MRO Magazine

Focus on Motors: How to install a drive system successfully; AC to DC to AC

Installing a drive properly can be a straightforward procedure. This can reduce startup time and ultimately save money. Everything from nuisance tripping to blown fuses can usually be traced to back t...

June 1, 2000
By George Galea

Installing a drive properly can be a straightforward procedure. This can reduce startup time and ultimately save money. Everything from nuisance tripping to blown fuses can usually be traced to back to an improper installation. It’s the same old story you hear: “It was working fine for a few days then it just stopped all of a sudden.”

Drive technology has come a long way in just a few short years. That’s why it is important to take a look at the manufacturer’s latest recommendations, or even better, take a training course on your drive equipment. Drives have found their way into many types of industries and they are far easier to use than their big box predecessors of the past. Sometimes users still deal with an installation the same way they did way back in the old days. Here are some things to keep in mind for today’s installations.

1. The site may still have some older technology on the lines, such as drive isolation transformers. This was a great item back when the best drive technology was the current source inverter and you would install a drive isolation transformer (DIT). These variable frequency drives (VFDs) used a SCR (Silicon Controlled Rectifier) converter for the generation of variable-voltage DC from the AC power line.

These SCR units were a bit of a pain to start up. They would have a very low threshold for the effects of noise on the lines and would be one of the biggest causes of noise effects on other plant equipment.

DIT’s are large, heavy, expensive, and inefficient. Current isolated gate bipolar transistor (IGBT), pulse width modulated (PWM) drives use a diode bridge rectifier and electronic isolation, eliminating the need for DITs. The added line impedance (which protects the VFD from voltage disturbances and lightning, reduces harmonic voltage distortion, improves power factor, and protects power factor correction capacitors from this non-linear load) can be duplicated with a small, lightweight, low-cost, and very efficient AC line reactor.

AC line reactors are often supplied in a packaged, pre-engineered system. Keep in mind, though, that an AC line reactor does not react the same as a DC choke.

2. Approach every installation as “application specific.” No two sites are ever identical, so when dealing with radio frequency (RF) or AC electronic devices, remember to have a mindset that recognizes the uniqueness of the site. Each one has its own characteristics and needs. One brand of drives has a different requirement from another. Never take an “it’s a piece of cake” attitude.

3. Read the manual: There are supposed to be pros and cons to everything, but if you think about the cons to reading the manual first, you’ll find there are none.

Here are the pros. If you follow the manual you won’t have to change anything once you’ve installed it. Let me give you an example of how you can avoid the little disasters that pop up on a job. Let’s say you are putting in a single-phase, 1-hp drive at 115 VAC input voltage. The output is determined by the drive and doesn’t come into play here. The specification on the drive’s label says Nominal Input Current 10.4/20.8 amps (the slash means 10.4 at 240v /20.8 at 120v).

You start to run wires for the 20.8 amps at 120 VAC because that’s way it reads and that may mean using 12 AWG wire. Then you put in a slow-blow, mini-style fuse rated for 15 amps because that’s what you have on hand and you figure it will do just fine. The drive will run – for a while. The problem is that the manual, if you read it, indicates you are wrong. Fusing must be fast blow, as all electronic equipment is, and you should use a class CC Bussman-type KTK-R.

That’s not all. The manual says that you must multiply the input current rating of the drive by 1.5 times, so 1.5 x 16.2 = 24.3 amps. Now this is when you must start pulling all the wire out and replacing it with 10 AWG wire to meet code (and don’t forget to put in the new fuse holder).

The same problem would have occurred if breakers had been used, except a higher incidence of nuisance tripping would have occurred. For reference, the manual for the breakers probably says something like “the breaker must be recalibrated after every trip to ensure warranty and meet industry regulations.” Leaving a breaker tripped in the panel contravenes the electrical code.

4. Make sure you match the drive and motor: Get the amps and volts rating of the motor from the nameplate. The same goes for the drive. Check the nameplate for amps and volts rating–don’t just go by horsepower. Make adjustments in the drive parameters if using a motor with a lower horsepower rating than the drive.

General installation guidelines

Take into account the environment and surrounding components in the same vicinity as the drive. Things like power factor correction capacitors are great for reducing the generated reactive current of motors without VFDs, which is a burden to the electrical system. These capacitor banks are often used to correct the poor power factor that results from this AC motor load. Drives correct all this and should not be within 300 ft (100 m) of the capacitor banks. If in doubt, install a properly sized line reactor or you will end up replacing a lot of fuses.

Consider what conditions the equipment will be experiencing over time when you are gone. Dust, heat and moisture will kill electronic equipment quickly. Protect against this by getting good information from a qualified distributor. Ask about enclosure sizing and NEMA ratings, and do a thermal cycle time study, if needed.

Remember, the output wires to the drive are carrying a frequency signal as well as power and should be in their own conduit. The input to the drive must be hard-wired. I have seen installations where a simple duplex wall plug was used to power a drive because it was rated for 120 VAC input power. The first problem with this scenario is anyone in the plant can just unplug the drive and shut down the system. Also, if that same drive was to be plugged/ unplugged more than once every two minutes, it would damage the electronics in the drive.

Use the manufacturer’s recommended parts. There is a reason why the manufacturer takes the time to publish a recommended part or accessory list in the manual and it’s not so they can make you spend more money. An example would be the use of coaxial Tee and Tap connectors for communications between a control device and the host or head control device. The price difference between a Radio Shack BNC Coaxial Tap connector and the proper one could be $90. Guess which one most people would be inclined to buy?

Well, you get what you pay for. The more expensive connector has an isolating circuit inside (L1 coil & L2 coil). This is in case the unit is unplugged from the communication coax cable–because coax makes an excellent antenna. If the cheaper connector is used, then once it is unplugged, the host control unit will see every noise signal in the plant coming in and will shut down. It’s not a very good experience to go through just to save a little money.

Control wiring

If you install a drive and it does not work properly, check the wiring. Poor control wiring usually results in a big waste of time for everyone on the job. Again, it’s described in the manual and there are usually very helpful people at the end of a toll-free phone number who will be willing to guide you with some good information.

It is crucial that you understand which end the power goes in and which is the output to the motor. I know it sounds simple but many people confuse L1, L2 and L3 (power in) with T1, T2 and T3 (PWM waveforms out to the motor). The result is a burnt-out drive.

Low voltage control wiring should be done carefully. High voltage and low voltage wiring should be separated. These control wires are susceptible to any noise generated in the plant (such as electro-magnetic frequencies or EMF). This noise is conducted, emitted or radiated from its source. Shielded wire must be used. The shields must be grounded on both ends of the cable for digital signal transmissions. One end is not gro
unded if the signal is a 0-10 volt DC or 4-20 mA signal (analog). If both ends are grounded, the ground loop will more than likely generate more voltage/current and that would corrupt the analog control signals.

Consider using an Inverter Bypass Safety Switch (such as the IBYSS made by Advance Controls Inc.), or a manual override switch. The bypass switch is very cost-effective for drives. It allows you to have power to entire system so it runs normally or to cut power to the drive or motor to make troubleshooting easier. The configurations come standard with a lockout and tag switch handle. Also, properly sized line reactors will prevent most of the damage a drive receives through line disturbances. Furthermore, many drive manufacturers will not sell a drive unless you buy a line reactor with the unit, especially in the 575-600 volt range.

In conclusion, when installing a drive system, read the section in the manual pertaining to installation, get the right tools, then take the time to do the job properly. There is no better feeling for an engineer or tradesperson when their system has a clean startup and runs smoothly.

George Galea is with Leeson Canada’s Industrial Controls Group. For more information, use the reader reply number.