Maximum recommended motor hp served at 480v??

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joshtrevino

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I have a project coming up at my plant in which we are installing a new boiler. One of my primary concerns is the boiler forced draft fan. The project is still in the preliminary phases, but I am told that the forced draft fan motor will be anywhere from 200hp to 500hp depending on the boiler chosen.

I have 2400v and 480v switch gear available to serve this load.

What is the rule of thumb for the maximum motor hp served at 480v?

Is there an IEEE or other recommended practice for this question?
 
joshtrevino said:
I have a project coming up at my plant in which we are installing a new boiler. One of my primary concerns is the boiler forced draft fan. The project is still in the preliminary phases, but I am told that the forced draft fan motor will be anywhere from 200hp to 500hp depending on the boiler chosen.

I have 2400v and 480v switch gear available to serve this load.

What is the rule of thumb for the maximum motor hp served at 480v?

Is there an IEEE or other recommended practice for this question?
Click to expand...
There is no applicable rule, because it is entirely application specific. I have seen plants that start at 250HP for MV and others, who do not have MV already available, go with 1000HP AT 480V. It also will depend on whether you go with X-Line, Soft Start, or VFD.

In your situation though since you already have 2300V and ostensibly MV qualified technicians, I would go MV above 200HP. NEMA size 6 and up LV starters get to be very expensive because the current is high and the sales volume is low for the mfrs, plus the smallest MV starter at 2300V is going to be around that size anyway.
 
joshtrevino said:
I have a project coming up at my plant in which we are installing a new boiler. One of my primary concerns is the boiler forced draft fan. The project is still in the preliminary phases, but I am told that the forced draft fan motor will be anywhere from 200hp to 500hp depending on the boiler chosen.

I have 2400v and 480v switch gear available to serve this load.

What is the rule of thumb for the maximum motor hp served at 480v?

Is there an IEEE or other recommended practice for this question?
Click to expand...

Depends on the size of the transformer supplying each voltage level.

480V is ususally limited to 3kVA due to the commonly available maximum secondary protection by switchgear. THat size may soon to change if we want to limit the available arc-flash duty for service on the swgr. 2400V is becoming an obsolete voltage, from the industrial standpoint, so equipment associated with it, including motors will not be as readily available in teh future, or even now. It still remains a relaiviely popular utility distribution level.

Our Corporate Standards used to limit the motors to be served @ 480V to 200HP, but now there is a qualifier to open it up the 200-500HP range to individual cosnideration if those motors are driven by a SSRV or ASD. (This is pretty much in line with the Petrochemical Industry Standards/Practice.) These larger motors often feed directly from the switchgear by a dedicated breaker, rather than form the MCC.


The secondary consideration is the overall cost. The distance of the motor form the starter can substanially sway that.
 
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joshtrevino said:
I have a project coming up at my plant in which we are installing a new boiler. One of my primary concerns is the boiler forced draft fan. The project is still in the preliminary phases, but I am told that the forced draft fan motor will be anywhere from 200hp to 500hp depending on the boiler chosen.

I have 2400v and 480v switch gear available to serve this load.

What is the rule of thumb for the maximum motor hp served at 480v?

Is there an IEEE or other recommended practice for this question?
Click to expand...

Its big enough that it might be something to go with the higher voltage on. A lot depends on factors that you have not bothered to tell us about.

I used to see a lot of stuff this size that they went with MV motors on. Now a lot of times they seem to put it on a soft start and go 480V.

I don't recall ever running across a 2400V motor though. Maybe I led a sheltered life.

In any case, somewhere around 200HP seems to be the line in the sand for where it makes sense to use MV motors.
 
The motor will be on VFD control (Powerflex 700 if 480v and Powerflex 7000 if medium voltage).
Our existing boiler is a 600hp, 2400v forced draft fan. The cost to install a new elevated building to house it, engineering, and installation was upwards of $450,000. The project engineer deems this cost unacceptable and so he is pushing for 480v. My existing 480v switch gear is about 200ft from where the boiler will likely go.

It sounds to me that a motor along the lines of 500hp will function at 480v. Is this correct?

What are the drawbacks of serving motors over the old rule of 200hp at 480v?
 
joshtrevino said:
The motor will be on VFD control (Powerflex 700 if 480v and Powerflex 7000 if medium voltage).
Our existing boiler is a 600hp, 2400v forced draft fan. The cost to install a new elevated building to house it, engineering, and installation was upwards of $450,000. The project engineer deems this cost unacceptable and so he is pushing for 480v. My existing 480v switch gear is about 200ft from where the boiler will likely go.

It sounds to me that a motor along the lines of 500hp will function at 480v. Is this correct?

What are the drawbacks of serving motors over the old rule of 200hp at 480v?
Click to expand...

So how do you figure going from 600HP to 500HP?

The drive-to-motor distance seems OK.

Do you have the capacity in the 480V switchgear to serve the new motor?
 
I have seen many motors over 200 HP at 480V. Not many, though a couple, over 600HP. Many plants have only one or two such motors and the cost of providing higher voltage is higher than the cost of feeding just a couple of big motors at 480. Once brought a 750HP, 480v compressor on line at a small plant in a pretty rural location in NY. Local utility requested to be notified prior to the first start to monitor their equipment upstream. No problems. The scuttlebut was that part of the deal made to locate the plant was upgraded electrical service that had not been provided yet so the utility agreed to allow the new load but was nervous about it. It was a Y-delta start, running amps was around 900-950.
 
Josh...

An answer is... Copper reduction!

Some time ago I presented a study that demonstrated considerable savings if a refinery's LV distribution system was 830V instead of 480V. The savings, of course, was related to copper reduction!

Regards, Phil Corso, PE
 
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The IEEE gray book mentions 1 HP (minimum) - 1000 HP (maximum) for motors at 480 volts.

There is mention of possibly serving larger motors (over 150 HP) from medium voltage (2400 V or 4160 V) to eliminate the voltage dip on the low-voltage system. In your case you mention a VFD so you should be good either way.
 
joshtrevino said:
The motor will be on VFD control (Powerflex 700 if 480v and Powerflex 7000 if medium voltage).
Our existing boiler is a 600hp, 2400v forced draft fan. The cost to install a new elevated building to house it, engineering, and installation was upwards of $450,000. The project engineer deems this cost unacceptable and so he is pushing for 480v. My existing 480v switch gear is about 200ft from where the boiler will likely go.

It sounds to me that a motor along the lines of 500hp will function at 480v. Is this correct?

What are the drawbacks of serving motors over the old rule of 200hp at 480v?
Click to expand...

I don't know that there was ever a rule about 200HP being the line in the sand where it made sense to use MV motors. It was a good place to start taking a look though. It might be that way in certain plants, or with certain utilities. I do not think you can make a general statement about this kind of thing. In any case, having a VFD on it makes it a lot more reasonable if your 480V system has the capacity. Not very many plants can just up and add 500 or 600 HP loads to their 480V power systems.
 
weressl said:
So how do you figure going from 600HP to 500HP?

The drive-to-motor distance seems OK.

Do you have the capacity in the 480V switchgear to serve the new motor?
Click to expand...


Existing forced draft fan motor is 600hp. This is staying in service with the new boiler that will have a forced draft fan motor whose size is not yet determined.

I am looking into capacity now on 480v switchgear.
 
Phil Corso said:
Josh...

An answer is... Copper reduction!

Some time ago I presented a study that demonstrated considerable savings if a refinery's LV distribution system was 830V instead of 480V. The savings, of course, was related to copper reduction!

Regards, Phil Corso, PE
Click to expand...


@Phil Corso
Could you elaborate a bit on this?
 
joshtrevino said:
I have a project coming up at my plant in which we are installing a new boiler. One of my primary concerns is the boiler forced draft fan. The project is still in the preliminary phases, but I am told that the forced draft fan motor will be anywhere from 200hp to 500hp depending on the boiler chosen.

I have 2400v and 480v switch gear available to serve this load.

What is the rule of thumb for the maximum motor hp served at 480v?

Is there an IEEE or other recommended practice for this question?
Click to expand...

Try to collect data sheets from the manufacturers of the fan from 200hp to 500hp to verify for suitability of operation at 480V, for the fan type of loads may place less stringent starting requirement on the system.
 
Josh...

Higher voltage => decreased current => smaller conductors => less copper => less cost!

I definitely lean toward the 2,400 V solution, especially if the motor is far from the source!

Regards, Phil
 
shamsdebout said:
It was not aware of this. Do you happen to know why that may be the case?
Click to expand...

Too many voltages. 2400V will remain a Utility voltage for mainly rural distribution, but in industrial facilities 4160V becomes the large motor choice unless it is in the multi-MW range, where 13.8kV will be chosen.

There are no motor starters available for that voltage either, you will be using 4160V equipment, so you're wasting your money.
 
joshtrevino said:
The motor will be on VFD control (Powerflex 700 if 480v and Powerflex 7000 if medium voltage).
Our existing boiler is a 600hp, 2400v forced draft fan. The cost to install a new elevated building to house it, engineering, and installation was upwards of $450,000. The project engineer deems this cost unacceptable and so he is pushing for 480v. My existing 480v switch gear is about 200ft from where the boiler will likely go.

It sounds to me that a motor along the lines of 500hp will function at 480v. Is this correct?

What are the drawbacks of serving motors over the old rule of 200hp at 480v?
Click to expand...
Drawbacks of servicing large motors at 480V:
  • Copper, as Phil Corso said. Copper cost, and the risk that it will attract thieves, is no longer a trivial issue.
  • Losses. More copper = more copper (I2R) losses. Losses inside of the VFD are based on current as well, so you will have more losses in the 480V drive than in the 2400V one.
  • Voltage drop; same issue as above, different problem that arises from it. Not much of an issue with using a VFD though.
  • Motor size. A 500HP 480V motor is going to be larger and heavier than a 500HP 2300V motor. Could affect foundations, bracing, etc.
Drawbacks of servicing large motors at 2400V:
  • Technicians must be MV qualified to work on the gear.
  • Higher initial capital outlay.

Advantages of going with MV for the VFD:
  • The PF7000 is inherently regenerative, so when you SLOW the motor down, the residual kinetic energy in it goes back into the line. Doesn't count for much, but it is something.
  • Fewer losses in the drive (and motor for that matter) means less heat rejection into the room and if the room air is conditioned in the summer, less cooling cost.
But still, power is power is power. If you ALREADY have a 480V system that has enough extra capacity to add 500 more HP to it, then in my opinion MV makes less sense. But if you will need to INCREASE the size of your 480V service to accomplish this, then MV makes more sense. Somewhere in between is the issue of the future expansion capacity. If you have the head room in your 480V system to do it now, but 5 years from now you might want to expand and will wish you had that capacity to work with, then consider adding it at 2400V now and bite the bullet.
 
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