Variable Frequency Drives - Toshiba

Variable Frequency Drives - VFDs / ASD's ~ Inverters for Electric Motors

Toshiba Drives from Sords Electric provide Excellent Motor and Pump control.

General discussion on uses and how they work.

You can divide the world of electronic motor drives into two categories: AC and DC. A motor drive controls the speed, torque, direction and resulting horsepower of a motor. A DC drive typically controls a shunt wound DC motor, which has separate armature and field circuits. AC drives control AC induction motors, and-like their DC counterparts-control speed, torque, and horsepower.

Variable Frequency Drives (VFD's)  also known as Adjustable Speed Drives, are used to vary the speed of an electric motor. They do this by changing the frequency of the electric power going to the motor. They work only with three-phase power. Today they are very economical.

In the United States, normal electric power is supplied at 60 cycles per second, sometimes called 60 hertz (Hz). At this frequency motors run at 1,800 rpm, 3,600 rpm, 1,200 rpm, or 900 rpm, depending on how they are wound. The number of poles in the winding determines its speed. For example, four-pole motors run at 1,800 rpm, and two-pole motors run 3,600 rpm.

The actual motor speed, as read on the motor nameplate, is a little lower than these theoretical figures because of slippage that occurs.

The speed of the motor changes in direct proportion to the hertz. Thus, a four-pole motor running at 45 hertz will turn 1,350 rpm, and a six-pole 1200-rpm motor at 40 hertz will run 800 rpm. A motor can be sped up, also: a four-pole motor running at 90 hertz will turn 2,700 rpm.

Most VFD's come with a preset limit of 60 Hz. This can be easily changed.

When a motor is slowed down, the cooling fan that is mounted on the motor shaft also slows down. Thus, motors have a tendency to overheat at low speeds like 10 Hz or 15 Hz. Feel the motor to see that it is not overheating. A Toshiba premium efficiency motor will overheat less. Low speeds are fine for a trial, but they may not be suitable for extended operation.

VFD's have a built-in circuit breaker that shuts down the motor if the amps get too high for the speed at which the motor is being run. This provides excellent (the best we know of) electrical protection for a motor and the machine it is driving.

It is best to have a VFD that is rated for more horsepower than the motor being driven. This gives more flexibility. However, where electrical overload protection is deemed important, the rating of the actual motor being driven should be loaded into the VFD. Otherwise the VFD might put out enough power, if called for, to burn up the motor.

It is very easy to install a VFD. They work only on three-phase power output, some Toshiba S15's have single phase power in. So there are four wires coming from the power control panel: white, black and (usually) red power wires and a green ground wire. The three power wires are hooked to the L1, L2 and L3 terminals. There are three output terminals, labeled T1, T2 and T3 (sometimes U, V, and W), to which you connect the power wires going to the motor.

When you turn on the motor, it may be running backwards. It is usually easy to change the direction of rotation with the VFD itself. Most VFD's have a simple toggle command for forward and reverse.

Unfortunately, when the motor is shut down and later restarted, it will restart running backwards again. To correct this permanently it is necessary to switch two of the power leads. It is a little tricky to change the direction of rotation of a motor with a VFD. Simply switching leads at the main circuit breaker in the motor control panel will not work. Instead, it is necessary to switch the leads coming out of the VFD, the ones going to the motor.

Once a VFD is wired up, there may be frustration trying to get the motor to start. The solution usually is to toggle from the Remote to the Local operation, then hit the Start button.

To change the speed (frequency), get into the frequency adjustment display (next to the actual frequency output display). Toggle the speed up or down, then hit the enter button.

Amps can be read by toggling the menu button to the amps display. Amps reading are a little peculiar with VFD's. They are no longer directly in proportion to the power being consumed. So, use them as a reference only.

Please do not let water and electricity to mix around a VFD. It is very easy to fry a VFD, and they are not worth trying to repair when you do. Be sure to have a plastic bag or sheet over the VFD. Protect the VFD from dripping pipes, rain, and wash-down water. Also, use GFCI's if necessary for human protection in wet areas.

VFD's are good for only one voltage, either 208-220-240 volts or 440-480 volts. Be sure you know what voltage you are working with. There are more sophisticated VFD models that work on both voltages.

For advanced students we offer the following: Basically, with a VFD set below 60 Hz, the motor drops maximum horsepower output and instead holds constant torque. Above 60 Hz, the horsepower is limited to the motor nameplate maximum, which means there is a reduction in torque. Some VFD's can be set for overload trip on either amps or torque; set it on torque for the best overload protection.

The input section of the drive is the converter. It contains six diodes, arranged in an electrical bridge. These diodes convert AC power to DC power. The next section-the DC bus section-sees a fixed DC voltage.
The DC Bus section filters and smoothes out the waveform. The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you'd measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414. The inductor (L) and the capacitor (C) work together to filter out any AC component of the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.
The DC bus feeds the final section of the drive: the inverter. As the name implies, this section inverts the DC voltage back to AC. But, it does so in a variable voltage and frequency output. How does it do this? That depends on what kind of power devices your drive uses. If you have many SCR (Silicon Controlled Rectifier)-based drives in your facility, see the Sidebar. Bipolar Transistor technology began superceding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology, which will form the basis for our discussion. Toshiba manufacturers their own IGBT's and thus has complete control and quality measures for their drives.

The drive's control board signals the power device's control circuits to turn "on" the waveform positive half or negative half of the power device. This alternating of positive and negative switches recreates the 3 phase output. The longer the power device remains on, the higher the output voltage. The less time the power device is on, the lower the output voltage. Conversely, the longer the power device is off, the lower the output frequency. The speed at which power devices switch on and off is the carrier frequency, also known as the switch  frequency. The higher the switch frequency, the more resolution each PWM pulse contains. Typical switch frequencies are 3,000 to 4,000 times per second (3KHz to 4KHz). (With an older, SCR-based drive, switch frequencies are 250 to 500 times per second). As you can imagine, the higher the switch frequency, the smoother the output waveform and the higher the resolution. However, higher switch frequencies decrease the efficiency of the drive because of increased heat in the power devices. 

Drives vary in the complexity of their designs, but the designs continue to improve. Drives come in smaller packages with each generation. The trend is similar to that of the personal computer. More features, better performance, and lower cost with successive generations. Unlike computers, however, drives have dramatically improved in their reliability and ease of use. And also unlike computers, the typical drive of today doesn't spew gratuitous harmonics into your distribution system-nor does it affect your power factor. Drives are increasingly becoming "plug and play." As electronic power components improve in reliability and decrease in size, the cost and size of VFDs will continue to decrease. While all that is going on, their performance and ease of use will only get better. 

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AC Variable Frequency Drives

AC drives have been around for many years in one form or another. Dating back into the 50’s and 60’s they we operated by vacuum tubes. Of course today we have IGBT’s (Insulated Gate Bi-Polar Transistors) that allow the usage of 460 Volt and higher voltages to be applied to motors. You may not be able to imagine the size of the older drives and how they occupied large amounts of space for even a small amount of Horsepower. Take for example an 100 HP drive would have required a room 12 or 14 foot long whereas today the same 100 HP would only require 3 to 4 foot of wall space, and they are getting smaller everyday. Another area of advancement has been in the performance area. The older drives were limited as to the load they could start. The stating torque was limited to on 100% of full load torque or less. Today drives can do 200% starting torque or more. This has benefited in the use of AC Drives on heavy equipment and difficult hard starting applications.

 

AC Drives have opened up many applications that use to be driven by DC Drives or large Mechanical Drives. Now the applications are limited only to a few very large systems that still require, mainly from a dollar standpoint, the use of the other types of systems. In the HVAC, Water, and Waste Water industries the use of AC Drives has not only brought about more efficient process control, but when used with centrifugal pumps and fans there is significant electrical energy savings achieved in operational costs. The drives actually pay for themselves in a short period of time through electrical energy savings. Still the main thing that the AC Drive has done is to optimize the control of a process. By optimizing a process to the right speed for the process, or being able to change the speed for various processes used by the same piece of equipment, the AC Drive saves in productivity and ease of operation which adds dollars to the bottom line of a business. Some examples of these would be in Mixing, Conveying, Cold Forging, and Presses.

 

Over the years we have seen AC Drives also become less expensive. This has lead to even more applications now using AC Drives. In some cases they have taken the place of Motor Starters. Because of the unique way an AC Drive starts an AC Motor by varying the voltage and frequency, the drive allows for a controlled acceleration of a load or soft starting. Even if the application does not need variable speed but would benefit from a controlled start the AC Drive has advantages over the basic Motor Starter. Even though the across the line Motor Starter will be less expensive, an AC Drive may save the user by providing less mechanical stress on the driven equipment. There are also Solid State Starters that will give a Soft Start, but they may have a problem starting some loads due to the nature of how they operate. I’ve often used an illustration to show the difference in across the line starting versus using an AC Drive. Have you ever had to push a car? You don’t back up 30 feet and run toward the car as fast as you can, because you will end up with something broken and in a Hospital and the car will not have moved an inch. The correct way is to lean against the car and gradually apply pressure until the car begins to move and then you can gradually pick up speed until you get the car off of the road. This is how an AC Drive starts a load, by applying voltage and frequency together in the right proportion that the load begins to accelerate. So, for a few dollars more you eliminate shock loads on coupling, gears, and other components in a drive train.

 

AC Drives may have many benefits for the user, but they still need to be applied properly depending on the application. If you think you have an application that an AC Drive would benefit your process then you should talk with an AC Drive Applications Engineer (I just happen to know one, ME). Don’t listen to a salesman say that this drive or that drive will work if they don’t know the application. And don’t be fooled that all AC Drives are all alike, they aren’t. One common mistake is sizing a drive by Horsepower only. There are some motors that have a given Horsepower, but their full load amps and starting amps are way above the amperage capacity of a drive of the same HP rating. Most distributors have inside sales people who only know part numbers and very little else. Know who you are talking with and discover if they are just giving you what you want, or are they offering you a solution for your application and a drive that will meet your needs, and one that will give you years of excellent service. My grandfather use to tell me that, “you get what you pay for”, and this is very true when it comes to AC Drives. Price alone should never be the deciding factor when purchasing an AC Drive.

 

AC Drives do wonderful things, but there are some problems that you should be aware of when you are making your selection. The installation and location of a drive are very important. There are some many potential problems that I don’t have time to cover in this format, but you should again find an expert and discuss the down side of AC Drives and some of the pitfalls that you might encounter when installing and operating a drive.

 

I’ve got 30 plus years in this business and if you need good application help please give me a call and ask for Syd, not many guys left that go by that name, or with that amount of experience.

www.sordselectric.com

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