Motor Amperage. 2

[eric34]

I have a Blower motor that we are having problems with. We monitor the pressure in the pipe going to the cyclone. If the pressure gets to 5 PSI we shutdown the feed, if the pressure gets to 7PSI we shutdown the blower motor. Last week we had a small hole in the pressure line going to the pressure sw causing incorrect readings, causing us problems. My boss now wants to install current xmers and program using the amperage readings.
My question is as the motor gets older, heats us, etc the amperage should go up. The motor on the blower system now gets very hot,and if pipe gets plugged smokes quite heavy.
The amperage should go up if the resistance is going down in the motor.

 

[waross]

What type of blower? It makes a difference.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eric34]

The blower is for are wood chips. Feed the chips using a chip feeder and the blower produces high volume, low pressure. If you are looking for the manufacture, I would have to get back to you.
The real only question I am looking for is how much would the amperage change if the motor is new versus one that has been hot and the resistance lower.
Does anyone know a formula I could use?

Thanks

 

[waross]

That would be a Roots type positive displacement blower. If the motor does not fail when it is overheated, it will probably be alright when it cools down. Every time it overheats, there is a greater chance of the motor failing.
I'll just mention re-dipping and baking, and hope some of our rewind shop members comment.
But if the motor is now overheating with a normal load, you have probably done permanent damage. I would change out the motor immediately. If it is run to complete failure there may be irreparable arcing damage done to the stator core when it does go. Normally the motor overload devices shut down a failing motor soon enough to avoid stator damage, but in your case it may be that the overloads are not functioning properly.
We have some members who are familiar with the more sophisticated motor protection relays. Wait for one of them to comment. Would this be about 75 Hp?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eric34]

250 HP is the motor size.
The Motor isn't failing under normal conditions.. The pressure line was the main cause and my boss was looking for better protection....The motor is running as we speak. I am afraid that the normal running amperage will change as the motor gets worn, overheated. .Then the amperages first set in the code will cause problems because the normal running amperage has changed.
We do have to make some changes in the code.We keep the motor running after 5 PSI to clear the pipe out. This causes the most damage. It only shuts down immedially if PSI reachs 7 PSI.
We do this cause a plugged pipe takes about 2 hours to clear.

 

[edison123]

If you're wondering that as the motor gets older, if the current will go up, the answer is no. The motor current is dictated by the load, not by its age or operating temperature. It's other way around - the motor age and temperature are determined by the motor current (which in turn in is dictated by the load).

 

[itsmoked]

That would be a Roots type positive displacement blower.

How do wood chips equate to a Roots?


To give a definitive answer we need to know what type of blower you have. Some will reduce current if they are loosing flow some will increase current.

You will also have a problem if you have both types of failures available. For instance if a hose could come loose you could get greatly increased current. But if instead the line gets clogged up you could get a greatly reduced current.

Without knowing what type of blower it is we can't begin to say.

By type we mean specifically the mechanical style not the brand name.

Perhaps we can figure out the mechanical type with a brand name and a model number.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[Marke]

I agree with edison123.

The major component of current on a motor of this size is work current and that will not change with winding temperature.
The magnetising current may be influenced by the temperature and resistance of the windings, but at that size, the magnetising current is typically in the order of 25% of the total current and at phase quadrature to the work current so you would not see any change.

Best regards,

Mark Empson
L M Photonics Ltd

 

[electricpete]

Yes, I agree with the others, no significant change in current for a given load as the motor ages .

Your concern seems to have originated with motor temperature. There can be increase in motor temperature (for same load) as the motor ages. This would be primarily to fouling of the internal (ODP) or external (TEFC) cooling surfaces. Less likely, temperature can also increase over time for same load due to vibration-related degardataion of core lamination insulation, or perhaps due to fan blade break.

BTW I wouldn't presume to know what your individual motor problem is. You haven't given us enough info to make any intelligent guesses.

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[edison123]

Even in no-load current, the resistance of the windings play only an minor role. It's the magnetic loading or rather mutual magnestism that plays a significant role in the no-load current.

I once had the case of motor where the rotor core was out of the stator core by about an inch on one side due to an earlier assembly mistake. The no-load current went from 50% to about 30% once the cores were axially aligned.

The extreme case is of course the current taken by the motor when the rotor is completely removed thus proving the mutual magnetism dictating the no-load current.

 

[waross]

Hi Keith;
The positive displacement blower is used to push air down the pipe. A star wheel type feeder drops the chips into the pipe AFTER the blower.
The chips DO NOT go through the blower. This is a common arrangement in Saw mills for moving wood chips. I have seen the same equipment used successfully to move chopped or crushed ice.
If you are familiar with the old 6-71 blowers designed for scavenging air on the Model 6-71 Detroit diesels, but used for generations to supercharge race car engines, then think BIG 6-71 blower, big enough to require a 250 HP drive and overloading at around 7 psig back pressure. Hearing protection is mandatory anywhere in the machine room.
I have seen centrifugal blowers successfully moving saw dust or planer shavings (with the product passing through the fan or blower), but wood chips are bigger and heavier. A typical wood chip for pulping may be 1 to 1 1/2 inches square and 1/8 to 1/4 inch thick. It takes a lot of air moving really fast to keep these chips moving.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Thanks Bill. I've seen a lot of saw dust and leather dust systems which all used centrifugal blowers. I can't imagine a 250HP Roots. Yikes!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

While we are at it. I had just such a blower a couple of years ago. Those Root blowers have massive rotors and high inertia. They also make a lot of noise because the air "trapped" and compressed between the two rotors more or less explodes twice per revolution.

Anyhow. The induction motors for these blowers failed quite frequently. Protection was there and had been tested. A consultant was checking the system and finally found out that there was a standing wave created in the blowpipe between compressor and an elbow just before the chip chute and that this standing wave consumed so much power that the motors mysteriously failed without affecting thermal protection.

I am hijacking this thread to put a question: What are your comments to that conclusion?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

eric34,
As the others have said, any change in current due to age and temperature is going to be so small as to be insignificant. But if I were you, I would not use current alone. Current varies with fluctuations in line voltage. There are much better load monitor relays available now that look at actual kW (HP) or what some people call "shaft torque" (which they determine based on Power Factor). They are much more accurate and dependable than looking at current alone.

Keith,
250HP is actually average for a roots blower, maybe even a bit small compared to most of the one's I have worked on. 400 - 600HP is common in the WWTP industry.

Gunnar,
How would any increase in motor load cause failure without affecting the thermal protection?




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[Skogsgurra]

That's right, Jeff!

That consultant had a vivid fantasy. I went to have a look and saw about ten DOL starts per hour. Changing the PLC program (the blowers supplied chips to a boiler infeed) so we had less frequent starts saved the motors. The frequent starts caused dynamic forces to destroy the insulation.

So much for fancy theories - and expensive "specialists".

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

So that was a test then?

Funny definition of a "Specialist" I read once:

One who, when hired to determine why the cart is moving, discovers a horse but insists on continuing to look for a zebra.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[eric34]

I am looking into a load monitor, they seem to be what I am looking for but am not fimilar with the brands. Can anyone out there suggest a good, reliable unit that is ethernet ready?
Network capable is a must.

 

[jraef]

Siemens makes one called the Simocode, but it is only able to communicate with Profibus, their choice of fieldbus networks. If you have multiple units you may be able to justify a small HMI running a Profibus network that gives you an Ethernet port for upwards connectivity, but that's expensive for one unit.

There are dozens out there that have Modbus RS485 comm ports, but you would need a gateway / protocol converter to port it up to Ethernet.

The only one I know of with a direct Ethernet port is Load Controls Inc. But there are probably more.

http://www.loadcontrols.com/products/products.html

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376