# vfd



## MechanicalDVR (Dec 29, 2007)

I've never worked on a crusher set up but I'd have to agree on the air quality issue around a vfd. There are a couple guy shere normally that work on crushers. You'll get your answer when they pop in.


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## John Valdes (May 17, 2007)

VFD manufacturers design and build special enclosures. You can buy dust resistant drives with a NEMA 12 rating for example.
Take a look in a motor and drive catalog and see. 
Weg makes crusher duty motors and drives to go with them. So do others.
The aggregate business requires robust equipment. Most manufacturers even have crusher duty lines of equipment.

In most circumstances the drive is put in a NEMA rated enclosure of choice. This allows a chassis mount drive in a NEMA 12 enclosure as an example.

Talk to your motor shop or find a distributor in your area for help with sizing and installation requirements.


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## JRaef (Mar 23, 2009)

garfield said:


> Is the vfd oversized for a crusher or just equal to fla plus service factor? What type of enclosure is it in? Is there filtered ventilation? What I see is a semi trailer for the generator and electrical set right by the crusher so lots of dust. I assume the vfd would not like the dust.


What they said.

Plus, usually people buy VFDs sized just for the motor nameplate FLC, not the FLC + SF. That's because if you talk to the motor mfrs, they will tell you that a motor that is rated for 115% SF is _*de-rated to 1.0SF if run from a VFD*_. So no point in over sizing the VFD for something you can't actually use.


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## John Valdes (May 17, 2007)

Rob (micromind) works with aggregate companies and may have something to add to this conversation.
He always has very good insight, and real world knowledge on this subject.


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## micromind (Aug 11, 2007)

VFDs are actually rated in amps, the HP rating is there because most people would not be able to size them properly with just an amp rating. 

They also usually have a normal duty HP rating and a heavy duty one. Normal duty would be something like a pump or fan that is never overloaded. A rock crusher would be heavy duty. If the heavy duty amp rating of the VFD is higher than the full-load amps of the motor, you'll be ok......usually. 

As noted, service factor doesn't apply to motors fed by VFDs, it's always 1.0 The service factor amps doesn't mean anything with a VFD.

Replacing a mechanical starter (or a soft-start) with a VFD is a bit of a trick though. If the motor is subject to brief overloads, depending on the severity of the overload, the next larger size VFD might be needed. The reason being that the utility effectively has unlimited current, it can supply locked rotor current if needed. All VFDs have a limit on the current they can supply. It's nowhere near breakdown torque or locked rotor current.

This means that if, lets say, a 100 HP motor operating on utility power is required to produce 200HP for a few seconds, it will. The same motor operating on a VFD most likely cannot and it'll slow down. 

Here's an example; A Balder 100HP TEFC 4 pole (1800 RPM) motor has a full-load current of 112 amps. A typical 100HP VFD has a maximum continuous current of around 125 amps. No problem so far......The breakdown torque of this motor is 250%. This means that it can produce 250HP right before the RPM suddenly drops. But in doing so, it now draws 382 amps. 

The utility can easily produce 382 amps, a 125 amp VFD can't. Most VFDs have an overload capacity of 150 - 200%. Depending on how the rock is fed, a crusher can easily overload a motor though an impactor is not as severe as some of the other types. 

Odd as it may sound, most VFDs are ok with dust. I suspect because there are fans blowing air across the heatsinks, dust doesn't build up much. 

Also, insure that the generator is connected 277/480 and the center of the Y is solidly grounded. Some are ∆ connected, a lot of these are ungrounded or grounded B. If it's anything other than 277/480, you'll need to cut the MOV leads (if the manufacturer allows) or they'll blow up.


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## JRaef (Mar 23, 2009)

micromind said:


> ...No problem so far......The breakdown torque of this motor is 250%. This means that it can produce 250HP right before the RPM suddenly drops. But in doing so, it now draws 382 amps.
> 
> The utility can easily produce 382 amps, a 125 amp VFD can't. *Most VFDs have an overload capacity of 150 - 200%.* ...


Just a slight correction: 
Everything you said is correct, up until the last part. Most Heavy Duty (Constant Torque) rated VFDs will be rated to deliver 150% current for 60 seconds, but also can deliver 200-220% current for about 3 seconds, basically the same amount of time the motor can take it without getting into the thermal damage curve. So with a good Vector Drive, you _CAN _produce Break Down Torque (BDT), *on Design B motors*. 

But if you have a "Crusher Duty" motors, the BDT is often higher, so as you said, the current it will draw will be higher as well. The 250% BDT / current level you mentioned is what you would expect on a Design A motor and many, if not most of the "crusher duty" motors are Design A, Baldor being a typical case. There is however no "official" definition of what "crusher duty" means, so let the buyer beware. 

So if you need to get the extra BDT and current for a Design A motor, you do sometimes need to over size the VFD, usually by one additional size. So on a 100HP Jaw crusher where someone needed a VFD (rare), I give them a 125HP VFD. It will deliver 156A x 2.2 = 343A for BDT. But that said, BDT is what is used mostly to RE-accelerate a motor after a step change in load, but occasionally to accelerate quickly from a stop with the most possible torque. So on an impact crusher, which works on speed, you rarely need that kind of extra power. On a primary (jaw) or gyratory (cone) crusher however, it's more important.









This chart is the official definition of these torque-speed curves from NEMA. Current, once the motor is already running, will follow these curves for the most part.


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