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.

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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.