Permanent Magnet Motors Introduction and Trends
Introduction of Author:
Ashish Deshpande has been working in the domain of electrical motor design and air conditioning for last 11years. His area of interest is Interior Permanent Magnet motor design and development. He is continuing his master degree from Bharati Vidyapeeth, Pune. This article is related to his dissertation work which is guided by Prof.Mrs. S.S. More. His dissertation topic is Design of Permanent Magnet Motor (PMSM) for Air Conditioning Compressor by Optimum Utilization of Induction Motor Manufacturing Set Up. You can contact him on ashishonmail@rediffmail.com
Induction motor is preferred choice of manufacturer and customer primarily due its robustness, easy manufacturing, and low maintenance. But is this the most efficient motor? Answer is obviously no. Efficiency is function of losses, so let us revisit the Induction Motor losses as shown in figure1. Now if compare these losses with permanent magnet (PM) motor, we would notice that there is rotor copper loss which is almost 15 to 20% of overall losses.
As these losses goes down the cooling requirement of motor can be reduced.
Figure1 : Losses Comparison of IM and PMSM
PM motor offers following benefits over Induction Motor,
· No rotor copper losses
· Cooling of the motor only for the stator copper and iron loses, cooling requirements goes down
· Different size/ volume and arrangements of magnets on the rotor will lead variety of desired characteristics.
· Higher efficiency because of fewer losses.
· Compact high density magnets can reduce the overall size of motor
The indicative comparison of various parameters is covered in Conclusion.
If we compare various motor technologies as against design, performance and manufacturing parameters, following table helps to select the appropriate technology. The scale between Excellent and Positive is larger to magnify the difference in various technologies.
Excellent: 10
Positive: 3
Weak: 1
Table 1: Motor Technology Comparison
Parameters | IM | PMSM | BLDC | Variable SRM |
3 | 10 | 10 | 3 | |
Noise (low) | 3 | 10 | 3 | 10 |
Converter Complexity | 1 | 1 | 3 | 1 |
Power Density | 1 | 10 | 10 | 3 |
Power Factor | 3 | 10 | 10 | 3 |
Torque Ripple Low Speed | 1 | 10 | 3 | 1 |
Starting | 3 | 3 | 3 | 3 |
Manufacturing | 3 | 3 | 3 | 1 |
Total Score | 18 | 57 | 45 | 25 |
IM: Induction Motor
PMSM: Permanent Magnet Synchronous Motor (or BLAC)
BLDC: Brushless DC (Uses Permanent magnet)
SRM: Switched Reluctance Motor
Above comparison clearly shows that PMSM will be the future technology.
What is Difference between PMSM and BLDC motor?
There is confusion or misconception about Brushless DC and Permanent Magnet AC. Many people think that BLDC (Brushless DC Motor) is permanent magnet motor. But there are two separate technologies available, BLDC and BLAC (Brushless AC Motor). BLAC motor is nothing but PMSM.
The significant difference in operation of these two motors is the stator winding design and in turn back EMF generated.
Following are several terms interchangeably used in BLDC and BLAC literature.
· BLDC: Square wave motor, Trapezoidal wave motor
· BLAC: Permanent Magnet Synchronous Motor (PMSM), Sinwave motor
The BLDC needs separate manufacturing set up for stator to develop BLDC motor for Induction Motor manufacturer. Whereas the BLAC (PMSM) motor can utilize the existing induction stator set up including winding machines.
Figure 2 shows the winding distribution for BLDC and BLAC.
BLAC (or PMSM) stator is similar to Induction motor stator and uses distributed winding. Whereas BLDC motor uses concentrated winding unlike Induction Motor. So BLAC stator can easily built on Induction motor manufacturing set up, but for BLDC motor new manufacturing set up is required.
Figure 2: BLDC and BLAC (PMSM) Stator Winding
In case of BLDC motor, the winding is energized in normally 60 electrical degrees. This generates pulsation in torque. So BLDC motors are prone to vibrate more and needs attention if the application is sensitive to sound and vibration. The advantage of BLDC motor is simple construction and less expensive controller. BLDC stator can be easily build on DC motor manufacturing set up. In case of PMSM there are very less torque pulsation and sound quality and vibration are at par with BLDC and IM. But PMSM need complex field oriented controller and start up algorithm making it bit expensive than BLDC.
Depending winding type, rotor mounting method, back EMF waveform,
Permanent Magnet motor can be classified as shown in figure 3,
Figure 3: Classification of Permanent magnet motor
Both BLDC and BLAC have got the permanent magnet on rotor, various rotor configurations are available depending on magnet mounting on rotor. Two major rotor configurations as shown below (figure 4),
Figure 4: Permanent magnet rotor configuration
BLAC (PMSM) with Buried Magnet or interior magnet rotor is most popular configuration to replace Induction Motor. The Interior magnet rotor makes motor to operate in field weakening region or above base speed as shown in figure 5.
Figure 5: Operation Range of IPM motor
Spoke type or C type interior designs are manufacturing friendly.
Sensors are used for sensing rotor position to drive current in stator winding. Resolvers, Hall sensors, optical encoders are few of the sensors available with these motors. Following figure 6 shows the typical BLDC motor block diagram.
Figure 6: Block Diagram of Typical BLDC Motor with Sensor.
In certain applications sensors can not be used, like hermetic motor for air conditioning compressor. So there are sensorless PMSM motors which estimate the rotor position and voltages from mathematical model of motor. There is ready made solution from IC manufacturers for PMSM. In India also there are few IC companies who offer this technology from there counterparts in US or Japan.
Trends in Motor and Drives Technology
Following figure shows the overall technology trend in motor-drive segment.
Figure 7: Motor and Drives Technology Trend
With the development of MOSFET and IGBT switches, the power electronics
widened the applications for high current and voltage applications. Especially
Inverters technology developed rapidly and now most efficient and sophisticated
Space Vector PWM technique is available.
In case of motor technology, Induction motor posed lot of challenges to new
technologies due to its simplicity, low maintenance and cost. For next couple of years Induction motor may be back bone for most of the electromechanical energy conversion applications. But with energy crisis the need of efficient technology is posing great threat to Induction Motor. PM motor is primarily developed from demanding energy efficiency requirements. Also BLAC (PMSM) is widely accepted technology for Hybrid Electric Vehicles, so the development and research is on rampage. Following is electric machine technology trend.
Figure 8: Motor Technology Trend
Conclusion:
As Interior Permanent magnet (IPM) rotor configuration allow the field weakening operation, it is considered to be Induction Motor replacement. Also IPM offers better protection against demagnetization of magnets due to stator magnetic field. It provided barriers to stator flux and avoid demagnetization. So IPM is proffered configuration.
Surface mount PM rotor configurations is suitable where field weakening is not required and motor runs at constant speed. The magnets are glued over the rotor surface, hence there is restriction on rpm for surface mount PM rotor configuration.
Instead of simple PWM, Space Vector PWM controllers are efficient and hence it is preferred. To reduce switching losses and hysteresis losses, the number of poles needs to selected for Permanent magnet motor.
As PMSM or BLAC develops less torque pulsation, it is used in the applications where noise and vibrations need to match with Induction motor. The BLDC can be used where there is not stringent requirement on noise and vibration. PMSM needs field oriented control and start up algorithm so its controller is expensive than BLDC. Depending customer specification BLDC or BLAC can be selected.
For induction motor manufacturers, it easy to switch to BLAC or PMSM type motor as stator manufacturing facility can be utilized as it is. BLAC is getting popular for Electric Vehicle application.
With reference to Induction Motor, PMSM motor parameters are compared in following charts. Volume and weight of equivalent will be reduced by 20 to 25%.
Figure 7: Volume of IM and PMSM
Figure 8: Weight of IM and PMSM
Figure 9: Efficiency of IM and PMSM
Above comparison is for rating 5hp and above. Following chart shows IM and PMSM efficiency comparison for 5hp and below ratings.
Figure 10: Efficiency of IM and PMSM (5HP and below rating)
These performance and dimensional parameters are studied and compared from
various motor manufacturers catalogue and brochures. This comparison is
indicative and sets expectations from PM motor. User may get more benefits by using PM motor as indications are based on conservative data.
PMSM technology provides good opportunity to reduce energy
Consumption and get more star ratings developed by BEE for many appliances and applications. The challenges to build permanent magnet rotor is also reducing as many Chinese suppliers are providing sophisticated permanent magnet rotor manufacturing set up. Even though initial cost of PM motor will be high, the pay back period will be less than IM. PM motor can be future back bone for many electromechanical conversion applications.
.
Note from EVHUB.BIZ
This article was published in http://www.evhub.biz for sharing the importance of Permanent magnet motors. Permanent magnet motors are being used in various electric vehicles - 2 wheelers, 3 wheelers and heavy vehicles due to the advantages explained in the above article.
