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Pony Motor Starting Without Pony Motor

Pony Motor Starting Without Pony Motor

Pony Motor Starting Without Pony Motor

Good Day Folks.

I will not resurrect a thread form 2017 since doing so tends to annoy people, so I will post the URL for the previous discussion and reference the points brought up in the thread.


In this thread, member Skogsgurra related a novel method employed to soft start 11kv synchronous motors using a 690v feed into a 3300V, 250KVA transformer, using a VFD to slowly bring up the speed on a defibrator at a pulp mill. Full excitation is used from the outset.

I hope Skogsgurra sees this post and can respond to it, but any input is greatly appreciated.

What I would like to know is, are defibrators usually direct coupled to their motors, or were these motors brought up to speed, then coupled to the load. Also, what was the general HP range of these motors. I am assuming these machines are available in a wide range of hp ratings, so I do not suppose anyone else could hazard a guess as to their size.

I am curious about the setup, since we are currently investigating how we can implement a soft start on our 8MW, 13.2KV main synchronous motor which is directly coupled to a 4 rotor, integrally geared centrifugal compressor. We supply exhauster service to our main client with this machine, and they are asking whether there is a way we could provide the flexibility of operation to start and stop this machine several times a day. They are in a cost reduction mindset, and obviously an 8MW exhauster is a large component of the running costs associated with their test campaign. Some of periods of the test would not require the flow provided by the exhauster.

This compressor currently has a DOL starting setup and the currents involved, and the mechanical stresses involved in each start are considerable, which is why we try to limit the machine to one start per day, unless there is a startup fault. If that occurs, there is a timeout to recover the thermal capacity of the motor and another start is attempted. This is not desirable, but we do have contractual commitments to fulfill for our client, so we make an exception in these thankfully rare occurrences. The shorting rings on the rotor have been repaired at least once before for cracks near the holes where the bars go through, so obviously we would like to avoid stressing the motor as much as possible. The machine was built in 1984/85.

Management has investigated the purchase of a VFD and it was about $2,000,000, but I am assuming this is for a full voltage, full power unit. If we could install a low voltage, partial load VFD to accomplish a soft start, it might be far more cost effective. Unfortunately, a compressor coming up to speed will probably present more of a load than a defibrator without material inside it, so it might be not as easy a task to accomplish.

Thank you for any guidance.

RE: Pony Motor Starting Without Pony Motor

Skogs has sadly shuffled off his mortal coil and is probably sorting out motors for his God in heaven.

I'm sure someone else will be along shortly...

Politicians like to panic, they need activity. It is their substitute for achievement.

RE: Pony Motor Starting Without Pony Motor

So ... whether you can start your compressor application via VFD depends on how the excitation field is applied to the synchronous motor. There are three possibilities, given the age of the machine (mid 1980s vintage). Either it is fed from a DC source through a brush-and-collector assembly, or it has an AC-DC brushless exciter, or it has an AC-AC brushless exciter. Given also that the existing machine is a line-start (with probable voltage dip during the initial few seconds), chances are good it's one of the first two methods.

If the excitation is provided by brush-and-collector (from outside source), then you can probably start the Synch motor successfully on a drive. This is because the value of field current is - strictly speaking - not tied to rotational speed.

If it is the AC-DC brushless version, you can't - because you can't supply the required field current at or near zero speed. The thyristors doing the rectification require a minimum voltage across them to turn on ... which is proportional to the rotating speed of the exciter rotor and hence the main synchronous rotor. Higher speed = more volts available.

If by some chance you have an AC-AC version, then you could operate on a drive ... but might need to modify some characteristics of the excitation package. The modification would come because you might need a bit of "voltage boost" when operating down below about 6 Hz line frequency to overcome some drive control issues.

Starting via a drive does not put any additional thermal stress on either the rotor or stator winding - provided the machine gets up to speed in a reasonable interval (15-20 seconds or so). Longer than that and you might need to investigate thermal stress on the rotor winding during start, since the rotor won't necessarily be moving enough air to effectively cool itself.

Last - I suspect the management investigation result was indeed a "full bore" VFD, with the intent that the compressor need not ever actually shut down, but operate at several speed points, corresponding to required throughput for the compressor/exhauster. A drive sized solely for starting would probably be cheaper - but may be 60% of the full model. It would not be a reduced voltage version unless you had additional equipment (like Skogs' intermediate transformer), so the price will still be high regardless. Besides - with a line voltage drive, you can just use a simple bypass switch to transfer the system back to line voltage for "normal" running.

Converting energy to motion for more than half a century

RE: Pony Motor Starting Without Pony Motor

If they are splashing on a full bore VFD, then why not go truly full bore and replace the synch motor with cage induction motor and run the compressor at the required speed with added benefit of eliminating synch motor field winding, its gear, controls and its future failures. That way, they can start and stop at will 'saving power'.


RE: Pony Motor Starting Without Pony Motor

Well, that's very sad news about Gunnar, may he rest in peace.

It looks like this will be a no go, since we have a brushless exciter with thyristors, so zero rpm excitation will be impossible.

I am sure management did a fair impression of the fainting banker in the Community Chest once they received the quote for a full chat VFD, so I doubt there will be any more action on that front.

I am thinking the easiest route would be a Korndorfer autotransformer starter arrangement. We have another 8MW compressor at our second facility across the city which uses this design. In the four years I worked there I started the compressor hundreds of times, and I never saw power at startup exceed 1MW. I'm not sure if this would be an "easy" enough start to allow multiple starts per day. I know the autotransformer didn't sound too happy with its lot in life, but it has been at it since the late 1950's, so it seems like everything was engineered properly the first time, even the manual exciter controller with it's bicycle chain driven multiple tap & wiper arrangement, pulling it's power through what looked like a large toaster.

Our compressor peaks at 8.1MW at startup, after about 20s, with the 1200rpm motor at 979rpm. The amp draw falls very quickly from there as it approaches full speed, even before excitation voltage is applied.

I suggested a DC drive pony motor installed parallel to the main 11" drive shaft, which would engage a gear installed on the shaft to spin the motor up that way. It appears DC drives are a lot cheaper, however, seeing as how the compressor will be requiring 1.5MW at full rpm, I am thinking 2000hp DC motor would probably not be an economic solution, no matter how cheap the drive might be.

RE: Pony Motor Starting Without Pony Motor

Do you apply AC or DC to the motor to excite the field?

Have you considered a soft-starter. That size is doable with a soft-starter and maybe cost about a 1/10th of a VFD.

The VFD can be used, it'd be similar to starting ATL then applying the field. The issue with a synchronous motor being started on a VFD without the field is that the motor will draw a bunch of reactive power, which means the VFD has to supply more current.


Either it is fed from a DC source through a brush-and-collector assembly, or it has an AC-DC brushless exciter, or it has an AC-AC brushless exciter.

All brushless exciters output AC on the rotor which is then rectified to DC. The exciter can have either AC or DC applied to it. It's either AC-AC or DC-AC. The AC-AC is built like a wound rotor motor and the DC-AC is built like an rotor-stator swapped or inside-out 3-phase generator.

RE: Pony Motor Starting Without Pony Motor

The exciter is running DC @37V and 2A.

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