inverter or contactor

hi,i am trying to decrease energy consumption of a whole conveyor system, and all conveyors are drıven by contactors.so it causes very high current flows when start up of motors. i wonder if i can decrease this consumption by using inverters for the motors or not. i'll be pleased if you tell your experiences about this.

i made an experiment but i am not sure the results are reliable.
i wrote a program with a plc and run the motor for 20secs, and than stop for 5secs periadically for 24 hours, with a contactor and a single-phase inverter. the 1.1 kw motor with delta connection by an inverter caused 4,27kw consumption and with a star connection by a contactor caused 4,68 kw consumption.my powermeter has three phase connection so i had to connect it between invertor and motor,and that's why i can't rely it. because inverer does not generate sinusoidal wave form and i think my analog powermeter is designed for 50HZ sinusodial wave.

Hello tny.

Is this a real simulation of the task. With so many starts ?
If so, you could consider reducing mechanical wear and also wear on the contactors by replacing the contactors with VFDs or softstarters.
Apart from that, if the operating conditions are the same, then a VFD or softstarter will not save any energy. On the contrary, a VFD in particular incur extra losses, some in the VFD and some in the motor.

edit: I'll modify the above somewhat. Because you can minimize the starting current, then there will be a small energy saving. However, by reducing the starting current, you also extend the time of the starting current. The end result will not be very different.

I read an interesting article about energy savings in conveyor applications. Some years ago energy saving motors promised a small saving, but they didnt really become as popular as hoped. The return in investment was too little.
Now, a much more interesting development is happening. Actually, the greatest energy loss in a motor+gear combination is in the gear. The most typically used kind of gear is with sliding meachanism ("snekke-gear" i dont know the english term, "snail-gear" ?), whereas the new gears are with rolling mechanism which has less than half the friction losses. The prices of the latter has been dropping so much that they become profitable from 1kW an up (as far as I remember).

Vfd's set to ramp the startup will help reduce your inrush current (current spikes during startup) another means to reduce overall inrush is to start each motor or group of motors with slight delays from each other usually 0.5 sec. Ie motor group one starts then 0.5 sec later start motor group 2. Inverters and VFD's both work on Pulse width modulation which will throw off the accuracy of meter readings True RMS is supposed to help aleviate that however I never rely on it. I've installed IGBT long lead reactors to aid in converting back to a more true sinusoidal wave form.

First, you are right that the output of the VFD is not a good way to measure power, unless you have instrumentation designed for the application. You could use an anlog output of the VFD to transmit a kW signal, or you could monitor the input side of the VFD.

VFDs are wonderful devices, but they are not magic. They reduce energy if the load is variable and mechanical power requirements decrease with decreasing speed. If you have the same load and speed on a motor with a VFD or across the line then the power required by the motor won't change.

If you are trying to limit inrush from starting and ramping to full speed then a reduced voltage solid state starter (RVSS) is slightly less expensive and just as effective.

no it's not the real situation but the motors start and stop aprox once at least in ten minutes.
how about the startup current (some says demarrage current)? i know that it is 6 times greater than nominal current. i think there will not be a situation like this with an invertor. (i say inverter because strangely soft starters are more expensive than inverters )
i heard that in japan, the %98 of the motors are driven by inverters. they noticed something but what? (may be mechanical wearings as you said)

the gears that you mentioned seems good, how can i find detailed info?what is its exact name?

tny said:

...because strangely soft starters are more expensive than inverters...

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Are you sure about this??

In my experience soft starters are significantly cheaper.

With only 1 start per 10 minutes, you should forget about the inrush current.

the gears that you mentioned seems good, how can i find detailed info?what is its exact name?

Click to expand...

Unfortunately I dont know the proper english terms.
The typical ineffective type is with the fast input shaft driving a wheel that transfers the power to the output shaft via a sliding thread.
It is cheap to manufacture, and you can get a high exchange ratio with only one stage. You also find this kind of gear in the small motors in toys.
The more efficient type is with rolling gears, possibly with several stages to achieve the same exchange ratio.
Goto the large manufacturers of motor+gear combinations. SEW. Danfoss. ABB etc.

We also are looking at a conveyor upgrade, and we are having a tough time justifying a drive on each motor. The simple fact of the matter is that in a 10 minute period, your startup power is such a small fraction of what you're ultimately using it's irrelevant. Trust me on this, if you want to cost justify all new inverter rated motors and drives to run them, you're going to have to increase efficiency. That is, efficiency of the conveyor system to move more packages in a given time frame. We simply found that there was just not enough power savings or mechanical stress type savings to justify this. Of course, ours was strictly for a conveyor system, and the hardware was cheap as well, which is what limited our mechanical stress savings capabilities. Good luck.

Russ

Lenze GSS is an example of the old type of gear. Efficiency is 0.75..0.90.

Lenze GKS is an example of the more efficient gear. Efficiency is between 0.92 and 0.95.

strangely soft starters are more expensive than inverters

Click to expand...

That all depends on what you are pricing out and who you are getting your quotes from. In my experience the cost of Drives approches the cost of soft starts in the low HP range but generally the soft starters should be less expensive simply because there is less to them.

If the inrush current from all those motors pushes your plant peak usage into a more expensive utility charge category, then using VFDs may save some money.


That's a big IF, however...

The two types of reducers mentioned are worm gear (inefficient) and helical (more efficient).

We use VFD's on any motor 1 1/2 HP and larger. The VFD's are about half the cost after the rebate we get from out utility provider. I would check with your power company to see if they have a program for VFD's.
As OkiePC stated, If your peak demand takes a hit when these motors start, you will pay for this hit for a long time. It all depends on how your power company monitors the peak and how they charge for the peak.
Racer

Tom, increasingly, under 10hp 460V, the cost of an inverter is very nearly the same as the cost of a softstarter. Not much to be saved there.

As for starting inrush and energy consumption, an inverter beats a softstarter every time since a softstarter still forces a motor to start dissynchronously. As a result, the very lowest inrush you can hope for on a softstarter is about 275% FLA. On the other hand, with an inverter, it is not uncommon to start the same load at 40% FLA. This is due, of course, to the synchronous start and the total lack of inrush current.

As far as this particular thread however, at these KW's, there is precious little energy cost to be recovered and I would expect little or none. That would be assuming no peak demand charges being mixed into the situation.

Thanks for clarifying the cost issue, Dick. Most of my work is on 100 hp + so I wrongly assumed the pattern was consistent. (You'd think I was old enough to know better, wouldn't you?)