Replacing Old Electric Motors with New

I have replaced one mill motor similar to the one you describe, pedestal bearings etc. To be honest we went to several manufacturers with the specifications we required and waited for them to do the work. It was then just a case of picking one that suited us best and had the performance guarantees.

Sorry I can't be of more help.

One thing I will add, nobody on the maintenance staff was sorry to lose the pedestal bearings.

The largest electric motor I've ever spec'd was 15 HP/50 Hz. So, take my opinion for what it's worth.

But, I will attest that Fred has it right.

We bought a LOT of motors when I built process equipment, and I got to know how they are really built.

Send your requirements out for bid and see what industry has to offer.

Good Luck.

I followed up on a replacement similar to what you are looking at. We had replaced an old (1947) GE DC Drive Motor on a Big Betts VTL. The nameplate on the old motor was 50 hp. The local A-B distributor had spec'd out a new 50 hp replacement motor, much smaller in package size and shaft size. The lathe operator couldn't make nearly as aggressive a cut with the new motor as the old one would do day in and day out. I put meters on the drive and tweaked a bit to optimize it. At full rated power, it was only half the motor our Grandpas had made.

Fred is on the right track, you need a conversion factor to take into account the difference in philosophy. It was indeed overbuilt. Back in those days, they built a motor that would win an argument, build a country, or in the case of that old GE motor, win a war.

I'd try to get a performance guarantee from the drive supplier. They will be more likely to make sure you have enough motor.

Robert

I would agree with the above comments but make a further observation in that in some instances it was possible to continually overload older motors and "get away with it". Considering there are now many hand held instruments now available that will measure electrical power utilised directly, I also would check the power (or if no suitable meters check the voltage & current) being taken by the motor and use this figure rather than just what is on the name plate.

You might even find that a smaller rated motor would do and save some energy (efficiency of motors being best at nominal power conditions)!

Why not using a gear motor if you only need 394 RPM ?

Hey guys, Thanks for all the comments and Fred is right on. Like I say, I have been working with and on this motor for 25 years. I have replaced a number of large mill type pedestal motors up to 1000hp in the past but they were all DC and there was a coorelation in DC concerning shaft size...etc. Some of the original motors were from the 1920's!!! The main reason that I cannot use a gearmotor for a replacement is that they are not made in this HP size. I've never heard of one or seen one being advertised and I usually watch what is available. Also, There is a gear reduction after the motor. This is for a slitting rolling stand in a steel mill. Very slow. Also the motor is across the line start and there is no "drive" associated with it. I would like to change that in the future but that is not on the burner right now. I am mainly concerned with the running torque of the newer motors verses the older motors. The original motor was a 1.25 S.F. and you never see that anymore. Unfortunatly there was only one of these motors made and I've been given a ballpark quote of 1/2 million for a new one. I am sure that one of the newer frame motors will work fine. I will need to make a transition base for it and a new coupling half but thats about all. Fortunatly we are only pulling 750 HP out of it right now so I think that the newer motor should work. I just hope we don't start seeing snapped shafts!..Thanks all.

Jim

You might want to consider a complete teardown and recondition at a qualified motor repair facility. Some of the bigger shops GE etc. are very experienced and do excellent work. Why discard something that is apparently working well for you to take a chance on something that may or may not work as well. I find that with many industrial products constructed today that quality is not always a first priority.

I have already contacted some motor shops in the area. The motor is wound with form wound coils and a sample is necessary for the coil shop to make coils. They are talking 4 weeks down on straight time and 2-3 weeks down on full overtime rush basis. We have already cut out 2 groups of coils due to insulation failure on two seperate occasions and I know that this motor has got one foot on a banana peel. Unfortunatly, A steel mill cannot shut down for 3 weeks let alone 4. I use large motor shops all the time and we really have no control over their timing. The motor I am looking at using as a replacement is not a new motor and is heading for the motor shop before coming here to be retrofitted. It is a Reliance which was a pretty good manufacturer and I am guessing that the motor is about 20 years old. We will not be discarding the old motor. I will probably have it rewound with all the bells and whistles but it will be somewhere in the area of $80,000.00. I will feel better knowing that I have a spare available as we never did before. I just can't take the 3-4 week hit that a rewind is going to take. Big boss won't like that!

Jim

As the new motors are smaller than old ones,the inertia of rotor will be lesser than old one. If we consider the rotor as an energy storing flywheel,the new rotor will have less energy stored and therefore less efficient in handling load fluctuation or variation or even starting. Even the rotor shaft will be smaller and will it withstand the torsional forces required during starting?. Depending on the application,choice of the new motor should be done.

Justifiably remark Pnaban, GA from me.

You make some good point, pnaban. The rotor will probably be somewhat smaller, although most of the size reduction in modern motors is in the stator, not the rotor. High-grade electrical steel has allowed designers to significantly reduce the mass of the stator punchings. The lower reluctance in the steel also means less copper in the windings.

Mass reduction in motors of this size is often accomplished by using a hollow shaft, but that may actually work against him in a cycling torque application like this one. One good thing in GopelT's application is that mill motors are usually started without process load, so the starting torque is minimal.

If you are going to go with a new motor I recommend getting the motor mfg's representative in to look at the application and if they want to bid ask for a guarantee that the motor to be supplied will work without problems. Put the monkey on their backs, CYA is always best.

Why don't you give full nameplate details of motor such as-make,country/year of manufacture,model/type, rated Amp,No-load Amp,PF,SF,Standard(BS/NEMA/VDE/IEC etc),Design code,Code letter(LRA/FLA),type of starter used etc?.

Yep. I want to see the Locked Rotor KW on that big bad boy!

Hey Pnaban....You are spoiled. You guys actually have nemeplates on your motors? I have all the specs that came with this motor but you have to understand that this old motor was made during the days that all mill type motors and I daresay all motors over 400 series frames were made to order and there were no standards above a certain frame size. There might have been standards within a company (GE's MCE and MCF motors for example.) Therefore useful info that is normally found on a nameplate goes out the window on these guys. I can tell you that it is 1000 hp, 132 FLA, 394 rpm and 1.25 SF. There was only one of these made.....EVER. That was confirmed in 1986. We use an Allis chalmers FVR movable starter for this fellow and it is configured across the line start at 4160 volt. Really dims the lights. For those of you that have dealt with steel mill motors you know what I am talking about. Our mill was built in 1893. I have had the pleasure of working on some really unique motors because of this. Until recently we had 2 1000hp Kramer sets here made by GE in 1928. Ever heard of one of those? Thanks for all the good advice. I picked up a couple of tidbits I hadn't considered. We will be doing this job pretty soon and I would like to post the results later so you guys know what happened.

Thanks,

Jim