How Variable Speed Drives Save Energy

Most motors are designed to rotate at a set speed based on the number of magnetic poles built into them and the supply voltage and frequency applied. They cannot alter their speed. Even for fixed speed systems, it can be difficult to find a motor that rotates at the correct speed for its designated application. Therefore, extra parts such as gears, dampers or variable speed drives are often required.

Variable speed drives provide effective speed control of AC motors by manipulating voltage and frequency. Controlling the speed of a motor provides users with improved process control, reduced wear on machines, increased power factor and large energy savings.

Most applications can be grouped into the following torque categories:

• Constant torque load applications such as conveyors often require a starting torque close to the rated torque of the motor, and show only small changes as they approach rated speed
• Linear torque load applications such as screw compressors have a more linear torque requirement that increases proportionately with speed
• Variable torque load applications like fans and pumps have torque requirements that increase in proportion to the square of the speed and reach 100% torque just below rated speed

The most significant energy savings can be achieved in applications with a variable torque load. The cube law relationship between speed and power means that reducing a fan’s speed in a variable torque load application by 20% can achieve energy savings of 50%. Therefore, for most motion control applications, reducing motor speed is often the easiest way to get large energy savings.

Square law fluxing

Altering the speed of variable torque loads can yield power savings proportional to the speed reduction cubed. This is because power is a product of torque and speed, with variable torque loads having torque requirements that increase in proportion to the square of the speed. In linear voltage-frequency mode, AC drives can provide 100% torque up to rated motor speed. This means that, in variable torque applications, there is always plenty of spare torque capability available.

The square law manipulates the linear voltage to frequency relationship so that the torque capability in the stator windings of the motor is reduced, reducing losses.

Dynamic V to F mode

Dynamic V to F mode is designed for applications where power loss in the motor should be kept to a minimum under low load conditions. In this mode the drive will vary the voltage to frequency characteristic it applies to the motor depending on load level, so that at light load levels the voltage on the motor is reduced for a given frequency. This reduces the magnetising current which reduces losses in the motor.

PID control

AC drives can significantly reduce energy consumption by varying the speed of the motor to precisely match the effort required for the application. To vary the speed of the motor dynamically, a closed-loop regulator that considers the measured output of a process is required. Common applications where this is used include pressure, level and temperature control. The most common method of regulation is the PID (Proportional-Integral-Derivative) control loop. All Control Techniques drives have either single or multiple PID loops integrated within the product.

Active Front End Systems (AFE)

The input stage of an AC drive is usually an uncontrolled diode rectifier, therefore power cannot be fed back onto the AC mains supply meaning excess energy is converted to heat. AFE systems use a controlled rectifier bridge to enable energy to flow bi-directionally between the mechanical system and the AC supply. This provides far greater efficiency levels than standard drives in applications where electrical braking of the mechanical system would otherwise result in significant amounts of energy being wasted as heat.

Common DC bus systems

DC bus systems offer another approach to reducing running costs by circulating energy between braking and motoring drives. An application example where this is particularly effective is systems where material is unwound and then re-wound. In winding applications it is important to maintain tension in the material. To achieve this, the unwinder is constantly braking and the winder is constantly motoring. In a perfect system, the net energy required is just enough to supply the system losses.

Combining an AFE with a common DC bus system offers maximum flexibility, allowing energy to flow both between drives and bi-directionally with the line supply. It is common in these applications for a single AFE unit to be used with many DC bus connected drives.