Duty Cycle of Motor 5

[PThiessen]

Hi, I'm an engineering student working a summer internship, and my current project has completely stumped me. I'm trying to take regular conveyor belt and automate it, however, it won't be in continuous operation; it will only be moving one 250lb cylinder about 6 feet every 60 seconds or so. So the motor needs to be in operation for about 6-8 seconds every minute.

So far, I haven't found that motor suppliers post their specs for how often you can start and stop a motor, so the next best option I was looking at was getting a motor that runs all the time, and throwing an electromagnetic clutch on it to engage and disengage when needed. The only problem with this idea was that all the electromagnetic clutches I found were in the $900+ price range.

So I guess my two questions are: do motors exist with that kind of duty cycle and where might I look for them?
Or alternatively, is there a better/cheaper option than an electromagnetic clutch to solve the problem?

Thanks,
P. Thiessen

 

[GregLocock]

What is wrong with the many solutions that google proposes?

http://www.brithinee.com/knowledge/Allowable_Starts.htm

looks like a good place to start.

$900 in the context of a line stopper sounds like peanuts to me. If you look at production machinery you'll notice that it often appears to be overdesigned, which incidentally is why we are using 60 year old stamping presses to build bodies for modern cars, but I don't think that is an accident.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[dvd]

The flywheels on presses store kinetic energy. Similar to a water tower, trickle in at low rate and remove for short duration at high rate. There are lots of different ways to capitalize on the philosophy of storing energy.

 

[Artisi]

Have you considered hydraulics, you could have a constantly running motor with bypass on the hydraulic circuit.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Tmoose]

NEMA MG-1 has something generic to say about starts per hour.

1.33 here for instance. Click on the + to see NEMA's reply.

https://www.nema.org/Products/Pages/FAQ.aspx

So also should someone in the motor manufacturer's engineering department or technical (probably not sales) support.

60 starts per hour (SPH) is a bunch. Some stuff will heat up from the inrush current, and there won't be much time for the heat to dissipate.
A more appropriate question for the motor mfr is minutes between starts (sometimes called MBS).

 

[LittleInch]

The issue is heat in the motor.

Why don't you replace the enclosed fan on the motor with a separate electrically driven fan which is on all the time?

I think you'll find a fan able to deliver the same air flow for less than $900.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ScottyUK]

The majority of the heat from a start ends up in the rotor, which means that the motor needs a period of running to allow the heat to transfer from the rotor into the internal air and thence to the stator and outer casing. You could cool the casing all you want but the rotor would still get in trouble if the cycle time is short and there's no period of running in between.

You'd be better off using a VSD to control the motor as this will put far less heat into the rotor, or use a PM motor with an AC drive. The latter solution will likely cost you more than $900 though.

 

[PThiessen]

Thanks for all the great tips, I've been researching them all further and seeing how I might be able to apply them.

TMoose, the article you posted lists mainly for larger motors, but there is a tendency towards smaller motors having shorter MBS. The motor I plan on using only needs about .3 HP, which is much smaller than any of the motors listed.

Do you think that the heat created by such a small motor could safely be ignored?

Alternatively, this article:

http://www.dartcontrols.com/2013/09/ac-and-dc-motor-drives-is-one-the-clear-choice/

seems to suggest that a DC motor is better for applications with quick starts and stops, but doesn't mention if a high start/stop cycle would be damaging to the DC motor.

In the context of this project, would a DC motor possibly be more practical?


Thanks,
P. Thiessen

 

[Tmoose]

" ..... Do you think that the heat created by such a small motor could safely be ignored?
............. In the context of this project, would a DC motor possibly be more practical? "

I would settle for nothing less than a written approval from the motor manufacturer.

 

[dvd]

At some point the acceleration and deceleration curves of the motor need to be considered. You are required to accelerate and decelerate the cylinder and all moving parts of the conveyor. Decide on how to control the motion, unless you just want to start the motor and let it figure out how fast everything is going to accelerate and decelerate. You may have a problem with coasting to a stop in the right spot.

A booklet that I have referred to for several years on motors - the Cowern Papers is a good reference on AC (and DC) motors.