Voltage Drop when starting large motors 3

[BOBDELL]

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?

 

[peebee]

For a temporary band-aid solution to keep your PLC limping along until you solve the real voltage-dip problem, I'd run out to CompUSA and get an APC UPS.

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.

 

[BOBDELL]

I have ordered a cvt to solve the plc problem,should be here in about a week.As far as solving the voltage dip problem, I am having a hard to sell the company on the idea of installing RVS on these 2 300hp motors,their reasoning being that we have been starting them DOL for years.Like I mentioned before the voltage problem only became apparent after I recently installed a PLC on this machine

 

[vonbad]

Maybe after the fact. But you said the UPS "(it says it has baterries)". APC puts out a pretty good unit. It does contain a battery that is DISCONNECTED at time of purhase you have open the back and hook up a wire to connect the battery. This is down so the battery doesn't die waiting to be purchased. Also, it's been my experience that the units usually last about 3-4 years and it doesn't depend on how many times the battery cycles.

Of course, the replacement battery is 90% the price of a new UPS, go figure. Better off buying a new one at that point.

 

[jraef]

Good though on the batteries vonbad. Thats the kind of little nuisance that will get me every time.

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"

 

[RRaghunath]

I really don't think it is a must to go for reduced voltage starters for the 300HP 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.

 

[VicP]

Be careful when selecting a UPS for a PLC. Many cheap UPS's will not give any protection against voltage dips.

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.

 

[aolalde]

I agree with VicP, in reality it is needed a high response voltage regulator integrated with the UPS.

 

[mc5w]

Come to think of it, a lot of large motors are equipped for part winding start not so much for control of starting current but to divide the load between 2 smaller contactors that are less expensive. If you go over 250 HP on 480 volts with a single contactor you are talking about an extreme jump in cost - two 150 HP contactors are usually cheaper.

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.

 

[afterhrs]

BOBDELL,
When is the last time your facility had an electrical infared survey completed?

Regards,
Afterhrs

 

[BOBDELL]

We had an inared survey done about 10 months ago,and I have had all the problems that showed up in the scan, repaired

 

[afterhrs]

I am curious to know if you added an additional 1kva
transformer to the control cabinet when you added the
SLC-5/04

 

[badasselectric]

I worked at a mine that blew up 2 size #6 starters in one night! the starters were on a 300hp vent fan on a bore hole. they started the fans in the wrong order and the chimney effect on the 300hp fan caused it to spiin backwards, when they tried to start it across the line it blew the door off the starter. they called the shift electrician who went to the warehouse checked out the spare starter changed it out and tried to start it again. it blew up and blew the door off again. by the time we got there we we saw the blown starter, told them they had to shut off fans and start in proper sequence. then we went to the warehouse to get the spare and found out we were out of size #6's. so we spent the rest of the night making one out of two.
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

 

[itsmoked]

Wow, sounds like a fun place to work badasselectric!

 

[jraef]

I love that MRO proceedure...

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"