Speed Control of Induction Motor
Induction motor is the workhorse of the industry. It is cheap rugged and provides high power to weight ratio. On account of high cost-implications and limitations of D.C. System, induction motors are preferred for variable speed application, the speed of which can be varied by changing the supply frequency. The speed can also be varied through a number of other means, including, varying the input voltage, varying the resistance of the rotor circuit, using multi speed windings, using Scherbius or Kramer drives, using mechanical means such as gears and pulleys and eddy-current or fluid coupling, or by using rotary or static voltage and frequency converters.
Variable Frequency Drive
The VFD operates on a simple principle. The rotational speed of an AC induction motor depends on the number of poles in that stator and the frequency of the applied AC power. Although the number of poles in an induction motor cannot be altered easily, variable speed can be achieved through a variation in frequency. The VFD rectifies standard 50 cycle AC line power to DC, then synthesizes the DC to a variable frequency AC output. Motors connected to VFD provide variable speed mechanical output with high efficiency.These devices are capable of up to a 9:1 speed reduction ratio (11 percent of full speed), and a 3:1 speed increase (300 percent of full speed). In recent years, the technology of AC variable frequency drives (VFD) has evolved into highly sophisticated digital microprocessor control, along with high switching frequency IGBTs (Insulated Gate Bi Polar Transistors) power devices. This has led to significantly advanced capabilities from the ease of programmability to expanded diagnostics. The two most significant benefits from the evolution in technology have been that of cost and reliability, in addition to the significant reduction in physical size.
Variable Torque Vs. Constant Torque
Variable speed drives, and the loads that are applied to, can generally be divided into two groups: constant torque and variable torque. The energy savings potential of variable torque applications is much greater than that of constant torque applications. Constant torque loads include vibrating conveyors, punch presses, rock crushers, machine tools, and other applications where the drive follows a constant V/Hz ratio. Variable torque loads include centrifugal pumps and fans, which make up the majority of HVAC applications.
Why Variable Torque Loads Offer Greatest Energy Savings
Invariable torque applications, the torque required varies with the square of the speed, and the horsepower required varies with the cube of the speed, resulting in a large reduction of horsepower for even a small reduction in speed. The motor will consume only 12.5% as much energy at 50% speed than it will at 100% speed. This is referred to as the Affinity Laws, which define the relationships between speed, flow, torque, and horsepower. The following law illustrates these relationships:
Flow is proportional to speed
Head is proportional to (speed) 2
Torque is proportional to (speed)
Power is proportional to (speed) 3
Additional benefits which are readily seen include: the reduction and/or elimination of motor starters, less stress on the AC motor windings and bearings, and a decrease in stress and wear on the pump or fan itself. This all equates to a smoother, longer lasting and more efficient operation process. When looking to apply VFD control to an existing pump, a basic overview of the application should be investigated. If the original design philosophy was set for the worst case maximum flow condition in a future requirement, or if the original designer used a typical 20% oversizing criteria, there is a great potential for energy savings. However, if there have been expansions, and near full flow requirements are already in use, the potential savings may be limited. Proper evaluation is critical to accessing and correctly applying VFDs.
Flow-generating equipment like fans, pumps and compressors are often used without speed control. Instead, flow is traditionally controlled by throttling with a valve or damper. When fl ow is controlled without regulating the motor speed, it runs continuously at full speed. Because HVAC systems rarely require maximum flow, a system operating without speed control wastes signifi cant energy over most of its operating time. Using VFD to control the motor speed can save up to 70% of the energy.
The mathematical relationship between motor speed, volume or flow, pressure/head and horsepower is not linear. A motor that runs at 100% of its potential speed, volume and pressure will require 100% of its rated horsepower. The same motor run at 80% speed and volume will reduce pressure to 64% and power required to 51%. Sizing a motor that is just big enough for the task will mean that it must run near its maximum load. Sizing a motor larger than the maximum will allow it to run with less power use. If electricity costs $0.08/kWh, a 100-hp motor that is run at 100% speed, 12 hr/day, 360 days a year will cost $27,139 to operate. The same motor run at 60% speed will cost $5,970 a year. Some system fan motors or pump motors are routinely adjusted to save electricity. Motors can be designed to be two-speed or equipped with variable-speed drives (VSDs), but the best way to control fan volume is with a variable-frequency drive (VFD). A VFD controls the rotational speed of the motor by adjusting the frequency of the power supply. It will add capital cost to the equipment. but it could very easily pay for itself in a short time.
The most obvious candidate for a VFD is a spray booth. In a powder booth the VFD can be used to increase fan volume during clean-up and decrease it during spray operation. Cleaning is more effective if the fan pulls a little harder because it helps to keep powder in the booth. During operation, less air is needed so electricity is saved. The fan can be adjusted to suit a particular operation.
A VFD is even more valuable in liquid spray systems. Precise adjustment allows the booth to move just enough air to maintain a clean and safe environment. Air volume can be low when filters are clean, then gradually increased as filters load with paint solids. This approach uses less electricity and exhausts less air. Less exhaust air means that less make-up air is needed, so an added benefit is reduced heating or cooling costs.
VFDs can also be used to accelerate oven purge cycles. The fan runs at a faster speed during purge and then slows to the normal volume when purging is complete.
Several other factors can impact motor efficiency. These include supply voltage, phase imbalances, location of capacitors and correct installation and maintenance. Motor supply voltage should be maintained with a maximum deviation of 5% from the nameplate value. Minimizing phase imbalance within 1% will avoid motor de-rating, and installation of capacitors as close to the motor as possible will maintain high power factor.
Adopt a proper motor maintenance strategy for motors. Control ambient temperature to maximize insulation life and motor reliability, and locate motors in well-ventilated areas. Keep them clean and lubricate according to manufacturers' specifications using high-quality greases or oils to prevent contamination with dirt or water.
Optimize transmission efficiency by assuring proper installation and maintenance of shafts, belts, chains, and gears. When replacement becomes necessary, install energy-efficient units. Always have burnt-out motors rewound by a qualified expert.
Flow control methods in comparison to speed control
Other typical ways to control the flow are: Throttling control with dampers or valves. Using inlet vanes in centrifugal fans to restrict the flow of air into a fan. Using fluid or eddy current couplings to control the torque between the fan and the motor. On/Off control. Pitch adjustment with axial fans, where the angle of the fan blades is altered to change the flow. The downside of traditional flow control is that none directly affects the main power consumer. There are possibilities to decrease the power consumption of some of these components, but none are as effective in energy efficiency as using speed control with a VFD. For example On/Off control will generate much mechanical stress and pressure peaks due to both the extra starts and stops and the current peaks into the electrical supply when the motor is started without the use of VFD. Fig. 3 below compares the power consumption using throttling control with valve or damper and speed control.
How Drive Changes Motor Speed
Just how does a drive provide the frequency and voltage output necessary to change the speed of a motor? That's what we'll look at next. Fig. 6 shows a basic PWM drive. All PWM drives contain these main parts, with subtle differences in hardware and software components.
Figure 6, Basic PWM Drive Components
Although some drives accept single-phase input power, we'll focus on the 3-phase drive. But to simplify illustrations, the waveforms in the following drive figures show only one phase of input and output.
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 superseding SCRs in drives in the mid-1970s. In the early 1990s, those gave way to using Insulated Gate Bipolar Transistor (IGBT) technology.
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