Derwin
07-08-2008, 10:20 AM
A fellow member sent me information about this new concept motor that seems to be very interesting. Have any of YOU heard about this, and what do you think? Here is the information:
Split-Rotor Field Weakened
Permanent Magnet
Brushless Motor
Overview
Permanent magnet (PM), brushless motors are recognized to possess the highest torque density and efficiency of any motor. However, PM motors have a significant shortfall in that they produce Bemf that limits the voltage available for driving current through the windings at high speeds with limited ability to weaken the permanent magnet field efficiently. For this reason the power and torque output quickly drops off at speeds above the peak power output speed. In order to make these motors suitable for many dynamic applications three sacrifices are generally made. First, the Bemf or voltage constant (Ke) and hence, the torque constant (Kt) are kept quite low, resulting in larger than desired current draw during acceleration and under heavy load. These high current draws result in larger than desired I2R winding losses. Second, electronic field weakening is implemented in the drive electronics to achieve higher speeds by injecting non-torque producing, waste current into the motor to weaken the permanent magnet field. Both of these compromises result in undesirable current draw from the sources and in-efficiencies in the motor. Thirdly, motors are designed to run at high speeds and then use gearing to obtain desirable speeds, resulting in efficiency, cost, and maintenance penalties.
A new motor technology known as split rotor field weakening (SRFW) eliminates the need for the above sacrifices. The SRFW motors provide the following advantages over conventional PM motors:
· Constant power or stepped performance speed range many times that achieved by electronic field weakening without injecting waste current into the windings.
· Use of much higher Ke and Kt values for large reductions in current draw from the source.
· Significant I2R winding loss reductions.
· No expensive, specialized, electronics required. Operates off most standard sinusoidal drives.
· Operational speeds, torques, and speed ranges that eliminate the expense, in-efficiencies, and maintenance of geared reductions.
· Automatic compensation of motor parameters based on applied load/commanded current to the motor.
The Long Lived PM , Brushless Motor Challenge
What is desired is a PM, brushless motor that has the ability to change its relationship of torque to current (Kt) from a very high value during acceleration or low speed/high torque demanding operation, to a very low Kt (also low Ke) during high speed operation. Achievement of this would allow motors to be designed for minimal current draw, without the sacrifices mentioned above, yet achieve high operating speeds. It is also desirable to have this change take place without special electronics or external means for triggering.
The Solution - Split Rotor Field Weakened (SRFW) PM Motor
The above challenge can be overcome by starting with a fairly conventional PM brushless motor winding and modifying the rotor. As shown below the rotor is split into two sections, each carrying a portion of the permanent magnets. The two rotor sections are held in a weakened or Bemf cancelling position as shown below via a magnetic coupler shown on the end of the rotor. The magnet ratio of the two sections can be designed to cancel any amount of the Bemf at high speeds. The rotor is designed such that the load is attached to one end of the rotor while the other is attached to the encoder and used for commutation phasing. Typically the phased portion of the rotor (primary) contains a larger portion of the magnets than the loaded (secondary) rotor section.
With an increase in load and torque produced by the motor the magnetic coupler is collapsed and the like poles of the two rotor sections move closer to alignment. Continued load and torque will collapse the coupler to a stop position at the end of the coupler where like poles are aligned and the motor is in full torque mode.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=588&stc=1&d=1215526602
The beauty of this approach is that it is the loading and subsequent applied torque that forces the increase in the motor torque constant. How this torque increases with loading is a function of the magnetic coupling design. What is important to note is that the split rotor concept can be applied to a wide variety of applications to greatly extend the performance beyond traditional PM brushless motors. Very different applications will require very different magnetic coupler performance characteristics.
The Key is in the Coupling
While splitting the rotor is what provides a new level of dynamic performance for PM brushless motors, the magnetic coupler together with its rotational stop provides specific tailoring of the motor performance to the application. The coupler can be designed with discrete torque shift points for applications with two distinct operating conditions or with a linear torque profile for constant horsepower applications such as electric vehicles, power generation, web tension control, coil winding, and etc. In addition, the coupler can be designed for unidirectional or bi-directional full torque modes of operation. Some examples of applications and corresponding magnetic coupler profiles follow.
Constant Horspower One of the best examples of a constant power applications, and likely to experience the biggest benefit from the SRFW concept is electric vehicles. Miles per charge is key, hence being able to wind for the highest possible torque per current, while not giving up top end speed is extremely valuable.
For these applications without SRFW, using PM motors, it has been typical to wind the motor for high speed operation. This is done by using a very limited number of turns and maximizing wire gage for the available space such that large currents could be used to compensate for the low torque constant. By doing this a motors power could be maximized for a given size. The penalty is high current draw from the battery source and short battery range. The result of this approach is high power bragging rights, while it often places the torque at speeds where it is not real important, bumps up the low speed/high torque current demand, and results in the use of an in-efficient gearing system.
Referring to the Torque vs. Speed chart below, the torque vs. speed corresponding to the above description is noted with the 2X Current. The corresponding horsepower is shown further down in the Power vs. Speed chart. In this example the base speed is 2000 rpm and represents the point of maximum power. The speed range can be improved somewhat by introducing additional waste current, weakening the PM field. If the number of turns were doubled, hence cutting the current demand in half, the torque speed profile would be shown by the 1X Current profile. The corresponding HP profile is shown in the second chart below. From comparing these curves it is obvious why with speed range and torque goals, there is value in the high speed winding at the price of current draw and vehicle range. In addition to sacrificing high current draw the speeds are too high for direct use so additional in-efficiencies must be added through gearing.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=589&stc=1&d=1215526602
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=590&stc=1&d=1215526602
Lets now look at the low current draw motor from ab
ove with a split rotor and linear coupler to be described in more detail below. This torque vs. speed curve is shown in yellow. The corresponding HP is in brown. Notice how the maximum power is extended 2 ½ times beyond the base speed and actually increases beyond the base speed. While the peak power is not as high as the 2x current draw curve, the speed range is much greater, and the low speed torque is equivalent. Only a small band of moderate speeds give up actual torque. The big plus is that with the SRFW motor only ½ the current is required at the lower speeds. Since most driving is done in short stints, a large portion of the battery life is consumed during acceleration, or low speeds so this current reduction is very attractive.
The coupler for the above SRFW example must exert a linear torque profile as the two rotor sections transition from the fully weakened state to the full torque state with one magnetic pole of rotation. Since operation in reverse is minimum, for cost and compactness it makes sense to have the coupling perform unidirectionally only. Attached Chart3 shows an example coupler torque vs. angle profile for this application. T1 represents the torque that is holding the two rotor sections in the full speed mode of operation against a limiting hard stop. In this fully weakened state the torque available is dependent on the ratio of magnets within the two rotor sections. The load / applied torque must exceed T1 to begin increasing the motor torque constant.
T2 represents the second rotational stop where like poles are aligned in the rotor sections. T2 is the load/applied torque desired to move the motor into the full torque mode of operation. For the torque/speed and power/speed curve shown above, T2 would represent the peak torque value shown, however there may be value in having T2 fall below this torque. Also note that with the weakened stop set at complete mis-aligned poles the reverse direction will always operate in the fully weakened state. By moving this stop toward pole alignment in the opposite direction the torque constant would increase in the reverse direction.
Directional Load Variation One of the most straight forward and effective applications for SRFW is where an application requires high torque in one direction and high speed in the other. For this application the forward and reverse stops within the split rotor in conjunction with the split ratio, will determine the torque constant for the corresponding load direction. In the high torque direction the load/applied torque will collapse the magnetic coupler at a low value, moving the rotor sections to aligned poles with limited load. In the other direction the device will rotate back into the full are partial weakened state.
In these applications the coupler is designed merely to satisfy deceleration requirements and prevent unstable servo operation. Some applications may be suited with a high durometer, high temperature urethane dampening stop at the split rotor, rotational stops.
High Speed/Low Torque Low Speed/High Torque in Same Direction (Bidirectional) An example of this application would be attached to a linear actuator, operating as a press. A high speed/ low force approach is desired followed by a high torque/ low speed press. Finally, a high speed/ low torque return is desired. The high force may be in either direction.
For this application a stepped, bi-directional coupler is most desirable. An example of the coupler is shown below.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=591&stc=1&d=1215526602
Since the fully weakened state is not really an operating condition with zero torque produced, the split rotor sections can be equal in magnet count. The coupler goal in this application is to have as steep as possible profile near the fully weakened state. At a load of 110 in-lbs the goal was to have the rotor sections make a near step transition to the full torque mode. It was expected that less than 110 in-lbs was required for high speed movement of the tooling and also acceleration and deceleration. Once the press load was encountered, a sudden shift to over 1000 in-lbs was encountered and the coupler collapsed almost instantaneously.
This application is a straight forward demonstration of the advantages of the SRFW motor. The torque constant could be maximized without regard for the speed limit at available bus voltage. The stops were set to +/- 1 pole. The coupler torque vs. rotation was then designed to assure that at a desired torque, the rotor sections were cancelling enough Bemf to allow the desired speed of operation.
High Speed/Low Torque Low Speed/High Torque in Same Direction (Uni-directional) Runabout boats are good examples for this approach. Without variable speed transmissions, boat designers are challenged with selecting the appropriate gear ratio to provide out of the hole torque, without limiting top end, planing speed. Often times boat owners are not happy with their low end torque or top end speed so they change propeller angles to gain in one and sacrifice the other. SRFW for boats would act like a variable speed transmission without the gears. By setting the coupler to have a full torque stop and a fully weakened stop in only one direction with a stepped profile similar to the previous application, boats would operate at full torque prior to getting on a plane. Once on a plane, as the torque load decreases, the split rotor sections would mis-align for high speed operation.
PM Brushless vs. Induction
Currently there is a battle for position between vector driven induction and PM motors. PM brushless motors have higher efficiency due to the elimination of magnetizing current and copper losses in the rotor. In addition, PM brushless motors require less sophisticated control and typically provide better low speed, high performance torque control. Finally, PM brushless motors typically provide higher peak torque and power vs. size.
On the downside for PM brushless, prior to SRFW, vector driven induction motors have provided desirable field weakening control for broad speed range, constant power. In induction motors there are no magnets and the magnetic field is adjustable by controlling the voltage to frequency relationship. In PM brushless motors, prior to SRFW, field weakening was only minimally effective at the expense of wasted current. These field canceling currents are typically quite high due to the high energy of rare earth magnets.
Now with SRFW, PM motors offer performance advantages over vector driven induction motors on many fronts. The biggest disadvantage remaining is in the expense of the rare earth magnets.
SRFW Status
The SRFW concept has been demonstrated with physical hardware and is currently patent pending with provisional application 188915 US. Current lab hardware demonstrates a split rotor with a 3/2 magnet relationship on the rotor. With this rotor configuration a motor originally designed for a top end speed of 700 RPM at a bus voltage of 460Vac achieved speeds of 3000 RPM without any reduction in torque production once the coupling was fully collapsed.
No longer is it necessary to size a motor for speed requirements and to accept the hit on current to achieve desired torque. Now motors can be selected for torque density and minimal current draw. The rotor can then be designed to weaken according to speed requirements. In essence, the split rotor acts much like an automatic transmission without gears or elements of wear or maintenance.
Split-Rotor Field Weakened
Permanent Magnet
Brushless Motor
Overview
Permanent magnet (PM), brushless motors are recognized to possess the highest torque density and efficiency of any motor. However, PM motors have a significant shortfall in that they produce Bemf that limits the voltage available for driving current through the windings at high speeds with limited ability to weaken the permanent magnet field efficiently. For this reason the power and torque output quickly drops off at speeds above the peak power output speed. In order to make these motors suitable for many dynamic applications three sacrifices are generally made. First, the Bemf or voltage constant (Ke) and hence, the torque constant (Kt) are kept quite low, resulting in larger than desired current draw during acceleration and under heavy load. These high current draws result in larger than desired I2R winding losses. Second, electronic field weakening is implemented in the drive electronics to achieve higher speeds by injecting non-torque producing, waste current into the motor to weaken the permanent magnet field. Both of these compromises result in undesirable current draw from the sources and in-efficiencies in the motor. Thirdly, motors are designed to run at high speeds and then use gearing to obtain desirable speeds, resulting in efficiency, cost, and maintenance penalties.
A new motor technology known as split rotor field weakening (SRFW) eliminates the need for the above sacrifices. The SRFW motors provide the following advantages over conventional PM motors:
· Constant power or stepped performance speed range many times that achieved by electronic field weakening without injecting waste current into the windings.
· Use of much higher Ke and Kt values for large reductions in current draw from the source.
· Significant I2R winding loss reductions.
· No expensive, specialized, electronics required. Operates off most standard sinusoidal drives.
· Operational speeds, torques, and speed ranges that eliminate the expense, in-efficiencies, and maintenance of geared reductions.
· Automatic compensation of motor parameters based on applied load/commanded current to the motor.
The Long Lived PM , Brushless Motor Challenge
What is desired is a PM, brushless motor that has the ability to change its relationship of torque to current (Kt) from a very high value during acceleration or low speed/high torque demanding operation, to a very low Kt (also low Ke) during high speed operation. Achievement of this would allow motors to be designed for minimal current draw, without the sacrifices mentioned above, yet achieve high operating speeds. It is also desirable to have this change take place without special electronics or external means for triggering.
The Solution - Split Rotor Field Weakened (SRFW) PM Motor
The above challenge can be overcome by starting with a fairly conventional PM brushless motor winding and modifying the rotor. As shown below the rotor is split into two sections, each carrying a portion of the permanent magnets. The two rotor sections are held in a weakened or Bemf cancelling position as shown below via a magnetic coupler shown on the end of the rotor. The magnet ratio of the two sections can be designed to cancel any amount of the Bemf at high speeds. The rotor is designed such that the load is attached to one end of the rotor while the other is attached to the encoder and used for commutation phasing. Typically the phased portion of the rotor (primary) contains a larger portion of the magnets than the loaded (secondary) rotor section.
With an increase in load and torque produced by the motor the magnetic coupler is collapsed and the like poles of the two rotor sections move closer to alignment. Continued load and torque will collapse the coupler to a stop position at the end of the coupler where like poles are aligned and the motor is in full torque mode.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=588&stc=1&d=1215526602
The beauty of this approach is that it is the loading and subsequent applied torque that forces the increase in the motor torque constant. How this torque increases with loading is a function of the magnetic coupling design. What is important to note is that the split rotor concept can be applied to a wide variety of applications to greatly extend the performance beyond traditional PM brushless motors. Very different applications will require very different magnetic coupler performance characteristics.
The Key is in the Coupling
While splitting the rotor is what provides a new level of dynamic performance for PM brushless motors, the magnetic coupler together with its rotational stop provides specific tailoring of the motor performance to the application. The coupler can be designed with discrete torque shift points for applications with two distinct operating conditions or with a linear torque profile for constant horsepower applications such as electric vehicles, power generation, web tension control, coil winding, and etc. In addition, the coupler can be designed for unidirectional or bi-directional full torque modes of operation. Some examples of applications and corresponding magnetic coupler profiles follow.
Constant Horspower One of the best examples of a constant power applications, and likely to experience the biggest benefit from the SRFW concept is electric vehicles. Miles per charge is key, hence being able to wind for the highest possible torque per current, while not giving up top end speed is extremely valuable.
For these applications without SRFW, using PM motors, it has been typical to wind the motor for high speed operation. This is done by using a very limited number of turns and maximizing wire gage for the available space such that large currents could be used to compensate for the low torque constant. By doing this a motors power could be maximized for a given size. The penalty is high current draw from the battery source and short battery range. The result of this approach is high power bragging rights, while it often places the torque at speeds where it is not real important, bumps up the low speed/high torque current demand, and results in the use of an in-efficient gearing system.
Referring to the Torque vs. Speed chart below, the torque vs. speed corresponding to the above description is noted with the 2X Current. The corresponding horsepower is shown further down in the Power vs. Speed chart. In this example the base speed is 2000 rpm and represents the point of maximum power. The speed range can be improved somewhat by introducing additional waste current, weakening the PM field. If the number of turns were doubled, hence cutting the current demand in half, the torque speed profile would be shown by the 1X Current profile. The corresponding HP profile is shown in the second chart below. From comparing these curves it is obvious why with speed range and torque goals, there is value in the high speed winding at the price of current draw and vehicle range. In addition to sacrificing high current draw the speeds are too high for direct use so additional in-efficiencies must be added through gearing.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=589&stc=1&d=1215526602
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=590&stc=1&d=1215526602
Lets now look at the low current draw motor from ab
ove with a split rotor and linear coupler to be described in more detail below. This torque vs. speed curve is shown in yellow. The corresponding HP is in brown. Notice how the maximum power is extended 2 ½ times beyond the base speed and actually increases beyond the base speed. While the peak power is not as high as the 2x current draw curve, the speed range is much greater, and the low speed torque is equivalent. Only a small band of moderate speeds give up actual torque. The big plus is that with the SRFW motor only ½ the current is required at the lower speeds. Since most driving is done in short stints, a large portion of the battery life is consumed during acceleration, or low speeds so this current reduction is very attractive.
The coupler for the above SRFW example must exert a linear torque profile as the two rotor sections transition from the fully weakened state to the full torque state with one magnetic pole of rotation. Since operation in reverse is minimum, for cost and compactness it makes sense to have the coupling perform unidirectionally only. Attached Chart3 shows an example coupler torque vs. angle profile for this application. T1 represents the torque that is holding the two rotor sections in the full speed mode of operation against a limiting hard stop. In this fully weakened state the torque available is dependent on the ratio of magnets within the two rotor sections. The load / applied torque must exceed T1 to begin increasing the motor torque constant.
T2 represents the second rotational stop where like poles are aligned in the rotor sections. T2 is the load/applied torque desired to move the motor into the full torque mode of operation. For the torque/speed and power/speed curve shown above, T2 would represent the peak torque value shown, however there may be value in having T2 fall below this torque. Also note that with the weakened stop set at complete mis-aligned poles the reverse direction will always operate in the fully weakened state. By moving this stop toward pole alignment in the opposite direction the torque constant would increase in the reverse direction.
Directional Load Variation One of the most straight forward and effective applications for SRFW is where an application requires high torque in one direction and high speed in the other. For this application the forward and reverse stops within the split rotor in conjunction with the split ratio, will determine the torque constant for the corresponding load direction. In the high torque direction the load/applied torque will collapse the magnetic coupler at a low value, moving the rotor sections to aligned poles with limited load. In the other direction the device will rotate back into the full are partial weakened state.
In these applications the coupler is designed merely to satisfy deceleration requirements and prevent unstable servo operation. Some applications may be suited with a high durometer, high temperature urethane dampening stop at the split rotor, rotational stops.
High Speed/Low Torque Low Speed/High Torque in Same Direction (Bidirectional) An example of this application would be attached to a linear actuator, operating as a press. A high speed/ low force approach is desired followed by a high torque/ low speed press. Finally, a high speed/ low torque return is desired. The high force may be in either direction.
For this application a stepped, bi-directional coupler is most desirable. An example of the coupler is shown below.
http://www.flytheroadclub.com/forums/attachment.php?attachmentid=591&stc=1&d=1215526602
Since the fully weakened state is not really an operating condition with zero torque produced, the split rotor sections can be equal in magnet count. The coupler goal in this application is to have as steep as possible profile near the fully weakened state. At a load of 110 in-lbs the goal was to have the rotor sections make a near step transition to the full torque mode. It was expected that less than 110 in-lbs was required for high speed movement of the tooling and also acceleration and deceleration. Once the press load was encountered, a sudden shift to over 1000 in-lbs was encountered and the coupler collapsed almost instantaneously.
This application is a straight forward demonstration of the advantages of the SRFW motor. The torque constant could be maximized without regard for the speed limit at available bus voltage. The stops were set to +/- 1 pole. The coupler torque vs. rotation was then designed to assure that at a desired torque, the rotor sections were cancelling enough Bemf to allow the desired speed of operation.
High Speed/Low Torque Low Speed/High Torque in Same Direction (Uni-directional) Runabout boats are good examples for this approach. Without variable speed transmissions, boat designers are challenged with selecting the appropriate gear ratio to provide out of the hole torque, without limiting top end, planing speed. Often times boat owners are not happy with their low end torque or top end speed so they change propeller angles to gain in one and sacrifice the other. SRFW for boats would act like a variable speed transmission without the gears. By setting the coupler to have a full torque stop and a fully weakened stop in only one direction with a stepped profile similar to the previous application, boats would operate at full torque prior to getting on a plane. Once on a plane, as the torque load decreases, the split rotor sections would mis-align for high speed operation.
PM Brushless vs. Induction
Currently there is a battle for position between vector driven induction and PM motors. PM brushless motors have higher efficiency due to the elimination of magnetizing current and copper losses in the rotor. In addition, PM brushless motors require less sophisticated control and typically provide better low speed, high performance torque control. Finally, PM brushless motors typically provide higher peak torque and power vs. size.
On the downside for PM brushless, prior to SRFW, vector driven induction motors have provided desirable field weakening control for broad speed range, constant power. In induction motors there are no magnets and the magnetic field is adjustable by controlling the voltage to frequency relationship. In PM brushless motors, prior to SRFW, field weakening was only minimally effective at the expense of wasted current. These field canceling currents are typically quite high due to the high energy of rare earth magnets.
Now with SRFW, PM motors offer performance advantages over vector driven induction motors on many fronts. The biggest disadvantage remaining is in the expense of the rare earth magnets.
SRFW Status
The SRFW concept has been demonstrated with physical hardware and is currently patent pending with provisional application 188915 US. Current lab hardware demonstrates a split rotor with a 3/2 magnet relationship on the rotor. With this rotor configuration a motor originally designed for a top end speed of 700 RPM at a bus voltage of 460Vac achieved speeds of 3000 RPM without any reduction in torque production once the coupling was fully collapsed.
No longer is it necessary to size a motor for speed requirements and to accept the hit on current to achieve desired torque. Now motors can be selected for torque density and minimal current draw. The rotor can then be designed to weaken according to speed requirements. In essence, the split rotor acts much like an automatic transmission without gears or elements of wear or maintenance.