Advantages of using a Variable Speed Drive (VSD)

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Advantages of using a Variable Speed Drive (VSD)

A Variable Speed Drive (VSD) is also called a Variable Frequency Drive (VFD), Adjustable Speed Drive, Adjustable Frequency Drive, AC Drive, Microdrive or Inverter. A VSD is a type of motor controller that drives an electric motor – alternating current (AC) induction motor.

VSDs 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.

Since motors consume a vast majority of energy produced, the importance of the control of motor load demands increases as energy supply becomes ever more strained. Additionally, 25 – 70% of energy savings can be realised by using VSDs. One of the biggest advantages of using a VSD is the energy cost savings associated with speed control (more efficient operation). When you use belts, sheaves, or gearboxes to reduce speed, the motor still runs at full speed. A VSD can reduce the actual motor speed, which results in a reduction of the amps drawn by the motor. The amount of energy used is thus reduced and saves you money on energy costs. VSDs offer precise, energy-efficient control of force and torque. 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 can therefore save money by increasing the efficiency of AC motors and decreasing electricity use. The amount of energy that a VSD can save depends on the average load of the motor and the number of hours per day it runs.

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.

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.

When you operate your motors at the most efficient speed for your application, fewer mistakes will occur. Production levels will increase, while earning your company higher revenues.

Globally, VSDs are used across several industries, including infrastructure, industrial, power generation, and oil and gas. In the oil and gas sector, these drives help in reducing installation and operating costs, optimising pump operations, improving pump up-times, and boosting oil production. These drives are also often found in cranes and hoists.

Benefits of VSDs:

Below is a more detailed description of the top 9 benefits of using a VSD:

Controlled Starting Current and Acceleration

In situations where an AC motor is initiated full throttle it requires up to eight times the current just to get the motor and load started, but not all electric motors need a full jolt to get started.

Using a VSD a much lower starting current is needed since it allows for a slow / measured ignition by starting at zero speed and accelerating smoothly on a customer-adjustable ramp (frequency/amps/voltage control). A VSD measures the actual output needed at start-up and provides only the amount needed to start (smooth release of power).

This allows the motor to ramp up as needed and saves wear and tear (keeps a check on motor winding flexing and heat generation) that occurs when a motor gets repeatedly battered with the full force. This helps in extending the motor life.

This is also where some of the greatest electricity cost savings come into play since motors are prone to use intense amounts of energy on start-up. When industrial customers start these motors during peak hours of electrical consumption, they are likely to be charged with surge prices. However, with VSDs demanding lower starting power, this issue can be addressed.

Energy Cost Savings

Energy savings = cost savings. The overall reduction in energy does not stop after the initial start-up. A VSD saves energy while controlling all of the functions and output of the motor. In addition, it eliminates the need for mechanical drive components, which also helps reduce overall costs.

VSDs cut energy usage significantly in centrifugal load operations such as fans and pumps. Centrifugal fans and pumps follow a variable torque load profile, which has horsepower proportional to the cube of speed, and torque varying proportional to the square of speed (Affinity Laws). As such, if the speed of the fan is cut in half, the horsepower needed to run the fan at load is cut by a factor of eight (1/2)3 = 1/8. Using a fixed speed motor would require some type of mechanical throttling device, such as a vane or damper, but the fact remains that the motor would still be running full load and full speed (full power).

When you factor the rate of energy that goes into a certain process and the amount it accounts for in monthly energy costs, the savings could be exponential, especially when consumption levels are halved.

Energy savings can be sufficient to pay back the capitalised cost in a matter of months or a few of years.

Elimination of Expensive Mechanical Drive Components

Due to the ability of a VSD to adjust speeds as-needed for each application (deliver low- or high-speeds as required by the load), there is no need to include a speed-varying device within a mechanical-drive setup.

Therefore, you do not need to use a mechanical device such as a gearbox. This eliminates maintenance costs, as well as reducing floor-space requirements. Considering just how costly it can be to repair the complex mechanisms of a gearbox, the ability to cut that component from your arsenal could be one of the biggest benefits of VSDs.

Increased Motor Longevity

An AC motor and all connected devices can experience a mechanical shock whenever start-up is implemented across the line. When an AC motor is started across the line, it can take up to as much as seven-to-eight times the motor full-load current to start the motor and load. Every time a shock like this occurs, each device is subject to a damaging impact. This current spike flexes the motor windings and generates heat, which will, over time, reduce the longevity of the motor.

By contrast, variable speed drives slowly begin from nothing and gradually work up to higher speeds, all of which makes for a smoother start-up process for full loads (As the frequency and voltage build, it magnetises the motor windings).

Certain applications, such as bottling lines, greatly benefit from a slow ramp up in power which allows the conveyor belt to smoothly speed up rather than an abrupt jerk to full power.

Less wear and tear on a motor is always something to strive for. One of the main benefits of a VSD is thus that it grants your motor longer life.

Adjustable Operating Speed

Unlike the traditional stop-and-go motor, the use of a VSD enables altering and optimising processes by allowing starting at reduced speeds and allowing remote adjustment of speed by programmable controller or process controller. Control, in an industrial sense, is always a big bonus for production.

For machines that need a bit of warm-up time upon start-up, a controlled start-up option could be the ideal setting. Some machines and applications are comparable to aeroplanes in that both need time to launch and soar to full-flight mode.

There are also certain machines that do not need extra time to activate but do require some human input due to the variable demands of a given application. Whether it is an air-powered machine or pneumatic tool, it might need to be programmed remotely or handled manually. For applications like these, it is best to have a variable-speed drive that can respond to changing needs in speed and torque.

For even greater reductions in mechanical or power stress, a VSD is also capable of controlling the motor in unique patterns, e.g., an S-curve can be utilised on conveyors for easier speed up/slow down commands, which lessen the possibility of conveyor recoil that sometimes results from abrupt jumps and drops in speed. For most centrifugal pumps, VSDs match equipment capability to current use more efficiently than alternatives such as throttling valves.

You can use a VSD to control a pump and maintain a fixed flow rate or pressure in case the demand is not constant, e.g., when supplying cold water for air conditioning units you do not need the same pressure or flow when only one unit is running, so you slow down the pump speed, but when you need more units working you increase the speed.

Limits and Adjusts Torque

A VSD can limit and adjust the torque so that the AC motor never surpasses certain thresholds. This can protect machinery from damage and protect the process or product (because the amount of torque being applied by the motor to the load can be controlled accurately). In the process this also saves energy by holding back excess power that a set-speed controller would end up using wastefully.

E.g., a conveyor jam. If just an AC motor is connected, the motor will continue to try to rotate until the motor’s overload device opens (due to the excessive current being drawn because of the heavy load). Using a VSD, it can be set to limit the amount of torque (Amp/Current), so that the AC motor never exceeds this limit.

Controlled Stopping

A smooth landing is just as important as controlled (ramped) acceleration. A smooth deceleration saves on wear and tear in the same way as a smooth acceleration. Controlled (ramped) stopping lowers the number of products that end up ruined because of ruptures and mechanical deterioration caused by shocks and sudden disruptions. There is thus less risk of product loss from unexpected impacts that would otherwise be damaging to the mechanisms of a motor.

Also, considering that not all damage inflicted on machinery is caused externally, and the fact that damage to internal parts can often be costlier, machines that utilise VSDs are safer.

Reverse Operation

Using a VSD eliminates the need for a reversing starter, since the output phases to the motor can be electronically changed without any mechanical devices. The elimination of a reversing starter eliminates its maintenance cost and reduces panel space.

Using a VSD eliminates the need for a reversing starter, a VSD allows for the electronic ability of reversing, either by integrated reversing or by using an external switch connected to the VSD terminal control board. The elimination of a reversing starter makes not only makes reverse operations easier and less cumbersome, but it also cuts maintenance expenses and conserves room that might otherwise be consumed by a reversing starter.

Reduced Power Line Disturbances

Starting an AC motor across the line (and the subsequent demand for the motor full-load current) places an enormous drain on the power distribution system connected to the motor. When the supply voltage sags (depending on the size of the motor and the capacity of the distribution system) the voltage sag can cause sensitive equipment connected on the same distribution system to trip offline (due to the low voltage).

Items such as computers, sensors, proximity switches, and contactors are voltage sensitive, and when subjected to a large AC motor line started nearby, can drop out.

With a VSD, however, you do not have to worry about sagging power and the damage it can inflict on expensive machinery and electronics since the motor is started at zero voltage and ramped up.

For more information regarding VSDs, please refer to the following related blog posts:

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