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Choosing Between Brush and Brushless DC Motors

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Choosing Between Brush and Brushless DC Motors Choosing Between Brush and Brushless DC Motors Trade-offs include speed, efficiency, lifetime, maintenance, robustness, size, and cost. Designing a system that involves motion and performs as intended requires the right motor. Just as the conditions and specifications of motion systems vary from use case to use case, so does the optimal choice of motor. The most fundamental decision involves commutation type: brush or brushless. In some cases, mechanically commutated brush motors provide the best solution for the budget. In other cases, only brushless DC (BLDC) motors will do. Here, we provide a comprehensive review of the technology, along with tips and tricks for choosing the right solution for a given application. Motors 101 Ampere’s Law tells us that current passing through a wire generates a magnetic field. If An electric motor consists of two basic we shape that current-carrying wire into a elements: a rotor and a stator. Both rotor loop and place it in a magnetic field, the and stator generate a magnetic field, and interaction between the two fields will apply the interaction between the two fields a turning force, and consequently a torque produces torque that enables the motor to moment, to the wire (see figure 1). The do useful work. Although a wide range of turning force is known as the Lorenz force. design variations exists, including linear This is the basic principle behind the electric motors, for purposes of this white paper, we motor. will focus on rotary motors in which the rotor (armature) revolves and the stator is fixed. Allied Motion Inc. 495 Commerce Drive Amherst, NY 14228 Tel: 1 (716) 242-7535 [email protected] www.alliedmotion.com 1 Choosing Between Brush and Brushless DC Motors where q is the moving charge in the wire, v is the velocity of that charge, and BS is the magnetic field generated by the stator magnets. If we differentiate equation 1 and express it in terms of current I defined as I=dq/dt, we can restate it as dF =I dt (v × B ) [2] Figure 1: The interaction between the current L d s (red) and the magnetic field of the magnets, B (orange) applies a Lorentz force, F (brown), to the Distance is equal to rate times time, which loop. When this force is applied at a distance from means that we can swap a differential the axis, it generates a torque that turns the loop. length of our wire loop dl for vddt. Now, equation 2 becomes DC motors can be divided into two classes: brush DC motors and BLDC motors. Let's dFL=I dt (dl × Bs) [3] start with brush DC motors. For our simple model of a loop between two Consider a DC motor built of a fixed stator flat magnets, we can restate equation 3 as made of two permanent magnets, and a rotor wound with coils and mounted on FL = I (l × B) [4] bearings such that it is free to turn. The rotor consists of a crossbar wound with wire Or, converted to a scalar expression, as to form a coil but with the direction of winding changed from one side of the FL = IlB sinθ [5] crossbar to the other. By Ampere’s Law, current running through each set of In other words, force is a function of θ, windings generates a magnetic flux which is the angle between the current distribution, creating a pair of vector and the magnetic field vector. electromagnets. Because the wire is wound in opposite directions on each side, the It is instructive to take this one step further electromagnets have opposite polarities. and examine the expression for torque in τ When we place our rotor in the uniform this context. We can express torque as magnetic field generated by the stator τ = r × F [6] magnets, the interaction generates a Lorentz force, FL, given by where r is the length of the moment arm =q(×) [1] (see figure 2). Then τ = r F sin θ = r Il B sin2θ [7] Allied Motion Inc. 495 Commerce Drive Amherst, NY 14228 Tel: 1 (716) 242-7535 [email protected] www.alliedmotion.com 2 Choosing Between Brush and Brushless DC Motors is zero or 180° is known as commutation, or commutating the motor current. The simplest way to accomplish motor commutation is with a set of mechanical switches, called, as you might expect, a commutator. A mechanical commutator consists of a number of switch contacts attached to the rotor, with each pair of contacts wired in series with a rotor coil. One or more sets of complementary contacts called brushes, which are affixed to the motor housing, contact the commutator Figure 2: The torque applied to our current- to deliver current to the coils through the carrying loop falls to zero as the angle θ falls to commutator (see figure 3). The brushes are zero. connected to a voltage source so that one brush is positive and the other is negative. Brush DC motors The commutator and brushes form a sliding switch assembly that energizes the rotor From equation 7, it is clear that the force coils, reversing the direction of the current and, hence, the torque on the wire loop are that passes through each set of windings as at maximum when the resultant coil the rotor turns, switching the polarities of the magnetic field vector and the magnetic field electromagnets. vector from the stator are orthogonal to one another (θ = 90° or 270°). As the force causes the loop to rotate, the angle θ decreases, eventually becoming zero. Analysis of the vector relationships should convince you that torque becomes zero as well. In the real world, of course, the moving coil has some angular momentum that carries it past the vertical. If, at the exact same time, we reverse the direction of the current in the loop, the polarity of the electromagnets also Figure 3: In a simple, single-coil brush DC motor, reverses. Now, the interaction of the two a split ring fixed to the rotor and in contact with fields again generates torque, causing the brushes acts as a sliding switch to reverse the coil to continue its rotation. The torque polarity of the current in the winding. The process returns to peak magnitude when the loop is is called commutation. once more horizontal so that the rotor Up to this point, we have been describing magnetic field is positioned orthogonal to permanent-magnet brush DC motors. In the stator magnetic field. This process of these designs, the stator consists of switching current direction when the angle θ Allied Motion Inc. 495 Commerce Drive Amherst, NY 14228 Tel: 1 (716) 242-7535 [email protected] www.alliedmotion.com 3 Choosing Between Brush and Brushless DC Motors permanent-magnet segments mounted in a Brush DC motor performance steel tube. Permanent-magnet brush DC trade-offs motors are simple, robust, and easily controlled. They can operate on DC or Brush DC motors are simple, rugged, low- rectified AC power sources. They have cost options for many industrial applications. linear speed-torque curves. They are widely available in a variety of configurations (see figure 4). They do not When built with rare-earth magnets, require onboard electronics for permanent-magnet brush DC motors can be commutation, making them a good choice smaller and lighter than those built using for high-temperature, high-radiation, and other field magnet types or wound-field high shock and vibration environments. stators. The trade-off is increased cost and They are well-suited for portable the risk of demagnetization at high applications because they can be powered temperatures. Alternatively, other magnet directly by battery, although typically they materials like samarium cobalt, nickel iron require intervening switches and/or a control boron, or even ferrite can get the job done mechanism, such as series resistors, for more economically but at the price of motor activation and control. greater size and weight for the same performance level. The wound-field brush DC motor, as the name suggests, generates the stator magnetic field using windings rather than permanent magnets. These motor types are more rugged than permanent-magnet designs, remain magnetically stable even at high temperatures, and can operate on DC or AC power sources. Figure 4: Examples of brush DC motors of various Wound-field brush motors are available in types and frame sizes. The largest motor shown several configurations: separately-excited here has a diameter of three inches. (field winding powered by a separate power At their simplest, brush DC motors operate source than the rotor winding), shunt-wound from a fixed DC power supply to provide (field winding connected in parallel with the constant-speed operation. They can be rotor winding), series-wound (field winding used in conjunction with DC motor controls connected in series with the rotor winding), to provide variable-speed operation. Paired and compound-wound (combination of with a servo drive/controller and feedback, series and shunt windings). Many of these brush DC motors can operate as servo configurations have nonlinear speed-torque motors to yield precise, accurate, curves, but that may be beneficial for the repeatable position or speed control. application, as in the case of using series- wound motors as traction motors. Brush DC motors work best in moderate- to low-speed applications, from a few hundred Allied Motion Inc. 495 Commerce Drive Amherst, NY 14228 Tel: 1 (716) 242-7535 [email protected] www.alliedmotion.com 4 Choosing Between Brush and Brushless DC Motors to a few thousand RPM. They are good from improperly serviced motors. Users solutions for medical mobility equipment should always check the documentation and such as power wheelchairs, stair lifts, and warranty of a motor before opening the motorized patient beds.
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