Lightning killed my grinder

[HSJackson 13]

Had a fast moving storm a couple days ago and lightning destroyed the transformer on the pole that feeds our house and shop.  Killed our modem and ceiling fan in the house and now my Wuertz TW-90 in the shop doesn't work.  When I hit the start switch it runs with jerky speeds for a few seconds, then stops and the status light turns red.   Not sure if it's the motor or the VFD/control stuff that's fried.    Any electrically smart folks got any ideas what to check?

[Brian Dougherty 1,171]

Most likely the VFD.  The motor is a lot harder to kill than the electronics.

Hard to say if it is repairable or not, but I would guess it's easier to replace the drive than repair it.

[HSJackson 13]

Makes sense.  Guess I'll contact Travis and see what a box costs. Thanks!

[J.Leon_Szesny 18]

Jerky speeds?
does it seem like it "stutters?" or "jumps?"
might be a damaged commutator bar on the armature assembly.

 but I don't really count as "electrically smart folk" XD

[Brian Dougherty 1,171]

A motor that runs with a VFD won't have a commutator

 

[HSJackson 13]

Well I thought a commutator was a judge that reduced a prisoner's sentence, so y'all are way ahead of me on motor knowledge. 

[Ron Benson 91]

A quick test would be to run the motor after disconnecting from the VFD. If it runs as it should, that would back up the answers others have given about the VFD.

[Brian Dougherty 1,171]

Most likely, the motor will be 3-phase, and he won't be able to plug it into the mains in a home shop.

 

I'm not bashing on ideas to be a jerk.  Hopefully it isn't coming off that way.  

 

VFDs are a great thing for those of us stuck with residential mains power.  They allow us to run 3-phase motors and at variable speed to boot.  This wasn't terribly practical before, and if you wanted to run a 3-phase machine you needed to get a rotary or static 3-phase power converter.

 

The catch is that VFDs only work with a specific type of motor called "induction motors".  These are motors like the one on your furnace blower rather than what is on your shop-vac, router or skill saw.  Also, generally speaking, they only work with 3-phase motors, although there may be the odd exception.

 

The speed of this type of motor is not controlled by voltage the way you can control the speed of a "Universal motor" (Think router)  The speed of an induction motor is proportional to the frequency of the voltage. 

 

Think of it this way:  The AC current flowing through the motor creates a magnetic field that is also fluctuating.  The motor is wound so that as the field is decreasing in one location (pole), it is increasing in the pole next door.  When the new field starts to decrease, the next pole in line starts to increase.  This happens over and over again until the field comes back around to the staring point.   

 

This up and down of the magnetic field happens at the same frequency as the supply voltage.  All the rotor (armature) is doing is following the high point of the field from pole to pole.

 

The problem with a single phase motor is that all of the poles are increasing and decreasing at the same time because they are all energized by the same phase.  To get around that issue, they use a capacitor (or sometimes a start winding) to create lag in the voltage to some of the windings so that the motor thinks there is more than one phase.  However, this is not generally going to be compatible with a VFD.

 

[Ron Benson 91]

Thanx Brian - I am here to learn, so I take your comments as a source of knowledge. I hope that doesn't make you feel pressured... 

[Jonathanbradshaw 9]

After that description it makes me wonder if a capacitor start motor would run off a VFD if the capacitor were removed. 

[Alan Longmire 2,741]

48 minutes ago, Jonathanbradshaw said:

After that description it makes me wonder if a capacitor start motor would run off a VFD if the capacitor were removed. 

 

Nope.  VFD requires a 3-phase induction motor.  Trying it on a single-phase with the start capacitor removed would give you one third of the desired impulse.  If you have a VFD now, try this:  Get it running, and turn the speed control down to 5% or so.  Notice how the motor is now "stepping?" As in, it goes click every time the output from the VFD kicks in and energizes one leg of the three in the motor, rotating the armature 1/3 of a rotation?  IF you tried it with a single phase motor you lose two of those legs, and the motor can't move fast enough to stay running.  

 

Single-phase brush-type motors can be speed controlled by variable resistance, like a router speed control, a variac, or even a dimmer switch for low voltage stuff.  That's how you can use a vacuum cleaner blower on a router speed control for a forge blower.  Unfortunately, the 1hp and larger motors we use on grinders are brushless and need the full current.  

 

Brian can explain what I got wrong about this later, but that's the short and almost sort of right version. 

[Brian Dougherty 1,171]

59 minutes ago, Jonathanbradshaw said:

After that description it makes me wonder if a capacitor start motor would run off a VFD if the capacitor were removed. 

Nope.  A 3-phase motor has 3 sets of windings that are energized by each of the 3 phases.  Those phases are physically separated by a few degrees around the motor which is what allows you to have magnetic poles increasing in strength as the neighboring ones are decreasing.  With a single phase winding, all of the poles would be increasing and decreasing at the same time.

 

Edit: Alan was replying as I typed this.  His reply is pretty good, even if it does have a bit of an archeologist flavor to it.

[Alan Longmire 2,741]

13 minutes ago, Brian Dougherty said:

His reply is pretty good, even if it does have a bit of an archeologist flavor to it.

 

I was in physics before I was in archaeology, remember. 

[Brian Dougherty 1,171]

That's ok, I wanted to be an archeologist before I became an engineer!

[Jonathanbradshaw 9]

Thanks to both of you for the explanation. I’m a pharmacist, not an engineer. So my education in electricity is basic physics more than20 years ago. And I just can’t find the time to read about everything I want to know more about. 

 

Alan I do not have a VFD to experiment with. I’m currently trying to decide if I should retrofit my Bader B2 with a VFD and new motor, or buy a whole new grinder. I would really like to be able to slow the belt speed down a bit sometimes. 

[Alan Longmire 2,741]

I know the feeling.  My KMG has three-speed step pulleys, which are fine as far as they go, but my new disk grinder has a VFD which is way more fun.  I just keep telling myself that all the first generation of custom knifemakers ran single-speed grinders and got by just fine, so I don't need a VFD on the KMG.  But boy, wouldn't it be nice to have...

And no, the VFD for the disk grinder is too small for the motor size I'd want on the belt grinder.

[Zeb Camper 780]

The VFD on mine has me spoiled rotten for sure. 

[HSJackson 13]

Just an update, I ordered a replacement VFD from Amazon and it works great now.   At first it would only run about half the speed it used to, but then I actually read the info sheet that came with it and saw I needed to move a jumper on the circuit board from 1X to 2X position.  Guess there is no problem with the motor. 

Wonder there's a company or somebody that would want the old VFD to try to fix it?  There's a whole bunch of little integrated circuit chips on that thing so probably be a big task to run down the problem. 

Edited March 24, 2020 by HSJackson

[Wayne Coe 56]

Always unplug all digital equipment when not in use.  I unplug the grinders (all have VFDs+), my Miller Mig welder and my pyrometer.  As sure as you don't lightning will strike.