Varaiable frequency drive

Yes, as the need for additional speed is required there will be additional draw on the mains. You will see more as it ramps up to a faster speed, but will level off once it reaches the new speed. The control portion itself is negligible on power draw, it is the change of speed that affects it. From what I have noticed, our vfd's on average draw less amps than a conventional AC motor. Mainly because we oversize them for their normal workload for when they need to work harder.

Good luck in your quest!

Hmmm... interesting response. Oversizing a VFD will have NO EFFECT on power consumption of a load.

Power consumption is directly related to the load on the motor, plus a ;little bit of overhead in the motor to provide magnetization. Without a VFD, the motor is run at full speed but the power drawn is still primarily a function of the load on the motor. If however, the load is modulated by choking off the full output of the driver on a variable torque load (pump, fan etc.), then the affinity laws will take over when using a VFD, because the power consumed will vary by the cube of the speed reduction. So if, for example, a centrifugal pump is run at 1/2 speed, the power consumed will be 1/2 cubed or .5 x .5 x .5 or 1/8 (.125) of full power. By comparison, throttling back the output of a full speed pump by 50% using a valve will have reduced the power used by only 1/2 (roughly). So using a VFD in a centrifugal load application that can take advantage of reduced speed instead of mechanical output modulation CAN greatly reduce the power consumption of the system for a given load.

If the load is a constant torque application such as a conveyor, there will be no significant energy savings compared to running DOL with the same loading on the motor. In fact, if a motor is run at full speed through a VFD, there are going to be switching losses of around 3%, so in fact it may actually cost MORE to run via a VFD in some cases.

The current application that I am dealing with is conveyors. And on the longer sections that have to start and stop because of flow of different sections and sorting on and off sections, the vfd gives a more smooth restart of the conveyor rather than the abrupt on/off of a regular drive motor. We have torn motor frames apart with the ac motors because of that. The vfd gives a smoother start in that case. The oversize of the motor that I stated is small, maybe 1/4 hp for when the belt may have more than the normal amount of boxes on it due to heavy or backlogged flow. That way we don't overload them causing more downtime of the equipment.

I think I hear my village calling

D'oh!

Sorry, I thought you meant the VFD was oversized, it appears you meant the motor were oversized. makes more sense now. You are not saving energy, but you are definitely getting good value out of your VFDs.

We use VFDs mostly on pumps and fans (variable torque applications) and we find that there are savings with regards to power consumption. I've watched the current draw on motors running on VFDs and it does drop as the frequency goes down...but only to a certain point. At very slow speeds, the current starts to climb up again.

Furthermore, at low speeds, the cooling capability of the motor's fan becomes almost negligible causing the motor to overheat. If you expect be running at low speeds for long stretches of time, you may need to install auxillary cooling fans to keep the motor cool.

Wish to thankfully acknowledge that views and practical experiences, so far shared by members,are immensely useful. Still more views,if available,are welcome.

Nvmani--

Thank-you for your interest and approach. In most applications the motor is oversized to the load, or is chosen with a "service factor" greater than 1.0 (this means that it can handle a short-term overload of the stated amount without excessive overheating). If your motor is oversized, regardless of whether this is a constant torque or a variable torque application, a VFD (variable freq. drive) can be expected to draw less power from the mains. This is because its electronics are matching the applied voltage and current to the load so the power factor is kept high and the magnetizing current losses (reactive power) are as low as possible.

However, the rectifier front end of the drive will impose a non-linear load on the mains, because the current is being drawn for only a portion of each incoming sine-wave cycle. This needs to be considered, so a good idea is to also look at the surrounding equipment and decide if you need line reactors or not.

A good rule-of-thumb is to use a supplementary cooling fan as an earlier post said if the motor is going to be run at less than 1/3 of its nameplate speed for any significant amount of time. Also, as mentioned in other threads on CR4, a motor that is designed for use on a VFD is often needed (but can be avoided in most applications if you use a load reactor).

--JMM

The energy conservation still stands. Besides the motor and driver heating, most of the electrical power (hopefully) is transferred to the the shaft. The VFD receives a 50/60 Hz frequency which is "squirted" in squared pulses (occasionally rectified) which pulses approximate a sinusoid of the required (new) frequency. The problem with the "approximated" sinusoid is that contains a bunch of parasite (higher) frequencies that get disipated in the motor as Eddy currents (that's why the permanent magnet-synchronous motors are prefered before the classic "cage-rotor" asynchronous motors). On the receiving side, i.e. the power supply, VFD can have a very detrimental effect on the "phi", i.e. power factor. The same parasite [high] frequencies can reflect back in the power line and disipate due to the line resistance (reactive power disipation). Actually, most of the VFD use a coarse sinusoid approximation, i.e. just an alternative square pulse that has a lot of 50/60 Hz junk from the original supply.

I hope I answered your original question... and not proved my lack of reading skills.

Nobody has used a square wave output on a VFD in 20 years. They are all PWM now. While there are harmonic currents, the level is nowhere near the energy savings you get from using the VFD. Many drives now are capable of actively correcting for these harmonics; if you are willing to pay for that capability.

As to the "phi" (by which I assume you mean cos phi, or Power Factor), modern PWM VFDs in fact will in fact correct the motor displacement power factor to .95 as seen by the incoming power source. That is the power factor that the utilities will charge extra for if it exceeds their specifications, so from that standpoint a VFD will help to avoid additional energy costs in that way too. The harmonics create a distortion power factor of course, but fortunately for users, the utility's metering systems will not pick that up. They may measure separately for it at installation and insist on some sort of mitigation because it can affect their distribution equipment life, but the solution is usually something simple such as filters.

Perfectly said JRaef!

One more thing though, just exactly what do you mean by "equipment distribution life"?

What could possibly be the direct effect of failing to integrate harmonic filter with VSD on your DCS as whole?

Cheers,

While discussing on VFD I find one issue like 'starting of motor' to discuss. As discussed 'VFD gives a smooth start' we can choose a soft starter which is on during starting and bypassed during running, gives a better economy than VFD used for starting. Now, my question, what is the starting current w.r.t full load current we should consider for DOL, VFD & soft start by thumb rule. I have some previous information, but want to recheck from your.

If all you want is a smooth start and/or a smooth stop, then go for the soft-starter, not the VFD. If you want to change the speed, then go for the VFD.

With a soft-starter or VFD, the starting current would still be high. The newer devices will have a current-limiter that you can program to limit the starting current. Basically speaking, whatever the breaker and overload rating that you use for direct-on-line starting you can also use for soft-start or variable-frequency-driven motors.

Not entirely correct.

The "rule of thumb" is that DOL will always take 600% current to start, a soft starter will take anywhere from 200 - 500% current (depending on load), and a VFD can usually start a load at 100% of motor FLA, rarely over 150%. Remember, only the VFD is altering frequency and voltage simultaneously so you can achieve full torque throughout the starting process AND take as long as you like. All other fixed frequency starting methods run up against thermal limits of the motor, so you must decrease the starting time by increasing current / torque before your motor overheats.

I was referring to breakers. You still need to use a breaker before the soft-starter or VFD. If you use direct-on-line starting, you can install a soft-starter/VFD between the breaker and motor without changing the breaker. As for the current settings on the drives, you should follow whatever the manual says.

yes, varying the speed is obtained by varying the operating curve (and as a result changing the the operating torque) and torque has a direct relation with the power consumption.

finally , as the speed setting decrease the torque decrease and as a result power consumption decrease and vice versa.

Islam

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