motor start / stop damages? 3

[electricaleng30]

Hello all,

We have a lot of 3 phase induction motors at our facility and some people think they may save energy by turning on and off those motors 3-4 times an hour. For the past few weeks I have been trying to find some good information on how often a 3 phase induction motor can start and stop per hour or day before it starts to sustain serious damage. I know that its probably different for different manufacturer, but it would be nice to have a bulk part and maybe links to different articles talking about the effect of doing so. Personnally I think the savings far outpast the damages and reduced life of the motors that will eventually result from those frequent start and stop, even with a soft start? Correct me if I am wrong. Thanks guys!

 

[waross]

Hello electricaleng30
This topic was well covered about a week ago.
See thread237-152831
yours

 

[electricpete]

Here's some good info from NEMA standards that I don't think was mentioned in the other thread.

http://www.joliet-equipment.com/allowable_starts.htm

There was a mention of 5 minutes off-time => secure a motor as I understood it. I think it may work in some circumstances but tough to generalize. As motors get larger and faster and carry higher inertia the limits get more restrictive.

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Eng-tips forums: The best place on the web for engineering discussions.

 

[itsmoked]

Well electricpete, that pretty much puts the bell on the cat!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[sreid]

Jus a thought; wouldn't the motor contactors have a much higher failure rate starting a motor 32 times a day?

 

[itsmoked]

I doubt it as the contacts would cool quickly. You would just eat up the "operations lifetime" in x years instead of y years.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[afc]

I have been using a motor of 5 kw 3 ph 380v for driving a flutting machine and it it start and stop at the rate of 0f 60 time per hour for at least 10 hours per day and it been runing without any problems

 

[itsmoked]

Up until you just made 'that' statement... Probably fail within a week now.. Very risky..

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electricaleng30]

Thanks a lot for the very usefull information guys!!!

 

[sacem1]

I have a motor that starts CW runs and stops to start again instantly CCW as much as 5 times per minute.

The power needed for continuos one direction motion was 5HP I installed a 15 HP motor and am using direct contactor starting with no power reduction. It has been working for a couple of months and no problems up to now except we fried the thermal protector (set it to low to protect the windings and we have replaced it with 2 sets in series of thermal overload protectors, if any one trips all the system stops.

Regards

SACEM1

 

[jraef]

SACEM1,
You may want to consider switching to a solid state thermal overload relay. The stresses on bimetal or melting alloy relay elements are probably what are causing your repeated failures.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[sacem1]

Thanks that is really a good idea

Regards

SACEM1

 

[itsmoked]

Nice call jraef! -a star-

Solid state thermals are so, SO much better! Truly superior!

Most motor starter makers have them that fit right in place of the damn bi-metal/melting pieces of junk. So do inquire for your make!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

I'm a big fan of the overload designs in which the insulated conductor passes straight through the toroid of the sensing CT direct to the contactor. Anything which reduces the number of joints, and hence the number of weak points to come loose or get hot, is my friend.

Siemens make one specific design in their 3RB10 range which covers the 50 - 200A range, which covers the majority of the LV drives which I deal with, and certainly the ones we have most trouble with. My site has a lot of starters which are one module size too small for the starter they are housing, so heat is our big enemy: we've had good results using these overloads - less starter problems and less dead motors. It's the S6 frame size on the following link:

http://www.sea.siemens.com/icc04/sec04/04-19_21.pdf

Big file - probably not worth it if you're on dialup!


----------------------------------

I don't suffer from insanity. I enjoy it...

 

[itsmoked]

Here's site that shows several of them:

http://www.pioneerbreaker.com/Default.aspx?Category_ID=b2e9469d-74b1-44e3-a3c2-6c1bd66364c6

you may have to cut/paste this link as I see pesky '?'.

http://www.sea.siemens.com/contrlbu/upld/files/p483.pdf

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[fsmyth]

How about a 4-pole version with separate sizing on
the poles (for D.C. motors - 2 at one setting and
2 at another)? Sounds like just the ticket for
replacing some existing overloads - some folks
here are using 3-pole start/stop contactors with
one side of the field run thru the contactor and
smaller heater, and the other side of the field
run thru an auxilliary.
<als>

 

[jraef]

fsmyth, would that be 3 phase DC motors?

Eng-Tips: Help for your job, not for your homework Read faq731-376