Direct-drive motors meet the high-torque needs of large inertia rotary loads. Increasing the pole count increases the rotor diameter, total flux, and torque moment arm which produces the higher output torque. The large hole in the center of the shaft comes from the magnetic pole design and may be u Contact online >>
Direct-drive motors meet the high-torque needs of large inertia rotary loads. Increasing the pole count increases the rotor diameter, total flux, and torque moment arm which produces the higher output torque. The large hole in the center of the shaft comes from the magnetic pole design and may be used for channeling wires and process fluids when necessary.
Traditional high-performance rotary machines such as tables, robots, turrets, and pedestal assemblies typically run on standard servo-motors connected to rotating members through transmissions. The transmissions are usually gears or belts that convert the high-speed, low-torque of servomotors to the low-speed, high-torque needs of the rotary machine. However, in some cases the transmission can become a limiting factor or introduce errors that some applications can''t tolerate. For these, direct-drive rotary motors might be the most efficient solution.
Direct-drive torque motors use unconventional magnetic-path designs to provide torque matched to the load. Their magnetic pole count is higher than standard designs, and the diameter of the rotor is larger — both of which add to the total flux of the machine as well as the torque moment arm generating higher torque output. As an added benefit, the magnetic pole design requires less space and provides a path for wires and cooling fluids to run through the center of the motor.
But magnetic-circuit designers have a particularly tough job with these motors because the steps needed to maximize torque output also produce undesired torques, such as cogging and torque ripple. Often, this can degrade system response, accuracy, and smooth operation. The goal is to produce large, undistorted torque and smooth output.
Without a transmission, the motor can reach higher accelerations with higher accuracy as well. Systems using direct-drive motors are not susceptible to the problems usually encountered by those using transmissions, such as gear chatter, belt stretching, and loss of accuracy from imperfect transmission component geometries. Furthermore, acceleration is limited only by the load and motor. Taken together, these advantages allow direct-drive systems to control motion with extraordinary speed and accuracy.
A final problem that can be solved with direct drives is resonance between motor and load. Because high-performance motion systems often rely on closed-loop control, they use high gain loops to obtain the best response possible. However, the compliant couplings usually used between motor and load cause oscillations of about 300 to 1,000 Hz. These oscillations occur at or near a frequency where the load and motor inertias resonate across the compliant coupling. Compliance in standard motors is high because the motor shaft is comparatively long and narrow.
As shown in the diagram, loads may be coupled to direct-drive motors on a large diameter, effectively eliminating resonance in servosystems. And while standard servosystems usually limit load inertia to no larger than 5 to 10 times the motor inertia, direct-drive motors have no such limit. The load inertia is often hundreds of times larger than the motor inertia with no negative effects.
Information for this article was contributed by George Ellis and Tom England, Kollmorgen, Radford, VA 24141, (800) 777-3786, Fax: (540) 731-0847, , [email protected]
Learn how a DC motor works to understand the basic working principle of a DC motor. We consider conventional current, electron flow, the winding, armature, rotor, shaft, stator, brushes, brush arms, terminals, emf, electromagnets, magnetic attraction as well as detailed animations for how the dc motor works.
DC motors look something like this above, although there are quite a few variations. These are used to convert electrical energy into mechanical energy and we can use these for example in our power tools, toy cars and cooling fans.
Wrapped around the T shaped arms of the rotor are the coil windings which carry the electrical current from the battery. As the current passes through the coils it produces an electromagnetic field, we control the timing and polarity of this magnetic field to create rotation.
The ends of the coils are connected to the commutator. The commutator is a ring which has been segmented into a number of plates which sit concentrically around the shaft. The plates are separated and electrically isolated from each other as well as the shaft. The ends of each coil connect to different commutator plates, they do this to create a circuit and we''ll see that in detail shortly.
The brushes rub against the commutator segments to complete the circuit. Electricity can then flow through a terminal, through the arm, into the brush, through a commutator segment, into a coil, then out to another commutator segment, into the opposite brush and arm back to the other terminal.
These components give us our basic DC motor. To understand how the DC motor works, we need to understand some fundamentals of electricity as well as how the components inside work.
Electricity is the flow of electrons through a wire. When lots of electrons flow in the same direction we call this current. DC electricity means the electrons flow in just a single direction, from one terminal of a battery directly to the other. If we reverse the battery then the current will flow in the opposite direction.
In these animations we''re going to be using two terms. That''s electron flow and conventional current. Electron flow is what''s actually occurring with the electrons flowing from the negative terminal to the positive terminal. Conventional current moves in the opposite direction from positive to negative. Conventional current was the original theory and it'' still widely taught and used today because it''s easier to understand. Just be aware of the two terms and which one we''re using.
As you probably already know, magnets are polarised with north and south ends. These types are known as permanent magnets, because their magnetic field is always active. When in proximity with another magnet, the alike ends push away and the opposite ends attract. So we get these pushing and pulling forces caused by the magnetic field of the magnets.
Magnets have these curved magnetic field lines which run from the north pole to the south pole and extend, curving around the exterior. The magnetic field is most powerful at the ends, we see this because there are more magnetic field lines closely packed together.
When two magnets are in close proximity to each other, their magnetic fields interact. Two alike ends will repel each other and their magnetic field lines will not join. However, two opposite polarities will be attracted to each other and the magnetic field lines will converge into a highly concentrated region.
When we connect a wire to the positive and negative terminal of a battery, a current of electrons will flow through the wire from the negative to the positive terminal.
When electrons pass through the copper wire, they generate an electromagnetic field around the wire. We can actually see that by placing some magnets around the wire. When we pass electricity through the wire the magnets rotate. When we reverse the current direction, the magnets will also reverse and align the opposite way.
The problem with the electromagnetic field in a wire is that it''s quite weak. But we can make it much stronger simply by wrapping the wires into a coil. Each wire still creates an electromagnetic field, but they combine into a much larger and stronger magnetic field, that''s what we use to create the coils in the rotor.
The coils of wire are known as windings. The simplest DC motor has just a single coil. These are a simpler design; the problem though is that they can align magnetically which jams the motor and stops it from rotating. The more sets of coils we have, the smoother the rotation will be, this is especially useful for low speed applications. Therefore we normally find at least three coils in a motor to ensure a smooth rotation.
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