The speed of standard induction motors can be controlled by variation of the frequency of the voltage applied to the motor. Due to flux saturation problems with induction motors, the voltage applied to the motor must alter with the frequency. The induction motor is a pseudo synchronous machine and so behaves as a speed source. The running speed is set by the frequency applied to it and is independent of load torque provided the motor is not over loaded.
Modern Variable Frequency drives (VFD or VSD) come in two major formats, V/Hz and vector. The V/Hz drive is a drive where the voltage applied to the motor is directly related to the frequency. In the ideal motor, the magnetic circuit would be purely inductive and keeping a constant V/Hz ratio would maintain a constant flux in the iron. The real motor has resistance in series with the magnetising inductance. This has no bearing on the operation at line frequency, however as the frequency of the drive is reduced, the resistance begins to become significant relative to the inductive reactance. This causes the flux to reduce at very low frequencies and so it is difficult to get sufficient torque at low speeds. For many applications, this low torque is not a problem, but there are some that do need a high torque from a low speed. Early drives were designed with a voltage boost to provide a measure of torque increase at low speed.
Vector VFD have a mathematical model of the drive in software and by measuring the current vectors in relation to the applied voltage, they are able to maintain a constant field at all frequencies below the line frequency. These drives need to be tuned to the motor and typically include a self tuning algorithm that is enabled at commissioning to determine the component values for the mathematical model. If the motor is replaced, the drive needs to be retuned to learn the characteristics of the new motors.
Vector VFD come in three major formats, closed loop, open loop and direct torque control. The closed loop controllers were the first vector controllers and are still the best option for accurate control at zero speed. The open loop vector and DTC are suitable for applications requiring good control above 3 – 5 Hz.
Quite a number of modern VFDs can operate as V/Hz, open loop vector or closed loop vector just by changing a parameter. – closed loop requires a shaft encoder to give accurate speed feedback.
The major differentiation between modern VSDs are the enclosure, auxiliary functionality, programming and user interface. Low cost drives are often very poorly filtered and can create major RFI (EMC) issues. Some drives include no filtering and must be installed with external filters, and others include all the filtering required.
DC and AC reactors help to reduce the noise generated by the drive, and to improve the distortion power factor of the drive. Because the drive rectifies the incoming supply, the current waveform is very distorted and so the harmonics are high. Low cost VFD without the reactors have a very poor power factor. NB Most variable frequency drive manufacturers quote the COS (phi) as better than 0.95 implying a high power factor. While the displacement power factor is high, the distortion power factor can be less than 0.7 Distortion power factor can not be corrected with capacitors, but can be improved with expensive filters. There are “active front end” drives or “regenerative” drives that have an inverter stage on the input as well as the output and these can draw sinusoidal current from the supply resulting in a high power factor. It is possible that this technology may become a mandatory requirement at some time in the future.
VFDs are typically used in some form of automation process and so they are now including additional functionality and controls to simplify the automation process. There are a number of programmable inputs and outputs and relays and most drives also include a PID loop and a motorised pot is also common. PID information.
Vector VFD and some V/Hz drives can be set up for speed control or torque control. Torque control is used in tensioning applications such as paper machines where the master controls a winding drum and the diameter increases as the drum fills up. This requires other drive feeding the paper to run at different speeds. Traditionally, this was achieved by DC machines as they naturally operate in torque mode.
DesignThe VSD power sections comprise an AC rectifier to convert the incomming power from AC to DC. This is followed by a power DC Filter which comprises a number of high voltage high current DC capacitors commonly in a series parallel arrangment. The DC filter will commonly include one or two DC chokes in series with the rectified DC.
After the DC Filter, comes the Output inverter stage which is made up of a series of solid state switches. There are three arms for a three phase output with two switches on each arm. One switch connects the positive DC bus to the output of that phase, and the other switch connects the negative DC bus to the ouput on that phase. Control of the output switches produces a PWM output waveform designed to cause a sinusoidal current to flow into the motor. There are a number of schemes and algorithms for the generation of the output waveforms, one common algorithm is the space vector modulation technique. The waveform generation is usually done in firmware or in a special function chip.
AC to DC ConverterThe AC to DC converter is a full wave bridge rectifier, single phase or three phase depending on the input requirements. The rectifier can be controlled using a combination of SCRs and Rectifiers, or more commonly uncontrolled using rectifiers only. Because the output of the rectifier is connected to a large capacitive filter, there must be a means of providing the initial charge to the capacitors without damaging the rectifier. - The initial charging current for discharged capacitors connected to the full rectified voltage is very high and would cause rectifier failure.
The initial charge current is commonly limited by a series resistance in one of the DC outputs. This soft charge resistance is shorted out as soon as the capacitors are fully charged. The shorting device can be a relay or contactor, or it can be an SCR. The alternative means of limiting the charge current is to use a controlled bridge and slowly increase the output voltage applied to the filter.
DC FilterThe DC filter provides smoothing of the DC bus applied to the output DC to AC inverter. There must be sufficient capacitance to provide the smoothing required for the output current required. The capacitors must have sufficent ripple current rating to avoid excess heating and life shortening and voltage rating to withstand the maximum expected input voltages. There are two types of DC filter used, a capacitive input filter and an inductive input filter. The capacitive input filter comprises a capacitor bank and an inductive input filter has an inductor in series with at least one of the DC inputs to the capacitive filter.
With the capacitive input filter, current will flow from the supply, through the rectifiers into the capacitors only when the supply voltage is higher than the DC voltage. The result of this is that a very high current flows for a short time at the crest of the waveform only. This results in a very low distortion power factor, lot of harmonics and excessive heating of the rectifier and capacitors. The reason for the addition of the DC Bus Choke(s), is that a lower current flows for longer in each half cycle reducing the harmonics and increasing the distortion power vactor. Another advantage of the DC Bus choke is that it helps to decrease the amount of switching noise that leaks back on to the supply, reducing EMC radiation.
The filter values are very different for single phase inputs and three phase inputs due to the magnitudes and frequency of the ripple currents. For a single phase input, the ripple frequency is twice line frequency and for a three phase input, the ripple frequency is six times the line frequency.
DC to AC Output InverterThe AC output inverter for a three phase output stage comprises six solid state switches. In small low voltage and low current VSDs, the output stages will typically be MOS FETs and in larger VSDs, they are typically IGBTs.
The output switches operate at a high frequency, typically between 3 KHz and 16KHz, and are controlled to produce a PWM output waveform which causes a sinusoidal current to flow in the motor. There are many different pwm schemes and algorithms with different advantages. One common waveform generator scheme is the Space Vector Modulation algorithm. SVM is covered here.
The output voltage must provide both variable voltage and variable frequency control.
Each switching element needs to have a driver circuit that is isolated from the control electronics and is able to provide sufficient energy to fully control the switching elements. In some cases, this would mean three isolated supplies to run the three top switching elements, and one isolated supply to run the bottom switching elements. The circuitry must be capable of withstanding very high rates of change of voltage with minimum delays. Care must be taken to prevent the upper and lower switch on one phase being on at the same time, this includes through the switching stage. This requires an interlock delay between one switch turning OFF and the other switch turning ON.
BrakingRapid slowing of the load can require energy to be removed from the load. This energy goes back into the drive and will result in an increasing DC bus voltage. If the bus voltage goes too high, the drive will be damaged.
The excess energy can be dumped out into large resistors provided that the drive is fitted with a braking module, or can be fed back into the supply if the drive has an active front end. If there are multiple drives in operation but with different duty cycles, it is possible to common all the DC bus circuits and the excess energy can then go into driving other motors.
The Braking resistors need to be sized to suit the drive (resistance) and to suit the load (Brake energy).
