George A. Shrokman, Jr. - The Industrial Door Geek
GS Control Systems LLC - Door Controls Engineering and Application
Does anyone in the door industry have more experience with inverters than us?
Inverters (sometimes called Variable Frequency Drives) enhance the operation and increase the mechanical life of any door system. The smooth accel and decel rates provided by the drive are only one of the many enhancements a drive can make to a door system! A high quality inverter can get more starting torque out of a motor than a standard motor starter. And that same inverter can use Dynamic Braking to help stop the door.
It is only recently that inverter technology has advanced far enough to allow low cost manufacturing of basic units. There are a few drives that actually cost the same or less than high quality motor starters.
Most of the door manufacturers use low cost inverters. Doors that are counterbalanced don't require much more than the soft start / stop technology and a few programmable frequencies to operate the door a different speeds. When you get into doors larger than let's say 20' wide x 20' tall, you need a few more options handle the special starting torque requirements and inertia stopping power.
Price and Quality in a drive makes a big difference when you get into large doors.
I continually see door manufacturers try to use the same drive on large doors as on small doors. The argument is that if it works on the small one it should work on the large one!
There is a big difference between the start up torque required get past the "friction" point on a 40' wide x 30' high door vs. a 10' wide x 10' high door. Your calculations have to take into consideration the effects of a small breeze against the curtain. On sectional doors you must consider what happens when the springs get dirty or weak. Many manufacturers try the inexpensive drives on a large door, run in to problems, and just say you can't do it.
It can be done. We do it all the time.
We already went through that big learning curve!
The first units we built were very simple. They were for use on rolling steel doors in an automotive manufacturing facility. They specified expensive drives that had many more features than what was required to run a door. But they used these units throughout the manufacturing facility, so they wanted the same drives on the doors.
We then started developing control systems using inexpensive drives. They worked fine on counterbalanced or sliding doors. But then we started experimenting on non-counterbalanced doors, or doors that were not adjusted properly.
We had what we thought were very simple test parameters, and we went through 7 drives before we found a drive that could meet our simple requirements!
Our test was very simple. We had a non-counterbalanced door operating at a closing speed of 18" per second. On the bottom of the door is an electric reversing edge. The edge could compress 1" before the full load of the door was on the edge exceeding UL325 requirements.
OH What Fun! All of the drives ran the door up and down just fine. But then we had the dreaded "ladder" test. We took a short ladder, set it under the door and tried stopping it without putting the drive into a "fault".
We had a lot of fun! We would get a drive, put it in, and then have someone from the drive manufacturer there when we ran the test. After all, they said it would work! It may sound mean, but it was fun watching them change all kinds of parameters trying to get the door to stop.
We learned a lot watching everyone change parameters. One parameter set wrong can change the complete operation of the door.
Finally we had a engineer who determined the drive needed a special software modification to make the motor stop immediately and under load without going into a fault. They named it the "Shrokman" parameter, and you could order it with their drive. Our 15 minutes of fame!
Low cost drives simply don't have the torque output capabilities and vector capabilities required to control the large inertia requirements.
High quality inverters contain "E-Stop" systems that allow you to stop, pause and reverse a high speed door quickly. Many door manufacturers use low quality drives, and if you try to perform a quick stop when the reversing edge trips you get a drive fault.
One of the most important parts of a drive is the software.
You can have two drives that are identical, and different software will make them control a motor differently. The software is installed in the manufacturing process, and cannot be changed in the field. You can change the "parameters" in the field via computer software, but the main software I am talking about is in a chip on the main board.
The drive software determines how the drive takes the incoming voltage and 50/60 hertz waveform and transforms it to enhance the motors operation.
If you have good data on your door, you can take all of your requirements and read through the 200 to 300 page drive manual to make sure it will work. Don't go by the literature or what the sales engineer says!
Not all Inverters have the same bells and whistles.
Many of the basic inverters only have a soft start / stop built into the unit. Some only allow two frequency settings, so you only have two speeds. As you spend more money, you may get 5 selectable speeds and some dynamic braking capabilities. After that you get into variable starting torque, unlimited ramp up and ramp down times, adjustable volts to hertz ratios etc. Some drives have over 300 settings you can tweak to get peak performance from your motor.
I feel the most important feature on a drive is how it effects the torque output of the motor.
Motors on big doors have high torque requirements. A drive that can output 220% of normal starting torque for 2 seconds out performs a standard drive that outputs 150% for 1 second. Two seconds is a long time even when you are moving a door at 12" per second. A door moving at 24" per second has traveled 4 feet in two seconds!
There not only is a big cost difference, but also an even greater performance difference. You can see it in the performance, but how do you get that across to the salesman who is complaining about cost? Actually, being a salesman myself and in the business of selling $8,000 dollar control panels, I tell them to go back to selling "Fuller Brush" from door to door. Most of the young guys don't even know what I mean!
The encoder needs to feed both the Door Controller for positioning AND the Inverter for torque control.
I have not found a drive that can give us full torque control at zero RPM without feeding a pulse directly into the drive. We can get high start up torque, but we need the full torque control with large doors to really enhance their operation.
Our tests have proven that the best sequence of operations with a high speed door is to have the drive slow and stop the door with dynamic braking, hold it in position, and then release the brake to hold the door, and then shut down the drive.
If you can make all of that happen and safely operate the door without having it over travel or free fall let me know. I would really like to see it in operation.
It is important to monitor the current output of the drive in your plc or your controller on large doors.
Many people just don't understand the destructive power of a 3 or 5 horsepower motor and gear reducer. If that door jams while going up, you need some way to read the high current and shut it down.
Designing a high torque output system on a door brings in other problems. Let's say you know that when you have a door with a wind load that you need 150% start up torque and 100% running torque. That drive is going to work itself to give you that output. But what happens if the guide rails have been pinched, and the door door curtain does not want to move? What are you going to damage?
Because of all of the variations you have with a very large door, we use an analog output from the drive into the PLC to monitor the amperage. You can use high settings in the drive, but when you get wind load conditions, you need to know the difference between a spike in the current and a natural current build up.
This system is expensive, but it works! How do you put a price on that?
Super-Charging with an Inverter
Super-Charging can be a very controversial subject. We only do this with Eurodrive motors, since they are the only motor manufacturer that we found that has any technical data published showing that you can do it. If you know of other manufactures that do this, please let me know.
George A. Shrokman, Jr.
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