soft starter and VFD on a hoist 4

[FuzzyQ]

Hi,

A question regarding the soft starter and VFD on a intake gate hoist. Hope to get your help.

The hoist is of 3hp and 25hp at low and high speed respectively. When the hoist starts directly at high speed, there could be voltage drops. so I'm seeking the solution of a soft starter.On the other hand, somebody provided VFD control for the two speed motor. My understanding is that VFD control is actually able to reduce the start inrush current.It works as a reduced voltage starter but provides other functions, which makes it more expensive than a soft starter, right?

The upsteam transformer is 3 single phase transformer 25kva each. Is the transformer capacity one issue that I need to consider for the motor start application?

Plus, what kind of the load the hoist is? constant torque or high torque or others? Those will be a thing of picking up the start method, right?

Thank you very much for your reply. I appreciate that.

 

[jraef]

Many questions here. In order of your asking:

VFD control is the best method for keeping starting current as low as possible. A VFD changes the voltage and frequency at the same time, so the motor is able to put out full torque at all times without the need to raise the current above FLA. A soft starter only controls the voltage, the frequency remains the same. So when a soft starter is used, current is always going to be higher than FLA on start-up, in fact typically not less than 200% FLA, usually 350-450% for something like a hoist.

A VFD for a 25HP motor is going to cost more than a soft starter, probably in the range of 4 to 5 times more. Keep in mind that if you motor down, as many hoists do (see below), the soft starter will also need a reversing contactor whereas the VFD can reverse the motor all by itself. However, a hoist is not a good application for a soft starter anyway. When lowering the load with a hoist, typically the motor is run in reverse and the motor's regeneration counter torque is used to prevent free-fall. With a hoist, you must consider the failure mode of the controlling device. If a soft starter loses control of an SCR during lowering operation, you lose the motor's ability to regenerate and you can drop the load. Maybe on an intake gate hoist this isn't a problem, only you can answer that, but generally for this reason soft starters are not used on hoist controls. VFDs however don't have that problem. If a transistor fails during lowering, there is still regeneration torque available to the DC bus through the others and therefore the dynamic braking system (which you will typically need by the way).

A hoist is a constant torque application, that is why your existing 2 speed motor is rated for 2 separate HP ratings based upon speed. Torque remains the same, so if speed is reduced, then HP is reduced. That is essentially how the VFD is going to work as well. Usually a hoist is geared so low that it is not considered a "high torque" load at the motor shaft.

If you are concerned about your hoist working with limited transformer capacity (although I would not think that yours is), I would recommend using the VFD.

JRaef.com
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[LionelHutz]

I really wouldn't recommend using a soft-starter on a hoist unless the hoist is geared so that it can't back drive the motor. This would typically be a worm-gear type of drive.

A 2-speed reversing soft-starter would have a lot of contactors in it. You could likely use the VFD with the motor permanently connected in the high speed without any contactors. Simpler to control and maintain.

 

[cbarn24050]

Sorry jref but your completely wrong with regard to the safety issue on vfds. If a transistor fails the drive will shut down all the others effectively disconnecting the motor any braking torque will dissapear very quickly. A softstarter on the other hand may or may not provide some braking torque on thyristor failure. Any real hoist will have a mechanical backup system so its not really an issue anyway.

 

[LionelHutz]

VFD's tyically go BANG and then they're dead taking out the line side protection at the same time.

A soft-starter may short SCR's and continue to operate the motor or may have a failure to ground which takes out the line protection. If the SCR's fail then the motor won't stop when you give the soft-starter a stop command.

 

[Skogsgurra]

cbarn,

I wouldn't be that harsh. Of course the VFD shuts off when a transistor fails. But that also means that the main contactor drops out and that the brake control activates the brake. So a VFD is a very good choice - compared to a soft starter.

Gunnar Englund

http://www.gke.org

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

 

[FuzzyQ]

Thanks all for your reply. Those clarified the issue pretty well.

Seems like a VFD control is a effective but expensive solution. Suppose I don't use either VFD or soft starter, what would be the inrush current at the time I switch from low speed to high speed? Would that be the same as I start directly from high speed?And what would be the disadvantages of using contactors, as pointed out by LionelHutz?

Could you give a bit more explainations regarding the torque, current, frequency and starting time? Thanks,

 

[cbarn24050]

Hi skoggs, I never said a soft starter was a better choice, it isn't, I was just pointing out a big flaw in his thinking.