VFD & Energy Saving-1: Cost of a Soft Start

I think you are confusing the efficiency of the VFD with that of the mechanical drive components, and unnecessarily adding the latter into the cooling requirements.

The VFD should be around 97% efficient (varies by mfr). If you keep the existing starter as a bypass contactor when the conveyor comes up to speed, you can avoid almost all of this 3% loss, too.

For this application, be sure that the VFD has constant torque capability (conveyors need full or nearly full torque to start.)

Idea of a bypass contactor is a good idea.

Regarding VFD efficiency as

"The VFD should be around 97% efficient (varies by mfr)" I put some of my observations

Example -1.We have 1800 KW Fan 690 V operated. Through transformer 6300 V are converted to 690 V. This fan when I see (on the display of the drive) shows 1270 KW. When I see this Power from the MV panel (Power going out of the transformer) is 1450 KW. so where this power goes i-e 1450 - 1270 = 180 KW.

It means drives take 1450 KW of input power and inverters are going to out 1270 KW. Another explanation is to that whenever Air Conditioning units stop for a an hour there is significant rise in Temperature of Substation.

Example -2:

Same case as above a 2000 KW 690V operated motor. Drive display shows 1340 KW and Power going out of MV Panel (Siprotec Relay 7SJ series) is 1510 KW.

Example -3:

A 550 KW fan is driven by a VFD. ACS800 Panel shows 180 KW and on MV panel (Siprotec Relay 7SJ series) Power was 203 KW.

Example-4:

A 26KW, 400V, 50 Hz, Y, 48.7A, P.F 0.84, 1460RPM is driven through a VFD. Drive shows 22 % of Power (5.72 KW). When we measure the Power before VFD, current on three phases was 14.2, 14.4, 14.5. Voltage was 230 V (with respect to ground). Now

Input Power to VFD = 3 * 230 *14.4 *0.8 = 7.94 KW

Efficiency = 5.72/7.94 = 71 %

Although P.F is high but I am taking it 0.8

If you are concerned about throughput efficiency and you are not planning to vary the speed, do not use a VFD as a soft starter. Use a soft starter, they are different. A VFD will have losses even at full speed, a soft starter will not and it will cost roughly 20% of the price of a similar VFD (percentage varies with size, but this applies to the size you stated).

If you can vary the speed, and it is a centrifugal load such as a pump or fan, the savings in throughput energy reduction more than makes up for the small losses in the VFD. If it is not a centrifugal load, you will not save energy but you may consider varying the speed for other reasons, some of which may end up with net efficiencies in other areas. But only a careful evaluation of all aspects can determine that.

"A VFD will have losses even at full speed, a soft starter will not and it will cost roughly 20% of the price of a similar VFD (percentage varies with size, but this applies to the size you stated)."

As you are quite right there. Application is not a variable torque Load i-e Fan or Pump etc. Application is a conveyor Belt. It is approximately 420 m long and it is fully covered.

If we install a Soft Starter and Power Fails (due to some reason) and Belt at the time of failure is full of material. When the Power comes back, We have't an empty belt. At that stage could Soft Starter Provide enough torque to run the Belt.

In our neighbouring Factory, they put Soft Starter on Bucket Elevators (Constant Torque Load). The bucket Elevators took material high up to 105 m. When ever Power fails same sort of situation occur. After each half hour they gave command to the motor, Five to six Buckets get empty and tripping occur. To recover the system to the normal state it took about 6 to 9 Hours.

_{As you are quite right there. Application is not a variable torque Load i-e Fan or Pump etc. Application is a conveyor Belt. It is approximately 420 m long and it is fully covered.}

_{If we install a Soft Starter and Power Fails (due to some reason) and Belt at the time of failure is full of material. When the Power comes back, We have't an empty belt. At that stage could Soft Starter Provide enough torque to run the Belt.}

_{In our neighbouring Factory, they put Soft Starter on Bucket Elevators (Constant Torque Load). The bucket Elevators took material high up to 105 m. When ever Power fails same sort of situation occur. After each half hour they gave command to the motor, Five to six Buckets get empty and tripping occur. To recover the system to the normal state it took about 6 to 9 Hours.}

You have several issues here.

Do you NEED to vary the speed? If so, you need a VFD, not a soft starter. The other issues become less relevant.

The scenario you describe at your other plant shows a lack of forethought. A good quality soft starter should offer the ability to increase the starting torque in the form of a 2nd ramp profile that you can select as a "loaded restart" scenario. So for example if you normally use 350% current limit and a 20 second ramp to start unloaded, if you have a shutdown, an operator selects Ramp 2 and the soft starter provides 450% or 500% or 550% current for 3 seconds or 5 seconds or 1 second, whatever it takes to accelerate the loaded conveyor. Either that, or use a bypass DOL starter for that scenario.

A VFD is NOT necessarily going to solve that problem. The difference in accelerating a load with a VFD vs a Soft Starter is, you can trade a longer acceleration time with a VFD without overloading the motor. But if you need full Break Down Torque from the motor to accelerate it fully loaded, only a Vector Drive is going to be capable of that, and only for a limited amount of time, and you are STILL going to need high current to accomplish it. The amount of line current will be less however, so that helps if you have a severely limited supply source. That is about the only legitimate reason to use a VFD as a soft starter.

Re: Bypassing a VFD. Be careful. Most PWM drives cannot be simply bypassed by closing a shorting contactor as you do with a Soft Starter. Remember that in a VFD, the line is converted to DC and re-created back into a NEW SOURCE of AC power going to the motor. There is NO relationship to the line phase angle and the load phase angle. So if you connect them out of synchronicity, you create huge voltage spikes that will fry the VFD transistors. it's the same as connecting two generators that are not synchronized. Some VFDs have ways around this, it's called "synchronous transfer" where the VFD monitors the line input sine waves and makes its output sine waves match before allowing the contactor to close. But very few VFDs have that capability and the ones that do are usually very expensive. The only alternative is to have a set of interlocked isolation contactors and you disconnect the VFD output from the motor before closing the bypass starter. But this inherently must be an open transfer scheme, so in the case of a conveyor, it will most likely slow down in the transition and a spike will occur when you reconnect. Something to consider.

'The difference in accelerating a load with a VFD vs a Soft Starter is, you can trade a longer acceleration time with a VFD without overloading the motor. But if you need full Break Down Torque from the motor to accelerate it fully loaded, only a Vector Drive is going to be capable of that, and only for a limited amount of time, and you are STILL going to need high current to accomplish it. The amount of line current will be less however, so that helps if you have a severely limited supply source. That is about the only legitimate reason to use a VFD as a soft starter.'

Really a VFD does not start like a soft starter or else you could simply make the soft start ramp longer? A VFD accesses the other end of the slip curve, the pullout torque, which is why it's so good and an efficient way to start. The soft start accesses stall and pullup torques which are much lower generally, hence the danger of overloading or overheating the motor during prolonged starting. Also, long conveyors take some time to start in the conditions described, the front might start but the tail end is much later! And high stiction loads whilst it's all breaking free.

'The only alternative is to have a set of interlocked isolation contactors and you disconnect the VFD output from the motor before closing the bypass starter. But this inherently must be an open transfer scheme, so in the case of a conveyor, it will most likely slow down in the transition and a spike will occur when you reconnect'

Of course you could overspeed the conveyor a little to take account of the small slowdown so the re-connection of the mains is at the correct rotational speed still on a decent part of the slip curve so that current is normal where the supply meets this speed, couldn't you?

Of course you could overspeed the conveyor a little to take account of the small slowdown so the re-connection of the mains is at the correct rotational speed still on a decent part of the slip curve so that current is normal where the supply meets this speed, couldn't you?

Yes, through trial and error and if the load was EXACTLY the same every time. I have seen it work at commissioning, then 5 days later it no longer works.

Maybe the load compensation (slip) may help to keep the rotational speed more similar in different situations?

If it's a little bit too fast it's OK so long as you don't go above the synchronous speed of the mains frequency.

However, I haven't done this on a conveyor application, most leave the VFD in circuit!

GA This is also my opinion for this type of application, the VFD is far more suitable and sure than a soft start in starting in all my experience in quarries and mines etc.

Not so interested in motors (ops, slip of the toungue :) but putting up VFD definition might simplify....

(Source is an internet downloaded book/handout pages called Motor Control Centers:)

VFDs are also referre to as AC drives. A typical AC drive receives 480 VAC, 3 phase, 60 Hz input power which is used to START and STOP a motor and control the operation of the motor throughout the SPEED RANGE.

Reduced-voltage soft-start motor-starting controls, such as the Furnace Nordic reduced-voltage controllers, provides a SMOOTH START while minimizing the high starting current and TORQUE associated with across-the-line motor starting.

Hoping that would help, if not, then I might have a second input here. Hehehehehe....