Home made brake for 3 phase motors.

garbo

Member
On a couple of vfd's with bypass we have been having major problems restarting a return fan from a running AHU [ AIR HANDLING UNIT ]. 2 of the VFD'S are less than 7 years old but are obsolite so the vendor can not help. At the Hospital where I work we can not shut down both AHU'S or ramp them down too far due to negative or positive pressures required in some patient rooms. When the return fan [100 HP ] goes down the 2 125 HP AHU'S cause the return fan to run at over 800 RPM in the wrong rotation. Even when we do a quick ramp down to 30% of full speed the return fan still runs pretty fast. Building Engineers tried adjusting dampers to slow return fan but that only helped a little. Years ago we use to wire in a full wave 30 amp cube rectifier from the 480 volt line side of a starter and thru a timer to give DC power to 2 motor leads for 0.5 to 1.2 seconds to stop explosion proof motors. Those motors were only 1 to 7.5 HP. Any body have any sugestions and what ampere for a rectifier if I go that route. Does any company sell a kit for this? Thanks tom grabowski
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
On a couple of vfd's with bypass we have been having major problems restarting a return fan from a running AHU [ AIR HANDLING UNIT ]. 2 of the VFD'S are less than 7 years old but are obsolite so the vendor can not help. At the Hospital where I work we can not shut down both AHU'S or ramp them down too far due to negative or positive pressures required in some patient rooms. When the return fan [100 HP ] goes down the 2 125 HP AHU'S cause the return fan to run at over 800 RPM in the wrong rotation. Even when we do a quick ramp down to 30% of full speed the return fan still runs pretty fast. Building Engineers tried adjusting dampers to slow return fan but that only helped a little. Years ago we use to wire in a full wave 30 amp cube rectifier from the 480 volt line side of a starter and thru a timer to give DC power to 2 motor leads for 0.5 to 1.2 seconds to stop explosion proof motors. Those motors were only 1 to 7.5 HP. Any body have any sugestions and what ampere for a rectifier if I go that route. Does any company sell a kit for this? Thanks tom grabowski
Not clear on exactly what you want. If you want to electrically HOLD the motor so that it does not turn the whole time the AHUs are running, you CANNOT do that, it would have to be a mechanical brake. Anything electrical is going to be good for short durations and low duty cycles, otherwise you can burn up the motor.

But if what you are after is to stop the fan from back-spinning BEFORE you run it the proper direction, that's different. You can buy DC Injection Brakes (DCIB) for just about any size motor. But a DCIB and a VFD cannot live harmoniously on the motor leads at the same time, you will need FULL isolation of each unit from the other, preferrably with mechanically interlocked contactors.

Before you spend a lot of money on that though, it might be a better idea to use what you would spend to upgrade your VFDs to modern ones that can handle that exact situation.
 

Besoeker

Senior Member
Location
UK
Before you spend a lot of money on that though, it might be a better idea to use what you would spend to upgrade your VFDs to modern ones that can handle that exact situation.
Yes, that would seem to be the way to go.
If I were Tom, I'd a bit miffed if my VFD vendor decided to walk away from a project that was just seven years old even if obsolete. Seems like an opportunity missed to offer, and possibly supply, new kit.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Yes, that would seem to be the way to go.
If I were Tom, I'd a bit miffed if my VFD vendor decided to walk away from a project that was just seven years old even if obsolete. Seems like an opportunity missed to offer, and possibly supply, new kit.
I agree, definitely a missed opportunity even if the VFD is obsolete.

But unfortunately I'm not at all surprised... So many so-called "professionals" are just collecting paychecks and taking the path of least resistance to get pesky customers out of their hair.
 

iwire

Moderator
Staff member
Location
Massachusetts
I agree, definitely a missed opportunity even if the VFD is obsolete.

But unfortunately I'm not at all surprised... So many so-called "professionals" are just collecting paychecks and taking the path of least resistance to get pesky customers out of their hair.
Well ..... there is also the fact that too many companies display no loyalty nor pass profits on to employees so it is often a case of companies getting what they pay for. It is hard for a person to care about the company when the company does not care about them.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Well ..... there is also the fact that too many companies display no loyalty nor pass profits on to employees so it is often a case of companies getting what they pay for. It is hard for a person to care about the company when the company does not care about them.
No argument from me on that point for sure.
 

sii

Senior Member
Location
Nebraska
Well ..... there is also the fact that too many companies display no loyalty nor pass profits on to employees so it is often a case of companies getting what they pay for. It is hard for a person to care about the company when the company does not care about them.
Could you explain this to my employer? Somebody needs to.
 

garbo

Member
VFD Brake

VFD Brake

Not clear on exactly what you want. If you want to electrically HOLD the motor so that it does not turn the whole time the AHUs are running, you CANNOT do that, it would have to be a mechanical brake. Anything electrical is going to be good for short durations and low duty cycles, otherwise you can burn up the motor.

But if what you are after is to stop the fan from back-spinning BEFORE you run it the proper direction, that's different. You can buy DC Injection Brakes (DCIB) for just about any size motor. But a DCIB and a VFD cannot live harmoniously on the motor leads at the same time, you will need FULL isolation of each unit from the other, preferrably with mechanically interlocked contactors.

Before you spend a lot of money on that though, it might be a better idea to use what you would spend to upgrade your VFDs to modern ones that can handle that exact situation.
Thanks for the quick reply. I would only feed the 100 HP motor for the shortest amount of time to bring it to a stop then start the VFD on hand. Our VFD'S can start a motor that is turning at a few to maybe 100RPM. They label it a flying start. I will look into an injection brake. I would use auxilary contacts on the VFD output contactor and say 0.25 second on delay timer for for energizing the ouput contactor after the motor comes to a stop. The motor is very hard to get to due to the fan is bolted onto the motor shaft and motor is inside a 3 or 4 foot round duct about 14 feet off of the floor. I hate to tell you how much a 3 man crew charges to set up to change the beast. I am not that happy with the newest and greatest VFD'S. On the old VFD'S FROM 25 TO 125 HP I change out a 220 volt muffin fan in 20 minutes any never shut the VFD off. I use a 20" box fan to keep it from overheating. New VFD'S the fan is mounted all the way in the back [ behind 2 boards] . If you have 2 screw guns and done about 20 the service guy you might get it down to a hour.Thanks garbo
 

garbo

Member
Obselete VFD'S

Obselete VFD'S

Yes, that would seem to be the way to go.
If I were Tom, I'd a bit miffed if my VFD vendor decided to walk away from a project that was just seven years old even if obsolete. Seems like an opportunity missed to offer, and possibly supply, new kit.
Some drives are only 2 yers old and are Obsolete. We have an order in for $20,000 wotrh of spare parts [ mostly boards ] for over a month now but only received 2 boards so I think we are just going to go with new VFD'S. thaks Tom
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Thanks for the quick reply. I would only feed the 100 HP motor for the shortest amount of time to bring it to a stop then start the VFD on hand. Our VFD'S can start a motor that is turning at a few to maybe 100RPM. They label it a flying start. I will look into an injection brake. I would use auxilary contacts on the VFD output contactor and say 0.25 second on delay timer for for energizing the ouput contactor after the motor comes to a stop. The motor is very hard to get to due to the fan is bolted onto the motor shaft and motor is inside a 3 or 4 foot round duct about 14 feet off of the floor. I hate to tell you how much a 3 man crew charges to set up to change the beast. I am not that happy with the newest and greatest VFD'S. On the old VFD'S FROM 25 TO 125 HP I change out a 220 volt muffin fan in 20 minutes any never shut the VFD off. I use a 20" box fan to keep it from overheating. New VFD'S the fan is mounted all the way in the back [ behind 2 boards] . If you have 2 screw guns and done about 20 the service guy you might get it down to a hour.Thanks garbo
It's too bad (sad) that you don't get good service from your drive supplier. The more I think about this the more I realize this isn't likely something so difficult to solve. What you are describing is called "Anti-Windmilling". I was implementing Anti-Windmilling in HVAC VFDs 20 years ago without having to add external devices, it's a really common problem.

For example is there no way to add a "Dynamic Braking" package to your drive, and/or does it maybe even have DC injection braking already in it? I find it hard to believe that a VFD less than 7 years old has no features you can use for this. "Flying Restart" is for something different, it's for when the motor is already moving in the correct direction and you don't want the VFD to artificially slow it down if it's already part way to the speed you want to go to. So if your vendor brought that up when you described your problem, that indicates that he really doesn't understand your problem. That's like going into the auto mechanic saying you are having trouble stopping at a stop light and he explains to you how your gas pedal works.

What's the brand name and model? I'm up for the challenge now...
 

iaov

Senior Member
Location
Rhinelander WI
I?m intrigued also. There is a lot of inertia in a big AHU fan when they are sitting at rest much less running backwards. This seems to me like a problem to be handled with some brains (a PLC) to handle ramp up and down times, and the timing of events as they relate to these ramp times. Most of the big AHU?s I?ve worked on recently have used ?soft starts? (AB SMC) but I have not had to deal with a problem of this nature. This may require some additional sensing tech being added to the process. Just some thoughts.
 
It's too bad (sad) that you don't get good service from your drive supplier. The more I think about this the more I realize this isn't likely something so difficult to solve. What you are describing is called "Anti-Windmilling". I was implementing Anti-Windmilling in HVAC VFDs 20 years ago without having to add external devices, it's a really common problem.

For example is there no way to add a "Dynamic Braking" package to your drive, and/or does it maybe even have DC injection braking already in it? I find it hard to believe that a VFD less than 7 years old has no features you can use for this. "Flying Restart" is for something different, it's for when the motor is already moving in the correct direction and you don't want the VFD to artificially slow it down if it's already part way to the speed you want to go to. So if your vendor brought that up when you described your problem, that indicates that he really doesn't understand your problem. That's like going into the auto mechanic saying you are having trouble stopping at a stop light and he explains to you how your gas pedal works.

What's the brand name and model? I'm up for the challenge now...
Isn't there a good chance that the motor is not inverter rated? Bearing isolation on that size would be in order, or shaft grounding brushes.

Anothe phenomena occurs with anti-windmilling application is that the bearings get chewed up. That would be especially a concern over complex, multi-feed air handling system that are badly tuned and subject to buffeting du to the bad damper control settings.
 

iaov

Senior Member
Location
Rhinelander WI
Yes sir all very good points. I suspect this is not an easy nor inexpensive problem to fix. I also suspect that there are air handling guys who have already invented this particular wheel.
 

garbo

Member
brand of obsolete VFD

brand of obsolete VFD

The 200 or so VFD'S we have in our 3 millon square foot campus are Danfoss. The 100 HP 480 is a model VLT6012. We still have some Reliance VFD'S but they are the pits. Troubleshoot one down to a bad contactor [50 HP ] but they replaced it with a Danfoss. We just got six 18 pulse ABB 40 to 75 HP VFD'S but they look hard to PM. They have fuses buried behind one of the 2 reactors. The 40 HP ABB 18 pulse drive with line and load reactor and bepass is in a 6.6 foot high cabinet 3 feet deep. We have a 250 HP danfoss going in that is only twice that size [ but not 18 pulse ]. I faxed Danfoss about this but they have not replied. I had to call their national service department a couple of times and the were very helpfull. Both VFD companies [ ABB &Danfoss is coming to our Hospital this year for a few days of trainning. Years ago I went to a 2 week drive class and 4 days at Fincor].Thanks
 

garbo

Member
VFD

VFD

I?m intrigued also. There is a lot of inertia in a big AHU fan when they are sitting at rest much less running backwards. This seems to me like a problem to be handled with some brains (a PLC) to handle ramp up and down times, and the timing of events as they relate to these ramp times. Most of the big AHU?s I?ve worked on recently have used ?soft starts? (AB SMC) but I have not had to deal with a problem of this nature. This may require some additional sensing tech being added to the process. Just some thoughts.
We have 2 seperate companies that work on the building system automation. The one for this are not very good. I got called in one weekend spent over a hour tracing unmarked wires thru at least 4 relays then into their locked control box. When the serviceman on called showed up I told him the problem was in their system but he did not believe me. The drive worked when I jumped out his relay but he still said that the problem was on our equipment. His relay was feed by a EP switch. We asked him to change it out but that he said that was a fitters job. wow need a specialist to remove a single air line.Thanks
 

Besoeker

Senior Member
Location
UK
For example is there no way to add a "Dynamic Braking" package to your drive, and/or does it maybe even have DC injection braking already in it? I find it hard to believe that a VFD less than 7 years old has no features you can use for this.
I'm with you on that. I'd be surprised too if a drive of that age doesn't have features that can cope with the situation described. And, if it doesn't, it was the wrong drive for the application in the first place.
If garbo can't get support from his supplier, maybe he needs to bypass them go to the OEM.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Wow, hard to believe, but that drive is apparently incapable of Dynamic Braking. Maybe because it was sold as a cheap HVAC drive? Sometimes marketing departments have too much power.

It does however have 2 useful features that might work, Freeze Reference and DC Injection Braking (but at only up to 50% of the unit rating). That should work though. From what I could gather, "Freeze Reference" is a way of overriding all other commands and having the drive output go to a fixed frequency no matter what, commanded by a digital input. The only thing that apparently releases it is a DCIB command. So I would experiment with a Freeze Reference speed command that allows the drive to slow down the fan no matter what is happening with windmilling, but without causing a DC over voltage fault from regeneration (because you don't have Dynamic Braking, that will need trial and eooror). Then once it is down to about 15-20% speed, give it a Brake Command (DCIB) to stop it right before giving it a Run Command to run it up to whatever normal speed you want in the correct direction.
 

rcwilson

Senior Member
Location
Redmond, WA
I had good luck using a manual sequence to restart windmilling 150 HP drives on an Air Cooled Condenser. (An ACC is a big horizontal radiator 75 feet off the ground with 35, 19-foot diameter fans to cool and condense steam turbine exhaust).

If a fan was off when adjacent ones were running or in high ambient winds, the downdraft spun the fan about 30-40% speed in reverse. Attempts to start the fan in forward direction tripped the VFD on DC Bus Overvoltage despite various torque and current limit settings. The operators' work around was to start the fan using the across-the-line bypass starter, let it run at full speed for a couple minutes to cool the winding, then switch to the VFD. That start shook the structure considerably and probably didn't help the gear box.

Experimentation led to a successful VFD control sequence:
If fan is spinning in reverse:
1. Start VFD with -10% speed reference = 10% speed in reverse.
2. Hold for 20 seconds or until speed feedback = -10%.
3. Ramp speed down to 0 and back up to forward setpoint using standard ramp.

We were able to program this sequence in the DCS but had to do some software tricks to prevent shutdown around zero speed. Our speed reference signal was 4-20mA for -90 rpm reverse to +900 rpm forward.

We demonstrated the sequence using manual control by having the operators do a manual start with -90 rpm reverse speed command for 20 seconds, turn off for about 10 seconds, then start with +30% speed and finally switch to auto. The off time between forward and reverse was needed to satisfy some protection settings. Sometimes that delay allowed the fan to get spinning backwards again.

If your Danfoss or ABB units can take a reverse input maybe you can implement similar logic.

One advantage was our process was not affected by a single fan running in manual at a different speed. That may not be the case for your hospital HVAC.
 

Besoeker

Senior Member
Location
UK
Wow, hard to believe, but that drive is apparently incapable of Dynamic Braking. Maybe because it was sold as a cheap HVAC drive? Sometimes marketing departments have too much power.
Most of the drive systems we do are for industrial applications and don't have dynamic braking as standard.
Some of the smaller ones do have the brake chopper fitted as part of the power circuit but they still need an external resistor to dissipate the energy.
The last one we did was a 200kW unit and we had to buy a separate brake chopper module and a fairly big dynamic brake resistor.
 
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