Save Money with Variable Speed Drive now!

Variable speed drives are basically a green energy savings product that matches the amount of work or load on a motor to the amount of energy it needs to power that amount of work.  This reduces excess energy from being wasted.

We use a lot of energy in this country and most of that energy is used to move air and water around a building.  About half the electricity in commercial buildings is just used to move air and water around, so a variable speed drive is a big way to save energy there.  If you look at a typical pump motor the life cycle cost of a pump, 90% of its life cycle costs is the energy it consumes and only 10% the actual cost of the pump motor… a variable speed drive can cut that in half.

Most motors are oversized to deal with your worst-case scenarios- your peak loads.  A variable speed drive allows you to run that motor at the load it needs to be instead of running it at peak load all the time.  Another benefit is it has a built-in soft start capability. So those combinations of things are going to give you savings; not only on energy, but also extending the life of the motor.

So what is a variable speed drive?  Some of the different names that are used in the industry are a variable speed drive, an adjustable frequency drive, a variable speed drive, or an adjustable speed drive.  The technology has been around for quite a few years but only has started to make some headway in HVAC and pumping applications in the last several years.  The size and cost of electronics has made variable speed drives applicable to a wider range of motors and increased the opportunity for savings.

All variable speed drives are going to take 3 phase AC power and convert that 3 phase power to DC power inside the drive and pulse it out in a simulated AC wave form to the motor.  The motor still thinks it has AC power but the DC power conversion now lets us control the speed of the motor without harming it.  Now we are in control to save energy and money.

The basic concept with the savings for variable speed drives is your speed and your flow are more or less proportional.   But the energy consumption is cubed. If you're running your motor at full speed 60 Hertz, you don't have any savings- but any reduction pays the reduction cubed.

But if you're even able to take that oversized motor down to 90%, maybe even still running at a constant volume let's say, cause you haven't changed out the entire system; but you just don't need to run at full speed most of the year. However, if you can run at 54 Hertz  or a 10% slow down output from the variable speed drive, your savings is 27%. How is that?  Well, if you cube 90% or .9 by multiplying .9 X .9 X .9  -the net result of that is .729.  So now you're only using 72.9% of the energy you were using.  The difference between that and 100% is your savings so with a 10% reduction in speed you save 27%.   If you're at half speed or 30 Hertz it's even more. Multiplying .5 X .5 X .5 is .125 so you're only using 12.5% of the electricity.  Below this you're really not going to get any significant additional savings.

What a variable speed drive does is match the amount of energy your motor needs to match the amount of work that is being done.  This saves energy- a lot of energy!
Great applications are air handling units ,industrial  process cooling pumps, chilled water pumps, hot water circulation pumps, cooling tower fans, return air fans, chillers, air compressors, circulation and supply pumps, combustion blower fans, injection molding machines- there a lot of good applications and you might even qualify to get one with no investment and have it paid for out of savings through a Green Energy NegaWatt Savings Agreement. The first step is for an energy service consultant to review your various motor opportunities and get an audit to determine savings.

Follow VSD Facebook Note:

Post by VSD - Variable Speed Drive.

What is Flux Vector Control in VFD?

Flux vector control is another method of speed control and this feature is also available on some standard VFDs which normally operate under V/F control.

There are two methods of flux vector control:

• Open Loop (sensor less vector).
• Closed loop (with encoder feedback).

Flux vector control principles:-
The torque producing and magnetising current vectors are independently controlled. Therefore providing accurate speed / torque control. This is enabled by motor equivalent circuit data of the motor. Motor parameters are entered in the VFD followed by an auto tune function. VFD then uses the auto tune data to produce a software model of the motor, for a closely matched speed / torque control.

Vector control in comparison with V/f provides:-

• Improved speed control.
• Dynamic torque response. (Response to rapid changes of input).
• Higher starting torque.
• Accurate speed control.

Closed loop control Can provide full torque at zero speed With encoder feed back.
Sensor less vector control can provide accurate torque control down to 3 - 4 HZ

Typical Speed accuracy :-

0.3% is achievable with open loop (Sensor less vector).
0.03% with closed loop (Encoder).
The closed loop vector option has additional cost elements i.e. encoder and its installation. Sensor less vector can be used for most applications, however when speed and dynamic torque control at very low speeds are required then must consider the application closely prior to selection of the of the system.
Motor Current (I) is made up of two Current Components
(I) Reactive or Inductive Induces magnetic field in stator and rotor. This is the flux producing current (magnetising Current).
(I) resistive is the torque producing current.
VFD can vary the I Reactive, hence total current and power factor angle.

VFD Operation

Although VFDs offer many application benefits such as energy efficiency, reduced stress on motors and equipment, diagnostic capabilities, and process control integration—the primary reason to use a VFD is to control motor speed. The bonus is that lowering motor speed usually increases energy efficiency.

For example, using VFDs to lower speed or flow by just 20% can potentially reduce energy use by 50%. Although a VFD is typically about 95% to 97% efficient—its ability to directly vary motor speed instead of using dampers, flow controls and blocking valves usually results in an overall increase in system efficiency along with lower maintenance costs.

A VFD rectifies the AC line voltage into DC voltage, which is applied to a DC bus. The DC bus voltage is “inverted” into pulsed DC, the RMS value of which simulates a sine-coded AC voltage. The result is a pulse-width-modulated DC output gated through insulated-gate bipolar transistors.

The output frequency of a VFD typically varies from 0 to the AC input line frequency. However, on some applications, the frequency can exceed the line frequency to beneficial effect. Like RVSSs, VFDs use CTs to measure motor current. Sensing current allows VFDs to implement motor overload monitoring, stall prevention, and torque and current limiting.

VFD speed control error

First, speed error is generally due to changes in torque demand. In an induction motor, this error is mostly slip. So the question becomes, how well does the drive compensate for torque induced slip speed changes. With a good vector drive, this can get down in the range of one-tenth of motor slip without an encoder. If you need better than that, an encoder is required. Note here that the error is a result of torque changes. If your torque doesn't change, you won't have much speed error to start with. Second, in some applications, especially those involving web products and tension control, cumulative error is just as important as actual error. For example, even if you are very accurate with actual error, if it is all negative or all positive, eventually you are going to have too much or too little tension. No encoderless system will assure non-cumulative error. For that you need an encoder. Third, speed reference error is often overlooked. That is error either in the speed signal going into the drive or error in the drive translating the input command into an actual output speed. Usually, the majority of this error is due to the analog input terminal analog-to-digital conversion. A 10 bit resolution A/D input will not be nearly as accurate as a 14 bit resolution input. This is a matter of purchasing a drive with the input resolution adequate for the intended purpose.

Vector VFDs

A standard VFD (lets call it a Scalar Drive) puts out a PWM pattern designed to maintain a constant V/Hz pattern to the motor under ideal conditions. How the motor reacts to that PWM pattern is very dependent upon the load conditions. The Scalar drive knows nothing about that, it only tells the motor what to do. If for example it provides 43Hz to the motor, and the motor spins at a speed equivalent to 40Hz, the Scalar Drive doesn't know. You can't do true torque control with a scalar drive because it has no way of knowing what the motor output torque is (beyond an educated guess). These problems associated with the scalar VFDs inability to alter it's output with changes in the load gets worse as the speed reference goes down, so the "rule of thumb" in determining the need for which technology to use is that scalar drives work OK at speed ranges between 5:1 (50Hz applications) or 6:1 (60Hz applications). So if your application will need accurate control below 10Hz, scalar may not work for you. A Vector Drive uses feedback of various real world information (more on that later) to further modify the PWM pattern to maintain more precise control of the desired operating parameter, be it speed or torque. Using a more powerful and faster microprocessor, it uses the feedback information to calculate the exact vector of voltage and frequency to attain the goal. In a true closed loop fashion, it goes on to constantly update that vector to maintain it. It tells the motor what to do, then checks to see if it did it, then changes its command to correct for any error. Vector drives come in 2 types, Open Loop and Closed Loop, based upon the way they get their feedback information.

VFD and Motor System

Variable frequency drive also called variable speed drive (VSD), frequency inverter, AC drive etc. It is an electric device to change utility power source to variable frequency to control AC motor in variable speed operation. There are several ways to define a VFD. Base on main circuit working methods, it can be divided into voltage type VFD and current VFD;

VFD in pulp & paper machines

Overall
The system is with the control method of PLC combined with VFDs. Each drive point is with buttons for site speed regulation. The block diagram of the system is shown as Figure 1.


Figure 1. System Block Diagram

The VFD is Gozuk EDS1000 series. The whole system can provide the following functions:

• The drive points of each individual shall be able to start and stop separately and shall be adjustable for their speeds;
• The complete machine shall be sync speed increasing and decreasing;
• Speed regulation of one individual can result in a sync speed regulation at this individual and its subsequent individuals;
• The speed shall be automatically recovered as original operating speed after paper tensioning;
• The maximum design speed shall be adjustable according to production requirements;
• The system shall be with emergency stop function.

Control of Speed Chain
According to the paper machine and production process flow, papers on the paper machine prolong longitudinally at wet end due to traction action; papers continue to prolong longitudinally when the dry end starts; after the water content of papers reduces, such prolonging reduces; and when papers enter paper calendaring and reeling machines, they prolong once again due to traction. Therefore, in the whole production line of the paper machine, the speed of each individual is different so as to keep tension for paper web. Meantime, the speed of each individual of the paper machine shall be adjustable so as to avoid loose of paper web or breaking caused by tensioning, with speed regulating range of each individual: 10-15%.



Figure 2. Calculation and Control of Speed Chain

After transmitting the speed command of drive points to VFDs, visit location register to determine the node number of the sub-register. If the node number is not "0", conduct corresponding processing for this node until the whole chain is processed completely. After that, check the node number of brother registers and process another chain. Thus, it is only required to initialize the location register to form any branch speed chain.

As shown in Figure 1, the first individual point of the paper machine is regarded as the main node of the speed chain, namely its speed determines the working speed of the whole paper machine, so regulation of its setting speed is to regulate the speed of the complete paper machine. For example, in PLC, if speed regulation signal is detected, then change speed unit value; the speed at "1" point is just the operating speed setting value of the first VFD, which is sent to the first VFD for execution and to the second for calculation. The speed value of the first individual is multiplied with the ratio of the second individual (b1×a) is the setting value of the second VFD. If the speed of the second individual cannot meet operating requirements, this means the ratio of the second individual is not suitable. You can regulate the ratio of the second individual (b1) to meet required operating requirements. This regulation is equivalent to be with a high-accuracy gear box in PLC so that any stepless speed regulation is available.

During normal production, if the ratio is suitable and it is required for paper tensioning or releasing for some reasons, press appropriate buttons of this individual, and then PLC will add one positive or negative offset to appropriate speed chain to realize such paper tensioning or releasing functions. In the figure, the "2" point includes speed values for commands for speed regulating and paper tensioning and releasing etc., which are send to the second VFD for execution and to the next step for calculation at the same time. And so forth, the control system of speed chains is formed.

Reeling Tension Control
Tension control shall be used for the paper reeling part of the paper machine. If the paper machine has high production requirements for papers, the tension close-loop control can be added in the calendaring part. The tension control is with two methods available for selection: 1) the close-loop control with direct tension detection; 2) the close-loop control with indirect tension calculation and testing. The VFD for tension control shall be with EDS1000 series VFD.

1. Close-loop Control with Direct Tension Detection
The EDS1000 VFD module has two levels: one level for universal functional modules such as PID control, multi-step frequency, and automatic energy-saving operation etc.; another level for special functional modules such as location control, textile application, and constant-pressure water supply application etc. The EDS1000 VFD also has rich programmable modules with complete functions and flexible programming, including: 1 two multi-function comparators that can define faults by themselves; 2 two logical units that can carry out calculations such as "and", "or", and "xor"; 3 two timers that can realize various time-delay functions; 4 one counter that can preset values and can save data after power off; 5 four arithmetical units that can add, subtract, multiply, and divide and can calculate absolute values.

Besides, EDS1000 variable frequency drive also has built-in process PID with complete functions, which is essential in close-loop tension control. Details are shown below:



The input and feedback channels of PID have many categories for selection, and the feedback signals also can be set as many types of calculation results that are calculated from analog values. The PID can be preset and has two sets of parameters that can be switched over during operation. Users can freely carry out programming for resources of EDS1000 VFD, not only able to use its programmable functional modules to coordinate with special functional modules, but also able to use these two types of modules to realize special functions for various industries. The programmable functional modules of the EDS1000 VFD are like a group of jigsaw puzzles which can form numberless ideal patterns in users' hand. This makes it be able to provide solutions platform and integration solutions for various industry requirements and thus it is very valuable for reduction of system cost and increasing of system reliability. more...