# VFD not enough torque?



## Yellow World (Oct 11, 2017)

Hey everyone,

Ran into a bunch of issues recently. The output from some of our machines had quality issues, so the management decided to let them run slower. I've installed VFD (Delta VFD-M) to all of them. Mostly, the machines are equipped with motors with a kind of disc brake attached to them, which are operated electrically by a magnet. The motors drive a steel belt, which then pulls the material through different kind of grinders and brushes.

First issue: 
These brakes need 90VDC, they have a rectifier which needs 220V single phase on the primary side. Normally, the rectifier is just wired to two phases in the terminal box of the motor. If now the motor is driven by a VFD, the brake won't open anymore, even if the VFD's output is set at 220V/60Hz, which is, what the brake would normally get. Seems like the recitfier can't handle the rectangular output from the VFD? 

This issue is actually easy to fix by putting in a separate relay and wire the brake directly. But I'd still like to know why the brake won't open on VFD output.

Second issue:

We have some motors type DEMAG KBA90B4, which have a mechanically operated brake attached. Once the motor gets voltage, the shaft is kind of moved forward and the brake releases. If the motor is operated without VFD directly via contactor, it works perfectly well. Once a VFD is getting involved, the brake won't release. Tried to play with the VFD's settings, set the ramp-up-time to minimum, increase the maximum output voltage a little, nothing worked reliably. The VFD would throw out an error about every 10th to 20th time it's started. Tried different wiring, too. Wired the VFD directly to the motor or with an contactor attached after it. The problem won't get better. Any thoughts on this one?

Third issue:
After installing the VFD on the first kind of machine (those with the electrically controlled brakes), it feels like the motors got too weak to get the steel belt running afterwards. After a few seconds they'll go into overload-error. This problem also didn't appear all the time, but two to five times a week, but it's annoying. Been playing with the same settings as I've mentioned above, but without much luck. Been thinking about putting bigger VFD, like a 3HP VFD for a 2HP motor and install an external thermal overload on the output side to protect the motor. 

Now management has already decided to decrease the speed of the motors permanently and they've ordered new pulleys with a different ratio. The newly installed VFD can go back to their packaging boxes and sleep until they're used somewhere else. For me, the issues are gone - at least for the moment. But I'd still like to know if there's anything I can do if I run into similar issues in the future. Maybe somebody has some input for me?


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## Peewee0413 (Oct 18, 2012)

Cant release a brake with a VFD output. Need to use a relay. 

Sent from my VS995 using Tapatalk


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## Peewee0413 (Oct 18, 2012)

Peewee0413 said:


> Cant release a brake with a VFD output. Need to use a relay.
> 
> Sent from my VS995 using Tapatalk


Also can't power a contactor coil with a VFD. Read up on it.

Sent from my VS995 using Tapatalk


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## Yellow World (Oct 11, 2017)

Thanks for this outstanding reply. But this was not my question. You might want to spend some time to read my post above again - but this time slowly.


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## Cow (Jan 16, 2008)

Yellow World said:


> Thanks for this outstanding reply. But this was not my question. You might want to spend some time to read my post above again - but this time slowly.



He may have misread your post, but gave you an honest reply to the questions he thought you asked.


Why the negative condescending attitude?


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## Peewee0413 (Oct 18, 2012)

He did ask "But I'd still like to know why the brake won't open on VFD output."

He's too stupid to realize he asked it though.



Sent from my VS995 using Tapatalk


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## Peewee0413 (Oct 18, 2012)

Cow said:


> He may have misread your post, but gave you an honest reply to the questions he thought you asked.
> 
> 
> Why the negative condescending attitude?


So many people come in here asking questions and talk crap to the people whom they are begging for help.



Sent from my VS995 using Tapatalk


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## Peewee0413 (Oct 18, 2012)

motor brake like you said is in won't release with a VFD.. It's not the motor has no torque, its the brake is still engaged.

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## micromind (Aug 11, 2007)

A VFD cannot power a mechanical brake directly or even through a diode bridge. 

The easiest way to go is to have a separate DC source control the brake through a really that is controlled by one of the VFD digital outputs that is programmed for 'run' or something similar. 

This way, as soon as the VFD starts the motor, the brake releases. 

Note.....this doesn't work with a crane hoist because the load will cause the motor to run backward until the VFD can supply enough torque to move the load. For a crane hoist, you need a VFD that has torque proving.

A VFD cannot provide the same motor torque as starting it across the lines, unless it's way oversized. And even then, it can only get close. 

2 reasons, one is that the motor will draw roughly 600% of its full load amps when started across the lines. Most VFDs can provide 150 - 250% though some can go higher. 

Then other is even if the VFD is oversized enough to provide the full 600%, it is not a true sine wave.


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## Peewee0413 (Oct 18, 2012)

Post by OP last year; "Been interested in electricity ever since I can remember, I'm currently preparing for the biiiiiiig test in March next year to become a licensed electrician/plumber. Afterwards, if everything goes well, I'll try to find a job as an entry level electrician/apprentice to learn the ropes of the trade in real life. 

Hope that one day I can be the one here who posts something to point someone else to the right direction. But until then, I guess I'll need lots of directions from you guys here first! " 


I hope he cleans crap better than he wires a motor.

Sent from my VS995 using Tapatalk


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## John Valdes (May 17, 2007)

You cannot use the brakes as are intended. You must feed the brake relay separate using a digital output on the VFD control board. You will toggle a relay rated for the brake voltage.
You will set this VFD output to open/close (release) the brake circuit when the VFD is told to start.
You say the brake is 90 volt DC. VFD output is simulated AC not DC. You must get the brakes separated from the VFD. The VFD is a variable voltage control and would screw with brake coils.
Now why you cannot get the brake to open with identical power peaks my curiosity as well. But do you run the VFD at this output 100% of the time?

Your second problem about starting.
How big is this load? Are you using a VFD that can provide full torque at almost zero speed? Are you using a feedback device?
Until the motor ramps up to rated speed, torque is reduced and may be whats causing your starting issue.
If you are running at 50% of base speed and your load is sufficient the drive will go into over current. Is the motor directly turning the load or do you use gearing?
At low speeds the motor does not have full torque. And may not turn the load. Or trip out. Like yours is doing.

Mechanical intervention like you plant is doing is going backwards. But it does tell me speed (low speed) is your issue.
The only way to correct this is 

1) Get brakes on their own circuits. Independent from the VFD. Use VFD control to operate brakes remotely.
2) Run motors at a higher RPM to help eliminate torque loss. Some times a gear reducer is required.
3) Use a torque VFD (control) or use a feedback device with a drive that will accept the signal. Low speeds with a standard VFD will not operate well for high inertia loads.


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## frenchelectrican (Mar 15, 2007)

Yellow World .,

Please understand that what the guys posted the correct answer on the electric brakes on the VSD you can NOT tie the electric brake circuit directally to VSD motor circuit .,, that you need to wired separated and some VSD do have aux concnants that you can add relay to brake circuit. as you mention 90 volt DC that will be on separated supply. 

As far for other situation the guys got it right on the target with the situation what you posted.


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## gpop (May 14, 2018)

Sounds like you started with a badly engineered plan. Most of us who have been around drives would have known this was a bad idea and would have pushed back on engineering. 

If production required speed control that is constantly adjusting then we would make a vfd work but in your case you just have a lazy engineer that deiced it was easier to install a drive then to calculate the pulley ratio. 50% of drives are requested by lazy engineers and could easily be replaced by adjustable pulleys.


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## MDShunk (Jan 7, 2007)

gpop said:


> 50% of drives are requested by lazy engineers and could easily be replaced by adjustable pulleys.


Amen. Every pump I put in is on a drive, not necessarily for speed control. It's "I don't want to do the math, so we might have to slow it down", or more popularly, "Crap, that pump's too small. Let's run it at 90hz". I want to check our spreadsheet at work, but I'm nearly certain we're approaching 1,500 installed VFD's. At least half run at a single speed that never changes that could have been sheaves or a different ratio gearbox instead. What's a bigger waste is a drive running 60hz that never changes speed.


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## Yellow World (Oct 11, 2017)

Good morning,

Didn't mean to be descending. Maybe I've written too much at one time so it's misleading. Sorry for that.

As I've stated above, the usual procedure for me is to actually put in a separate relay to power the brake. So there are two wires going to the motor: one is three phase output from the VFD, one is single phase going to the rectifier, which then supplies 90VDC to the brake. I've figured that out already.



Yellow World said:


> This issue is actually easy to fix by putting in a separate relay and wire the brake directly.


Maybe should have made this point more clear.


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## Yellow World (Oct 11, 2017)

John Valdes said:


> You cannot use the brakes as are intended. You must feed the brake relay separate using a digital output on the VFD control board. You will toggle a relay rated for the brake voltage.
> You will set this VFD output to open/close (release) the brake circuit when the VFD is told to start.
> You say the brake is 90 volt DC. VFD output is simulated AC not DC. You must get the brakes separated from the VFD. The VFD is a variable voltage control and would screw with brake coils.
> Now why you cannot get the brake to open with identical power peaks my curiosity as well. But do you run the VFD at this output 100% of the time?
> ...


1) As I've just mentioned above, I do use a separate relay for the brake. The voltage that goes to the rectifier comes from the main power and not from the VFD.

2) That's what the company actually did, but with the smaller pulleys we don't need the VFD anymore.

The VFD have been installed to figure out if we can get the quality issues fixed by lowering the speed of the machine permanently. So we've tried different outputs varying from 52 to 60Hz. 60Hz just to let it go back to original speed and compare the difference with the lower speed.


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## Yellow World (Oct 11, 2017)

micromind said:


> A VFD cannot power a mechanical brake directly or even through a diode bridge.
> 
> The easiest way to go is to have a separate DC source control the brake through a really that is controlled by one of the VFD digital outputs that is programmed for 'run' or something similar.
> 
> ...


That's a nice explanation, I'll keep that in mind.


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## Yellow World (Oct 11, 2017)

Peewee0413 said:


> He did ask "But I'd still like to know why the brake won't open on VFD output."
> 
> He's too stupid to realize he asked it though.
> 
> ...


Special sorry to you, didn't want to offend you. My first reply was actually meant to be kind of fun, but I guess my English has some problems. 

Maybe I didn't make it clear enough, but my question is, why does the brake not open if the VFD output is set at 220V/60Hz, which is actually the same as the usual supply voltage?


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## jcaden (Oct 30, 2016)

When a conventional contactor/ starter energizes a motor, it provides Full voltage at 60 Hz. When a VFD starts a motor, it starts out at 0 Hz. and zero voltage. It then ramps up to full voltage at 60 hz. Using the keypad, read the voltage at 30 Hz. and see.


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## gpop (May 14, 2018)

MDShunk said:


> Amen. Every pump I put in is on a drive, not necessarily for speed control. It's "I don't want to do the math, so we might have to slow it down", or more popularly, "Crap, that pump's too small. Let's run it at 90hz". I want to check our spreadsheet at work, but I'm nearly certain we're approaching 1,500 installed VFD's. At least half run at a single speed that never changes that could have been sheaves or a different ratio gearbox instead. What's a bigger waste is a drive running 60hz that never changes speed.


They wanted me to install 56 drives that will be used to run at 25 hertz for 4 hrs every 3-4 years then run at 60 hertz the rest of the time. After a lot of back and forth they finally agreed to installing a vfd on a roll around cart that can be wired in for the 4 hrs when required.


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## gpop (May 14, 2018)

Yellow World said:


> Special sorry to you, didn't want to offend you. My first reply was actually meant to be kind of fun, but I guess my English has some problems.
> 
> Maybe I didn't make it clear enough, but my question is, why does the brake not open if the VFD output is set at 220V/60Hz, which is actually the same as the usual supply voltage?


Voltage with out hertz equals heat.

A vfd controls both voltage and hertz. So roughly 3.6v per hertz at 220v/60hz now the drive does all sorts of tricks like boosting voltage at start knowing that the motor will get warm but its more likely to be around 120-130 which isn't enough for the brake. 

Even using a output to control the relay/brake can be a problem as the load can slip (run backwards) before the drive/motor gets enough torque to control the load. Thankfully most drives have programming that can account for this but it requires someone with a very good understanding of the programming. 

Most drive manufacturer's offer a programming service on new installs. I haven't used the service in years but if i was asked to install a drive on equipment like yours i wouldn't have hesitated and called in a drive specialist.


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## micromind (Aug 11, 2007)

MDShunk said:


> Amen. Every pump I put in is on a drive, not necessarily for speed control. It's "I don't want to do the math, so we might have to slow it down", or more popularly, "Crap, that pump's too small. Let's run it at 90hz". I want to check our spreadsheet at work, but I'm nearly certain we're approaching 1,500 installed VFD's. At least half run at a single speed that never changes that could have been sheaves or a different ratio gearbox instead. What's a bigger waste is a drive running 60hz that never changes speed.


Sadly, this is getting to be more and more popular. 

I can't think of how many VFDs I've installed simply because the engineers are too lazy (or often, too stupid) to properly design the system.


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## Yellow World (Oct 11, 2017)

gpop said:


> ...
> 
> Even using a output to control the relay/brake can be a problem as the load can slip (run backwards) before the drive/motor gets enough torque to control the load. Thankfully most drives have programming that can account for this but it requires someone with a very good understanding of the programming.
> 
> Most drive manufacturer's offer a programming service on new installs. I haven't used the service in years but if i was asked to install a drive on equipment like yours i wouldn't have hesitated and called in a drive specialist.


Running backwards is not a problem here, the belt goes all the way horizontal. I guess some of our machines are a little more "tight" than others, so they need higher torque to start.

Actually we've been thinking about calling in a technician from the Delta. But this entire installation thing was like "quickly install, quickly test, quickly confirm, quickly fabricate new pulleys and then wait for someone to clean up the mess". From installation to deinstallation took around a week. Not too much time to try out different settings. But obviously, the problems weren't big enough to call in someone who knows the answer. 

But since there are still a bunch of similar machines left in our company, that still don't have VFD installed, I guess the chance is high that I'll run into those problems again one day. That's why I'm asking here.

Thanks for the input guys, I really appreciate it!


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## just the cowboy (Sep 4, 2013)

Some basic rules of the road that bosses and new people need to know.
VFD's are not a fix all for machines, they may cause more problems than correct.
VFD's do not fix machine issues or save money if not designed correct.
PLC programs do not need to get changed to fix a machine.

Just a few things to keep in mind, most of us learned this the hard way before there was forums you could ask questions on.

Cowboy


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## John Valdes (May 17, 2007)

gpop said:


> Voltage with out hertz equals heat.
> 
> A vfd controls both voltage and hertz. So roughly 3.6v per hertz at 220v/60hz now the drive does all sorts of tricks like boosting voltage at start knowing that the motor will get warm but its more likely to be around 120-130 which isn't enough for the brake.
> 
> ...


I was stubborn as a young electrician and was hesitant to call for help. I thought it made me look like I did not know what I was doing. Sometimes that was the case.
Later in my career I had a super ask me why I did not call for help on stuff. That a five, 10 minute phone call might save hours of troubleshooting.
This man was correct.
Once I learned to ask for and receive help is when my career really took off.
This was how I learned about drives, motors and made contacts in this industry. It helped me on the job and opened doors I thought would never open for me.

Before long I was the guy taking the calls.

It was good to know I was not alone anymore. I had help if I needed it.



just the cowboy said:


> Some basic rules of the road that bosses and new people need to know.
> VFD's are not a fix all for machines, they may cause more problems than correct.
> VFD's do not fix machine issues or save money if not designed correct.
> PLC programs do not need to get changed to fix a machine.
> ...


I can only imagine how things would have gone for me had I had this resource many years ago.
People pay for this freely given knowledge. We get it for free and we give it back for free.
I have seen several guys fix things in real time using this forum and or directly contacting it members.


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## JRaef (Mar 23, 2009)

The horse for #1 has been sufficiently beaten after death.


#2 is that Demag crane motors are different, they are not 'standard" motors. The rotor and stator are in a tapered cone shape. So unpowered, the rotor shoves into the stator and stays put, a different way of "braking" the load that is inherently fail-safe. The movement that you detect is that the magnetic center of the rotor is purposely made to be off to one side so that as soon as it is energized, the rotor moves over to create the air gap necessary for the motor to work. 









So applying a VFD to a Demag crane motor is not something that should be done without the direct involvement of Demag, because if you ramp the energy into the motor, it will not move the rotor enough and either leave you in a locked rotor condition or cause significant friction. Demag sells their own VFD for this that has the necessary programming to make it work correctly, but I don't think they offer a lot of help in allowing you to do it on your own.


#3 is likely that you are not using a Vector Control drive. A standard V/Hz drive will produce torque, but not accurately under changing load conditions. A Vector controlled drive has a feedback loop involved that looks at what the motor is told to do vs what the motor is actually doing so that it can tweak the output to make sure the motor is performing. There are two basic "types" of Vector Control, SVC (Sensorless Vector Control) is the simplest and is now available in almost all drives, FVC (Flux Vector Control) is better, but needs an encoder feedback from the motor. Chances are that if you bought a reasonable drive in the past 5 years, it is SVC capable. But that also requires two things to make it function; you must PROGRAM the drive to use it (some come that way by default, others do not) and you must perform an "Autotune" procedure between the drive and motor. For SVC to function, the VFD must know the motor equivalent circuit in detail. You can program that information manually, but most people can't get all the info needed. So the VFD mfrs will provide a feature called "Autotune" (some call it "Motor ID") wherein the VFD will test the motor to get this information. You MUST perform that routine or the VFD will not be able to make the motor work properly, and any time you change the motor, you must do it again, even if it is the same brand/model of motor. Every motor is different.


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## jaacrane (Mar 8, 2020)

I’ve dealt with a lot of Demag over the years on crane systems and we’ve modernized them with new VFD controls that are NOT Demag. Demag has a tendency of connecting their motors in low voltage to use higher Hz in order to get more rpms. Not all VFDs may work with these types of motors but it’s possible. We generally use PEI which are intended for crane applications and I’ve never had an issue with and without the mechanical brake. On this type of VFD you are capable of increasing a pulse torque setting to do an initial voltage to pull the rotor away from the brake and make it run or float. 
With the electric brake question, yes you can use a contactor to control it but also, Demag brakes require a module they use which will generally introduce high voltage to open and then reduce to the 90vdc. This part is tricky depending on what the application is. For instance in a trolley forward or reverse motion you can get away with just using a regular rectifier off of 120vac. But in a hoisting motion, you need a quick release reaction so that you don’t have issues with the feedback.


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