Motor starting
Motor starting
(OP)
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?
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?





RE: Motor starting
The ultimate solution would be an adjustable frequency drive. That would definitely work, but would be much more costly than a soft starter.
RE: Motor starting
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Motor starting
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
RE: Motor starting
I think it's starting OK, they just don't like the voltage dip. At least that is how I interpreted it.
RE: Motor starting
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...
RE: Motor starting
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.
RE: Motor starting
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
RE: Motor starting
RE: Motor starting
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
RE: Motor starting
JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
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RE: Motor starting
RE: Motor starting
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
RE: Motor starting
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
RE: Motor starting
RE: Motor starting
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...
RE: Motor starting
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
RE: Motor starting
"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.
RE: Motor starting
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Sometimes I only open my mouth to swap feet...
RE: Motor starting
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 ?
RE: Motor starting
The compromise is to provide high accelerating torque to the motor load, but only for a few seconds.
By the other hand the higher the current the larger the voltage dip. As I was trying to explain above, the initial voltage dip is limited only by the generator reactance, the VR and Governor react after some cycles are sensed.
Yes if you could feed the motor with a dedicated generator, the transients during the motor start will be avoided to affect the load sensible to voltage drops.