Variable Frequency Drives
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
Vector drives 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 drives 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 drives 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 drives without the reactors have
a very poor power factor. NB Most 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
Drives 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 drives 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.
The 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.
to DC Converter
The 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.
The 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
to AC Output Inverter
The 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
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
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.
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
The Braking resistors need to be sized to suit the drive (resistance)
and to suit the load (Brake energy).