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Old October 7th, 2004, 07:19 PM   #1
Pierre
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50HP DC motor to AC !!!

I know of a companie which deal will printing machines.

They intend to replace on of there 50 HP DC motor and Drive with a 75 HP AC motor and Inverter.

This process is a slow start and acceleration, then contant load.

Will they really need to install a much stronger motor in AC or not?

The load is the inertia of printing cylinders.
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Old October 8th, 2004, 01:57 AM   #2
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I do not found any reasnoablel reason why,but in plastic extruders DC motors was verey common until few years ago and now they use AC motors
with encoder in closed loop.
Somehow they use one step above motor,I got some answers for that but non of them was reasonable.From the other side I dont see any harm to use bigger motor.The common answer it work better in low speed with full load,
If the DC motor work in Torque Mode you must to have closed loop.
if you work in Speed mode you maybe need just VFD.
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Old October 8th, 2004, 03:39 AM   #3
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I've never seen a pure 50 HP DC motor - as in a permanent magnet DC motor. Although some Fanuc red caps and Kollmorgens are 15+.

There is a blurring and confusion when a motor and drive are called a DC motor (above 1 HP).

I believe, and could/probably be wrong, that they are replacing a 50 HP with a 75HP motor and upgrading to a PWM/VFD drive. The PWM (inverter) will be much more efficient and less damaging to the motor in a slow start ramp up. Variable velocity/torque is what VFDs are designed to do.

Why are they upping the HP by 50%? The answer is in how many brushes have they replaced due to high amperage demands during the ramp-up and how many times they have been shut down while the commutator ring is being re-turned.

Now you've got me curious, what is the highest HP BRUSHLESS DC motor out there? We use Kollmorgens and have had only two failures in 14 years - same machine - a cruddy wire internally!

Rod (the CNC guy)
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Old October 8th, 2004, 04:25 AM   #4
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Rod

In plastic industry where DC motors in this size are very common.
the motor fail once in few years,field problems and brushes.are the common problems.In the worst case armature problem.
We use to replace this motors with ABB motors and VFDs which made for that perphes.Max torque in "zero" speed.
DC motors with permanent magnet is different story.
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Old October 8th, 2004, 05:59 AM   #5
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I have worked with Reliance VS and other DC for many years with motors generally from 10 to over 100HP. I am hoping DickDV may offer better details.

In the OLD days they used DC motors for variable speed and positioning control. In most applications large startup torque was needed with minimal or no slip...this is the key area. Ever noticed how large the older DC motors were?

Quote:
Motor torque increases with an increase in iron and copper, combined with current. It can then be said that it takes iron and copper to produce torque and torque makes products. Or to put it another way, what you purchase to make product is TORQUE and that is IRON and COPPER. The rate of doing work is power and HORSEPOWER is a unit of power.
The construction allowed torque values with less slip than todays AC motor of the same HP rating.

I have the actual formulas used for converting but the rule of thumb seems to be add 50% ....ie a 10HP DC motor would be replaced by a 15HP AC motor with VFD.

It does seem kind of strange because formulas state that AC and DC motors follow the same rules but I guess all things being equal are not always equal.
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Old October 8th, 2004, 07:01 AM   #6
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Once again, I proudly show off my ignorance.

You guys are great!
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Old October 8th, 2004, 07:40 AM   #7
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In this case its an Offset Press.

So, if its installed with say, 8 colors, you have 8 more mechanical sub-systems to drive with a single motor hence need for the 50 HP.

Like with all printing system, it start slow an accelerates in steps to the require output (in ppm)... to thousands of pages.

While the operator adjust water and ink and other stuff he steps up the central drive.

Once its up to speed the torque needed is way less than what he needs to accelerate from zero.

I understand what a dc motor can supply for such heavy movement but the need to increase by 50% is somewhat questionnable.

Since I come to the site its because I have no idea of it.

They will be testing it when its installed (in the next 14 weeks).

I will be checking on them. Out of curiosity for my own apprenticeship.

After all we are all apprentice in some way or another in this automation game.
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Old October 8th, 2004, 08:27 AM   #8
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I am speculating, but I believe DC motors generally have better low end torque capabilities than AC induction motors. That may be the reason for upsizing. By defintition at rated speed and horsepower both AC and DC motors will have the same torques.

Another option for low end torque is a NEMA Design C or D motor, which are designed for higher starting torque. That means higher current output from the VFD as well, so the VFD might need to be "oversized" compared to the motor nameplate rating to meet the current requirements. In that case, it just may be cheaper to oversize the motor and VFD to get the torque.
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Old October 8th, 2004, 08:29 AM   #9
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I have changed a few DC motors to AC and the rule of thumb was to go bigger like was discussed in the previous posts. Dont really know why we go bigger but it works... The other thing I did was since my DC motor was usually running lower than base speed (I think around 1200 rpm) I used a 1200 rpm AC motor.
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Old October 8th, 2004, 08:32 AM   #10
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Quote:
Originally posted by Tom Jenkins
...Another option for low end torque is a NEMA Design C or D motor...
Great,

As usual you come up with another good option.

We will be taking Amps reading on all phase of the startup and then "speculate" on it.

They have no reading of the actual motor, they thinck tu upgrade to a 60HP or a 75HP.

Its an older machine which is being retrofitted with newer components.

I'll keep this option in my back pocket. It could be an Ace.
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Old October 8th, 2004, 12:08 PM   #11
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Do not forget if you need Max torque in low speed you need motor with encoder on the shaft,force air ventilation,and closed loop vector VDF.
It big differnt in the over all cost.
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Old October 8th, 2004, 01:37 PM   #12
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Oh boy, I sure hate to pour cold water on so much of what has been said so far but, in my experience, here are the facts as I have found them in the real world.

First, a 50hp 1750rpm DC motor has exactly the same continuous torque range as a 50hp 1750rpm AC motor.

Second, as for overload capabilities, a NEMA Design B motor will deliver a max of 220% short time overload if across the AC line. On an inverter, generally the overload capability is much less due to the short time overload ampacity of the INVERTER. If you oversize the inverter compared to the motor, you could and I sometimes do take the motor all the way to the 220% limit. A DC motor is also capable of 300 to 400% short time overloads but, again, the DC DRIVE is the limiting factor since these are generally sized for 10-50% additional ampacity for overload purposes. Further, if you try to overload a DC motor heavily at slow speeds, you will very likely distort the commutator bars and an expensive repair will result. In practical terms, a DC motor quits around 220% just like an AC motor.

Third, as for motor cooling capacity, a TEFC DC motor has very limited low speed cooling just like an AC motor and for the same reason--low fan speed. If the DC motor is in an auxiliary cooled or TENV enclosure, then it will basically cool down to stall. And so will an auxiliary cooled or TENV AC motor!

Fourth, the use of an encoder or analog tach on DC systems has nothing to do with low speed torque. They are there for speed regulation so the speed remains constant regardless of motor load and a few other obscure variables in the drive. While a pulse encoder produces a near perfect signal to follow, the average AC or DC analog tach is good for about 1% error. Your resulting system speed cannot be better than that, can it? The main reason you see so many tachs on DC systems is that, without one, the speed error can go as high as 3-5% which is often unacceptable. With AC, the speed error is primarily a matter of how well the drive compensates for motor slip. Using 4 pole motors for illustration, it used to be common for the nameplate to read 1740-1750rpm. That's 3-3.5% speed error which the drive hopefully can reduce somehow. Today, the same size premium efficient motor will typically be labeled 1770 to 1775rpm. That's only 25-30 rpm (less than 2%) speed error for the drive to manage. I recently started up a 400hp system with a motor labeled 1787rpm!!! That's a better than 1% speed regulator before the drive even tries to help further! My point is simply that the choice of motor is a key component in an AC system's speed accuracy. Now, you don't have to have a closed loop flux vector drive to get excellent torque and speed control at zero and near zero speed anymore. Today, the best sensorless systems can do that job in the process reducing the speed error to around one-tenth of motor slip. Of course, if the speed has to be dead-on or the error has to be non-cumulative as in tensioning applications, then the AC system needs a pulse encoder too.

Finally, let's say you size an AC system at the same hp as the old DC system and you find that more starting torque or higher low speed cooling or less speed error would be desireable. Simply increase the drive train ratio between the motor and the load so, at maximum load speed, the motor is turning at the 90Hz speed instead of at the 60hz speed. All of those benefits come your way with this simple chance. You say the motor is direct coupled and the drive train can't be changed? Then do as suggested above and use a motor of the same hp but with more poles, as a six pole (1200rpm) rather than a four pole (1800rpm). All of those benefits are yours this way too. The only disadvantage with the slower motor is a somewhat more expensive and physically larger motor. But, for the price of a sheave or sprocket, it's cheap insurance if your project is in trouble!
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Old October 8th, 2004, 07:55 PM   #13
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Another thing to consider, and that I see quite often,is when the replacement IM's are oversized, the pf and efficiency is not as great as the motor never quite reaches full rated load power output. This situation may require extra pf correction which will also hurt the ol' wallet!!!
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Old October 9th, 2004, 02:34 AM   #14
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Dick

Thanks for your educated lecture.
What you wrote sound very reasonable.
But I am littel bit confused.
When I see machine which made in the last years and come with AC motors instead DC motors.all the motor are about 50% larger with closed loop.I talking on plastic extruders.and paper winders and rewinders.
Most of the European manufactures use ABB with closed loop.(when I asked diffrent brand they insist on that for the extruders)
I know DC motor can work with out feedback I canceled tach in DC motor and it still work fine.
Why according to your opinion they spend more money to use expensive systems.Maybe there is some more reason behind that or it just so simpel?
I have to replace 250A DC motor if I will use sensorless vector VFD it will save me a lot (I already have the motor), from the other hand if it wuold not work I will be in deep $hit.

Can I use what you describe(with out closed loop)to work in torque
mode, and stall the motor in zero speed,
What if I want to be in speed mode and stall the motor(I dont want to use mechanical brak every cycle).I mean keep the run on and 0v reference.
Can I do it.?

Thanks in advance
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Old October 9th, 2004, 04:36 AM   #15
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my 2 cents

In systems where speed control and response is important, the ratio of reflected load inertia to motor rotor inertia should be kept as low as practicable.

Speaking from bad experience with servo motors, a DC motor will have significantly higher rotor inertia than an equally rated AC motor. Simply replacing a 10 NM DC servo with a 10 NM AC servo will result in performance ranging from unimpressive to unacceptable due to the change in inertia ratio.

I'll take a punt and guess that there is a similar difference between AC & DC standard motors.

As DickDV suggests, changing the gear train can counter the problem since the inertia ratio reduces with the square of the gear ratio.

I'm currently involved in upgrading tissue rewinders from line-shaft to sectional drives. From a single line shaft drive of about 60 KW, we have section drives as large as 90 KW - not for the power, but to reduce the inertia ratio to a range that will allow the speed control and response required.
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