AC and DC Drive Schemes for Brushless Motors

Because they deliver an optimized torque per package size, brushless motors are widely used in many applications. As such, they are smaller in size and lighter in weight; they provide higher peak overload capacity with wider speed range capability; and provide long, reliable and maintenance-free life. If time up to speed is important in the application, brushless is eight to 10 times faster. Additional benefits can be achieved when powering a brushless motor with an ac drive scheme.

Basic Motor Operation

Motor rotation is based upon the fact that if a conductor carrying current is placed in a magnetic field, a force will act upon it. The simplest machine is the induction motor. It consists of windings on a peripheral housing, (stator assembly) and an internal assembly (rotor). The motor operates when power is applied onto the stator winding assembly resulting in current flow that sets up the first magnetic field. This in turn induces current in the rotor setting up the second field. It is the interaction of these two magnet fields which results in rotation.

The induction motor is basically a constant speed motor, with speed being dependent upon the frequency of the applied voltage. In the early days, in order to adjust an application's operating speed, various pulley diameters were initially employed.

Today microprocessors use various methods to manipulate the magnetic field to adjust speed. However, back in the early days, it took the development of the dc motor to allow speed to be adjusted quite easily - simply change the applied voltage and motor speed would vary. In the dc motor design, permanent magnets are used on the stator to set up the first magnetic field.

The rotor, which sets up the second field, consists of several windings and a commutator. Each winding consists of turns of wire set between steel laminations to concentrate the magnetic field. When power is applied onto the rotor, current passes through the windings, thus setting up the second field. As the two fields interact, resulting in rotation, they will normally align and rotation ceases.

However, the mechanical commutator switches power from winding to winding, thus maintaining the rotor magnetic field at the optimum angle with respect to the permanent magnet field to obtain maximum torque and efficiency from the motor.

In a brushless motor, the design layout incorporates the most beneficial attributes of each design to provide the best of both: the long life of the induction motor and smaller size of the dc motor.

In a brushless motor, the layout or design includes electrical windings on the outside stator housing, similar to the induction motor (however only three windings); and like the dc motor, the brushless design includes permanent magnets mounted on the rotor.

This design has intentionally eliminated the mechanical commutator; and since commutation in a permanent magnet design is the action of switching current from winding to winding for rotation to occur, the obvious question is how rotation in a brushless motor occurs.

The answer is via "electronic commutation." The motor plays no role, commutation is accomplished by the control, which is supplying power and running the motor.

Electronic Commutation Principles

To illustrate and explain electronic commutation, a brushless motor consisting of a rotor with a small feedback device will be used. The feedback device consists of a small magnet (located on the rotor shaft) and three Hall sensors (mounted on the stator). Edward Hall in 1879 discovered that a sensor would alter its output when subjected to a passing magnetic field.

Thus as the rotor shaft turns, the small magnet, with north and south poles, passes by and causes the three Hall devices to turn on and off. This provides information about the rotor position. Of course, other feedback devices may be used to detect shaft/rotor position, however this was chosen for simplification. This rotor position information is fed into the logic circuitry of the control.

The control's logic circuitry uses this information to turn "on" specific power devices applying power to the motor windings thus maintaining the magnetic field at the optimum angle for rotation.

Torque Development

A brushless motor's back-emf displays only the positive portion of the waveform. When power is applied to the first winding, the shaft will normally rotate to the 180 electrical degree position and stop. However, if power is removed from the first winding "R" and applied to the second winding "S" when at the 120 electrical degree position, the shaft will continue to rotate. This process of electronic commutation is repeated (power is applied next to the third winding "T" at 180 degrees) and rotation continues.

Note that each winding is conducting for 60 degrees, thus this would yield an effective or average back-emf (Ke) of:

AC and DC Drive  Schemes for Brushless Motors(1)

Where K'EOO is the peak value of the back-emf waveform, measured phase-to-phase (o- o). Since in metric, the voltage constant KE (volt/radian/second) equals the torque constant KT (N-m/amps), the equation for torque which the motor develops becomes:

T = 0.955 K'EOO I (T=N-m, K'EOO =v/r/s, I=amps) (2)

Where T is torque expressed in Newton-meters (N-m), and I is current (amperes). Keep in mind that the back-emf KE is easy to measure (whereas the torque constant is more difficult) and can be verified by simply back driving the motor at 1,000 rpm and measuring the voltage on the motor terminals - this is the motor's back-emf in V/Krpm (which is easily converted to v/r/s).

Notice the current as depicted in the slideshow below. Although shown as ideal without any ripple etc., it is flat or level. A level amount of power is applied over the 60 degree time period for each winding. By increasing the amount of power, this will make the motor speed up, and vise versa, less would result in lower speed. When "electronic commutation" of this type is used to power or drive a brushless motor, it is termed "dc brushless drive."

Same, but Different

Now suppose a sinusoidal current with a value of IRMS is applied to the brushless motor windings. The output torque developed is equal to the sum of the torques developed in windings R, S and T:

T = TR + TS + TT (3)

And the equation for torque becomes:

T = 1.22 x K'EOO IRMS (T=N-m, K'EOO =v/r/s, I=amps) (4)

Again K'EOO is easy to measure in the lab by back driving the motor, and is the peak value of the sinusoid measured phase-to-phase, and IRMS is the RMS current. When electronic commutation of this type is use to drive a brushless motor, it is termed "ac brushless drive."

dc Brushless vs. ac Brushless

From equations (2) and (4) above, the prime comparison feature of dc brushless and ac brushless may now be accomplished, but first a simple conversion is necessary. As indicated, it is easy to measure the brushless motor voltage constant in volts per 1,000 rpm (V/Krpm) with either a meter or scope. Then simply take the measured value of V/Krpm times 0.00955 to convert to v/r/s and use equations (2) and (4).

Now, as an example, ask how much torque will the motor develop, if the drive can provide 10 amps? Assume the sample motor is back driven at 1,000 rpm and the voltage (i.e. the back-emf) measured is 75 volts peak/Krpm measured phase-to-phase.

First determine the torque developed if the dc drive scheme is used:

Step one: convert V/Krpm to v/r/s:
KEOO = 75 V/Krpm x 0.00955 = 0.716 v/r/s

Step two: using the dc drive equation (2) the torque developed is:
T = 0.955 x 0.716 x 10 = 6.83 N-m
Or: 6.83 N-m x 8.85 = 60.5 lb-inch

Now determine the torque developed if the ac drive scheme is used:

Step one: convert V/Krpm to v/r/s:
K'EOO = 75 V/Krpm x 0.00955 = 0.716 v/r/s

Step two: using the ac drive equation (4) the torque developed is:
T = 1.22 x 0.716 x 10 = 8.73 N-m
Or: 8.73 N-m x 8.85 = 77.3 lb-inch

Same Motor, More Torque

Comparing these two drive schemes, reveal that the sc drive will provide 28 percent additional torque. The differences between these drive schemes arise in the control algorithm and the feedback device. This added torque is a benefit improvement in numerous applications, as it means additional performance is delivered to the machine/equipment. Another key point: although either the dc or ac drive scheme was employed, the motor in the example was the same. In other words, the same motor can be powered by either drive scheme. When the motor is driven with a dc drive scheme it is termed a dc brushless motor, and when that same motor is driven with an ac drive scheme it is termed an ac brushless motor.

Application Determines dc or ac Brushless

The drive and the feedback device used in the application, will determine whether the motor is termed either a dc or ac brushless motor. Typically when Hall sensors are used, the motor will be termed a dc brushless, since this feedback can be used in applications like fans, pumps and conveyors, in which adjustable speed is the prime requirement. When resolver feedback is used, the motor will be termed an ac brushless motor, as it is used in positioning applications. When an incremental encoder is used, the motor can be either a dc or ac brushless, since it is used in either adjustable speed or positioning applications. Some dc drive schemes, when initially turned on, will hunt in order to learn rotor position for commutation.

When an absolute encoder is used, the motor will be an ac brushless, since the application demands the highest precision. The application requirements will determine which drive technology to use, though applications requiring adjustable speed will make best use of the dc drive scheme; applications requiring positioning, lower drive currents (or more torque for a given current) will make best use of the ac drive scheme.


About the Author:
John Mazurkiewicz' experience began in research and development of servo control systems, and then progressed into application engineering, technical marketing and new product development. John has held key positions, the most recent as Product Marketing Manager for Baldor Electric (Ft Smith, AR) in which he was responsible for introducing numerous new products. He has recently retired from Baldor.
AC and DC Drive  Schemes for Brushless Motors
AC and DC Drive  Schemes for Brushless Motors

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