VFD and Start/stop controls

[alexmin]

I've upgraded my PM1236 lathe to VFD as I've already mentioned.
My VFD/motor connected directly to 220V. I am using VFD panel to control the lathe. The big electric box mounted behind the headstock is completely unused now.


PM1236 has very useful forward/off/reverse lever that I want to get working again with VFD setup.

It would be also nice to get the lathe's safety controls working again.
("mushroom" emergency stop, off switch on the foot break etc)


Any advise is appreciated.
My VFD: KBAC-27D

 

[unterhausen]

surprised that vfd doesn't have a reverse switch, not that I ever use mine. Looks like you just need momentary switches if you don't already have them. There is a jog switch, which would be nice, don't have that hooked up on mine yet.

 

[65535]

You should be able to wire up the existing switch to the VFD as a logic control. You don't want to switch the 3 PH out from the VFD.

Barry may chime in.

 

[alexmin]

surprised that vfd doesn't have a reverse switch, not that I ever use mine. Looks like you just need momentary switches if you don't already have them. There is a jog switch, which would be nice, don't have that hooked up on mine yet.

It does have a forward/reverse switch. What I want is to hook up the forward/reverse lever on my lathe to my VFD to as a "logic control"

 

[precisionworks]

I take the path of least resistance ... operate the drive from the touch panel buttons

Remote operation isn't hard to do, but does require RTFM (once for Will Quilles, multiple times for me ... and I've read dozens of docs from at least that many manufacturers, so practice is not the answer).

Some points to consider before you decide to do this ... The front panel touch buttons will no longer work - every drive I've looked at will operate from only one command source, be that local or remote. That means speed up/speed down will have to be controlled by a remote speed pot.

If I were attempting this, I'd call tech support and ask how to proceed. I phone them often, so they are used to talking with people who don't know what they're doing

Tech support, from every manufacturer I spoken with, is superb. I'm unfamiliar with KB Drives & couldn't offer any meaningful advice.

 

[alexmin]

Barry,

I'll RTFM. :mecry:

I am pretty much ready to bit a bullet and get a 10HP VFD (part number ESV752N02TXD) from Wolf Automation.
Input: 120-240V 3 Phase
NEMA-4 enclosure

I understand that I also will need dynamic braking module. 10 HP breaking module costs $242 Is there any cheaper alternatives?

Thanks guys for all your input.

 

[wquiles]

Alex,

What you want to do is exactly what I did with my knee mill in which I wired an ON/OFF switch, variable pot for speed, a FWD switch, REW switch, and an Emergency-Stop switch. This "fancy" diagram is not pretty, but it shows the idea. The big "box" in the middle is my FWD/Stop/REW switch. Then it is in the OFF position, neither of the two contacts is closed, so the VFD does nothing. When I turn the lever one way or the other, the specific contact is "closed" and the VFD then proceeds to turn the motor in the appropriate direction:

To make this work, the VFD should have various "logic" level inputs and one for a variable resistor (for speed control). The logic levels should allow at least 3 different conditions:

- stop
- running forward
- running backwards

and if you have the pot, when running forward or backwards it should run at a percentage from full speed.

In the AC Tech Vector Drives that Barry and I like so much, the RUN/Enable switch is normally open, so to connect an E-switch, you simply connect them in series -> this means that both of them have to be closed before the machine will run. It also means that if either one opens, the machine also stops. If your lathe is like mine where the side gear cover has an enable switch, and yet another one for the chuck guard, you simply keep wiring them all in series. ALL of them have to be closed/enabled before the machine will run - very straight forward.

The lathe already has two wires that provide a "connection" when doing forward/reverse on the factory level (on the carriage), so all you need to do is wire those to the VFD as logic signals (no power of any kind). All that you are doing is either opening or closing a contact at the VFD pannel, which then lets the VFD know to run in forward or reverse.

What makes this more "interesting" is that these signals can be normally open, normally closed, or a combination. This is why neither Barry nor I can give you specific advice since the specifics are unique to each VFD - this is why you need to read the f&%^#*& manual - to find out what/how your particular VFD is expecting to see.

Lastly, technically speaking you do not "need" to buy a dynamic brake module - the system will eventually stop on its own. However, if you vary the speed too much too quickly, you can cause a fault in the VFD drive and the system basically shutdown to protect itself. If you do have the dynamic breaking module, then the VFD sents out that back-EMF energy to the external module (basically big, power resistors!), which allows to stop/change directions, and change speeds almost instantly.

Will

 

[unterhausen]

It does have a forward/reverse switch. What I want is to hook up the forward/reverse lever on my lathe to my VFD to as a "logic control"

I skimmed the entire manual and didn't see any mention of reverse other than the picture of the front panel. My skimming skill stinks though.

 

[65535]

What you have is a pretty basic and simple drive.

Which may NOT have a reverse installed. In which case you'll have to shut down the drive to reverse the motor every time you want to. (You probably could get away with leave the drive on and use the switch on the motor power wires. Need a DPDT switch to switch two of the motor wires.)

You'd be better off getting a new VFD with logic inputs that can use the FWD/STP/RVS switch.

 

[alexmin]

What you have is a pretty basic and simple drive.

Which may NOT have a reverse installed. In which case you'll have to shut down the drive to reverse the motor every time you want to.

I am glad you told me. Otherwise I would keep using the forward/reverse switch :nana: especially on my KMG belt grinder. (My VFD is shared between multiple machines)

On a more serious note it is odd that the manual does not cover forward/reverse switch or variable pot for speed control that came with my unit.
BTW I have also installed a remote on/off foot pedal and highly recommend it to everyone. Foot pedal is very useful on a drill press or with belt/disc grinder when both hands are holding a part.

 

[precisionworks]

I also will need dynamic braking module.

Most likely, you'll new a few modules, depending on the inertial load being stopped & how fast you need it to stop :nana:

This is a typical situation for my lathe ... the 70# Set-Tru chuck is spinning 2000 rpm while working on an aluminum part, the 60# rotor (inside the motor) is spinning 1750 rpm, and the gears inside the head stock are spinning at who knows how many rpm ... best guess is that a single DBM will not stop the chuck as fast as you want. Two might do it, three probably would give the result you want. If you need an instant stop with a large inertial load, get out your checkbook & be prepared to write in a big number

any cheaper alternatives?

None that can be recommended. It is possible to make up a bank of large power resistors, but they aren't cheap:

Not only that, but the calculation is interesting:

Sizing the Dynamic Brake Module

Gather the following information:



1. The nameplate power rating of the motor in watts, kilowatts, or horsepower.



2. The nameplate speed rating of the motor in rpm.


3. The motor inertia and load inertia in lb-ft2.


4. The gear ratio, if a gear is present between the motor and load, GR.



5. Review the Speed, Torque Power profile of the application.



Equations used for calculating Dynamic Braking values will use the following variables.


ω(t) = The motor shaft speed in Radians/second, or
N(t) = The motor shaft speed in Revolutions Per Minute, or RPM
T(t) = The motor shaft torque in Newton-meters, 1.01 lbft - 1.355818Nm
P(t) = The motor shaft power in Watts, 1.0HP = 746 Watts
-Pb = The motor shaft peak regenerative power in Watts

Step 1 – Determine the Total Inertia

JT = Jm + GR2 X JL
JT = Total interia reflected to the motor shaft, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2
Jm = Motor inertia, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2
GR = The gear ratio for any gear between motor and load, dimentionless
JL = Load inertia, kilogram-meters2, kg-m2, or pound-feet2, lb-ft2 -- 1 lb-ft2 = 0.04214011 kg-m2


Step 2 – Calculate the Peak Braking Power


JT = Total inertia reflected to the motor shaft, kg-m2
ω = rated angular rotational speed,
N = Rated motor speed, RPM
t3 - t2 = total time of deceleration from rated speed to 0 speed, in seconds
Pb = peak braking power, watts ( 1.0 HP = 746 Watts)

Compare the peak braking power to that of the rated motor power, if the peak braking power is greater that 1.5 times that of the motor, the deceleration time, (t3-t2), needs to be increased so that the drive does not go into current limit. Use 1.5 times because the drive can handle 150% current maximum for 3 seconds.

Peak power can be reduced by the losses of the motor and inverter.


Step 3 – Calculating the Maximum Dynamic Brake Resistance Value


Vd = The value of DC Bus voltage that the chopper module regulates at and will equal 375Vdc, 750Vdc, or 937.5Vdc
Pb = The peak braking power calculated in step 2
Rdb1 = The maximum allowable value for the dynamic brake resistor



The choice of the Dynamic Brake resistance value should be less than the value calculated in step 3. If the value is greater than the calculated value, the drive can trip on DC Bus overvoltage. Remember to account for resistor tolerances.

(I didn't make that up, it was copied & pasted from one of the many websites discussing DBM)

For most users, it make sense to buy what the manufacturer suggests. Try one DBM & see if the results are "good enough" ... if not, add a second DBM & see if that does what you want. At some point you'll get the machine to stop in the time frame desired.

CAUTION: People have tried various means of cheaply replacing the DBM or the power resistors ... when they fail, the drive is destroyed. Unless you really spend the time to understand what is needed, it's cheaper to buy the factory DBM than it is to replace the drive.
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