Help me understand SEW-Eurodrive gearmotor calculation 1

[EighthBen]

Dear Engineers,

I will be responsible for calculating the parameters and selecting the appropriate gearmotors in our company. However, I almost have no experience in doing that. We are buying SEW-Eurodrive gearmotors, but due to policy of our company (it is a long story under this situation which I shall not bother you with), we must buy gearmotors through another company in another country. The "seller" is only the reseller, having no qualification in calculations, therefore I must give them a complete order page (which is generated in SEW Drive configurator) with full information. So I have to learn how to select an appropriate gearmotor without a help from official representative of SEW. The only source I have is old calculations of previous engineers, and also - current working motors in our factory that I can inspect - see the working current, get information about their lifetime (some of them are working for years, some lasted for 1-2 weeks), and so on.

I started with studying the SEW manual “Project planning for drives”:
[link download.sew-eurodrive.com/download/pdf/10522913.pdf]download.sew-eurodrive.com/download/pdf/10522913.pdf[/url]

When reading the document, I face some questions that I can not understand because of lack of experience or knowledge, and I can not find these answers in the internet. I will post short-accurate-exact questions in this thread, and I hope somebody shall find a few moments to explain them. I believe these will be easy for professionals of the field, but they are not even near to being easy for me as a beginner. I appreciate your help in advance.

 

[EighthBen]

I shall begin with some very basic question. We usually use AC motors with gearboxes, and SEW offers two basic series of them: DR and DT . In any situation I had, I was able to choose between them (I mean there always is a similar motor of DR _or_ DT series to choose from with very similar parameters). As I understand, DT is older generation motors, maybe with less possibilities in selecting the high efficiency class, and narrower variety of accessories, e.g. encoders? This is only my guess, is this correct? What are the main differences between them?

 

[Parchie]

@EighthBen, try getting help from SEW. They'd be more than willing to help you.

 

[ZiPick]

I agree that it looks like the DT motors are older, possibly becoming obsolete. Check out this PDF

http://download.sew-eurodrive.com/download/pdf/16891619.pdf

It's a guide to converting DT motors to DR motors. If either work, go with the newer.

 

[EighthBen]

Thank you for your comments. ZiPick - yes, I know this document, but it says nothing about technical differences between two motors, only info about converting.

Would you have any comments between prices of IDENTICAL (same kW, same accessories, same RPM) motors DT versus DR?

 

[EighthBen]

So, after exploring some more documents from SEW I found out that DR is a new generation motor, which is better than DT motor, because:
[ul]
[li]used new technologies in manufacturing[/li]
[li]it has better efficiency, possible to order premium efficiency motor[/li]
[li]it has a modern design for integrated encoder - which saves some space[/li]
[li]may have 3 versions of brakes depending on needs[/li]
[li]DR is up to 200 kW, DT - only up to 120 kW[/li]
[li]possible to order compact size motor[/li]
[li]many possible flange options[/li]
[li]can have temperature measuring devices[/li]
[/ul]
...and many other options

There is a small site for DR motors:

http://www.seweurodrive.com/dr-motor/

There is also a PDF, which shows how many new features can be ordered in DR motor, while not available in DT: sewmotortruth.com/SEW_DR_Motor_Series.pdf

So, this is enough for comparison i think, except it would be really interesting to compare the price range of DT and DR if somebody has an opinion on that. Thank you in advance

 

[controlsdude]

I have run into same issue where the company you work for wants you to work direct with a company due to some deep discount which is bypassing the local distributor. But in all cases you as an engineer need the local distributor to ask your detailed questions and support. I would just contact the local distributor yourself, get your questions answered so you can properly do your job. Bean counters and purchasing agents can sometimes create these nightmares.

 

[EighthBen]

Now I have dedicated some time on reading other literature from SEW, watched various lectures on youtube on similar topics, googled for other examples from other manufacturers, and a lot has became clearer now. Anyhow, some answers I didn't find. This is one of them:

In the document I have mentioned previously - “Project planning for drives”:
download.sew-eurodrive.com/download/pdf/10522913.pdf

Page 71 - there is an explanation how to calculate the appropriate starting frequency of the motor in behalf of ratio loaded/unloaded cycles:

In this example, it is allowed to start the motor 112 cycles/hour in loaded condition, and 247 c/h in unloaded. In my opinion, if this motor would work in the mixed load (sometimes loaded, sometimes not) then the permitted starting frequency would be somewhere in between. But according to this calculation (and all the other examples in this manual from SEW) this formula is present, which represents a lower number of mixed condition starts than the fully loaded condition starts. How can this be explained from physical point of view? Because now it means that the motor can be started 112 times in fully loaded condition, but only 76 times fully loaded + 1 time unloaded. I don't understand this, how can it be?

 

[tygerdawg]

My 2 cents' worth, take it at face value. My whiskey glass is empty, so...I refuse to accept respnsonsity...rospnegbit...oh, what the hcek. Nevermind.

I worked on contract for a few months for SEW and got to know these calculations intimately. But it's been a long time. With the "planning" document, you're as good as you can get. The German approach is very rigorous. Admirable in some ways.

That calculation is about "starts per hour" and is all about heat dissipation. In order to ...ahem....optimally...size the motor for an energy-sensitive location such as Germany and NOT like the US (...yet, anyway), then these "starts per hour" must be analyzed. Take your best shot. If your best shot is no good, simply solve the problem by upsizing the physical size of the motor so that the bigger mass will handle the multiple starts per hour (which, essentially, is a short circuit and therefore creates a bit of heat that must be dissipated).

Some years after my stint at SEW I had the opportunity to do a gearmotor sizing exercise. I chose Dodge as the supplier. American. Murcan. Much more brute force unsophisticated get-it-done type of calculation. That's when I discerned the truth about an alternative path to going home on time. Upsize that b*tch and call it a day: it's just torque, and the bigger motor will last forever because it never overheats.

Aw heck...my whiskey glass it empty.

TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[EighthBen]

tygerdawg , thank you for your input. To add up, I would say - shame for SEW, this document should had been improved so much since it is 15 (fifteen !!) years old, it has lots of mistakes, haziness, and some of motors in the examples do not exist any more...

Anyhow, I completely understand the number of motor starts equals to short circuit in the winding, dissipating more heat. What I don't get is - how can it be that you can start a motor 112 times per hour fully loaded, but only 77 times in mixed mode (say 50 loaded ant 27 unloaded). How can these 27 no-load starts be equivalent to 112-50=62 loaded starts?

Again - 27 no-load starts will give the same amount of heat like 62 loaded starts IF motor is started in mixed mode
BUT - if there is only no-load starts, then it can be as much as 247
?
I say this might be a mistake, but the same formula is in so many examples ...

I really must know everything I can on this topic. I have seen too many broken gearmotors due to inappropriate calculation, and production lines staying off work waiting for gearmotor replacement ...

Thank you in advance