Variable Speed Drives (VSDs)
What is a Variable Speed Drive?
A Variable Speed Drive (VSD) takes fixed-frequency AC supply and converts this to a variable-frequency AC supply. This controls the power use and mechanical power output so that the motor can run at the most efficient speed for the motor and the process.
Control of the motor speed can be based on feedback from the process, e.g., flow rate, temperature, or pressure, so that process control is improved. This enables accurate control of the motor speed over a broad range.
VSDs thus provide extremely precise electrical motor control so that motor speeds can be ramped up and down and maintained at speeds required (by varying the frequency and voltage supplied to the electric motor). Doing so motors utilise only the energy required rather than running at constant (fixed) speed and utilising an excess of energy.
For a more detailed explanation on this, please see our blog post: What is a Variable Speed Drive?
Naming Conventions used for VSDs
Variable Speed Drives (VSDs) are also commonly referred to as Variable Frequency Drives (VFDs), Adjustable Speed Drives, Adjustable Frequency Drives, Inverter Drives, Micro Drives or AC Drives.
Although EMHEATER refers to Variable Speed Drives (VSDs) due to it being the popular naming convention used in South Africa, the EMHEATER product is in fact a Variable Frequency Drive (VFD), which has some subtle differences when compared to a Variable Speed Drive (VSD). Both VSDs and VFDs are used to decrease an electric motor’s unnecessary power consumption and optimise the efficiency and operation of electric motor systems, but:
- Variable Speed Drives (VSDs) controls the speed of a motor by changing the input voltage and can be used with both AC and DC
- Variable Frequency Drives (VFDs) controls the speed of a motor by changing both the voltage and frequency and can only be used with AC
For a more detailed explanation on this, please see our blog post: Variable Frequency Drive (VFD) vs Variable Speed Drive (VSD)
TCGC distributes the following EMHEATER VSDs:
Single-Phase to 3-Phase Converters
1-Phase (200-240V) Input
3-Phase (200-240V) Output
1-Phase (200-240V) Input
3-Phase (380-440V) Output
3-Phase Input and Output
3-Phase (200-240V) Input
3-Phase (200-240V) Output
3-Phase (380-440V) Input
3-Phase (380-440V) Output
3-Phase (460-480V) Input
3-Phase (460-480V) Output
3-Phase (525-575V) Input
3-Phase (525-575V) Output
Advantages of using a Variable Speed Drive (VSD)
Variable Speed Drives are probably the best method for controlling motor speed in response to varying process demands. These small decreases in the speed of pumps and fans or the pressure of pumps could lead to large decreases in energy use (and thus electricity costs).
Since motors consume lots of energy, the importance of the control of motor load demands increases as energy supply becomes more expensive and ever more strained. One of the biggest advantages of using a VSD is the energy cost savings associated with speed control (more efficient operation), with energy savings of 25 - 70% that can be realised by using VSDs.
Motor speeds can be revved up, slowed down, sustained indefinitely, or brought to sudden halts, with speed levels delivered as-needed rather than at uniform, arbitrary rates. By ramping up the motor speed slowly, VSDs also help save energy by reducing the in-rush current and mechanical issues associated with starting motors. A VSD also saves you money by extending the life of your motor. VSDs decrease the fluid flow into an AC motor, which in turn decreases the pressure and friction on the valves and other machinery inside the motor. This will help reduce the wear and tear on your motor, thus extending the life of your equipment. A VSD can therefore save money by increasing the efficiency of AC motors and decreasing electricity use.
When you operate your motors at the most efficient speed for your application, fewer mistakes will occur and production levels will increase, while earning your company higher revenues. The initial cost of a motor or VSD is generally only a fraction of the cost of operating the motor for its lifetime. A VSD usually pays for itself in a matter of months or just a few years.
Modern VSD benefits include power-factor correction and low power consumption, along with full-speed control and not just start/stop control. Other benefits include dynamic braking, PID control, built-in I/O and logic, circulatory control mode, multi-motor control, communication interfaces etc.
Below is a list of the top 9 benefits of using a VSD:
- Controlled Starting Current and Acceleration
- Energy Cost Savings
- Elimination of Expensive Mechanical Drive Components
- Increased Motor Longevity
- Adjustable Operating Speed
- Limits and Adjusts Torque
- Controlled Stopping
- Reverse Operation
- Reduced Power Line Disturbances
Further to this VSDs allows for an increased number of motor starts per hour and protects motors against possible low/high voltage/current supply, short-circuits, lost phases (in/out) and 3-phase imbalances.
For a more detailed explanation on this, please see our blog post: Advantages of using a Variable Speed Drive (VSD)
Other VSD Use Cases
Other than the noted benefits of using a VSD, VSDs are also often used as Single-Phase to 3-Phase Converters and customised to offer Solar Powered VSD products, referred to as Solar Drives. If you have a 3-Phase motor/device and do not have ready access to 3-Phase power, VSDs offer a workaround to get your equipment properly powered.
For more details on using VSDs as converters, please see our blog post: Single-Phase To 3-Phase Converters
Solar Drives are essentially designed to make use of this free power from the sun, as well as combining it with the benefits of using a VSD.
For a more detailed comparison between VSDs and Solar Drives (for Water Pumps), please see our blog post: Solar Drive vs Variable Speed Drive
For more details on using Solar Powered VSDs, please see our Solar Drives (for Water Pumps) product page: Solar Drives
Advantages of VSDs vs Soft Starters
VSDs provide all the Soft Starter benefits and more. Compared to Soft Starters, Variable Speed Drives (VSDs) offer much more flexibility in terms of features and settings and, depending on the application, much bigger cost savings, especially where speed control is required.
Not only do VSDs provide an extremely smooth and controlled start, but they also efficiently control motors to nearly any desired level of speed, torque, or position, including over-speed and full torque at zero speed. Special applications and safety features are also included, providing even greater benefit. In most cases, variable speed control greatly improves processes and yields impressive energy savings.
The biggest advantage of a Soft Starter over a VSD is however the lower initial cost of the product. If matching of the speed of the process is not required, then the initial cost of the product becomes a bigger factor, which might make a Soft Starter a more optimal choice. If the process can be optimised or additional costs can be saved on electricity by matching the process speed, or by reducing maintenance costs of the overall system (extending system lifetime), then a VSD will most likely be the optimal choice.
For a more detailed comparison on this, please see our blog post: Choosing between a VSD and a Soft Starter
For more details on using Soft Starters, please see our product page: Soft Starters
VSD Industry Applications
What are VSDs Used for?
VSDs can avoid pressure surges in pump applications, also known as water hammering, for conveyor belt systems it can smoothly start the system, avoiding jerks and stress on the various drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping as well as air pressure surges.
Although the majority of VSDs are installed in pump and fan applications, they can be incorporated into many other motor applications. VSDs can be adapted to serve a wide variety of applications in motor controls, here are the main applications of a VSD:
- Steel industries (Rolling mills and processing lines)
- Cement industries
- Sugar plants
- Paper and pulp
- Rubber and plastic
- Textile industries
- Machine tool applications
- Power sector
- Water supply
- Various process control applications
For a more detailed discussion on this, please see our blog post: VSD Industry Applications
For other industry applications, please also see our Industry Applications page: Use Cases – VSDs and Soft Starters
Please also see the following 4 ESKOM fact sheets on reducing costs using VSDs:
How do Variable Speed Drives (VSDs) Work?
VSD Working Principle
VSDs connect to standard AC induction motors with control capabilities for adjusting speed, torque, and horsepower. Modern VSDs contain three key components which can be used to describe its basic working principle:
- Rectifier: The first step in this process is to convert the AC supply voltage into DC using a rectifier.
- DC Filter (DC Link or DC Bus): The DC circuit contains the capacitor and inductor used for filtering (smoothing) the DC power output from the previous step which contains voltage ripples.
- Inverter: The basic working principle of an inverter is switching the DC on and off so rapidly that the motor receives a pulsating voltage similar to AC. This conversion is typically achieved using power electronic devices such as IGBT power transistors using a technique called Pulse Width Modulation (PWM). The output voltage is turned on and off at a high frequency, with the duration of on-time, or width of the pulse, controlled to approximate a sinusoidal waveform. The switching rate is controlled to vary the frequency of the simulated AC that is applied to the motor.
With a standard AC across-the-line motor starter, line voltage and frequency are applied to the motor and the speed is solely dependent on the number of motor stator poles. In comparison, a VSD delivers a varying voltage and frequency to the motor, which determines its speed. The higher the frequency supplied to the motor, the faster it will run. Power applied to the motor through the VSD can make the motor working speed lower than the nameplate base speed or increase the speed to synchronous speed and higher.
For a more detailed explanation on this, please see our blog post: How do Variable Speed Drives (VSDs) Work?
TCGC offers the following EMHEATER standard Spare Parts:
Keypad | Control Card | Power Card | IGBT Model | Fan | Capacitor | Rectifier | Fan Board | Keypad Extender
TCGC offers the following additional items to be used for EMHEATER products:
Dynamic Braking Systems | Aluminium Housed Braking Resistors | DC Chokes/Reactors | AC Chokes/Reactors | AC EMC/EMI Filters (Noise Filters) | Sine Wave Filters | Enclosures | Custom Built Panels | Remote Monitoring and Control Devices