)

I don't have the formula handy for working with metric units but 1 l/min = 0.264 GPM and 1 Bar = 14.5 PSI.

Different pumps will have different efficiencies. A good quality piston pump will be 90-95% efficient. Gear & screw pumps are typically in the 60-85% range.

I worked on a system that had a similar problem in which a soft starter was specified but the hydraulic design of the HPU did not incorporate a true unloading valve. The instant the pump motor starts to turn, itwill begin to pump volume. As soon as the motor began to rotate it pumped oil and began to raise pressure, so I had the same problem with trying to soft start a pump motor under load. I wound up setting the ramp times for the soft start to their minimum setting to try and get the motor up to full speed as quickly as possible in the brief time that it took for the pump to compress the oil in the line. That seemed to take care of the problem. In this particular case, our company didn't supply the hydraulics so we had no control over the design of the HPU, and the soft start was spec'd by the customer, but was totally the wrong choice for this application (a full voltage starter would have worked better.

 

[elevmike]

Greg,

Let me correct your formula a bit:

Motor HP = (Pump Volume (in GPM) * Pressure (in PSI))/(Over-All System Effeciency * 1714) Their's a lot more to the system than just the pump.

Tom,

When moving 200+ GPM thru a pump & valve & pipes etc.. this creates friction, which in-turn creates heat. We commonly cycle (bypass) the oil to maintain proper viscosity in cold weather applications.

Dick,

Thank you for clarifying that for me!

Mike

 

[APB]

A couple thoughts

Softstarts work by keeping the current below some multiplier of the FLA. At least on the ones I have worked with, this number is adjustable. Typically 3-3.5x FLA. Normal line start is around 6xFLA.

I suspect you can get or modify a softstart with two possible settings. One for EMG, one for line power. Actually, I am not sure you need two limits, as the pump is working ok on line starts.

So maybe all you need is some smarts in the softstart to know if you are on EMG or line. Then use the softstarts bypass contactor for Line starts and the SCR for softstarts. While you can purchase softstarts with all the features, you want to get a) Shunt trip in teh event of an SCR failure, and b) a bypass contactor for normal running. SCR's loose 2-3% to heat. Without the bypass contactor you will be warming up the cabinet and wasting electricity.

As for the possibility of unloading the pump. Good pumps are pressure compensated to unload the pump during times of no load. This reduces flow and HP as well as heat generated. There is often a small hydraulic line to do this. Any chance you can trick the pump into unloading during the starting process?

There is another trick used for heavily loaded conveyors. They sometimes use a fluid clutch that allows the motor to start up easily while allowing the load to start slowly. Expensive, and another part to go bad.

Do you have the make and model of the EMG. Perhaps they have a web site that has sizing instructions. This might provide some supporting detail when you have your discussion with owner. Also see if you can find a short description (Ordinary English, not Engineer speak) that describes what happens when you start a motor. There may even be something in the NEC.

 

[elevmike]

APB,

The generator powers other loads in the building also. I am not privy to imformation on those loads. The generator guy is suppose to do an audit of all loads, not me. The generator has more than enough capacity to handle the elevator alone.

The elevator pump only runs in the up direction; and only for 30 to 40 seconds at a time. The starters we usually use have no bypass contactor as the running time is so short.

I must admit, before this thread I have never heard of a "bypass contactor" due to the impractability of one in our applications. However for this job a bypass contactor may be a solution. I would simply leave the current contactor in place and use that for the "bypass contactor", and the soft start will only function on generator power.

I will present this proposal to my customer.
In the event that he does not accept this I will propose the two ramp starter proposed by Dick or Alan.

I want to thank all for your suggestions and comments. I for sure learned a few new things that will assist in this issue.

Mike

 

[Doug-P]

Rick,
Doug, ....

In this case the load from the smaller motor (after start) would require a higher HP main motor, ultimately defeating the purpose of my efforts.

Mike.

It's OK, my feelings weren't hurt.

It took me three reads to get what you meant in the statement above. Not to flagellate an expired equine but - one COULD put a clutch between the two motors that's only used on starting the larger. Once it's up to speed the clutch and the 'pony' motor disengage and draw no more power. Again, this adds more control, unless it's a purely mechanical sprag clutch. Additionally, I don't doubt that there are a lot of issues, it being a machine that carries people, that the rest of us are completely unaware of.

p.s. I know what it is to compensate through ladder logic for others' shortcomings/oversights. I feel your pain!

 

[Alan Case]

Though it is no use to you I am sure the SMC flex range has a built in bypass contactor. Regards Alan Case

 

[elevmike]

Alan,

I have recived your email, printed out the manual (all 132 pages) and am currently in the prosses of reviewing it.

Doug,

I checked you link. I have one of these also! It's out of the box and has been sitting on the shelf for years. I dont know who bought it or what to do with it..till now.

Thanks to all..again.

Mike

 

[Vic]

Mike

If you purchase a Soft Start, be sure it will operate over a wide frequency range, at least 45-65 Hz. As you mentioned in an earlier post, some starters do not tolerate frequency shifts. I was caught on this myself, years ago. Most moderm starters should be ok.

 

[elevmike]

Vic,

I'm studing the docs on the starters suggested on this thread. We have had problems with the AB starters due to frequency shift. However maybe they corrected this by now. If you know of a soft start that can tolarate this please let me know.

Thanks Mike

 

[APB]

Are you sure the AB starters are softstart or are they normal starters with solid state thermal overloads? The solid state overloads are more sensitive to tripping, particularly with phase voltage/current imbalance and repeated starting over a short period of time. A quick change in frequency might create a short term problem with imbalanced fields and currents as the motor tries to change speed. The old ITC elements seem to tolerate quick stop/starts better. The NEC requirement is for the overloads to be rated for 125% of FLA continuous for motors with SF not less than 1.15 (sec 430). I will admit to abusing that rule- once. I changed my mind after the motor caught fire.

Depending on the model and mfg, you can change out the SS overloads. Not sure about AB starters.