Motor startup power consumption...

[Rock Crusher]

Let me try and word this correctly.

I work at an industrial facility. I crush rock at a rock quarry. When we are not crushing, for whatever reason, they want me to turn the motors off. They range in size from 40hp to 400hp. They want this done to save money. Understandable.
I want to figure out how long a motor has to be off to save enough money (ie. use power) to make it worth shutting it off compared to keeping it running. I know they draw some serious amps when starting up.
Thank you.

 

[broadgage]

If the machinery is to be unused for more than a few minutes then it will save money to turn the motor off.
Electric motors draw substantial extra power when starting up, and this needs to be allowed for in the circuit design.
This extra power is however used only for a few seconds, perhaps up to 30 seconds, and therefore has little effect on the utility bill.
The extra energy used for starting might be 5 or 6 times the normal demand for a second or so, and up to twice the normal demand for a few seconds.
Very frequent starting and stopping is to be avoided as it may cause the motor to overheat, and will certainly result in extra wear on the starting contactors, and may result in extra wear on gears, belt drives and the like.
Some motors are designed for only a certain number of starts an hour, this data might be on the nameplate, or may be available from the manufactuerers.
In the absence of any specific data I would suggest no more than 4 starts an hour.

 

[Jraef]

There is no simple answer, sorry.

Depends on a number of things, such as the idle current, loaded vs unloaded run time, power source and type of crusher. Most impact crushers work on speed and inertia, so once it is spinning at full speed, idle power consumption is relatively low. In general an unloaded impactor is going to take maybe around 10% power to keep it idling, so that will affect your return on deciding to turn it off. Cumminution (grinding) crushers have a lot of friction even without product in the machine, so running those even unloaded takes quite a bit of power, maybe 3 times what the unloaded impact machine would. So OFF is almost always the best energy saver, but depending on what you have the time frame to where it pays back will vary.

In either case, the high starting current is not always a problem. You don't pay for current, you pay for power, and power includes power factor in the equation. Starting an AC motor from a stop means very high starting current, but at a very low power factor. So the net energy cost is only a little higher that just running it. But starting from a stop does still consume more energy cost because you have to re-accelerate the mass without doing any useful work. So if your loaded run times are short, it may not pay to turn off in between.

If you are generating your own power on site, ANY unloaded run time means fuel consumption. So Off is usually better in that case.

PS, don't fall into the "Peak Demand Charges" trap foisted by a lot of soft starter suppliers. 99% of utilities do not charge for instantaneous peak demand, they use a sliding demand window. So only the total highest demand kVA during any 15, 20 or 30 minute window of time is going to be considered in determining the penalty assessment. In that case, reducing the starting current is not going to make a significant difference unless you can stretch it out beyond the demand window, which you cannot. Soft starters are good for lots of reasons, but not that one for most people.

 

[ptonsparky]

I put a softstart on a 250 hp a few years ago. Early spring. No one told me I was not to test run that motor until after April 1st. I think we started and ran it 4 or 5 times that day until we got the parameters set. Changed their billing rate for the whole month. No one said anything to me until a year or so later and by then they were laughing about the look on the managers face each time we had hit the start button.

 

[Jraef]

I put a softstart on a 250 hp a few years ago. Early spring. No one told me I was not to test run that motor until after April 1st. I think we started and ran it 4 or 5 times that day until we got the parameters set. Changed their billing rate for the whole month. No one said anything to me until a year or so later and by then they were laughing about the look on the managers face each time we had hit the start button.

But that was likely because you started them 4 or 5 times, and the cumulative energy consumption of each one during the demand window pushed it over the edge. They may have had a very low demand threshold for the off-season or even a temporary instantaneous demand factor negotiated, very common with a utility when you have seasonal loads. There's no way you would be able to know that but somebody did, they should have scheduled that better with you.

 

[tish53]

I also work at a rock quarry and we shut down conveyors if we have more then a few minutes of idle time but typically leave the crushers running, due to the mechanical stresses starting puts on the equipment.