VFD connection 4

[electrolitic]

To use a VFD just for starting and braking a 690V, 300kW motor that drives a high inertia load.After reached a proper speed at starting, motor is connected directly to the line short circuiting the VFD. Question: Is it necessary a device that disconnects VFD-Motor connection during direct line operation stage? Thanks.

 

[Skogsgurra]

Yes, you will destroy the VFD if you connect mains to the output directly.

If it were not for the braking, a soft-starter would have been perfect. You are aware that the braking probably needs regenerating? Or a big resistor.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[electrolitic]

Thanks Gunnar
It is a saw drive. Depending upon the stopping time requirements I will use braking by internal losses of inverter or a brake resistor.
So, one CB (draw-out type)for protection and maintenance disconnection, one short-circuit contactor and another contactor for inverter disconnection. Open transition always.
Motor with encoder for speed detection and "catching on flight" for braking.
Is the conception correct?
Thank you again.

 

[LionelHutz]

Yes, the idea is correct. But, it sounds very complicated to me. I would use a soft-starter with DC injection braking myself. Unless, the inertia is too high to be able to start the motor with a soft-starter.

With a high inertia you will need a braking resistor for the VFD or else it will take a long time to stop. Typically, without a braking resistor you really can't stop a lot quicker than if the motor and load were coasting.

 

[electrolitic]

Thanks Lionel
The rotary saw inertia does not permit the use of soft-start device. Using soft-starter I would apply a DC converter, current controlled, to inject DC current at stator. Motor is IC 041. IC 061 would be better in case of DC injection (heat dissipation).
Normal starting/stopping is always commanded with the blade out of material to be cut.So, the load is motor, gear and blade. This is the reason why I would like to try the braking without resistor, a dec ramp causing no overvoltage at DC bus and taking a reasonable time(greater than starting time but better than idle stopping)
Please, correct me where I am wrong.
Gunnar answered that VFD would be destroyed if connected its output directly to the mains. Unfortunately he did not say why. I would like to know.
Thanks again

 

[itsmoked]

It would be destroyed because the inverter's output is not synchronized with the mains. The output power elements would hence be feeding the mains out of phase and would catastrophically fail.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

I don't understand why you can't use a Soft Starter utilizing its built in soft stopping function? Soft starters have automatic bypass circuitry built in and handle the contactor control etc, etc.

If you use a VFD why do you feel you need to bypass it?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[electrolitic]

Itsmoked
a)The bypass of VFD: As it works just to accelerate/decelerate the machine and not to cut the material I think it can be smaller in size/power.
b)Inverter output not synchronized: The idea is to provide an open transition.Inverter to be turned off before mains be directly connected. The question remains: Even in this case it would fail?
Thank you

 

[Skogsgurra]

Yes. There are free-wheeling diodes parallel to the switches. You have to disconnect the inverter output.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

A VFD without a braking resistor will provide very little braking capability relative to your high inertia load. I seriously doubt you will notice the difference compared to coasting to a stop. If someone told you otherwise, they are mistaken. The kinetic energy in the load must be converted into something, it does not magically disappear.

So you have three real choices: dissipate the energy in the load (i.e. friction brakes), dissipate it in the motor, i.e. DC injection braking, or dissipate it outside, i.e. dynamic braking using the drive. If you already need the VFD for some other reason, dynamic braking through external resistors is the most effective at a reasonable price. Regenerative braking is the most effective, but is going to be very expensive for a load that large. If you don't need the VFD to change speed, either is a very expensive solution.

The most reasonable cost without the VFD is a soft starter and a DC injection brake. Yes, the kinetic energy is absorbed by the motor, but saws are generally not high duty cycle applications. DC injection braking is no more stressful than having a second start cycle, so when looking at the duty cycle of the motor, count each baking cycle as if it were another start cycle. For example, if the motor is rated for 6 warm starts per hour, that equates to 3 starts and 3 brakes. Just use a soft starter that has a programmable start-per-hour protection feature and move on with life. I have yet to meet a saw load that cannot be started with a soft starter by the way. I have applied hundreds if not a thousand or more soft starters with DC injection brakes on band saws, chipper saws, circular saws, edgers, you name it. I have never had a problem with any of them.

 

[electrolitic]

Jraef
Thanks. Very good answer!
I will check carefully the inertia moments involved, friction losses, acc/dec times (typically 30/60sec), modern soft starters capabilities.
So I will compare the cost of soft starter + current controlled DC converter + additional switches (CB converter AC side, DC contactor) versus VFD + braking module and braking resistor.
Thank you. I am also thanking Skogsgurra, LionelHut, Itsmoked.

 

[LionelHutz]

What kind of saw? The only kind I have ever seen that can be problematic are band saws due to the inertia in the wheels.

Look for a soft-starter which has the DC braking control built-in. You have to add a "free-wheeling" SCR or diode module to the soft-starter to get braking. No contactors, DC converters, circuit breakers or anything else like that required. A free-wheeling SCR or diode is connected between 2 of the motor leads and keeps a constant DC current flowing in the motor.

Do not fall for any manufacturer that will provide a soft-starter with a DC brake which uses a contactor for braking or nothing else but the six phase control SCR's for braking. They really just don't work for anything more than an unloaded motor (and even poorly in that case).

Benshaw builds them and I believe there are a couple of other manufacturers that will also do braking with a free-wheeling diode or SCR but I really don't know who since who does what keeps changing. Motortronics might build one too.

Keith - the soft-stop function in most soft-starters refers to it ramping down the voltage. This can only be used to increase the deceleration time of the motor.

 

[jraef]

Motortronics does build a full wave DCIB unit, but it is not part of their soft starter, it's a separate unit. I'm not sure if they have fixed their problems with it however, it had a tendency to be subject to catastrophic failures in applications where there were spikes from other nearby equipment such as large Y-Delta or open transition autotransformer starters. But if you are not in the US (and with a 690V motor I doubt you are) I would not bother looking at their's or Benshaw anyway, your support will be difficult to attain. There are many other soft starter manufacturers with worldwide support who offer braking options. Find one with good support to your location and who has a good Applications Engineer with experience in braking applications.

 

[LionelHutz]

Jraef, yes a manufacturer with a strong local presence is the best but the proper type of injection braking is very important. If electrolitic's good local manufacturer only sells soft-starters with 6-scr or contactor braking then he should pass on the braking option. If they don't have a good purpose built DC injection brake then that option is out too. Buying something like a DC drive to get the DC braking will likely be cost prohibitive. I would recommend a manufacturer with more worldwide availability but I just don't know who else builds suitable DC braking anymore.

 

[AusFeMan]

electrolitic,

Any decent VFD/VSD should be cpable of handling all you accel/decel issues, I use predominantly AB (Allen Bradley) Powerflex drives (here in Au), and yes they are not always cheap, but take the guess work out of speed, and brake control issues. in terms of bringing the load to a stop most times regen braking has worked fine for me, and dependant on stop time and inertia of load DC injection braking, or braking resistors work best. Be careful with both as i recently cooked a resistor set (that was underated for the load), and too much DC inj heats up the rotor & stator a lot.
By the way 690V supply???, is this fed from mains? here in Aus we have 415Vrms 3 phase supply @ 50Hz. R U using a step up TRansformer?

Hey is anyone familiar with Motor over-flux breaking?

 

[Marke]

Hello electrolitic

I have done this many times using a soft starter, using the DC braking on a soft starter, or using the soft starter to plug brake the saw.
Plug braking will give you better performance near full speed and DC will give you better performance near zero speed.

The major issue with DC and/or plug braking, is that just like the start, you are dissipating the full speed kinetic energy of the load in the rotor of the motor, and the supply current is much higher than it would be with a VFD.

DC braking with a soft starter is half wave and produces more supply harmonics than full wave DC braking and is not as effective close to full speed.

Some soft starters can provide the control for the braking so that you use reversing contactors, plug brake at high speed and then switch to DC at low speed. - You get the best of both worlds.

Best regards,

Mark Empson
L M Photonics Ltd

 

[ozmosis]

electrolitic
is the braking requirement a safety requirement? If it is, then the methodology will need to be in accordance with safety standards locally to you.

 

[waross]

If the application isn't required by safety regulations, Taking another two or three cuts after the motor is turned off often works well to stop the saw.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Just a reminder, this thread was 6 months old...

 

[ozmosis]

well spotted Jeff. Didn't even look myself! I guess the saw has stopped by now.