Voltage Drop when starting large motors
Voltage Drop when starting large motors
(OP)
I amhaving a problem with my 120v power dropping to 98.5v when starting up large (300 HP across the line 480v) motor in a sawmill. The lighting trans is fed from the same 480v distribution as the 300 hp motor. Total connected on the 1500 Kva transformer bank is 3132 H.P. Fla 4055, Any suggestions?





RE: Voltage Drop when starting large motors
Barry1961
RE: Voltage Drop when starting large motors
All the same, voltage dip is a result of having a low impedance load (a starting motor) and a high output impedance (transformer). You can decrease the dip either by decreasing the output impedance (increasing transformer size), by increasing the load impedance (smaller motor), or by limiting the starting current electronically.
The last option is the most realistic, and it can be accomplished by a soft start with bypass contactor. I am surprised anyone would start a 300 hp across the line--it would be impressive to watch it pull in, though.
If your voltage dip is severe (and 17% is severe), you risk stalling the motor, continuing to draw starting current without motor rotation.
William
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
More information is available at ht
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
Welcome to the best engineering forum on internet.
You can also try auto-transformer starting (with 60%, 80% taps), if your starting requirement isn't critical.
RE: Voltage Drop when starting large motors
Check your peak demand, both KW and KVAR. If you can get your utility to install a meter such as the General Electric KV2c that can do apparent power calculation, do that to assess what size transformer you need.
One expansion option is to add a second service and then move about half your load to it. This avoids increased short circuit current availabilty and having to have a total outage to change out the transformer.
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
Would a soft start not serve the same function as a starter motor?
William
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
After this incident was when I noticed the problem with the PLC going down once in a while when the machine was being started up.It has taken me a long time to actually find out was happenng,as the problem was very intermittent.
I am wondering if this voltage sag is causing a problem with the motor starters not pulling in properly due to low voltage supply to the starter coils(120v from a control transformer in the machine MCC)Maybe I should look into installing a cvt for the control trans.All the large coils for motors are controlled with interposing relays,and the relays are controlled by the PLC.
RE: Voltage Drop when starting large motors
If that size 6 starter started to chatter because of low voltage it would probably blow. I would use a small UPS on the PLC as it has other advantages over a CVT and won't add more heat to the environment. I have used them many times in abusive locations. I had one job in a leather factory where a sissorlift which was used every couple of minutes would bring down a PC near it every other time it operated. I put the PC on an UPS. That solved the problem totally. The UPS had to do its thing every couple of minutes. Ran for years. AND! kept the PC up on occasional blackouts.
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
Welcome to the forum. As you have already seen there are a lot of valuable contributions availablehere, and the difficult part of your adventure is already over; the part where you learned to recognize the problem for what it is. Now all you need to do is sort through the options available to you to fix it. By the way, there used to be a DANDELL that frequented this forum a lot, any relation? (just kidding of course)
First off, a CVT is an OK solution to a small short duration voltage dip, but has limitations when it comes to a long term sag or a deep low voltage conditions. If your size 6 contactor indeed chattered and blew up (my suspicion as well), your voltage dip was probably in excess of a level that a CVT can maintain at. It would help and you should do it first, but you may still get a problem now and again. Fix the cause (motor starting issue) as soon as possible.
As to which technology, IMHO a reduced voltage solid state (RVSS) starter is your best solution. Resistor starters are a big no-no, high fire risk (glowing red hot resistors and sawmills do not mix). Start-delta (aka Wye-Delta in the US) may cause more problems that it solves due to transition spikes when the starter shifts from a wye pattern to full voltage delta, and besides, wye-delta motors are not as common in the US so replacements are an issue. Autotransformer starters can be used with standard motors, but still give you problems with transition spikes and are now MORE expensive than RVSS starters anyway. Starter motors (aka Pony Motors) add complexity, safety and service issues that most lumber mills cannot afford to add to a machine that is often the heart and soul of the entire operation.
RVSS starters offer you full adjustability, no transition, and if selected correctly, 25+ years of service without replacing contacts etc. I wrote an FAQ that will give you some insight as to what they are and how they work.
FAQ237-1064
In addition to that info, let me also offer some opinions on how to select one.
1) Make sure you get a NEMA 12 enclosed starter with a bypass contactor. This will keep the sawdust out of it, and the bypass contactor will never wear out since it does not switch the motor on or off, it just keeps the SCRs from oveheating with the door closed.
2) Buy the heaviest duty rating you can afford. Smaller cheaper starters look attractive up front, but remember "there is no free lunch". They are smaller and cheaper for a reason. Many skimp on heat sink material and so cannot handle the heavy duty nature of most sawmill machinery loads. A good starting place is to ask about the "Overload Capacity" of the starter, usually expressed as a percentage of FLA rating and time. This is a combination of the SCR sizing and heat sink design, both of which are important to the successful operation and longevity of the starter. 500% overload for 30 seconds should be your minimum requirement, more is better. Many are rated for only 350% for 30 seconds. They are OK for some pumps and fans, but typically are inadequate for heavy machinery. Be wary of manufacturers who do not publish these ratings or tell you it doesn't matter, they usually have something to hide!
3) Lastly, look for good motor protection features. Most of the best RVSS starters on the market now include very good motor protection, as good or better than many very expensive stand-alone protection relays. The extra $50-100 they may cost you will be money well spent.
"Venditori de oleum-vipera non vigere excordis populi"
RE: Voltage Drop when starting large motors
Power companies like to deliberately overload oil filled transformers partly because they do not want to spend money for a bigger transformer. This is poor economy for any kind of commercial or industrial load. Heat pump heating/cooling systems for houses also need a full sized transformer. Tell them the they cannot use a 2 ounce bird to airlift a 1 pound coconut. ( From Monty Python and the Holy Grail. ) Tell them that for X dollars per month they have no business skimping. About once a year Cleveland Electric Illuminating Company blows up an oil filled padmounted transformer.
How they usually get away with this is that oil filled transformers up to 500 KVA 3-phase (167 KVA 1 phase) are usually about 1.8 to 2% impedance as compared to dry transformer typically 5.4% impedance. The oil units also have a lot of heat capacity compared to dry units which for cyclic load allows short term overload.
We also had a construction site this past summer where we had to have CEI change out a transformer that never carried any load. The customer got around to adding up their load list for MRI and CAT scanners and so forth dictating an upgrade from 150 KVA to 500 KVA.
A 1500 KVA 480 volt 3-phase transformer can carry 1800 amps of continuous load. For your connected load you might need 3 of these even with power factor correction capacitors.
The type 12 enclosure would also need a dedicated airconditioner to get rid of heat and keep the dust out. In your situation I would use a 2 piece system with the condenser unit located outside with a low ambient speed control for the condenser fan. OR, install packaged water chillers outside and run chilled water to your control cabinets to run dedicated air coolers for each cabinet.
RE: Voltage Drop when starting large motors
It sounds like you THINK that you currently have an APC UPS, but either it's not a real UPS, or it's broken (bad batteries?) or something. Even a cheap CompUSA UPS should be able to easily support your PLC during motor starts.
In the long run, though, you should definitely solve the voltage-dip problems using one of the solutions proposed above.
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
Of course, the replacement battery is 90% the price of a new UPS, go figure. Better off buying a new one at that point.
RE: Voltage Drop when starting large motors
I would also venture to say the this may have been a problem all along that may have been masked by the UPS, but now the batteries have failed from overuse. Short discharges and recharges will kill most batteries if repeated often enough. If this has been happening every time you start the motor, I would think that would do it. Consumer UPS's like that are made to protect against occasional brownouts and power losses, but are probably only good for a few dozen cycles at best.
BOBNDELL,
I find it frustrating that your management seems to feel reluctant to invest in protecting their machinery and their power system. Try explaining to them that starting a 300HP motor X-Line is like popping the clutch in second gear on a very expensive sports car. Sure you can get away eith it for a while, but eventually you will be replacing not only the clutch, but the flywheel, transmission, drive shaft, u-joints, rear differential etc. etc. Short sightedness is so rampant in industrial users it is scary sometimes. I would bet that the downtime alone has cost more than both soft statrters put together!
"Venditori de oleum-vipera non vigere excordis populi"
RE: Voltage Drop when starting large motors
From the numbers indicated 98.5V is about 82% of 120V. Generally speaking, the 480V motors would have been designed to start with terminal voltage of 80% rated. In practice, if the requirement on load side is not stringent, the motors will be able to start without any problem even at 75% voltage.
Thus, the present arrangement seems OK from motors point of view.
This leaves the problem of PLC not able to tolerate the voltage dip. For this, UPS is the best bet, I would consider. The CVT, if based on servo motor changing the transformer taps, may not be able to respond fast enough (during motor starting) as required to help the PLC.
RE: Voltage Drop when starting large motors
Allen-Bradley's PLC-5 power supplies are rated at 97-132 Volts. The PLC processor will stop at 95-96 volts. I have verified this by connecting a Variac to the PLC and turning down the voltage until the PLC faults.
Most cheap UPS's have no brownout protection. They do nothing until the voltage drops to a preset level, then the UPS switches to battery and inverter mode. The specs for the APC model Back-UPS CS 350 states that the "Input voltage range for mains operation" is 98-140 volts. In a voltage dip situation, the PLC may shut down before the UPS cuts in.
More expensive UPS's have boost taps on a transformer, which are switched in as the voltage falls. This helps to regulate the output voltage in a voltage dip situation. The best solution is a "full time" UPS which continuously powers the output through the batteries and inverter. These are also the most expensive.
rraghunath
The Sola CVT is a ferroresonant transformer, which does not have taps that must be changed. The voltage regulation response time is very fast.
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
Part winding start does not really control starting current and most motor manufacturers state that the second contactor MUST be closed withing 1 or 2 seconds of the first.
I have also wired up a 100 HP 480 volt motor that theoretically could do either part winding start, wye delta start, or a combination of both.
RE: Voltage Drop when starting large motors
When is the last time your facility had an electrical infared survey completed?
Regards,
Afterhrs
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
transformer to the control cabinet when you added the
SLC-5/04
RE: Voltage Drop when starting large motors
Anyway I wonder if conditions were right with other loads that when your operator tried to start the motor it stalled and pulled the voltage way down causing other contactors to drop out before the starter blew and and voltage returned to normal. just a thought. badass
RE: Voltage Drop when starting large motors
RE: Voltage Drop when starting large motors
1) Something big and expensive blew up
2) Replace it with the last remaining backup unit
3) New one blew up
4) Call someone who might know what is wrong.
"Venditori de oleum-vipera non vigere excordis populi"