150 hp motor

[puckman]

We have a 150 hp motor 480volt started across the line . We also found it being feed with 3/0 cu xhhn-2 cable in 2" gal conduit and walked off a distance of over 290' . When this motor starts it drops out hps fixtures in the plant. I had calculated a vd of 7%. I could be wrong. This is a 15 year old unit. Does anyone think increasing to 4/0 would make a differance {help} in the starting problems? Or is a soft start the way to go as was suggested here?

 

[mcclary's electrical]

We have a 150 hp motor 480volt started across the line . We also found it being feed with 3/0 cu xhhn-2 cable in 2" gal conduit and walked off a distance of over 290' . When this motor starts it drops out hps fixtures in the plant. I had calculated a vd of 7%. I could be wrong. This is a 15 year old unit. Does anyone think increasing to 4/0 would make a differance {help} in the starting problems? Or is a soft start the way to go as was suggested here?

Iv'e seen up to 200 hp's started accross the line, but it all depends how much power is available. I would definately use a soft start in this day in age.

 

[augie47]

Unless I'm missing something or you have 90? terminations, the 3/0 does not meet NEC. Overlooking that for a moment, I don't think increasing the wire size would have any effect on your problem. Some type of reduced voltage starting, soft-start-etc. would most likely be the only solution, IMHO>

 

[iwire]

If you increases the conductor size the effect will be that the lights in the plant will dim more as the motor circuit will now allow more current to flow.

That 290' run of 3/0 is already working like a soft start by reducing voltage during start up.

 

[Jraef]

If you increases the conductor size the effect will be that the lights in the plant will dim more as the motor circuit will now allow more current to flow.

That 290' run of 3/0 is already working like a soft start by reducing voltage during start up.

I was going to say the same thing; it's an ARVS; "Accidental Reduced Voltage Starter"! Those cables are likely acting like a Primary Resistance Starter, dropping the voltage to the motor during start up because of the high current through undersized cable. The cables are probably getting very hot during start up as well. If you re-size them, you will solve that issue but just shift the problem further up stream. You need to use something else to reduce the starting current; a soft starter is the least expensive and most reliable solution.

 

[ptonsparky]

Was this always a problem or just recently? POCO tranformers are not big enough and most likely the SE, along with the feeder that you already know about.

 

[puckman]

This has been a problem from day one from what Iam told.
The mcc this is in is drawing less than 165 amps . It is feed with 6 - 600's from main gear, I don't know what the main switchgear is drawing, we do have 4 other mcc's. The utility trans is 750kva.

 

[Power Tech]

150 amps across the line.:-?

Soft start.:roll:

With it you get all the great motor protection.

 

[Power Tech]

If you increases the conductor size the effect will be that the lights in the plant will dim more as the motor circuit will now allow more current to flow.

That 290' run of 3/0 is already working like a soft start by reducing voltage during start up.

Good point. I wonder if a capacitor would help?

 

[ptonsparky]

Seems odd to me. I have a customer that starts a 200hp accross the line and the lights don't flicker. Thats after the first 500 of misc is on line. 480v. I suppose the primary set up is different.

 

[Chenley]

I'm with the guys above and throw a soft start on it. Pretty sure I would have done that on day one.

 

[weressl]

150 amps across the line.:-?

Soft start.:roll:

With it you get all the great motor protection.

Saying that soft start will solve his problem is irresponsible. How do you know if the motor is sized for the starting torque in the first place?

Compare the breakaway torque of different manufacturers and different models and you will be surprised in the results.

A reduced voltage SS is designed to limit the inrush(magnetizing) and starting current but it iwll also limit the available torque. While now he only dimms or drops out some lamps but the motor at least starts and run, it is possible that adding the SSRVS will not be able to start the motor, even with the maximum allowable setting and kickstart functions enabled.

Then there is of course the staring voltage at the termianls that can be improved upon by oversize wiring.

Just please stop offering simple solutions whose succes you have no way of knowing.

 

[chris kennedy]

Just please stop offering simple solutions whose succes you have no way of knowing.

OK, I won't do that ever again.

 

[benaround]

The motor has been starting for the last 15 years, I think it has the starting torque. In this

time of saving energy coupled with the aging grid system, IMO any 150hp motor should be

on a soft start or whatever, it would be irresponsible not to.

If the wire is too small, change it to the right size.

Or it could be some 1 out of 100 goofy problem.

 

[weressl]

The motor has been starting for the last 15 years, I think it has the starting torque. In this

time of saving energy coupled with the aging grid system, IMO any 150hp motor should be

on a soft start or whatever, it would be irresponsible not to.

If the wire is too small, change it to the right size.

Or it could be some 1 out of 100 goofy problem.

The excessive voltage drop IS the evidence that the motor HAS a problem with accelerating the load, eg. insufficient starting torque. A SSRVS would most likely worsen this condition.

Since this condition exist from day one there is no evidence offered so far that the mechancial design was executed properly.

I calculate the voltage drop for the given data and NEC 430 Ampere table - which is conservative - @ 1.54% or 7.38V, which is more than sufficient to start the motor.

If the single 750kVA transformer @902FLA is fully loaded there may be a problem, but not if the load inertia is average.

 

[wirenut1980]

The excessive voltage drop IS the evidence that the motor HAS a problem with accelerating the load, eg. insufficient starting torque. A SSRVS would most likely worsen this condition.

Since this condition exist from day one there is no evidence offered so far that the mechancial design was executed properly.

I calculate the voltage drop for the given data and NEC 430 Ampere table - which is conservative - @ 1.54% or 7.38V, which is more than sufficient to start the motor.

If the single 750kVA transformer @902FLA is fully loaded there may be a problem, but not if the load inertia is average.

weressl: You calculated 1.54% voltage drop for starting? Seems low to me. I don't think there is enough information to calculate this anyways since we don't know the source impedance, 750 KVA transformer impedance, NEMA starting letter of the motor. The fact that HPS lighting is going out indicates a voltage drop of at least 10% at motor starting.

What measured values tell you if you have insufficient starting torque?

To the original poster: Have you measured the starting current to see if it makes sense? How long does it take for the motor to get from start to running load?

 

[weressl]

weressl: You calculated 1.54% voltage drop for starting? Seems low to me. I don't think there is enough information to calculate this anyways since we don't know the source impedance, 750 KVA transformer impedance, NEMA starting letter of the motor. The fact that HPS lighting is going out indicates a voltage drop of at least 10% at motor starting.

What measured values tell you if you have insufficient starting torque?

To the original poster: Have you measured the starting current to see if it makes sense? How long does it take for the motor to get from start to running load?

I told you exactly what was the basis of the voltage drop and it is accurate fur NEC FLA. As I pointed out there is insufficient data to do a starting study that not only would need the missing data you cited, but also the motor impedance and load inertia.

HPS lights won't drop out at 10%, the voltage drop is much greater.

When you calculate voltage drop, 10% running voltage drop usually give you sufficient terminal voltage to start a properly sized NEMA motor. (That is at nominal supply voltage.)

 

[wirenut1980]

I told you exactly what was the basis of the voltage drop and it is accurate fur NEC FLA. As I pointed out there is insufficient data to do a starting study that not only would need the missing data you cited, but also the motor impedance and load inertia.

HPS lights won't drop out at 10%, the voltage drop is much greater.

When you calculate voltage drop, 10% running voltage drop usually give you sufficient terminal voltage to start a properly sized NEMA motor. (That is at nominal supply voltage.)

Ok, I guess I don't understand how knowing the voltage drop along the conductors for running load amps is going to help determine anything? If the motor is not sized correctly, then the FLA would be larger than nameplate? Is that what you are getting at?

By the way, I said at least 10% and I used the attached EPRI document as my minimum percentage. If the lamp is old, 10% may take it out.

 

[puckman]

When we started the motor we set the fluke meter to the inrush current and at start up it pulled 802 amps we also put a volt meter across 2 legs and the voltage was down from 471 from 482 at the starter. When running the motor pulled I believed 76 amps. Ican't say how long it to took to get up to speed. I am told it is similar to a large sumbmurgable pump/motor. It is surpose to work the opposite of a pump and push air at the bottom of the tank. Don't know exactly how it works. It starts every hour and runs for 20 minutes. Thanks again

 

[Cold Fusion]

...In this time of saving energy coupled with the aging grid system, IMO any 150hp motor should be on a soft start or whatever, it would be irresponsible not to. ...

Frank -
Tell me how this soft start will save energy.

As I see it. ATL starting is maybe a second at LRA falling off quickly to running current in maybe 10 seconds. Running current in this case is about 40%FLA

A soft start may be programmed for 30 seconds at 3xFLA

I don't know how to tell that the first case takes more energy than the second case.

cf