Motor starting

[biminijoe]

Trying to overcome motor starting problem on axial piston pump hyd system.

50 hp Squ. Cage Motor 480 VAC NEMA CODE F

Generator is 188 KVA stand-by - 169 KVA continuous

Power distriubution system experiences unacceptable voltage drop when motor is started.

Is it possible to reduce the starting load enough with a soft-start device? Which method/type should I use ?

The hydraulic systen is not under load while the motor is starting other then some fluid recirculating back to tank. Is this still considered staarting under high torque?

 

[dpc]

A solid-state reduced voltage starter is the best option cost-wise, provided it can provide you with enough starting and accelerating torque while limiting the current demand enough to get the voltage dip to something you can live with. You need to determine the speed-torque curve for the driven equipment and I can't help you much there. There will probably be other coming along shortly who are familiar with the pump you are talking about.

The ultimate solution would be an adjustable frequency drive. That would definitely work, but would be much more costly than a soft starter.

 

[Skogsgurra]

dpc is always right on - but I think too cautious with your money. A 50 HP VFD isn't overly costly. And it is the better solution, too.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

That size generator should start a 50 HP hydraulic motor especially with the bypass open. Do you have a large base load on the generator tries to start the motor?
If the load on the generator is less than about 50 KVA I would check the governor settings. An improper dead-rack setting or excessive droop will cause these symptoms. A faulty turbo may also be at fault.
Or, if you just have too much base load and not enough spare capacity to start the motor, see the previous posts.
All good advice.
respectfully

 

[dpc]

waross,

I think it's starting OK, they just don't like the voltage dip. At least that is how I interpreted it.

 

[ScottyUK]

That size set is about where the AVR starts to become a serious item instead of a toy - might be worth checking what type of AVR is fitted. A 3-phase averaging type will give you the best response if it is correctly set up.

Is my European memory correct that a NEMA type F motor is pretty enthusiastic about starting current? My tables are at work - what is the multiple of FLA?


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Sometimes I only open my mouth to swap feet...

 

[biminijoe]

Thanks for all responses so far ....

There can be as much as 40 to 80 KW on the line when the motor is started. Yes it is the voltage dip take is the problem

I could spend some more money on the generators and upgrade the voltage regulators, add a permanent magnet field excitation and set up for parallel with another generator but I wasn't sure to start with the gens or try to reduce the load.

I am calculating a 250 KVA start surge when the 50 motor starts.

 

[waross]

Agreed dpc;
In my experience with generators in that size range I have found that a bad voltage dip is often associated with engine problems. Either a worn out engine or governor problems or settings.
If the set has adequate capacity to start the motor, the Automatic Voltage Regulator does a pretty good job of holding the voltage up. However, if the engine slows down when the motor starting load is applied, the Under Frequency Roll Off feature of the AVR will reduce the voltage in proportion.
Or, the situation may be normal and the folks are over-reacting to a normal voltage dip.
I support yours and Gunnars suggestions, but I have several times had similar motor starting problems on gen sets that were engine/governor related.
Respectfully

 

[CJCPE]

Locked rotor code F is the designation for 5.0 to 5.6 KVA per horsepower.

 

[itsmoked]

A flooded piston pump will always be a nasty starting load even bypassed because you still have to get all that stuff moving.

A VFD would be a great way to go as it will also reduce the starting loads on all components and piping etc. Also you can then start controlling things like the flow meticulously if desired. You can stop bypassing to control pressure which is a mighty waste!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Soft starter of VFD will work fine. At 50HP 460V a soft starter is going to be about 1/2 the cost of a VFD.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[aolalde]

Nothing is wrong with the Voltage regulator or the Governor, the voltage dip is proportional to the load current and generator reactances. The options to reduce the voltage dip are; Increase the Generator size or reduce the load current. The most effective solution has to be the above recomended VFD to have complete control on the motor starting.

 

[polvalrot]

i would like to mention the soft starting possibility through the generator itself.THis is an option, only when the generator is dedicated to one motor only.

So this is not our case, unless you acquire another generator only for starting the motor (or also a transfer breaker system to transfer each of the started motors to the other generator and keep the 2nd generator for the starting procedure).

AVR often have the option of soft starting individual motors, through the soft voltage build up on generator terminals, in the prefered time, with the generator and motor breaker closed from the begining of the procedure.

Limitations are the thermal trip of the breakers (although you can "chop the voltage" and consequently the current from the Stator Current Limiter through the AVR), the possibility of rotor demagnetization (so you will need a voltage flashing plate - a battery)

best,
chronis

 

[waross]

Hello aolalde;
This ain't my first Rodeo. I have twice encountered generator problems with extreme voltage drop and an inability to start a motor that was within the capability of the set. One mechanic went so far as to do a complete rebuild on a perfectly good engine, and when he was finished the set still wouldn't start the motor. Dead rack adjustment on the governor both times. Operation was normal on both sets after governor adjustment.
I agree that this is probably not the problem here but it is a possibility if a mechanic without geneset experience has been near the set.
respectfully

 

[ccjersey]

This may be a "closed center" system where the pump maintains system pressure, but is destroked when there is no demand for flow. So no bypass etc. That should make a difference in SS vs VFD if there are almost no economics to speed control.

 

[ScottyUK]

aolalde,

You must be privy to some information which the rest of us aren't to make such a sweeping statement in the absence of any data regarding the depth and duration of the voltage excursion. Have you become a manager since last time you posted?


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Sometimes I only open my mouth to swap feet...

 

[waross]

Sorry for my rant.
Further to ScottyUK's post, what is unacceptable to one group may be an a minor event for another group.
I realise that this is an electrical forum but a generator is a mix of electrical and mechanical systems. There are several mechanical issues that will produce symptoms that will fit the broad parameters of the original post. Include a partially plugged fuel filter, or a non functioning turbo.
Also, the engines are not made with a specific power output for a generator set. The nearest suitable sized engine is selected and the power output is matched to the alternator with the dead-rack adjustment. Some sets may be "Boosted" by opening up the dead-rack adjustment. This will give them some extra torque to help with motor starting surges, but some other method must be employed to prevent sustained overloads.
That said, I do agree that it is probably time for a soft start.
respectfully

 

[aolalde]

"There can be as much as 40 to 80 KW on the line when the motor is started. Yes it is the voltage dip take is the problem."
"I am calculating a 250 KVA start surge when the 50 motor starts."
"Generator is 188 KVA stand-by - 169 KVA continuous"

To me the above information by biminijoe is stating a transient dip when the current increses sudenly around 200% on a Stand-by generator. The transient voltage dip is then corrected by the VR and Governor reaction but the initial voltage drop is causing the problem.

 

[ScottyUK]

I totally agree, but how wide and how deep is the dip / sag? Poorly tuned governor or AVR parameters, or hardware problems, can cause a deeper or wider dip than could be achieved with a well tuned system in good condition. Without any data we are all just speculating.


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Sometimes I only open my mouth to swap feet...

 

[biminijoe]

Thanks to all for some good leads ....

I am going to double check generator mechanical operation, and AVR and woodward control on govenors.

The motor does start... its just that distribution system voltage drops more then desirable. The then the generator takes about 15 seconds to stabilize voltage.

Once I am confident of the generator performance is that motor load just too big? I can live with some voltage drop but I am worried about an in-rush of 250 KVA on a generator with a standby rating of 188 KVA.

I like the idea of isolating the start-up load on one generator. That can be done with a bus-tie breaker on the switchbd and moving the location of the hyd pump breaker. If I did that would I still have to lower the starting load to protect the generator ?

I suspected the flooded piston pump does cause alot of the start-up torque and thus high start current. It is a closed loop system.

Is there a type of motor starting more suited to high torque applications ?