Motor stop start power efficiency and maintenance 6

[Johnnykool]

Hi, have machine that receives product downstream it has hoppers that collect scrap and output is dependant on production upstream. Hydraulic pumps and conveyors are constantly running waiting for product. Sometimes cycling constantly but mostly waits (5min to 1hr) until hopper full.

Question
1. When does it come advantages to stop and start motors while waiting for product? With no soft start or sensorless vector drives motor are started with IEC Euro-style motor contactor/starter. Experience has shown me that these starters contactors are garbage compared to NEMA.

2. Power surges caused by starting cause electricity power rate peaks: how do you figure out if this will cost effective?

3. The ware and tare on the motor shafts could cause unforeseen maintenance issues: How do you figure out if it worth all this.

Any direction or help would be appreciated not in any rush to cause more downtime for the sake of conservation.

Thanks all

 

[waross]

I'd say any wait over 5 minutes dserves a stop.
Power peaks. Utilities often us a slow responce demand meter that often is based on the heating curve of a typical transformer. A demand meter may take 15 minutes or more to respond completely to a step change in load.
What this means is that a small motor starting is almost invisible to the demand meter.
A properly adjusted belt drive needs very little maintainance. A loose chain drive will impart a lot of shock loading when it starts.
yours

 

[ScottyUK]

(IEC) starters contactors are garbage compared to NEMA

Mechanically I don't think they're much worse than NEMA types when comparing similar physical sizes. IEC contactors suffer when compared with NEMA contcators of similar electrical rating: the IEC contactor is usually smaller and lighter, so it has less iron and less copper, and tends to run hotter and nearer the limit. I usually oversize IEC contactors by a full frame size and I'm fussy about which brands I use: Telemechanique's latest range is steaming crap which doesn't last five minutes; Siemens range is pretty good and puts up with arduous duty without failure. To a certain degree, you get what you pay for: a NEMA contactor is a lot more expensive than the equivalent rated IEC type, but if you go up an IEC frame size the cost difference and life expectancy is similar.

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

 

[itsmoked]

I never thought of that!!! Nice point Scotty.. I like how IEC ones look and believe it or not I like my equipment to look visually stunning or at least sharp! NEMA devices don't! Most look like they're outta the dark ages. I will be considering your point from now on.

Thanks.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Good point Scotty. I see a lot of dual rated devices and as you say, the NEMA ratings are more conservative.
Thanks.
respectfully

 

[wschel]

Most motors are rated for a limited number of starts per hour. Don't forget to take that into consideration.

 

[Johnnykool]

Thanks great responses, one more question.

What about hydraulic pumps 20hp 2000psi?
5 min then stop as well?
Just seems like I'm opening a new can of worms of mechanical failures.

 

[itsmoked]

Wow that is thrashing it... 5 minutes on 20Hp.
How about a VFD running the 20HP then just slow it way down to 10-20% then ramp it back up for work. It will avoid the big thermal whack of a dead start (softstart or DOL).

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

wschel and itsmoked have good points about starts per hour and the hydraulic pump.
For a 20 horsepower you should check the manufacturers specs for the number of starts per hour or the number of starting seconds per hour. Small fractional hourspower motors with a 1.15 service factor will take a lot of abuse on a conveyor. As the motors get bigger they have less tolerance for abuse and the repairs get more expensive.One way of coping with starts per hour is to determine the minimum allowable interval between starts and set a timer to keep the motor running for that length of time. For instance if you are allowed 4 starts per hour, set the timer for 15 minutes. That way it can be restarted at any time and not exceed the 4 starts per hour.

A typical hydraulic pump setup often has a high pressure bypass that opens when there is no flow in the hydraulic system. The load on the motor (and the money you pay for it) is much reduced when running on bypass. I suggest that if the work schedule will result in too many starts per hour leave the hydraulic pump running. Otherwise shut it down as soon as you're finished with it.
respectfully

 

[Marke]

Hello Johnnykool

In addition to the excellent answers you already have, I will add my pennies worth.

When you start an induction motor, there is an overload current drawn from the supply. Under full voltage starting conditions, this is typically 600 to 800% of the motor rating. The duration of the start overload is dependant on the load inertia as seen at the motor shaft.
During start, there is a high power disipation in the rotor of the motor. This energy dissipated in the rotor is equal to the full speed kinetic energy of the driven load and is independant of the type of starter employed provided that the acceleration torque is not severely compromised by too low a start current.
Typically, maximum demand metering has a long time constant of around 15 - 30 minutes. The short term overload from starting a motor will not normally register on maximum demand metering. If the total starting time is very long (due to a very high number of starts) then there can be an increase in the integrated maximum demand readings.
Motors and machines are rated with a maximum number of starts per hour. There are two major limiting factors in starts per hour. They are the thermal affect on the motor and the mechanical transient affect on the motor and driven load. Thermally, if the start time is very short, the number of starts per hour can be greatly increased. If the start transient torques can be reduced, then mechanically the starts per hour can also be increased.

The issue of keeping a machine operating as opposed to starting and stopping it is related to the cost of running continuously under no load conditions, and the cost of frequently starting the machine.
Using some form of reduced voltage starter correctly can minimise the mechanical problems associated with frequent starting, but will not reduce the energy consumed during start.

I would suggest that you determine what the losses are in keeping the machines running before you contemplate shuting them down. Induction motors are quite efficient and the major losses are usually mechanical rather than electrical.
If you do decide to shut the motors down and frequently restart, I would recommend that you consider the use of a reduced voltage starter such as a soft starter to minimise the mechanical wear and tear associated with full voltage starting.

A variable frequency drive is a way to reduce off load mechanical losses by slowing he machine down, but will increase the on load losses. I have seen installations where the onload loss increase is higher than the off load reduction in losses when totalled of the operating cycle.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[waross]

Hello Johnnykool
Check out electricpete's post in thread237-153412
respectfully