“Pro's & Con's of Electric Motor Types” - Mining · PDF fileSynchronous Reluctance...

12 June 2014 ***Confidential*** ©2013 Advanced MotorTech LLC Pinellas Park FL USA ***Confidential*** 1

ADVANCED MOTORTECH LLC

4951 71st Avenue North

Pinellas Park, FL 33781-4428

Phone: 727 - 412 - 8200

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www.AdvancedMotorTech.com

By: Dr. Keith Klontz and Dr. Howard Li

9 May 2014

Induction

Wound-Field Synchronous

Permanent magnet (AC)

Synchronous Reluctance

Switched Reluctance (w/ Focus on Integral HP AC machines)

“Pro’s & Con’s

of Electric Motor Types”





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Advanced MotorTech

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Drawing Review & Package

Training & Tutorials

Theoretical Analysis

Finite Element Method Analysis

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Research

Development

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Testing & Data Evaluation

Manufacturing Process

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Energy Efficiency Economics

Patent Writing & Review

Product & Technology Comparisons

Re-Design, Upgrades, Modifications

Repair & Re-Building

Failure Analysis

Technical Writing & Publication

Selection & Application

High Efficiency or High Power Density Trades

Design, Purchase or Service Specifications

Prototype Project Management

Component Sourcing

Technology Reviews

Founded in 2005 to provide advanced

electric machine and power magnetics

technology to industry.

Member: SMMA, EASA, IEEE, EMERF Bd of Dir Technical Partner: JSOL Corp.,CD-Adapco

Business Partner: PowerSys





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Differences: • Phase Number • Rotor Construction • Starting Method • Speeds Built In • Rotor Excitation • Controller Type • Current Shape

Also, Flux Direction: • Radial • Axial • Linear • Tubular • Transverse





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PMAC Synchronous

Overview: Similarities and Differences of Electric Motors

• All motors operate by opposing magnetic forces at

the air gap causing motion

• Stator (Stationary) usually gets it magnetic field by

current thru coils;

• 3 phase: smooth flux variation (sinusoidal);

produces rotating magnetic field

• The key difference is in how the Rotor gets its

magnetic field

Switched Reluctance

Induction







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PMAC Synchronous

Overview: Similarities and Differences of Electric Motors

• How the Rotor gets its magnetic field:

• Permanent magnet motors—special material,

no current

• Synchronous motors—Coils on rotor with dc

• Induction motor—transformer principle,

stator is primary

• Synchronous reluctance—flux from stator

• Switched reluctance—flux from stator, plus

shape effect

Switched Reluctance

Induction







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Overview Similar and Difference of Motors

From: SPEED’s Electric Motors by TJE Miller

• Difference is in how rotor excitation is achieved: influences how motor is started





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Interior magnet Surface magnet

Many variations

Permanent Magnet (PM) Motor ― Rotor

• Synchronous speed based on

frequency: RPM = 120 *f /Poles

• Magnets are used for excitation

• Line start PM motor needs rotor

current

• Requires controller

• Higher efficiency and power density

than other motors





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Permanent Magnet Motor: Magnet Material: Enabling Technology

Demagnetization Curves for different types of magnet materials

So

urc

e: S

. T

rou

t/M

ag

neq

uen

ch

1940’s

1950’s

1970’s 1980’s

Histogram for different types of magnet materials





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www.AdvancedMotorTech.com • NdFeB (since about 1980):

– Large BHmax to >48 MGOe

– High Hc and Br (1.1 - 1.2T & 800KA/m)

– Feasible operating temperature 80ºC - 200ºC

– Linear characteristic, -10%/100ºC

– Medium cost

• SmCo:

– Good magnetism characteristics (1.0T & 700KA/m)

– High operating temperature 200ºC - 350ºC

– Expensive

• Ferrite:

– Normal magnetism characteristics (0.42T & 230KA/m)

– Normal operating temperature 200ºC

– Least expensive, lowest cost for unit of torque

• AlNiCo:

– Seldom used in machine

– non-linear BH

– Easy to demagnetization

– Better magnetism characteristics (1.2T & 80kA/m),but low Hc

– Very high operating temperature 600ºC

– Expensive

Permanent Magnet Motor ― Rotor Material





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• Operation of PM Motor

– Windings are located in the stator, and build the stator field in air gap,

looks rotating

– Air gap field: sinusoidal or trapezoidal

– Rotor Excitation: Permanent magnets produce the excitation field in air gap

– Principle: Two magnetic fields interact and develop electromagnetic torque

and rotate with synchronous speed

– Most PM motors require position sensor, speed sensor & controller

PM Motor ―Pros & Cons





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• Pro’s of Permanent Magnet Motors

– Efficiency, Variable Speed, Motion control, High Speed

– Inherently synchronous, needs help to start

– Rotor Excitation: permanent magnets (PM), requires no current, but not

controllable

– Lower rotor weight, Lower total weight, and inertia

– Application: many areas– hybrid vehicle, high accurate control, high

efficiency application


1500hp, 4 pole, line-start PM

motor

Induction cage

Ventilation hole Shaft

Permanent magnet





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• Con’s of Permanent Magnet Motors

– Control: Overload causes motor stop, need a controller

– Starting: need controller, or induction cage

– Temperature Sensitivity: Magnets weakens with temperature

– Safety: Magnets never ‘turn off’

– Magnet cost, new manufacture process

– Little control of magnetic field strength

– Brittle material, can crack & chip

Demagnetization due to eddy current and overheat

Broken magnet





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• Speed

Runs at synchronous speed. Precise speed adjustment by using controller (frequency)

• Efficiency

PM motor has a higher efficiency than induction motor and wound field synchronous

motor, because magnet loss is much smaller than that of excitation coils.

• Reliability

No rotor conductors to break; cooler rotor extends bearing and insulation life. Robust

construction for years of trouble-free operation; Must have banding for centrifugal forces

• Power density

Rotor physical size is smaller than induction and synchronous motors. Very higher power

density (kW/ kg); Beware: Weight is more proportional to Torque, not Power; P=T*N

• Cost

--RE magnet material is commodity, 95% ore is in China;

--PM motor more expensive than induction motor mostly due to lower volumes,

--High efficiency is good investment; low lifetime operating cost.





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Wound-Field Synchronous Motor ― Structure

Wound-field Synchronous Motor Two common variants: 1) Round rotor & 2) Salient Pole

Replace magnets with Coils





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Salient rotor

4 poles

Round rotor

2 poles

north

south

shaft

coils

Rotor

core

Synchronous Motor ― Rotors • Synchronous speed based on frequency

• Salient rotor with damper winding can be

used for synchronous motor to get line-start

• External sources using slip ring or exciter

(generator) provide power supply of rotor,

no induction

• Adjust excitation current to control Rotor

magnetic field strength, hence torque and

output power

Damper winding (bars)





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www.AdvancedMotorTech.com • Operation of Synchronous Motor

– Synchronous speed, but needs help to start

– Rotor Excitation: wound coils, needs power source on rotor, now rotor field is

controllable

– Gap field: usually sinusoidal, big air gap

– Two magnetic fields develop electromagnetic torque, rotate with synchronous

speed (fixed in rotor frame)

– Application: generator, large compressor, high power operation, synchronous

speed, High efficiency

– To start

• Damper bars as cage rotor

• Pony motor to start—Hydro Gen

Synchronous Motors





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• Pro’s of Synchronous Motor

– Generator with Voltage Control

– Controllable Power Factor

– Mature, well-suited for large sizes

– Highest proven efficiency for large sizes >10,000 hp, >10 MW

– High inertia, good ride-through capability

Synchronous Motors





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Synchronous Motor ―Pros & Cons

• Con’s of Wound-Field Synchronous motor

– Overload causes motor stop and lost synchronization

– Starting problem, very hard to start just by excitation winding

– Need DC power to rotor coils; has losses due to rotor current:

• 1) Slip rings: Regularly maintain brushes, bearing and slip ring

• 2) Exciter = AC Gen with diode rectifier

– Stable operating angle: power angle < 60 degree; need stability control





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Synchronous Motor ―Pros & Cons • Speed

Frequency of AC power determines the speed of synchronous motor.

--For 60Hz, speed will be 120*60/2p, 2p is pole number of synchronous motor.

--For variable speed job, must have Variable Frequency Drive (VFD)

• Efficiency

Higher efficiency than induction motor. Copper excitation winding of synchronous motor has

a lower loss than induction motor’s aluminum cage. Adjustable power factor will also

improve PF of industrial plant and decrease losses of power system.

• Reliability

Brushes and slip ring or Exciter will be an issue for long time operating. Regular

maintenance service is required. Wider air gaps than induction motors make them

mechanically better. Loss heat from both stator and rotor

• Power density

Higher power density than induction motor because of copper winding used on Rotor

• Cost

The complicated design makes the synchronous motor more expensive than induction motor.

However, above 10 megawatts, the higher efficiency of synchronous motors brings lower

operating cost.

Note: Some are now advocating wound-field synchronous motors for EV/HEV traction motors





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Induction Motor ― Structure

Stator

winding

Stator iron core

Electrical

port

Mechanical

port Rotor iron

core

Rotor Winding,

cage in this case

Electrical

port

Mechanical

port

Energy

Conversion

process





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• Wound rotor

– Windings embedded in slots, terminals brought to

(3) slip rings and brushes

– Connect resistors to windings’ terminals to adjust

the starting performance or regulate speed

• Cage rotor

– Bars embedded in slots and short circuited at both

ends by end rings

– Special case of wound-rotor design

– Bars and end rings like a squirrel cage

Cage rotor Wound rotor

Induction Motor ― Rotors





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• Properties of induction motor

– Winding build a fundamental rotating magnetic field

– Rotor Excitation: induced current in wound or cage rotor, Slip speed to get

transformer action

– So, “torque producing field” slowly rotates with respect to rotor

– More complicated due to transformer action used to create rotor excitation

– Never achieves synchronous speed

– 1) Wound Field: secondary of transformer

– 2) Cage Rotor: cage is on short circuit and generate short circuit current to build an

excitation field

– Two magnetic fields develop electromagnetic torque, rotate with synchronous speed

(fixed in rotor frame)

– Application: many areas, general drive

Induction Motor ―Pros & Cons





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• Pro’s of induction motor

– One of very few that can Line-Start without special controls

– Air gap field: sinusoidal, shaped by stator field

– Very robust, very durable, ubiquitous

– Rotor magnetic field strength controllable by stator current

– Mature speed & torque control (Vector Control, Field Orientation)

– Application: many areas, general drive

This technology is the benchmark that

others should be compared to:

“Is it better than using an induction motor?”






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• Con’s of induction motor

– Speed varies with load torque

– Low starting torque

– Low power factor

– Peak torque may not be enough at overload situation

– High starting current, soft-starting may be required

– For wound rotor, regularly maintain brushes and increase cost due to slip ring

and brushes

– Rotor heat due to losses of rotor bar and end ring; meltdown at locked rotor

– Rotor cage reliability: broken bars; limited number of starts in motor lifetime






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• Speed

Must run different than synchronous speed to develop torque.

• Efficiency

Even NEMA- premium efficiency units exhibit degraded efficiencies at low load with low

power factor. Full load has much better efficiency, but sometimes to avoid starting or

lock rotor issues, engineers always choose a big motor to drive a small load.

• Reliability

More loss heat are from both stator and rotor, and it degrades insulation, motor

operation and life time.

• Power density

Induced field generated by squirrel cage embedded in iron core limits power density.

• Cost

Can start without expense and losses of VFD. The simple design and huge quantities of

manufacture make them less expensive, but higher operation costs (electrical power) –

very good, but not best efficiency for VFD systems






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Synchronous Reluctance Motor―Pros & Cons

--Structure

Synchronous reluctance motor

New kid on the block: You will be hearing about this as the newest, greatest motor --It is good, but neither superlative





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Synchronous Reluctance Motor

Properties of the Synchronous /Reluctance Motor

The stator has a normal lamination and winding

The rotor is designed to maximize the saliency ratio Xd/Xq; iron vs. air

Rotor Excitation is from set of balanced, polyphase AC sinewave currents in Stator

Operates at Synchronous speed

Torque is from alignment of rotor iron with stator magnetic field

Note: varies from Switched Reluctance (SR) by shape of stator iron and shape of

current from power supply, hence stator magnetic field





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Pro’s of the Synchronous /Reluctance Motor

Efficiency and power-factor are comparable to induction motors

Saliency ratio of at least 10 is needed to match induction motors (difficult)

Torque/speed characteristic is inverse; good low speed torque

Torque/current characteristic is non-linear

Good overload capability





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Con’s of the Synchronous Reluctance Motor

Small airgap required

Difficult to design for high speed

--Limited mechanical strength against centrifugal force

--Balance problems

Axially-laminated rotor has potentially high iron loss;

Hard to manufacture rotor

Torque ripple is difficult to eliminate





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• Speed

Runs at synchronous speed. Needs VFD, so speed is variable, but torque is inverse

characteristic. Not well-suited for high speed due to structure

• Efficiency

Can be quite high; comparable to Induction, not quite as good as PM; Good controller

can give good efficiency at partial load; starting torque dependent of controller, typical

150%.

• Reliability

Some loss heat in rotor—lower than Induction, higher than PM; Simple and robust rotor

structure

• Power density

Rotor field generated by stator current, iron rotor core limits power density.

• Cost

Needs VFD. The simple design can make them less expensive, –very good, but not best

efficiency for VFD systems





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Switched Reluctance Motor ― Pro’s & Con’s

12slot/8 pole SR motor






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Switched Reluctance Motor ― Rotor

• Synchronous speed based on 2X frequency

• Rotor material: simple lamination

• Requires complex controller and feedback

sensor

• Simple rotor, no magnet and winding

Slot/pole configuration For 2 phase: 4/2, 8/4, 4/6, 8/12, 12/18, 16/24

For 3 phase: 6/2, 6/4, 6/8, 12/8, 18/12, 24/16

For 4 phase: 8/6, 16/12, 24/18, 32/24, 8/10

For 5 phase: 10/4, 10/6, 10/8, 10/12

For 6 phase: 12/10, 24/20, 12/14





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• Properties

– Operates at synchronous speed

– Laminated rotor, torque by alignment of iron parts

– Gap field: square, needs small air gap to get high torque

– Requires specialized controller, virtually none OTS

– Application: some good examples for high power, mostly for home application,

Step-motors, motion control

SR Motor ―Pros & Cons






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www.AdvancedMotorTech.com • Pro’s of Switched Reluctance

– Synchronous speed, Motion Control

– Simple, Robust rotor; no magnet and no winding –Big attraction feature

– Strengths: medium sizes, robust

– Excellent candidate for very high speed, > 50,000 rpm

– Stator magnetic field pass through rotor steel teeth and develop reluctance

torque between steel

– Only single phase’s switch is on at any time; Can have good fault tolerance


Dyson

Remember: Power = Torque x Speed





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• Con’s of Switched Reluctance motor

– Vibration and noise, torque ripple — Serious problem

– More lead wires

– Require a Special controller – Serious problem

– Not many commercial manufactures or OTS controller ; None in high HP

– Very small air gap

90dB





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• Speed

Frequency of AC power determines the speed of SR motor. For 60Hz, speed will be

120*60/P, P is pole pair of SR motor. Variable speed with controller

• Efficiency

Most of SR motors have similar efficiency with induction motor when having same speed

and power, but lower power factor

• Reliability

No winding, no magnet in rotor. Simple rotor structure and special operation bring very

high reliability. Phase winding is isolated in electrical circuit each other, good fault isolation

& tolerance.

• Power density

Higher power density than induction motor and lower than PM motor.

• Cost

Very simple design in both stator and rotor make SR motor cheaper in basic material cost,

but low quantities make the price high. Additionally, the SR motor needs a specialized

controller for operation. It is well-suited for variable speed, very high speed application.





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Summary Induction motor Synchronous motor PM motor SyncRel motor SR motor

Rotor and stator

cage rotor or wound rotor

Would-field rotor, round or salient rotor, damper winding

Magnet, surface PM or IPM, damper winding

Laminated rotor, normal winding for stator

Laminated rotor, concentrate winding for stator

Speed Must run less than synchronous speed; varies with load

Synchronous speed, need damper winding for line start

Synchronous speed, need induction cage for line start

Synchronous speed Half speed of synchronous speed

Efficiency Degraded efficiency at low load operation

Higher efficiency than induction motor

High efficiency than induction motor and synchronous motor

Similar efficiency with induction; better at partial load

Similar efficiency with induction or synchronous motors

Reliability More heat degrade insulation and life time, many years of proven performance

Brushes or slip ring are issues for long life operating, regular maintenance in fewer years

Lower temperature reduce maintenance, trouble-free operation for years.

Good reliability due to simple rotor

Higher reliability than others due to very simple rotor

Power density

Induction field & rotor iron limits power density

Higher power density than induction motor

Higher than induction and synchronous motors

Power density similar to induction motor and lower than PM motor

Higher power density than induction motor and lower than PM motor

Cost Simple design and high manufacture quantities lower price, but high operating cost

Higher price than induction, but above 10kW, high efficiency brings low operating cost

Higher cost, but higher efficiency gives low operating cost

Lower material cost , but low quantities. Mfr assembly difficult. Need a VFD controller

Lower material cost , but low quantities. Needs a special controller

Application Many areas, any power, general drive

Many areas, high power, synchronous speed

Many areas, small to high power, high efficiency

Newly available; target is Induction motor applications

Some good examples, mostly for home application; noisy





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“Pro's & Con's of Electric Motor Types” - Mining · PDF fileSynchronous Reluctance...

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Transcript of “Pro's & Con's of Electric Motor Types” - Mining · PDF fileSynchronous Reluctance...