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Not all motor technology performs the same

05 June 2017

Low voltage electric motor users have never had it so good. Users are spoiled for choice from an array of motor technologies that have reached the market in recent years.

SynRMs are available in sizes that match induction motor frames, or smaller if less space is required

Here, ABB’s Dave Hawley, takes a look at the pros and cons of four of the most widely used types.

 

Induction motor

Induction motors have been with us for many years: the technology is well understood, they are easy to control and to repair and they have become more efficient over time. The best way to save energy using an induction motor is to stop using them direct-on-line and start using variable speed drives (VSD) technology. 

Advantages

• Well known mechanical sizes, most popular and common motor design. Many applications such as air handler bed plates, for instance, have standard mechanical interfaces to these motors.

• Efficiency levels are improving with IE4 now available on larger machines.

• Easy to maintain and operate: can be readily re-wound and spares are widely available.

• Magnet-free: no generated electricity from freewheeling loads.

• Can operate direct-on-line or with a VSD. VSDs bring energy savings between 20 to 80 percent.  

Disadvantages

• The induction motor requires current to be induced into the rotor, hence a second magnet. This rotor current is a source of I2R losses within the motor, making it hard to achieve very high efficiencies in smaller powers.

• Smaller power motors are particularly difficult to make high efficiency. It is very hard to increase the cross sectional area of conductors and add copper whilst keeping motors small.

Making motors below 7.5kW efficient, therefore, is why other technologies like ECM, PM and SynRM are often considered – see below.

Permanent magnet (PM) motor

PM motors may be used to achieve the Eco-design directives for efficiency. The motors only work with a VSD to achieve a control point that saves energy. The drive can be connected to fieldbuses and building management systems (BMS). Sometimes encoders are required, depending on the VSD being used.

Advantages

• Permanent magnets mean no generation of a “second magnet” in the motor. This absence of heat, due to current in the rotor, means the motor is more efficient.

• 30-60 percent higher torque capacity and 30 percent better torque utilisation with faster acceleration and deceleration compared to induction motors.

• Compact design in non-standard frames with high torque density and less weight, fit into smaller spaces.

• Higher continuous torque over a wide range of speeds.

• Lower rotor inertia, higher dynamics.

• Higher operational efficiencies with no magnetising currents.

Disadvantages

• Can be between two to three times the cost of an induction motor.

• Can be hard to maintain and operate as permanent magnets on the rotor make the rotor hard to extract.

• The magnets can be attracted to nearby equipment such as lifting or handling machinery.

• Often de-magnetise over time, especially if over-heated or excess current flows.

• Not possible to over speed PM motors to achieve best efficiency characteristics of the system.

• Do not have standard IEC frame sizes, meaning some applications need to be mechanically re-designed.

• Generate electricity in a freewheeling load situation making them dangerous to work on, causing electric shock or death.

• Best efficiency point is near base speed, so when controlling some applications across a speed range, the efficiency drops.

• VSD control topology needs to be good for reasonable control of a fan or pump. As such, encoder feedback is quite often required.

• High noise levels are generated.

• A spinning motor will generate dangerous voltages within the terminal box.

• The only way to avoid electric shock from this regenerative energy is to mechanically lock the rotor during maintenance.

• PM motors only work with a VSD. 

Electronically commutated motor (ECM)

ECM’s are widely used in the building services industry. These are brushless DC motors that function using a built-in inverter/chopper and a magnetic rotor, and as a result are claimed to achieve greater efficiency in air-flow systems than other kinds of AC motors.

Advantages

• Medium to high efficiency.

• Consistent airflow over a range of downstream static pressures.

Induction motors can be made between 20 to 80 percent more efficient when operated with a VSD

• Gradual ramp-up to set-point flow rate at start-up.

• Longer motor life.

• Less motor noise because smaller package of a “fan array” compared to one large induction motor.

• Convenient package – simple installation.

Disadvantages

• Higher capital cost.

• Above 7.5kW do not have enough torque to overcome static pressure in the system (restricted by fan hub size).

• Can generate considerable harmonic distortion, especially at higher frequencies – up to 120 percent total harmonic distortion (THD) is not uncommon.

• Cannot ride through power dip situations.

• Cannot catch a spinning load.

• When one part of the system (fan, motor, controller) fails, the whole unit has to be replaced.

• Cannot be connected directly to fieldbus networks, like BACnet which is an industry standard, so not easy to gather telemetry or control the application from a BMS.

• Cannot program the controller to perform PID control or timed functions for the application, so no intelligence.

Synchronous reluctance motor (SynRM)

While the high efficiency offered by ECMs may be suitable for some applications, it is essential that the right motor technology is selected if unforeseen problems with harmonics, catching spinning loads, power loss ride through, etc., are to be avoided.

For instance the cost-effective synchronous reluctance motor (SynRM) offers the same efficiencies as ECMs but without the aforementioned issues. A SynRM controlled by a traditional VSD brings all of the advantages such as:

• Improved harmonic mitigation

• Adequate power drip ride-through

Fieldbus connectability

• Built-in control features to enhance the application

There are many other benefits of using a SynRM.

Advantages

• Same price as equivalent induction motors.

• Available in sizes that match induction motor frames, or smaller if less space is required.

• Up to 50 percent lighter than induction motors.

• Significantly quieter running than other motor technologies - a big benefit in HVAC applications where noise in the ductwork is a major consideration.

• Have superior efficiency over induction motors; achieve same efficiency as PM motors. ABB guarantees package efficiency.

• Does not introduce high harmonics onto the supply. The VSD can even be an ultra-low harmonic variant

• Can catch spinning loads, allowing power loss ride-through.

• Easy to optimise motor selection to the duty point. When operating variable speed the entire system can be adjusted to run most efficiently by selecting operating points that fall within the “efficiency islands” of that system.

• Easy to maintain and operate: standard motor re-winders can repair a SynRM motor with no more than a “copy wind” approach. 

• Magnet-free, therefore easier to handle than PM and no risk of de-magnetising. No magnets means no generated back EMF electricity from freewheeling loads.

• Only able to operate with a VSD.

• Absence of “slip” makes SynRM motor more responsive.

• Cooler running and quieter than other motors, giving longer maintenance intervals or better working environments. 

Disadvantages

• VSD operating motor can be slightly larger than a conventional induction motor selection, due to higher excitation current on heavy loads.

• Potential cost increase is countered by additional savings from the guaranteed package efficiencies.

• Motor runs synchronously (as does PM), so some re-scaling may be needed on BMS/SCADA. 

Comparison of some motor types compared to IE2 motors

 

HO SynRM

IE4 SynRM

IE4 IM

Typical PM

Motor size

Smaller

Same

Bigger

Smaller

Replacement

Not always

Yes

Not always

Not always

Repair

Same – copy rewind

Same – copy rewind

Same – copy rewind

More difficult

Efficiency

Same or higher

Higher

Higher

Higher

Ease of service

Same

Same

Same

More difficult

Price

Same

Same as IE3

Higher than IE3

Higher (2 – 300%)

Reliability

Same or higher

Same or higher

Same or higher

Magnets

Availability

1.1 – 350kW

11 – 350kW

Above 75kW now

-

Weight

Lower

Lower

Same or higher

Higher

Operating noise

Lower

Lower

Same

Similar

Package efficiency testing

Yes

Yes

No

No


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