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?
Eng-Tips is the largest engineering community on the Internet
Intelligent Work Forums for Engineering Professionals
Voltage Drop when starting large motors 3
- Thread starter BOBDELL
- Start date
This is splitting hairs, but you are actually talking about a voltage dip. Drop is due to IR in the copper.
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
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
- Thread starter
- #4
Tahnks for your input I kind of thought that the load that is on that trans bank is on the high side and starting those 300hp motors,Is definitely causing these dips. I will look into installing soft starts for these motors. In the meantime I need to install some thing to maintain the 120v to the PLC processor.When power dips the processor goes down momentarily.
Install a Constant Voltage Transformer on the 120 Volt supply to the PLC. We install a Sola CSV Series transformer on every PLC we install in a sawmill. Only the PLC power supply needs the transformer, not the power to the I/O.
More information is available at
More information is available at
- Thread starter
- #7
- Thread starter
- #8
3,132 HP of motors on a 1,500 KVA transformer seems to be an overload just for full load current. You seem to need more like a transformer rated 3,000 KVA with convection cooling and 3,750 KVA with forced air cooling.
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.
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.
You didn't mention what the load is. But if the load is amenable, how about a starter motor? Maybe a sprague clutched 10HP to run the 300HP up to some large percentage of running speed. I image a lot of sawmill motors run loads that have no load on them at start but possibly a lot of starting inertia.
- Thread starter
- #15
I am going to install a cvt to eliminate the problem of the PLC dropping out for now,,but I think I need to re-evaluate the complete service. I am sure it is overloaded.It's the same old story,the company thinks that power is like a well that never runs out of water,they keep saying (we only need another 100hp, then 6 months later,we only need another 60hp and on and on)I think my well is going dry.
- Thread starter
- #16
About 6 months I had a strange problem with this same 300 hp motor.When the operator tried to start up the motor after alunch break,the size 6 starter for this motor blew up for no apparent reason, and all motors on another MCC that were fed from the same service dropped out.The 300hp motor was meggered and it checked out ok.The starter was changed and we have been running ever since.
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.
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.
BOBDELL:
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.
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.
- Thread starter
- #18
-
2
- #19
BOBDELL,
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"
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"
Bobdell,
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.
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.
- Status
Similar threads
- Question
- Locked
- Question
- Locked
- Question
- Question