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LOT 30ECO-DESIGN OF ELECTRIC MOTORS

Final Meeting

Anibal De AlmeidaISR – University of Coimbra

Brussels, February 10, 2014

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Task 6

Technical Analysis of BAT

Provides general inputs for the identification of improvement potential when compared to the BaseCases.

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Small induction motors – IR2 Losses

• Increasing the cross-section of stator windings - This modification is where the largest gains in efficiency are achieved. High efficiency motors typically contain about 20% more copper than standard efficiency models of equivalent size and rating.

• Increasing the cross-section of the rotor conductors.• Use copper rotor bars – Due to the excellent electrical

conductivity of copper (57 MS/m compared to 37 MS/m), replacing the aluminium in a rotor's conductor bars with die-cast copper can produce a significant improvement in the efficiency of an electrical motor.

• Increase size of the end rings.

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Small induction motors – IR2 Losses

• Reduce rotor bar skew - rotor bars are slightly skewed which helps reduce harmonics Reducing the skew will help reduce the rotor resistance and reactance, thereby providing gains in efficiency. However, care must be taken not increase harmonics. Odd harmonics, particularly the third harmonic, can originate cogging.

• Reduce the air gap between the stator and rotor - A smaller air gap lowers the magnetizing current the motor draws to maintain the magnetic field across that gap. The motor will then require less current to drive the load and thereby reduce I2R losses.

• For single-phase motors, adding a secondary “run” capacitor

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Small induction motors - Magnetic losses

• Lengthening the lamination stack - reduces the flux density within the stack, therefore reducing core losses.

• Use of magnetic steel with better magnetic properties hysteresis losses and Eddy currents are reduced because the resistivity of the laminations is higher, reducing the magnitude of the currents.

• Reduce the laminations’ thickness - using thinner laminations decreases the cross-sectional area through which the eddy currents are produced, reducing the magnitude of the eddy currents.

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Small induction motors - Magnetic losses

• Ensuring adequate insulation between laminations, thus minimizing the flow of current (and I²R losses) through the stack and reducing eddy current losses.

• Annealing the core steel - After being annealed, the material becomes much easier to magnetize, which means the magnetic domains reorient more easily reducing hysteresis losses.

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Small induction motors - Mechanical losses

• Use low friction bearings – These bearings take advantages of better geometry, materials and lubricant to reduce friction by more than 30% when compared to standard bearings

• Improved cooling – properly designed cooling systems, such as optimized fans, can help reduce ventilation losses. Improved air flow can also help reduce the power required to move the fan.

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Small synchronous PM Motors

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Small synchronous PM Motors

Source: „Untersuchungen zur Energieeffizienz von Ventilatorsystemen“ (Analysis about the energyefficiency of ventilator systems)Institute of Air Handling and Refrigeration (ILK), Dresden/Germany, Fachbericht ILK-B-31-13-3839, Dated 24th June.2013

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Super-Premium Motors

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LSPM

• Hybrid motor with squirrel cage rotor fitted with high energy permanent magnets (NeFeB) making it suitable for direct on line start

• Interchangeable with induction motors (same output x frame ratio) and similar starting torque

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LSPM

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LSPM

• It is worth noting that the starting “kick” of LSPMs is quite violent, which can lead to accelerated mechanical wear of the motor and load bearings and/or gears (if any). This can be particularly critical in application with frequent start/stop cycles.

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Medium Synchronous PM Motors

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Medium Synchronous PM - Motors

Part-Load Efficiency

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Switched Reluctance Motors

An SR motor is a doubly salient design with phase coils mounted around diametrically opposite stator poles. Energisation of a phase will cause the rotor to move into alignment with the stator poles, so minimizing the reluctance of the magnetic path. As a high performance variable speed drive, the motor's magnetics are optimized for closed-loop operation. Rotor position feedback is used to control phase energisation in an optimal way to achieve smooth, continuous torque and high efficiency.

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SR Motor: Rotor

• Simple and robust laminated steel construction: no brushes, windings, rotor bars or magnets

• Minimal losses in rotor– no cage or rotor bars– indefinite stall possible, no limit to frequency of starts– reduced shaft temperatures and prolonged bearing life

The simple SR Drive® rotor has many advantages over conventional types which utilise magnets or conductors

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SR Motor: Stator

• No magnets: straightforward laminated iron construction• Simple coil windings: absence of phase overlaps significantly

reduces the risk of inter-phase shorts• Compact and short coil overhangs make efficient use of active

coil area

Compact end-windings permit construction of high-performance motors with unusually flat aspect ratios.

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SR Motor

• APPLICATIONS UP TO 75 kW: High speed centrifugal machines, compressors, washing machines, vacuum cleaners, vacuum pumps, HVAC, variable-speed drive systems, machine-tools, automation, traction, etc.

T(N.m)

n(rpm)

ideal

a

bc

Torque Speed Curve

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Synchronous Reluctance Motors

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Synchronous Reluctance Motors

The rotor design eliminates rotor I2R losses. These motors require an electronic controller (VSD) to operate.

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Synchronous Reluctance Motors

Potential efficiency increase due to rotor loss reduction in SynR Motors

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Synchronous Reluctance Motors

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VSD Efficiency Typical percent of losses for

passive front-end convertersFactors affecting these losses

Switching losses (output stage)

30 to 50

Motor-current and switching-frequency.

Line-rectifier 20 to 25 losses Line-current(nearly proportional to motor power).

Forward losses (output stage) 15 to 20 Motor current.Internal control circuitLosses (microcontroller, internal power supply, display,keyboard, buscommunication,digital and analogue ins/outs…)

5 to 20 Nearly constant.

Switching losses (line-side converter / active front-end only)

- Line-current and switching-frequency(nearly proportional to motor power).

Compound losses(line-side converter / active front-end only)

- Line-current(nearly proportional to motor power).

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VSD Efficiency

• VSD losses are mainly influenced by the switching frequency (the higher the switching frequency, the higher the losses in the drive) and the output current (which is function of output power and load). However low switching frequency can cause torque ripple.

• Transistors - IGBTs (Insulated Gate Bipolar Transistors) and MOSFETs (Field Effect Transistors) - have nearly completely replaced thyristors in inverter circuits below 1 MW. Overall losses, parts count, and driver cost are markedly reduced with these devices resulting in an increasingly competitive product.

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VSD Efficiency

It must be noted that the energy benefits from using a VSD always come from decreasing the losses of the system on the load side and that if this benefits can be achieved they surpass the losses in the drive itself.

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Harmonic Generation within VSDs

Typical input current waveform for a 1.5 kW three-phase drive (with supply voltage) and corresponding harmonic spectrum

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Harmonic Generation within VSDs

Typical input current waveform for a 1.5 kW single-phase drive (with supply voltage) and corresponding harmonic spectrum

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Harmonic Losses

• Motors fed from a converter present additional losses, higher than during operation on a sinusoidal system. These additional losses depend on the harmonic spectrum of the impressed supply quantity (either current or voltage) from the converter.

• Additionally, harmonics may cause significant damage to the motor by producing bearing currents and insulation voltage stress.