What is a Brushless DC Motor?

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MAGNETICS

A brushless DC (BLDC) motor is a hybrid permanent magnet DC motor. Figure 1 is a simplified illustration of how torque is generated in a permanent magnet DC motor.

( Figure 1 )

If current is caused to flow in the armature conductors, torque is produced. There is an application of a of physics which is expressed as:law

F = KBli, ( Equation 1 )

Where:
F = force
K = a constant
B = air gap flux density
l = length of conductor
i = current in a conductor.


If more than one conductor is carrying the same current (multiple turns per coil), then

F = KBliz, ( Equation 2 )

Where z = the number of conductors in the series. In a motor, the conductors rotate about a central shaft (see Figure 1). Then torque, T, = FR, where R = radius at the air gap. So,

T=KRBliz  ( Equation 3 )

Figure 1 shows the coil in the zero torque position. The maximum torque position is 90 electrical degrees from the position shown. As the conductors rotate from the maximum torque position, torque drops off in a sinusoidal fashion and becomes zero when the coil has moved 90 degrees.

A brush type motor has more than one coil. Each coil is angularly displaced from one another so that when the torque from one coil has dropped off, current is automatically switched to another coil which is properly located to produce maximum torque. The switching is accomplished mechanically with brushes and a commutator as shown in Figure 2.

( Figure 2 )

In a brushless motor, the position of the coils (phases), with respect to the permanent magnet field, is sensed electronically and the current is switched, or commutated, to the appropriate phases. The commutation is effected by means of transistor switches. A brush type motor may be converted into a brushless motor by bringing out all the leads that are attached to the mechanical commutator and providing switches for each lead; however, this approach would involve a large number of switches. Instead, a polyphase winding similar to that used in AC motors is utilized. In this design, the phases are “commutated” as a function of shaft position.

Two, three and four phase motor designs are common. BEI Kimco provides mainly 3 phase designs. This configuration optimizes performance even though it requires more electronic components. Three types of 3 phase windings are available: delta bipolar, wye bipolar, and wye unipolar. There three winding configurations and their transistor orientation are shown in Figure 3.


3 PHASE, BIPOLAR COMMUTATION

Figure 4 illustrates the sequential steps in the commutation of a 3 phase, bipolar system. Closing transistors (1) and (4) will enable current to flow through phase A and B. The permanent magnet rotor will then align itself in a zero torque, preferred position. If (1) is opened and (5) closed, current will flow through phases B and C, and the rotor will move 120 electrical degrees. Similarly, opening (4) and closing (2) will cause the rotor to move another 120 electrical degrees. (Note that the current through phase A is now flowing in the direction opposite the one at the start of this exercise).

Obviously, there must be some logic in the order and rate the transistors are switched. Hall Effect sensors are typically used in the logic scheme. Graph 1 may help to illustrate how this works. For instance, if one were to energize individual phases of a three phase brushless motor, one would generate, as a function of electrical degrees of rotation, a torque curve as shown in Graph 1. Each phase would be 120 electrical degrees apart. (It should be noted that electrical degrees is simply mechanical degrees multiplied by the number of pole pairs of the motor.)

( Figure 3 )

( Graph 1 )
 

Now, imagine the rotor in Figure 4 resting in its zero torque position (i.e. the 180 electrical degree point on Graph 1), with current flowing through winding A. If the rotor is physically moved back from its rest position, torque will build up in a roughly sinusoidal fashion and reach its peak at 90 electrical degrees. Since the objective is to have the motor run at its peak operating point, the position is either still another 30 degrees back from the peak torque point, or 60 degrees, which is the point at which the winding must be switched on. A sensor is located to trigger from a rotor magnet at this specific event.

If the rotor is allowed to turn back towards its original resting point, or “zero torque point,” but current is switched from winding A to winding B at 180 electrical degrees, the motor will operate on a new sine wave, or torque vs. angle, resulting in another point of peak performance. Again, a sensor is located in such a manner to mark this event. Similarly, the third sensor is set to trigger at 300 electrical degrees.

 ( Figure 4 )
 

These Hall Effect sensor settings, 120 electrical degrees apart from sensor to sensor, automatically sequence the switching of currents from one phase to another at the appropriate time.

Another important point to note from Graph 1 is the sign of the torque generated as a function of rotor position. If the currents in individual phases were switched at the proper electrical position, positive torque could always be generated, as illustrated in Graph 2.

Brushless DC Motor Torque Curves Switching Phases for Positive Torque

( Graph 2 )
 

With the proper selection of phase energization (i.e., the proper commutation scheme) the resultant torque output of the motor is as illustrated in Graph 3. The successful commutation of the brushless motor is knowing the rotor position in electrical degrees and having the proper commutation scheme.

( Graph 3 )

( Figure 5 )
 

Figure 5 provides connection and color code information pertaining to motor and sensor leads in BEI Kimco BLDC motors. The diagram also includes tables illustrating the proper sensor logic for clockwise and counter-clockwise shaft rotation.


“INSIDE-OUT” MECHANICAL DESIGN

A brush type motor has a permanent magnet stator and a wound rotor, as shown in Figure 2. The configuration of a brushless motor is reversed (i.e., a permanent magnet rotor and a wound stator). The wound member is referred to as the “armature.” Furthermore, there are two types of brushless DC motors; the style that has an outer rotating magnet assembly, and the “inside-out” style that has an inner rotating magnet assembly. Figures 6A and 6B depict the two motor types. The outer rotor and inner rotor features of brushless DC motor designs each have advantages and disadvantages. The ways in which the motor characteristics differ between the two designs are summarized in Table 1. TABLE 1 Comparison of Motor Characteristics Inner Rotating vs Outer Rotating Permanent Magnet Assemblies.

( Figures 6A&6B )

( Table 1 )



Inertia Considerations

One of the key elements of a proper motor selection exercise is an optimized load-inertia to rotor-inertia ratio. The recommended ratio is a maximum of 10 in rigid mechanical systems that utilize gear reducers or worm gears and a maximum of 3 for systems that include belt and pulley reductions. A motor with an outer rotor would therefore have the advantage of greater stability in a system with very high reflected load inertia. On the other hand, low rotor inertia enables attainment of higher acceleration rates, since the acceleration torque required in an application is the product of total inertia times acceleration rate, plus load/friction torque. The recommended load-to-rotor inertia maximums should not be exceeded whenever possible.

Another advantage of the lower inertia, inner rotor is the level of rotor balance attainable in the system. This advantage enables smooth operation at higher speeds (approximately 9,000 RPM to 12,000 RPM). Again, there is a tradeoff; the inner rotor has a high speed limitation of about 15,000 RPM (without special mechanical sleeving of the rotor), whereas the outer rotor has a limitation of about 30,000 RPM. For very high speed operation, the outer rotor has the clear advantage.

Torque & Power Output

For a given motor volume, the outer rotor has higher torque and power output. This advantage is particularly important at high speeds of operation, where the back EMF of the motor winding eats away at the voltage source, leaving little voltage available to pull current. However, the inner rotor motor can be cooled more effectively than its outer rotor counterpart. This is because the armature is external to the rotor, enabling direct heat dissipation through the motor outside diameter (O.D.) Addition of forced air cooling or heat sinking to the motor O.D. results in a dramatic increase in motor performance.

Number of Motor Components

In this category the inner rotor motor offers a couple of advantages over the outer rotor motor. The fewer number of parts in the inner rotor design means greater inherent reliability. It also puts money in the user’s pocket, since its design simplicity generally results in a lower cost.

< The inner rotor motor is the motor of choice where speeds of operation and inertia matching considerations allow.


TWO OFFERINGS: HOUSED VS. FRAMELESS

BEI Kimco offers BLDC motors in both housed and unhoused, or frameless, configurations. Frameless motors are utilized by Original Equipment Manufacturers (OEM’s) interested in fully integrating motor part sets into the finished product. Frameless motors are also utilized in systems that require high servo bandwidth, where the use of a shaft coupling device could introduce unwanted mechanical resonances. They are also used as a result of economic considerations. Figure 7 depicts a frameless motor and its housed brush motor counterpart.



( Figure 7 )
 

Housed motors offer the convenience of a complete motor package, including bearings, shaft, enclosure, and mounting provisions. In addition, BEI Kimco can provide housings that incorporate features of the customer’s original equipment. For example, the motor mounting flange may be cast to include a mounting plate that would otherwise have been supplied as a separate component. The result is a savings in labor, book keeping, and inventory expenses.


SYSTEM CONFIGURATIONS

Pound-for-pound, the typical “inside-out” brushless DC (BLDC) motor provides more output power, greater life and reliability, higher operating speeds, cleaner operation, etc., than its traditional, brush-type (PMDC motor) counterpart. High torque-to-inertia ratios; a stationary armature around a rotating permanent magnet assembly (instead of a stationary permanent magnet assembly around a rotating armature); and, of course, elimination of the brushes – make the BLDC motor the preferred choice when it comes to performance considerations. A BLDC motor is also the preferred choice from a cost consideration, when applications are evaluated on a total cost (i.e., over the life of the product) rather than up-front cost basis.

The main technical issue of replacing a brush-type motor with its brushless counterpart is the commutation electronics consideration. A typical BLDC motor is an eight-wire device. Replacing the two-wire PMDC motor with its eight-wire counterpart requires additional electronic circuitry and additional considerations at the systems level. These and other related factors have led to the development of the two-wire BLDC motor – a BLDC motor that contains integral electronic commutation, or drive control circuitry. The present tutorial is provided to familiarize the reader with the types of two-wire BLDC motors available in the market-place, variations thereof, and guidelines for applying these products.


TWO QUADRANT VS. FOUR QUADRANT CONTROL

In order to get a better appreciation of the two-wire BLDC motor application, it is first necessary to understand the different ways PMDC and/or BLDC motors may be controlled.>/p>

( Figure 8 - Two-coordinate Torque vs. Speed System )
 

Figure 8 shows a two-coordinate system in which the horizontal axis represents motor shaft speed and the vertical axis represents the output torque. In DC motors, output torque is proportional to current, and output speed proportional to voltage. Motor drive electronics that can provide positive current and positive or negative voltage to the motor terminals are therefore called two-quadrant drives, since they operate in quadrants I and II. Drives that provide positive or negative current and positive or negative voltage to the motor terminals are called four-quadrant drives, since they operate in all four quadrants of two-coordinate system.

( Figures 9A&B )
( Figure 10A&B )
 

Applications abound for two-quadrant and four-quadrant drives. For example, a turntable that needs to be rotated over a range of fixed speeds can be controlled with the two-quadrant system. The friction due to windage, bearing, etc., constantly applies a drag, or negative torque to the system, causing the table to slow down and eventually stop, in the absence of a positive controlled torque. Positive torque is needed to keep the turntable rotating at the desired speed, in one or the other direction (i.e., clockwise or counter-clockwise). If, on the other hand, it is necessary to stop the turntable faster than frictional torque allows, or if the turntable must be stopped and held at a fixed angular position, then the four-quadrant drive is the proper solution. In this case, negative torque is used to decelerate the system, and positive and negative torque is used to hold the turntable in place (against disturbing forces).


OPEN-LOOP VS. CLOSED-LOOP CONTROL

As alluded to in the previous section, it is sometimes necessary to control the rotational speed and/or position of the motor shaft and attached load.

Figure 9 depicts a system in which there is control of the shaft output. This type of system requires a feedback device which senses the specific parameter that needs to be controlled. Referring to the turntable example previously described, the table that runs at a set speed would need a motor with a feedback device that measures the rotational velocity of the motor shaft. A table that also needs to be positioned would need a feedback-device that senses rotational velocity and angular position of the shaft. Either system is referred to as a “closed-loop” system, as illustrated in Figure 9.

Figure 10 depicts a system in which control of the motor shaft is not necessary. This type of system requires no feedback device (other than for commutation purposes). Since the feedback loop is not closed, this system is called “open-loop”.


TYPES OF TWO-WIRE BLDC MOTORS

The primary differences between these two systems are the customers interface requirements and the locations of the commutation electronics.

The eight-wire system shown in Figure 11 includes commutation electronics that are provided as a separate item – sometimes in a separate enclosure and sometimes as a board that is to be mounted somewhere in the equipment. This type of system also includes a BLDC motor with eight leads. Three of the leads are to power the three windings (or phrases). The other five leads are for the three Hall Effect device outputs, the Hall voltage supply, and the Hall voltage return. The Hall Effect devices sense rotor position. Their output is used to commutate the brushless motor through the use of commercially available commutation I.C.’s. These chips decode the signals from the Halls and provide the logic with which the current-carrying transistors are switched on and off for proper phasing of the motor.

 ( Figure 11 - Block Diagram of an Eight-Wire BLDC System )
 

There are many variations of the “two-wire” BLDC motor presently available in the marketplace. However, the most popular and most widely used configuration is the two-quadrant, open or closed-loop speed control package. Speed may be controlled with an on-board potentiometer or by bringing out a third wire for remote speed adjustments. The third wire may accept a 0-5V analog input signal. Additional wires or digital communication ports could also be provided to add features, resulting in what the industry has dubbed a “Smart” motor. Use of these options or configurations depends on the desired overall level of control. Of course, there are also the ever-present cost-vs.-performance trade-off considerations. The main point to bear in mind is that, with its integral electronics packaging, the two-wire BLDC motor accepts, and operates off of a straight DC supply, just like its two-wire brush-type DC counterpart.


GLOSSARY

MOTOR PARAMETERS:

Peak Torque (Tp) – The torque that can be produced for 10 seconds without exceeding the maximum allowable winding temperature.

Continuous Stall Torque (Tcs) – This is the amount of torque that can safely be produced over an indefinite period of time under a stalled rotor condition. This value is measured with the motor mounted on an aluminum plate (6” x 6” x 1.8”) heat sink, to the maximum allowable temperature of the windings.

Moment of Inertia (Jm) – The moment of inertia of the rotating member, equal to Wk²/g where W = weight (oz), k = radius of gyration (in.) and g = acceleration due to gravity (386 in/Sec²).

Motor Constant (Km) – A figure of merit of the motor. The higher the value for a given volume of motor, the more powerful the motor.

Electrical Time Constant (TE) – This value is equal to L/R (inductance divided by resistance). It is also equal to the time it takes for the current to reach 63% of its steady state value when the winding is energized by a step input of voltage.

WINDING PARAMETERS:

Windings may be changed to optimize parameters of a given motor model for a particular requirement. This may be accomplished without changing the basic motor constants. The winding constants are listed below:

Resistance (RM) – The resistance between any two lines on a bi-polar motor of between line and neutral on a unipolar motor.

Torque Constant (Kt) – The torque that will be produced for a given current input.

Back EMF Constant (Kb) – The generated voltage as a function of speed. ◢ Save Save & Exit Cancel Copyright ©2016 CST English English Deutsch العربية Česky Ελληνικά Español Français Italiano Nederlands Svenska Slovensko Polski Português de Portugal Português do Brasil Русский 简体中文 繁體中文 עברית Lietuvių Suomi Dansk Bahasa Indonesia Magyar ไทย