Old vs New Motors
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  1. #1
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    Default Old vs New Motors

    The insulation on the leads on the 7 1/2 hp motor in the 1941 monarch are cracked and brittle so i investigated the cost of a repair vs the cost of a new motor. In doing so i learned some things regarding the advances made in the last 78 years.

    First of all, the old motor is huge by today's standards for it's rated hp. The guy specing a new motor for me told the the reason motors got smaller is the insulation is more efficient. Said another way, for a given resistance at a given voltage, the insulation material occupies less volume today than in the old days. He didn't say anything about performance at elevated temperatures or mean time between failure but i can imagine that got better as well. On the down side, the new motors don't have the same inertia due to the lower mass. This means that they don't handle sudden load changes as well as an old motor of equal hp. For this reason, the motor tech recommended i replace the 7.5 hp motor with a modern 10 hp motor.

    I've heard it said that because old motors were bigger than new motors, they developed more torque. I've never been able to reconcile this since power is proportional to the product of speed and torque. Since the speed of an induction motor is basically set by the operating frequency, two motors of the same rated hp operating at the same speed must develop roughly the same torque. However, as i mentioned above, the old motors had more running inertia so i think the assertion that old motors had more torque is really that old motors had more running inertia which is really the ability to provide instantaneous torque.

    A second thing i learned was that the frame cross reference charts are a bit counter intuitive. The old motor has a 284 frame and the cross reference charts call out 213T as the "equivalent" frame size. The hole pattern, shaft diameter and shaft distance to base dimensions of the 213T vs the 284 are all different. However a modern 284T motor has all the same dimensions as an old 284 frame. When i saked the motor tech why he couldn't give me a motor with a 284T frame, he indicated it would be much larger than 10hp. Bottom line is "equivalent frame size" really means "frame size for equivalent motor hp".

    At any rate, my RPC has a 10 hp idler so i declined the new 10 hp motor. the motor shop is evaluating a repair to my old motor now. Hopefully it'll cost less than the new motor.

    At this point, i'm not sure what i'll do. I do have a spare 5hp 3 phase motor kicking around so i may rig a second idler to start & run the monarch when i get it all together.

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  3. #2
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    I had my old probably early forties Cincinnati 12 inch pedestal grinder rewound.. It is 2 hp three phase.. The motor shop quoted me a price to rewind a 2hp 3 phase motor over the phone.. I brought the stator in and they quoted more money because they said the old motor was much bigger than a modern 2hp motor....I went on and had the rewind done and it was worth the extra cost I think... Cheers; Ramsay 1

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    The insulation on the leads on the 7 1/2 hp motor
    Suggestion - get some heat shrink tubing, apply it to the leads and run it until it dies (but don't obscure the T number tags)

    I have a 1917 Westinghouse 10 HP 865 RPM and that is what I have done as power for the 20" Heavy Greaves Klusman.

    It must weigh 400 Lbs
    Last edited by johnoder; 09-02-2019 at 07:16 AM.

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    Quite a while ago I was talking to an engineer from a motor manufacture, he said the newer motors are rated by electrical HP like the European motor not like the old USA motors that were rated using torque and speed and why the new motors are smaller so that you are actually getting less HP. Your getting the HP equivalent of the KW rating as used in European motors.

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    Quote Originally Posted by Froneck View Post
    Quite a while ago I was talking to an engineer from a motor manufacture, he said the newer motors are rated by electrical HP like the European motor not like the old USA motors that were rated using torque and speed and why the new motors are smaller so that you are actually getting less HP. Your getting the HP equivalent of the KW rating as used in European motors.
    I feel like I lost some power when I remotored my Monarch, maybe that explains it?

    Sent from my SM-G973U using Tapatalk

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    NEMA standards have changed 2 times. This covers the physical dimensions of the motor. I believe it was ‘52 and ‘64. The tech is correct, insulation has gotten better, but everything has gotten better. Of course, so has cost savings and pushing the limits if the motor. I have seen 5 hp 56 frame motors. They run for a while. But not for 50 years. Many old motors run at lower rpm, for many reasons. This drastically increases the size. We used to supply Dover with elevator motors that stood 4’ high and weighed 6000 lbs. they spun at ~50 RPM, and only about 20 hp. Sales declined, so we re-designed it into a 1750 rpm motor, at 1250 hp, and sell it today. The Dover version was to eliminate a gearbox, but you can see how large a motor gets when you slow it down.
    We do produce better steel, more accurate punching, and better insulation, but the old motors usually have longer life because they run cooler.
    Joe


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    I agree with posters here, especially Oder. At most I would have the motor shop dip and bake the old motor and attach new lead wires.

    Tom

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    One thing is that older motors have very well-placed insulation. So they suffer less from issues like corona and arc-over due to beugng driven by a VFD. Newer motors, other than VFD rated, may have less well-placed insulation, due to cost-cutting and replacing careful hand operations with machinery, and also because it is thinner, so the placement is more critical. There is a lit to be said for varnish filled cloth or paper insulation, although it will char over time. But it is thick and if well placed, may be less likely to fail from VFD induced ringing etc that may develop high voltages.

    Downside is that the older insulation may not take heat as well. VFDs do tend to heat older, thicker laminations more. In many ways older motors may be better for use with a VFD than a newer motor that is not specifically VFD rated.

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    Quote Originally Posted by Froneck View Post
    Quite a while ago I was talking to an engineer from a motor manufacture, he said the newer motors are rated by electrical HP like the European motor not like the old USA motors that were rated using torque and speed and why the new motors are smaller so that you are actually getting less HP. Your getting the HP equivalent of the KW rating as used in European motors.
    This makes a lot of sense. I can't recall typical motor efficiency but i always thought electric motors to be very efficient. If so, there would be little loss and the difference between the electrical power and shaft power would be relatively small.

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    Quote Originally Posted by johnoder View Post
    Suggestion - get some heat shrink tubing, apply it to the leads and run it until it dies (but don't obscure the T number tags)

    I have a 1917 Westinghouse 10 HP 865 RPM and that is what I have done as power for the 20" Heavy Greaves Klusman.

    It must weigh 400 Lbs
    Thanks John. I definitely thought of this. I did the same thing on an old coolant pump on my cleerman drill press. Unfortunately, the insulation is buggered up before the terminal number tags and all the way back into the motor housing. The motor ran fine when i bought the machine so i'm betting the fix will be for the motor shop to re-lead it. I'll be happy if that's the fix.

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    It's not to complicated , you are not looking at the simple fact that if you apply the same magnetic force to a 1' dia rotor pole as to a 2' dia rotor pole the 2' rotor will have aprox. twice the torque.

    We had a lot of older machines when I started here.Some had 15hp dc 800rpm powered by MG sets.One by one replaced the MG's with SSD's.Easy.
    When it came time to replace the motors quite a different deal.These were huge by todays standards,had ten B belts primary drive.If you had never seen any 15hp drives in the plant with 10 belts then you would question why?

    Super asked me what size to replace.Took my dial type torque wrench and measured at the input shaft and calculated that at 200RPM(normal run speed) would need 6HP,at 800RPM 24HP.
    He comes up with a 1750 30Hp.I told him it wouldn't work.Had to machine the pulley to work.It started ok but started tripping out under load.Next up was a 50HP 2400.Same deal make pulley.Didn't work(drive problems).I told our ace electrician at the time the power requirements and he said he had it handled.New motor comes in week later ,Ace had me mod pulley to fit and he proceeds with install.
    "
    Monday a week later super says to check machine,new motor tripping out."Where's Ace?"
    "He's tied up,its mechanical anyway"Checked one end to the other no mechanical issues.Dumb me finally checks to see what kind of motor he put on.25HP 1200RPM!

    I found a pair of 30HP 850RPM referb motors for less than the 25hp and they worked until we finally scrapped them.

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    Quote Originally Posted by ratbldr427 View Post
    It's not to complicated , you are not looking at the simple fact that if you apply the same magnetic force to a 1' dia rotor pole as to a 2' dia rotor pole the 2' rotor will have aprox. twice the torque.
    You are assuming constant flux density. The old motor insulation couldn't take the heat, so they were larger to compensate. Rewind an old motor with new wire and get a much stronger motor.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    You are assuming constant flux density. The old motor insulation couldn't take the heat, so they were larger to compensate. Rewind an old motor with new wire and get a much stronger motor.

    Tom
    How much stronger?

    I had to remotor my Monarch because a mouse drug some alum foil into the housing and out went the smoke when I tried to fore it up. It was cheaper to install a new 1750 3hp than have the old beast reworked, but if there's power to gain and I can restore it to its original configuration then you have my attention.

    Sent from my SM-G973U using Tapatalk

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    Can't really tell without a bunch of tests. It comes down to what was the flux density in the old motor windings? If the engineers couldn't pass the heat test, then why put in steel that can't be used?

    I haven't designed a motor, but I have designed contactors which are similar. Here is one way. You know how much pull is needed for the contacts. That defines air gap area and flux. For a given steel, the saturation curve is known, that helps specify the maximum flux. Knowing the flux defines the coil. The coil is usually the maximum copper that can be put in the allowed space which is usually smaller than what is wanted. So then you go with the best insulation that is affordable, same with steel. Now check the temperature. Too low, peel some turns, increase the rating of the motor. Too hot (usual) add turns, derate the motor, other tricks of the trade. Still not enough, start redesign. And so it goes.

    Another trick is to take device of the same construction, drive it the desired load and test for heating and pull. I doubt you have a similar motor, else you would using it.

    About the only solution I can think of short a lot of tests and measurements, is to wind the motor with high temperature wire and see what you can get out it. The steel can be over driven, that just adds to the heating.

    Feel lucky?

    Tom

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    I got a quote back from the motor shop to repair the old motor. Basically they will steam clean, bake, megger re-lead, change bearings, re lube and fix any insulation problems. It won't be cheap but it saves me a bunch of work adapting a new motor with a different frame size.

    Regarding the discussion on the topic of flux density inside the motor, i would be careful not to over-simplify this. It's a complex physics problem that requires a detailed understanding of the inductances in the motor windings, the physical dimensions of the components involved and, as i recall, the mutual inductance between the stator windings and the rotor under various transient and steady state conditions. The phasor diagram is complex and it's been too long since i've looked at it carefully. Having said that, it is fun to kick around these ideas.

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    Quote Originally Posted by marka12161 View Post
    I got a quote back from the motor shop to repair the old motor. Basically they will steam clean, bake, megger re-lead, change bearings, re lube and fix any insulation problems. It won't be cheap but it saves me a bunch of work adapting a new motor with a different frame size.

    Regarding the discussion on the topic of flux density inside the motor, i would be careful not to over-simplify this. It's a complex physics problem that requires a detailed understanding of the inductances in the motor windings, the physical dimensions of the components involved and, as i recall, the mutual inductance between the stator windings and the rotor under various transient and steady state conditions. The phasor diagram is complex and it's been too long since i've looked at it carefully. Having said that, it is fun to kick around these ideas.
    One test is worth a thousand opionions.

    Tom

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    Quote Originally Posted by marka12161 View Post
    The insulation on the leads on the 7 1/2 hp motor in the 1941 monarch are cracked and brittle
    If it was my motor..

    Open it up and try to find the attachment points for the motor leads. Attach new wires. Has to be done with a clear head.

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    I haven't done any testing nor made any electrical or mechanical measurements so my comment is based on simple use. When ever replacing an older motor with same speed, RPM and pulley size the new motor would stall when the machine with the replaced motor was use as it was with the older motor. If replacement was required I increased HP so a 5HP old motor was replaced with 7-1/2 HP if rewinding the old motor was not possible. Probably a little over kill but I never had the stall issue.

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    A motor that is rated at 1hp produces 1hp of mechanical energy. A motor rated at 746 Watts, the theoretical equivalent of 1hp does not produce 1hp of mechanical energy, it produces 1hp minus the efficiency rating of the motor, which is somewhere in the region of 80% to 85%. If the motor you buy is rated in Watts, as most are today, you need to know the efficiency rating, and calculate the losses to get a motor of equivalent hp to the one you are replacing. Sorry, but I dont buy tne "all modern motors are more efficient" story. Very high end expensive motors can be made more efficient, but the standard 3 phase motors have been at around 80% efficient for many years.Single phase are much less efficient. I dont go for the tale about "modern varnishes can run hotter" either. Firstly, a motor is designed to produce mechanical energy, not heat, if it is producing heat, it is dissipating energy as heat rather than torque and is therefore inneficient. I think the new high temperature varnishes allow manufacturers to produce over rated cheaper motors that will run hotter without actually burning out!

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    I agree with hermetic in general, yes better insulation and stator metal allows smaller cheaper and hotter motors.

    Disagree about this electrical watts stuff, as ive never yet seen an induction motor rated by watts input, one exception being Chinese cnc spindles

    Secondly, most 1hp single phase ac motors are about 67% efficient and always have been. You need to get to 3 to 5 hp to reach 85%. High efficiency expensive 5hp motors are 91%.


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