The technology of variable frequency drives (VFDs) has evolved into a highly sophisticated digital microprocessor control, along with high switching frequency insulated gate bipolar transistors (IGBTs) power devices. This has led to significantly advanced capabilities - from the ease of programming ability to expanded diagnostics. Further, the two most important advantages include
That of reduction in cost and increase in reliability, in addition to significant reduction in physical size.
Working mechanism of VFDs
Generally, the speed of an induction motor is given by:
Nr =( 120f / P ) (1-s)
where Nr is the motor speed, T is the supply frequency, p is the number of poles and s is the slip. Here, speed is mainly the function of f and p, while s depends on the load and, hence, cannot be used for controlling speed in wide variations. Consequently, by either varying the frequency f or number of poles p, the speed of induction motor can be altered. Speed range is usually defined as the ratio of the maximum operating speed to the minimum operating speed.
Before the advent of power electronic switching devices, it was not so easy to control f . The only way to control the supply frequency was to vary the number of poles. This is known as the pole changing method, which is used to control the speed. The drawback of this method was that it did not provide smooth speed control. Besides, the stator had to be wound for several poles that were used to achieve various speeds. This made the stator unnecessarily large and costly. As a result, this method did not find much industrial application. Also, this method could not be used for varying the speed of rotor wound induction motors. This was due to the fact that the rotor could not adjust to varying number of poles with wound stator.
In fact, varying the value of inserted resistance in rotor winding can also alter the speed of wound rotor induction motors. But, it results in highly inefficient operations and, hence, has hardly been used.
All VFDs have the same structure, which includes a rectifier, filter and inverter. The rectifier converts 3-0 AC line power to DC power. The components used in the rectifier are typically thyristors and/or diodes. The filter sits between the rectifier & inverter and provides harmonic and power ripple filtration using inductors and capacitors. These components work to respectively smoothen and regulate the current and voltage supplied to the motor. The inverter portion typically consists of thyristors or IGBTs that are carefully controlled to sequence the proper voltage and current to the phase windings of the motor, depending on the speed and load required. The smoother the DC waveform, the cleaner the output waveform will be to the drive. In case of IGBTs, control circuit is connected to the gate, but this circuitry is less complex and does not require polarity reversal.
Inverters provide the user with tremendous flexibility by controlling two main elements of a three-phase induction motor, i.e., torque and speed. Motor speed and many operating parameters may be infinitely adjusted for precise process control. Some of the other types of inverters include current source inverters, voltage source inverters; load commuted inverters, PWM inverters, vector control, etc. Each type of inverter has its own sets of advantages and disadvantages and should be selected according to particular requirements. The present economics favours PWM drive for applications under 200 HP.
The fundamental task, while selecting a motor in a variable speed application, is to match the torque speed capability curve of the motor to the torque speed requirement of the load. A motor can produce certain amount of torque continuously without exceeding the stipulated temperature rise up to a certain speed. The continuous torque producing capability gets reduced at the lower end of the speed range. At lower operating speeds, less cooling air flows through the motor or over its surface due to the reduced speed of the motors self-cooling fan and/or rotor fins. Additionally, a motor must dissipate additional heat while operating at variable frequency, as it operates less efficiently on variable frequency than on a purely sinusoidal waveform. It means that the full load torque capability of the motor on non-sinusoidal waveform will be lower than the torque producing capability on sinusoidal wave form.
A motor can be selected for an application so that its continuous torque capability curve will be above the torque v/s speed demand curve of the load throughout the operating range. Also, the intermittent torque capability of the motor must be enough to supply the torque required at start and accelerate the load and to drive the load momentary overload situation. Design changes that improve sine wave performance sometimes degrade variable frequency performance.
The decreased cooling capability, along with losses due to harmonics, combines to cause the motor to run hotter as its speed decreases. Therefore, a separately controlled fan is generally installed to ensure enough cooling of the motor under low speed operation. The motor insulation system should be designed to withstand higher peak voltages and fast rates of voltage change that are found in variable speed applications. Besides, high frequency ground currents that flow through the shaft & bearings and damage them should be considered. Further, the design should also include features that minimise the effect of non-sinusoidal waveform on mechanical vibration and acoustic noise. The inverter duty windings feature longer core lengths and a higher thermal reserve to allow speed turndown to 4:1.
As VFD sends out several thousands pulses each second, the switching frequency can pose as a problem. This is especially true if the cable between the drive and motor is more than 50 ft long. In such cases, a reflected wave and incident wave can meet at the motor terminals, effectively doubling the voltage that surges into the motor winding.
Even today, the application with the greatest amount of energy savings continues to be centrifugal pumps and fans. lf VFDs are retrofitted for controlling centrifugal pumps and fans, the potential for energy saving is as high as 30% depending on the original design philosophy of the pump or fan system, flow modulation method, duty cycle and electricity cost. Proper evaluation is critical to accessing and correctly applying VFDs.
Constant torque loads require the same torque regardless of speed. Examples include reciprocating compressors and conveyors. Although constant torque loads are suitable for VFDs, operation of these loads at low frequencies are limited, and the VFD must carefully be sized to ensure adequate starting torque as well as adequate torque & cooling at low speeds. The main problem with VFDs is that their torque producing capability also goes down with the decrease in speed. In vector control drives, an effort is made to resolve this problem.
Loads in which torque decreases with speed, usually involve very high inertia loads like vehicular drive or flywheel-loaded applications. It takes less torque to keep these loads turning than to accelerate them. Though such loads are difficult, they are not impossible for VFD applications. Custom-engineered solutions are often required to handle the extra heat generated during starting and stopping such loads.
As with any variable speed system, it is important to determine any mechanical resonance frequencies and to programme the VFDs to avoid steady operation at these speeds. These resonance frequencies - common in large fans, gears and in belt driven systems - can identify these frequencies by monitoring noise and vibration while instructing VFD to gradually increase the speed from low to high. When used as part of the regular system maintenance, this technique can reveal weaknesses in bearings, fan or impeller balance, bent shaft and other problems that may escape at constant speed. Hence, the nature of the variable frequency power source and the complete range of operating conditions must be carefully considered to ensure that the application is successful.
Motor VFD compatibility
To ensure that the VFD and motor are compatible, both motor and drive should be purchased from the same company or the VFD should be designed and tested by the manufacturer.
VFDs eliminate the need for energy wasting throttling mechanisms like control valves and outlet dampers. These are mainly installed on loads where torque increases with speed including centrifugal pumps, fans, blowers, compressors, etc. VFDs are used to improve process control, which in turn increases the efficiency of an operating system.