Medium Voltage VFDs

Medium Voltage Variable Frequency Drives (VFDs) are key components in the realm of industrial automation. They provide precise control and efficiency in the management of electric motors. In this article, we delve into what Medium Voltage VFDs are, key manufacturers in the industry, applicable specifications, and how to select an appropriate VFD. 

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Understanding Medium Voltage VFDs

At their core, Medium Voltage VFDs operate on the principle of variable frequency drive technology. They control the speed and torque of electric motors by adjusting the frequency and voltage supply to them. This dynamic control enables seamless adaptation to varying load conditions, optimizing energy consumption and prolonging the lifespan of equipment. Generally, the design of Medium Voltage VFDs is beneficial to motors ranging between hundreds to several thousands of horsepower. These drives operate at voltages from V, which is classified as medium voltage according to ANSI C84.1. While in some heavy-duty applications, voltages could be as high as 69kV AC. VFDs can either be of the current source inverter (CSI) type or voltage source inverter (VSI). However, the VSI is more popular due to its high reliability and low harmonic distortion.

Common Manufacturers of Medium Voltage VFDs

There are a host of Medium Voltage VFD manufacturers in the industry, but this section reviews some of the most common.

ABB

ABB is a global leader in power and automation technologies, offering a diverse range of medium-voltage VFDs. Drives range between 200 kW to 150 MW, and even more, depending on project demand. VFDs from this manufacturer are tailor-made to meet demands for applications in several industries including marine, chemical, power, water, mining, as well as oil and gas. Common features of VFDs from this OEM are:

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• Arc-resistant design.
• Fuseless design.
• Combination of air and water cooling systems.
• Softstarter.
• Constant network power factor across the entire speed range.

Siemens

Siemens has established itself as a prominent player in the medium voltage drive market, delivering innovative solutions via its SINAMICS Perfect Harmony product line. Some features associated with this product line include:

• Advanced Cell Bypass: Can bypass multiple failed cells in less than a quarter of a second to maintain a balanced output voltage.
• Synchronous Transfer: This serves to soft-start multiple motors in a series. In addition, it efficiently transfers them across the line without stressing the power grid. Synchronous transfer increases energy efficiency, as well as protects motors and equipment from excessive torque transients.
• Clean Power Input: SINAMICS Perfect Harmony VFDs meet the most stringent IEEE-519- requirements for voltage and current harmonic distortion. It utilizes an integrated sinusoidal converter to eliminate the need for harmonic filters and power factor correction capacitors.
• Process-Tolerant Protection Strategy (ProToPSTM): Unlike typical systems that trip the drive and shut down automatically due to a malfunction, ProToPSTM offers proactive control for applications where failure avoidance is critical. With a proven record of 99.99% process uptime, it protects systems from faulty sensors or data.

Schneider Electric

Schneider Electric provides medium voltage VFDs, ranging between voltage classes of 2.4kV to 6.6kV, while the maximum power rating ranges from 241 hp to hp. VFDs operate within a wide temperature range from -25℃ to 70℃. They can also operate at altitudes up to .84 ft without derating, and up to .68 ft with 1% current derating per 328ft.

Toshiba

Toshiba is an industry leader in providing Medium Voltage VFDs with features such as multi-level Pulse Width Modulation (PWM) with Neutral-Point Clamping (NPC). This enables a smaller footprint, lesser number of components, and lower cost than many competitors. Its designs are also robust, with the MTX being the world’s first drive specifically for outdoor mounting in remote applications.

How to Select Medium Voltage VFDs

Selecting an appropriate VFD for an application ensures maximum long-term payback and minimum initial cost. To get the selection process right, there are key factors that are divided into two major groups: Electrical Factors and Design Installation Factors.

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Electrical Factors

• Process Loads: In VFDs, process loads are classified into constant torque and variable torque. Constant torque (CT) loads include equipment such as conveyors, crushers, and grinders. While variable torque (VT) loads are typical of pumps and fans. Generally, VFD suppliers provide 10% additional overload capacity above full torque and ampere rating for VT loads. For CT loads, there is usually 50% additional capacity, with both additional capacities limited to a time of 60 seconds. Other factors to consider include the load profile, duty cycle for the process to be powered, and regeneration requirements.
• Drive System: For the drive system, it is necessary to define the system voltage, motor voltage, and VFD waveform. Inform the supplier if the motor to be powered is new or existing, as this will determine the drive input voltage. Although Medium Voltage VFDs without isolation transformers are cheaper, they are prone to malfunction and damage from lightning and line voltage surges. So, it is advisable to insist on having isolation transformers.
• Power System: When assessing power requirements, there are two roles that VFDs play. One is a power consumer, so the power supply in the facility must enable the VFD to operate reliably. The second role is as a neighbor to other equipment in the facility such as switchgears and switchboards. When VFDs draw power, they create harmonics – distortions in voltage and current. These harmonics could cause malfunction and extra heating of other equipment connected to the same power system. Choose a VFD that keeps harmonic values to a minimum. IEEE 519- recommends 5% or less of fundamental current, and 5% or less distortion on voltages produced by this current. Review the VFD supplier’s power factor impact on the facility’s power system, and take advantage of modern power factor correction technology.

Design Installation Factors

• Physical Environment: Most medium voltage VFDs operate indoors in a clean and corrosion-free environment. However, they could also function outdoors where the conditions often become extreme. Depending on the location and conditions, the influence of derating should be applied to its capacity. Air-cooled VFDs typically require about 0.85 tons of air-conditioning per 100 hp of load. As load requirements increase, it may be profitable to shift to water-cooled VFDs, especially if cooling water is available. It is necessary to prevent dust from accumulating on electronics and heat sinks on the equipment.
  • Auxiliary Equipment: Some applications require additional equipment to ensure reliability. For example, motor-mounted tachometers are a requirement when applications need to reach values higher than 150% of torque rating. It is necessary to review the possibility of including such auxiliary equipment with the supplier.
• Maintenance and Troubleshooting: Currently, computer tools for maintenance and troubleshooting have many desirable features. Some of these include built-in trending, fault logging, and notifications when trouble occurs.
• Connections: There are a variety of communication protocols available when connecting control systems to a VFD. The supplier needs to clearly define interface requirements.
• Restrictions and Requirements: Usually, medium voltage VFDs are tolerant of input and output connections and cables. Manufacturers recommend EMI shielding on output cables and proper grounding of the equipment.

Re: Motor voltage?

It would be a code violation to operate a 440 v rated motor on a 460 volt system:
110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

A motor operated at 10% about its rated voltage will draw excessive current, while its a 460 volt system, typical secondary voltage is 480 or higher.

AS the voltage increases, the magnetic field saturates and above 10% overvoltage the current increases.


More information from nema at:
http://www.nema.org/index_nema.cfm//E77E-D3BC-494F-A0D0CE6E55B99A74/

Please see additional comments at the end.

[ January 27, , 03:34 PM: Message edited by: tom baker ] Re: Motor voltage?

I was involved in the electric motor business for over 20 years ( motor rewind and sales up to 500 hp) and never had any problem with motors rated 440 volt on 460 or 480 volt networks. The pre nema frames and then nema frames were all rated440 volts. 460 volt was the standard for all motors later on when nema U frames were introduced. The nameplate service facotors on pre nema and nema frames was as high as 1.35. This allowed for much larger voltage and even frequency variations. The amount of core Iron on these older motors was substantially more than is used today. Newer motors use a higher grade of lamination iron and thinner plates allow for better heat dissapation wheras the old units relied on bulk to absorb heat and prevent the windings from overheating. Todays insulation on magenet wire is also far supeior to what was used in the distant past. In my opinion an older motor will operate just fine if operated within the NEMA standard of 10% above or below nameplate voltage. I disagrre that it is a code violation to use this motor on 480 volts. I believe the code contradicts itself on this issue. Nearly all thre phase motors today are nameplated 460 volts so if you interpreted the code to preclude use of these motors on 480 volt networks you would be hard pressed to find anything compatible with the 480 volt rating.I believe the code allows for this when it also mentions that elect4rical equipment be applied and used per manufacturers instructions. Anyway just my opinion as a motor shop guy. Re: Motor voltage?

Joe I agree with you I have used 440 and 460 volt motors on 480/277 systems a hundred times through the years I just wanted some other opinions on what people think.

I agree you will not find a 480 volt motor in the Grainers Cat. But a lot of supply houses do carry them.

I also think if a 440 volt motor say a 5 horse is driving a 5 horse load and you apply 480 to it would cause it to pull more amps in turn caus it to over heat and shorten its life. And also the users power bill will be higher.

Thanks for the reply Re: Motor voltage?

Ron I still caNT FIND ANY MAFG IN ANY OF MY CAT INFO BALDOR,LINCOLN GE,WEST.,DOERR RELAINCE etc that offers a motor at 480 volt. If you find one let me know. Actually when the voltage rises nominally say 10% or less above nameplate the current drops.It is only when you reach the "knee " of the saturation curve that cureent starts to rise and it does so dramatically until all the smoke comes out! Theoretically if you can keep the smoke inside the motor it will last forever. Thats what our job was as a rewind shop to put the smole backe inside. Ha ha.All that smoke is what kept us in businees wheas some peop-le hated the smell of a burned up motor I loved it It smelled like money in the bank. Actually voltage imbalance is worse than hi voltag. A 5% voltage unbalance between phase can cause as much as a 20 to 25% increase in amps at rated load. Hope Ive helped in some small way. Joe Re: Motor voltage?

Tom's reference to 110.4 seems to make the practice of using 440 or 460 motors on 480 a code violation even though we all seem to agree it is common practice.

110.4 Voltages.
Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.

If we look at Table 430.150 part of the text before the table says:

The voltages listed are rated motor voltages. The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480, and 550 to 600 volts.

For 440 to 480 we use the 460 column.

So does this text override 110.4? Re: Motor voltage?

I think we need to consider the definition of Nominal.

It is a loose term use to specify a particular operating voltage out of many and which may vary in that particular range.

The code specifies that you use the nominal voltage to do your calculations with not that the voltage has to be that very voltage, if it did we would be hurting in this area its never 208 0r 240. Its usually over the nominal.

Just my two cents. Re: Motor voltage?

Ok I may be wrong on allowing a 440 volt motor on a 480 volt system.
But, its clearly a violation to use a 200 volt (for 208) rated motor on a 240 volt system with utilization voltage for the motor of 230 v.

Motors are rated with a utilization voltage that is lower than the sytem voltage, 200 for 208, 230 for 240, to allow for voltage drop from the source.

What about 440? Hmmm, not sure, its a secondary voltage that went away a long time ago. 110 and 220 still exist at the big box stores, like this:
Homeowner: I need to install a 220 v plug for my welder.
Big box sales person: Sure you'll find the 220V plugs on aisle 6 right next to the 110 v outlets.

What I would be concerned with is the motor current does increase if the applied voltage is more than 10% above the motor rating. So a 440 V motor may ok in one location, but not another. Re: Motor voltage?

Yes its a great tool to our trade.I know I learn something about every time I open this forum up.

Tom I don't know where or which motors have an increase of current draw when you run it at a higher voltage most don't but some do.

From my experience I have found that motors which create their own load such as a fan motors do.

Many a times I have went on trouble shooting calls and had to up the heaters in the starter because of this.Usually when I measure the voltage it is on the high side.

To many people in this field are taught that if you increase the voltage on a motor it will automatically decrease the current draw or reduce the amperes and this is not always true.

Ronald Re: Motor voltage?

Ronaldc, in my last response I stated that if the motor was delivering CONSTANT HP and you increased the voltage, the current would go down. In certain situations, it is possible that the motor would draw more current with a given increase in voltage. Constant HP is delivered when the motor supplies constant torque at a constant RPM. A good example is a motor that powers a conveyor system that runs at the same speed all the time and carries the same load. The power output is always the same and any increase in motor voltage (pressure) will require less current (flow) to maintain the same amount of work. Many loads are not constant and are inverse as far as torque is concerned. A example is a centrifugal pump (or fan) motor as compared to a motor powering a flywheel for a stamp machine. Centrifugal pumps require low starting torque (hence less motor current) during start and gradually increase in load as the pump develops head pressure and starts moving fluid. After the pump develops it's running head pressure, the motor torque will remain constant as long as the head pressure on the pump remains the same. If the head pressure increases, the flow (and the required current) will go down. Reduce the head pressure and the opposite is true. The flwheel is the opposite and requires high starting torque (more current) to get it moving (inertia) and the required accelleration torque gradually decreases as the flywheel comes up to full speed and develops equilibrium with the load. At that point it remains constant. The motor powering the pump will draw less current at higher voltage (during start and run) with acceleration (basically)remaining the same. The flywheel motor will draw higher momentary current with a higher voltage due to the higher inrush and locked rotor current developed during starting as it tries to accelerate faster. I didn't get this from a text book. This is what I experienced in my 25 years designing, installing and serviceing industrial machinery. It may lack some refinement, and I look forward to discussing it more on this forum.
We're all here to learn.
steve