Writing at the Gibbon’s Group blog, Carl Brennand presents five benefits of VFDs that you may not know:
- Controlled acceleration
- Mechanical drive components not needed
- Adjustable torque limit
- Avoid dips and trips
- Multi-motor control
An electric motor and its driven machine are subjected to great mechanical stress when the motor is started ‘across the line’. When this happens repeatedly, all connected equipment will suffer wear and tear which may lead to premature failure. A variable-speed drive, on the other hand, ensures a smooth, gradual start-up which is essential for many processes such as production lines carrying delicate items and containers.
Mechanical Drive Components not Needed
Mechanical means of controlling speed in motor-driven applications, such as dampers and gearboxes, are expensive, inefficient and more onerous to maintain than variable-speed drives. Running a motor at full speed and then slowing down the driven machinery manually has been compared to pressing the accelerator pedal in a car and braking at the same time. A variable-speed drive ensures that
Mechanical means of controlling speed in motor-driven applications, such as dampers and gearboxes, are expensive, inefficient and more onerous to maintain than variable-speed drives. Running a motor at full speed and then slowing down the driven machinery manually has been compared to pressing the accelerator pedal in a car and braking at the same time. A variable-speed drive ensures that energy is used much more efficiently while reducing the load on machinery.
Please read the whole thing.
Chris Edwards, writing at the Institution of Engineering and Technology website, says that the motor industry is being proactive in finding ways to save energy costs.
The waste has come to the attention of the people running the motors. According to the IEA, 95 per cent of the life-cycle cost of a motor is in the energy it uses and, as a result, the purchase price of the motor fades into the background.
John Inskip, product specialist at Siemens Drives Technologies, says: “Industry as a whole is looking at energy efficiency, looking at how it can provide the same level of service with a reduced energy intake. Whether it’s airports, oil and gas, automotive or process control, they are all looking at ways to reduce energy usage.”
When running at or near full speed, the difference in efficiency between the best-performing and worst motors on the market is only a few percentage points. Over the motor’s lifetime this difference in itself can accumulate a substantial saving but the real savings in improving motor efficiency come from not having to run it at full pelt when not necessary.
The differences between motors and VFDs become more apparent when motors run at low speeds.
In injection-moulding, for example, there are long periods after the fluid has been pumped into the mould where the pressure needs to be maintained but there is no need for the motor to turn other than to simply hold a position at high torque. Changing the motor and its electric drive can make a big difference to energy consumption as some are less efficient than others at maintaining torque at a full stop.
One of the solutions for the efficiency problem is to use permanent magnet motors, such as the SyMAX from Marathon Electric. One drawback to using permanent magnet motors end users are discovering is the initial cost. Typical squirrel-cage motors are less expensive than their permanent magnet cousins, but less efficient.
“Within process control, there tend to be higher-power motors running more of the time, so there is more scope to save energy. People who run the plants understand the benefit of doing that,” Jones says.
Lomax adds: “For end users, it’s the energy cost that’s driving it. Take something like a baby-feeding spoon, where it’s included in every single tin of the milk, they are given out by the billion. You have to keep the unit cost as low as possible but the energy cost of producing it has been creeping up and up.”
Although process control is important, Jones says the most rapid pace of adoption seems to have been in building design, helped by the adoption of efficiency standards such as VDI 4707. A high proportion of elevator contracts have gone to systems based on advanced drives and permanent-magnet motors. “There are some benefits to using that kind of technology,” says Jones. “You can put it in the top of the elevator shaft, which means you don’t need an engine room. So you save a lot of space as well as energy.
Using a permanent magnet motor, in some cases, eliminates the need for a gearbox. By cutting one piece of rotating equipment out, there is a gain in energy savings. The marriage of a permanent magnet motor and VFDs result in a very efficient operating system.
The better efficiency of a variable-speed drive and a permanent-magnet motor can manifest in subtle ways. Lomax says: “On a lot of systems, the motor is putting heat into the fluid and then you have to have cooling as well. But if you are more careful putting heat in you don’t have to take it out again.”
He cites the example of presses used to form the flooring of vehicles, which needs very high pressures. Traditional designs run a pump at a constant speed to build up the pressure before a valve is opened. The running motor generates a lot of heat that demands large amounts of water cooling.
“Using a permanent-magnet motor you can bring on the pressure extremely rapidly. You can go from nought to 1,500rpm in a millisecond. With that, there is no waste at all during the idle times,” says Lomax.
Necessity, they say, is the mother of invention. As efficiency standards for electric motors continue to rise, so do the benefits of using VFDs in your operating systems.