Driving motors to better efficiency

Selecting the correct variable speed drive (VSD) for a motor application delivers both financial and environmental benefits. The latest generations of VSDs provide additional savings and performance advantages when driving motors.

Wayne Turtill, Product Manager at Mitsubishi Electric, looks at how to reduce energy usage by selecting the correct VSD technology.

VSDs are popular in applications for the speed control of motor driven devices like fans, pumps, winches, conveyors, and machines. They work by varying the voltage and frequency to a motor’s electrical supply. This enables them to precisely vary the speed and torque that the motor delivers to the process or  machine.

Driving motors to better efficiency

In many applications it is unnecessary for a motor to run at its top speed, particularly for loads like fans and pumps. For these duties, reducing the motor speed from 100% to 80% will cut energy use by up to 50%. It is also acceptable for most applications.

As an example, a typical mid-sized fan or pump motor rated 22 kW and assuming 93% efficiency has an input of 24 kW. Assuming it runs for 8 hours a day, five days a week and 48 weeks a year, then it consumes:

8 x 5 x 48 x 24 = 46,080 kWh of electricity a year

46080 x £0.15/unit = £6,912 a year, or about £103680 over its lifetime.

Installing a VSD to reduce speed by 20% cuts the electricity consumption by almost 50%. This would save over £3,456 every year over the life of this one motor, which is less than the cost of the VSD.

Further benefits from installing new generation VSDs

Mitsubishi Electric’s VSDs also include an Automatic Energy Optimisation function available to the end users. This feature analyses the output to the motor to reduce the voltage while maintaining the same operating parameters. It works by reducing the voltage until the VSD detects an increase in the current, at which time it increases the voltage to compensate. The solution uses a Proportional-Integral-Derivative (PID) algorithm without setpoints, allowing the system to reach the best operating voltage.

The adjustments happen a hundred times per second, supporting real-time continuous voltage control. Moreover, this happens during steady state operations as well as acceleration and deceleration, providing an optimum curve for the motor. As a result, even oversized systems running at full speed can enjoy energy savings of up to 12%.

Next-level energy intelligence

Different applications may need more dynamic motor performance characteristics. For these, Mitsubishi Electric’s E800 range of VSDs, uses Advanced Magnetic Flux Vector control.

On a conveyor, it can automatically switch the VSD into energy saving mode when running the conveyor unloaded and maintain this until loaded again. This leads to large energy savings compared to more conventional control methods and driving motors to better efficiency.

New generation VSDs are fully featured to enable businesses to benefit from added efficiency gains while still enjoying the flexibility and productivity advantages of traditional control.


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