Is there a device that can reduce downtime whilst increasing the lifespan of your electrical motor?
In our NEW series, we’re unpacking Variable Speed Drives.
In Part 1, we introduced you to the basic principle of operation of a VSD, and touched on their key benefits
In Part 2, we give an overview of the types of VSD’s, and delve into how a VSD works
Watch this space to catch the full series!
Types of VSD’s
The three common types of VSD’s are:
- Current Source Inversion (CSI) – used in signal processing and industrial power applications. It produces a clean current waveform but requires expensive inductors and causes cogging below 6 Hz.
- Voltage Source Inverters (VSI) – has a poor power factor, causes cogging below 6 Hz and are non-regenerative.
- Pulse-Width Modulation (PWM) – used most commonly in industry because of high input power factor, no motor cogging, higher efficiency and lower cost.
Pulse-Width Modulation VSD’s
A PWM VSD works by using a series of voltage pulses of different lengths to simulate a sinusoidal wave. These pulses are timed in such a way so that the time average integral of the drive outputs a sinusoidal wave. Insulated Gate Bipolar Transistors are the electronic components used to generate these voltage pulses.
The below diagram depicts a Sinusoidal Wave created by a comparator.
How Does a VSD Work?
The first stage of operation for the VSD is the converter. This converter consists of 6 diodes that allow current to flow in only one direction (indicated by the arrow in the diode symbol).
The below diagram depicts a VSD Converter and Output Waveform.
The electricity will alternate in the supplied phases but the diodes will only allow the peak phase to pass.
These diodes operate in the following manner:
- When A-Phase voltage is more positive than B or C phase voltages, then the diode will open and allow current to flow.
- When B-phase voltage becomes more positive than A-phase voltage, then the B-Phase diode will open, and the A-Phase diode will close.
- This process also applies for the negative side of the waveform resulting in six current pulses as each diode blocks and allows the phases to pass.
Assuming that the drive is in operation using a 480Vrms supply. Then the VPeak of the supply is 679V. As seen in the picture above, the VSD DC bus has a DC voltage with an AC ripple. The below diagram depicts a DC Bus and Voltage Waveform at the DC Bus.
The AC ripple on the DC bus is removed by adding a capacitor resulting in the voltage on the DC bus being approximately 650V. The actual voltage is dependent on the AC supply line, the level of voltage unbalance on the power system, the motor load, the impedance of the power system and any reactors or harmonic filters on the drive. The AC to DC conversion is done by the “converter” whilst the DC to AC conversion is done by the inverter. The below diagram depicts a Converter, DC Bus, Inverter and Output Waveform Supplied to Motor.
The inverter converts the rectified and conditioned DC back into an AC supply of variable frequency and voltage. This is normally achieved by generating a high frequency pulse width modulated signal of variable frequency and effective voltage. Semiconductor switches are used to create the output, the most common being the Insulated Gate Bipolar Transistor (IGBT). We can control the frequency by controlling the timing of the switches (IGBT). As seen in the picture below, a VSD does not produce a sinusoidal output but a rectangular waveform which is adequate for a motor. The below diagram depicts the Output Waveform from a VSD at Various Frequencies.
Protection of VSD’s
Short circuit protection of variable speed drives is obtained by using fast acting fuses in front of the drive to minimize the let through energy in the event of a fault.
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