DC Motor Derating 2

[edison123]

My client wants to buy a spare for existing 4 nos. 550 V, 1500 HP, 1000 RPM DC Motors. He has located a Reliance make, used DC motor in very good condition. This used motor rating is 700 V, 2000 HP, 850/1200 RPM.

Since he needs only 1500 HP at 1000 RPM, can he use this used motor at 550 V (which will reduce the speed) and with field weakening (which will increase the speed) to get 1000 RPM ? What are the possible ways this whole proposal go horribly wrong ?

 

[edison123]

The driven equipment is a rubber kneading and rolling mill (banbury), a wildly fluctuating load.

 

[DickDV]

Well, let's see here. The old motor develops 1500hp at 1000rpm. That calculates to 7875 ft-lbs torque at 1000rpm and probably at all lower speeds.

The newer Reliance motor is probably at its field weakened limit at 1200rpm and 2000hp. That calculates to 9000 ft-lbs torque so that part is covered. However, at the field-weakened limit of 1200rpm and 700VDC on the armature, a reduction to 550V on the armature would be expected to reduce the armature speed in direct proportion to 942 rpm. That's a problem.

Now, since you have some surplus torque in this motor, it may be possible to reduce the field current a bit further to get to 1000rpm at 550V. That would be a question for Reliance.

So, to summarize, your torque requirements are covered but you are a bit short on speed.

 

[edison123]

Thanks Dick. I also arrived at 942 RPM at 550 V with field weakened. As you say, if we push our luck, then we can possibly get 1000 RPM.

With armature control at 550 V, we will have lower pf at the ac side.

Now, the big question, what about sparking with such a low armature voltage and low field ?



Life is licking honey off a thorn

 

[TomG33]

Edison123,
The existing motor is rated at 1,000 RPM at 550 Volts. I am assuming that you mean the "base speed" (without field weakening) is 1,000 RPM.

Now the weak field setting of the reliance motor will give you 1,200 RPM at 700 VOlts. Doing a straight ratio calculation gives you 943 RPM AT 550 Volts. So clearly you will not be able to get the 1,000 RPM at 550 Volts without weakening the field below it's nameplate value.
This in itself may not be an issue, as we have operated motors slightly below the weakest field level for years without adverse effect.

Having said that, there are a few things that can "Getcha"

1
Is the motor to be used in the fourth or second Quadrant (regereration)?. It is well known that motors operating in weak field and in Regeneration tend to be a little Unstable (read un-predictible) in this situation. If you do not do heavy regeneration, Then this should not be an issue.

2
In the weakened field zone, the motor requires more armature current to achieve the same torque that it has with a Strong field. This is a reasonably linear ratio. Now the motor is rated at 2,000 HP, This gives you about a 33% margin but the whole thing hinges around how much the field has to be weakened to achieve the 1,000RPM at 550 Volts. if it is greater then 33% reduction You could be in Trouble.

The calculations are complicated because some DC Machines are designed for deliberate stalling, and cyclic overloading so they have a lot higher current capacity then The nameplate rated. If this is the case with the 2,000HP Motor then you may be OK.

What is the nature of the application, a Continuous proccess ? or a cyclic loading ?

Does it require the full 1,000 RPM or can you get by with the 943RPM?

Does it need the Full 1,500 Hp?

If you were to post the full nameplate data of both machines I could comment further but with the information supplied it is hard to say more then I have.

Regards
Tom Grayson

 

[TomG33]

Edison123 Wrote

"With armature control at 550 V, we will have lower pf at the ac side."

What is this about power Factor?

The power Factor depends on the ratio of the Maximum Bridge output volts compared to the actual output volts, Not on the motor nameplate.

SO it should be the same no matter which motor you use.

 

[edison123]

Thx Tom. I was hoping you, jraef et. al. will be jumping in in this one messy pool.

1. The load is a rubber mixing and rolling mill with wild fluctuations in load current. The present 1500 HP, 1000 RPM (base speed) motor sees the peak current of 2400 amps (rated is 2100 amps) for about 3 seconds in a cycle of about 3 minutes. The rest of the time it sees currents from 300 to 1000 amps only. The average loading (it may be a misguide) is only about 30% for a duty cycle. The maximum interpole winding temp was only 55 deg C with an ambient of 40 deg C. The field voltage/current = 178 V/ 25 A.

2. I do not have the field volts & amps for the proposed spare motor.

3. The client would like to have the present 1000 RPM for productivity.

4. We do not have the design data of the spare motor. Since it comes from Reliance stable, I was willing to bet on this horse.

Life is licking honey off a thorn

 

[edison123]

Tom,

I agree with you on the PF.

Life is licking honey off a thorn

 

[TomG33]

Edison123,
A cyclic load such as this changes everything.
If your duty Cycle is only 30% load, (backed up by the low winding temp rise) then the 2,000 Hp Motor should be able to live with this even with the field weakened below name plate.

We do our RMS Calculations over a 5 minute period, So your cycle on a three cycle look good. Not sure what Reliance do.

One other thing, I am imagining that this motor is supplied by it's own separate converter. If it is somehow powered parallel with another such motor, You may have load share issues.

FOr a motor to be an Effective SPARE it's mounting must be identical or easily adapted. If the distance from shaft to the bottom of the feet is greater for the larger motor then you will have some issues there to worry about.

Given this info, I would say the motor would be Ok For use as a "Spare".

Incidently, what sort of money is the 2,000 Hp Motor Going for?

 

[edison123]

Tom,

I have not even opened that can of worms yet about the mounting holes and centre height. May be I am being totally foolish without considering that. I am awaiting those details from the surplus vendor.

The price is 50K for as is where is and 96 K if they "remanufacture". Do you think it is a rip off ?

Life is licking honey off a thorn

 

[TomG33]

Edison
If they are USD then i would say it is a reasonable buy.

but the Extra 46 for the class 1 overhaul, seems a little high. I would shop around first before committing to the overhaul.

Tom

 

[edison123]

Thx Tom.

The price is in USD. Double the price for "remanufacture", which as I understand is a just an overhaul, seems steep.

Do you have any idea about these surplus motors on the web ?

Life is licking honey off a thorn

 

[TomG33]

Edison, Another thing you could consider
is Second hand GE MD for Mill duty Motors,. These are nameplated at 475 Volts, however designed to run at up to 600 Volts. a better match for your 550 Volts.

The MD820 is rated at 800 Hp amd 800RPM at 475 Volts (926HP and 926 RPM at 550 Volts)
Rated current is 1350 Amps and Rated Stall at 2700 Amps.
So you would have plenty of margin to handle the cyclic overload as stated.

They are used on Draglines, shovels all over the country, So You should be able to find them somewhere on Google.

Just a thought

 

[itsmoked]

http://www.royaleq.com/geshovelmotors.html

http://www.millmotors.com/motorsset.html