inverter 3

[richerdick]

i am looking to replace a system that uses a dc drive to drive a dc motor gearbox assembly with an inverter driven ac system. how do i go about designing such a system. what information do i need to take from the dc arrangment before deciding on an ac replacement i am aware that it is possible that there is a difference between the performance of ac and dc motors developping full torque at low speeds what is the normal first port of call. i can guess that i could use a torque wrench or guage to find what torque is required to drive the load and then measure the speed of the existing output shaft and work back from there. i would appreciate a few pointers of how to start i am sure that there are more than a few of you who have done this many times before

thanks

 

[itsmoked]

How about the speed? What speed you you need "finally"?

Your torque wrench to speed method is fine. Use a bending bar torque wrench. Do you have any feeling for the change in dynamic torque are the running speed? For example if your load is a fan the torque wench method is out.

If you can get the speed and POWER consumed by the existing motor we can probably make a good guess on the AC replacement you would need.

BTW: Please use proper capitalization around here, or you will catch hell, as trying to read your post sucked, and savvy non-english readers will just not bother to help you.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Right. Punctuation matters. Please.

There is usually a nameplate on DC motors. Saying what nominal current, voltage and speed are. You can get a very good estimate from those numbers. The volts times amps give you nominal input power at rated speed. That will give you the operation point that the original drive was designed for. You can safely use those data to select the new drive.

There are a few caveats, though. The first one is that cooling may be inadequate at low speeds. That is usually overcome by adding a separately driven fan. If you do not intend to run your machine at low speeds for extended periods - say, around ten minutes - then this may not be an issue.

The available torque at low speeds is another thing to consider. There used to be a problem area below around five Hz, but many drives do a lot better than that these days. Check with the manufacturers. If your application involves hoisting, you need to be extra careful. An encoder or tach may be needed so that your drive doesn't run away when lowering the load.

Using armature current to measure the actual load at different speeds is what I would do. I would then know fairly well what the machinery needs - not just what the original designer thought was needed.

Gunnar Englund

http://www.gke.org

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

 

[richerdick]

Ok thanks, point taken about punctuation. I can see it made for hard reading. I will be back witrh more questions shortly I'm sure.

 

[richerdick]

On first inspection the motor i wish to replace is rated at just above 3000rpm 2kw and 8A.
Fortunately the gearbox it is mounted to seems to be Fairly standard my plan now is to investigate the opperating speed of the current set up then try to find a two pole motor (because speed at 50hz will nearly match).
slightly larger than 2kw
If I find the torque vs speed curve for the motor i choose I can use my torque wrench method to determine whether the motor will be able to provide the torque at the lowest required opperating speed.

I would then look perhaps to select a 3kw drive to ensure it has enough power.
does this sound like a reasonable approach or am i over simplifying things. i am aware that i may have to look for forced ventalation if the motor is to run for prolonged times at low speeds.

 

[itsmoked]

You are on the right track.

Sounds like you have the replacement sized pretty well too.

Remember the DC motor was only about 80% efficient so you probably don't need to go over much with your AC replacement since AC motors are 90+%.

The torque wrench method is pretty dubious verse the numbers you've just given. I doubt your T-wrench will give any better data because of inertial, lubrication, and windage error, the wrench will miss.

Remember if your chosen motor is a little large you won't be using a bunch extra power so don't worry about it.

Are you producing these things or just upgrading a single tool of yours?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

I am curious as to how you will measure the load torque with a torque wrench. I am always willing to learn new tricks.
respectfully

 

[itsmoked]

Easy Bill...

Turn the motor shaft with the wrench, read the torque reading while in steady motion, the faster and the smoother the better. Cross fingers, Hail Mary, add a fudge factor of [∑]e^[π] and "call it".

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[richerdick]

Thanks again for your help.

This project is just to replace a single DC drive motor set up that has been in place for some 15 years. the spares have now become obsolete and i am tasked with updating in order that we can have spares in the event of a failure. More to the point I was hoping to learn a little from the project that I may apply in the future to similar tasks on a larger scale.

The torque wrench method was just how i supposed other people aproached the problem. The dc motor is old and from a manufacturer i dont recognise so I don't supose I would find much info other than that on the name plate.

How would you suggest i ensured the motor i selected could provide adequate torge to the gearbox?

This is the first project of this type i have undertaken

Thanks again

 

[DickDV]

richerdick, let's start over here and think this thru with the data you have. First, rather than simply replace your old drive/motor system with an AC equivalent and possibly duplicate errors in that system, it would be better to understand your load's torque and speed requirements and go with that.

Since you have an existing DC motor, you can find load torque quite easily since motor current is directly proportional to load torque. Notice that I did not mention motor kw or hp. Sizing a drive/motor system can best be done with torque and speed, leaving hp to come out wherever if finally does.

Since I am more familiar with hp and ft-lbs, I coverted your 2kw to 2.68hp, multiplied it by 5250 and divided the result by 3000rpm to get 4.7ft-lbs of torque. This is the motor's continuous torque rating from 3000rpm down to whatever slower speed it can cool itself. If it has a shaft fan, that point would be about 20% speed or 600rpm. If it has an auxiliary blower, it will cool all the way to zero speed. But, the more important question is "how slow does my load have to run at full torque" That will determine the type of motor cooling required.

Next, observe the DC motor amps carefully for enough time to capture all of the operating conditions in the load. Watch particularly for overcurrent when starting, accelerating, and for any sudden overload conditions in the load. That will tell you how much torque overload capacity you will need in the new system.

Once you know the load's torque requirements and speed range, take the highest continuous torque level and the highest speed and size your AC motor to cover that both. Select the cooling method that will cover for the slowest continuous torque condition.

Using the highest shortterm overload torque level, figure the motor amps required. Size your AC drive using, first, the continuous current rating of the motor and, second, the highest shortterm overload amps to cover for the overload torques. For example, you may need a four amp drive continuous but 5.2 amps shortterm. That would require a drive with a larger overload capacity than the common 10% variable torque or normal duty rating. You would have to select the nest larger size to cover the overload amps.

One other thing. You need to understand what the speed stability requirements of the load are. For example, when the load torque changes from a small torque to a large torque, does the speed have to remain absolutely stable or can it sag a little. Selecting a V/Hz drive will get you about 1% speed droop from no load to full load. A sensor-less vector drive will get you about .5% droop, and a full flux vector drive (requires an encoder on the motor shaft) will get you essentially no droop.

That's the process I use. Hope it helps you make your selection. Just one last comment. Don't be taken in by the notion that "just oversize everthing and you'll be ok" is a good way to go. You have to understand your load's speed and torque requirements and, once you meet them, any oversizing is just waste.