출처: 알리 바지, 코네티컷 대학교 전기 공학학과, 스토스, CT.
DC 기계는 AC 전류와 전압이 필요한 AC 기계와 달리 DC 전류 및 전압으로 작동합니다. DC 기계는 DC 전류에 의해 제어되는 두 개의 자기장을 발명하고 활용하는 최초의 기계입니다. DC 기계에 필드와 아마추어라는 두 개의 필드가 있기 때문에 적절한 필드 여기를 사용할 수 있는 경우 동일한 기계를 모터 또는 발전기로 쉽게 재구성할 수 있습니다. 필드는 일반적으로 stator 측에 있고 뼈대는 로터 측에 있습니다 (AC 기계에 비해 반대 또는 내부 아웃). 필드 여기는 영구 자석 또는 권선 (코일)에 의해 제공 될 수있다. 현재가 뼈 대또는 로터 코일에 적용되면 DC 소스에서 코일로 전달되어 브러시를 터치하는 회전 로터에 고정된 고정 및 슬립 링이 장착된 브러시를 통과합니다. 로터 뼈가 전류 운반 루프이고, 스테이터 또는 필드 자석에서 외부 필드에 노출되면, 힘이 루프에 가해된다. 루프가 베어링을 사용하여 모터의 양쪽에 “매달려”있기 때문에 힘은 다른 방향으로 이동하기보다는 로터의 샤프트를 회전하는 토크를 생성합니다.
이 회전으로 인해 자기장이 정렬되지만 동시에 슬립 링이 브러시의 측면을 전환하거나 “통근”하는 것이 통근 프로세스라고 합니다. 이 통근이 발생하면 로터 코일의 전류 흐름이 반전되고 자기장은 서로 반대하여 동일한 회전 방향으로 더 많은 토크를 일으킵니다. 이 공정은 계속되고 로터 샤프트가 회전하여 모터 동작을 제공합니다. 발전기 작동에서, 기계적 회전은 자기장 하에서 이동 코일로 인해 유도된 후 로터 샤프트및 전류 흐름이 로터에서 흘러나온다.
이 실험에서 논의된 기계에는 영구 자석이 아닌 필드 권선이 있습니다. DC 기계 작동에서 중요한 통근 프로세스는 미끄러짐 링과 브러시를 사용하여 로터(아마추어)에서 외부 세계로 에너지를 전송하는 데 는 회전선이 회전하고 회전하는 전선이 비틀어 부서지기 때문에 외부 세계로 에너지를 전달합니다. 그러나 이러한 브러시및 슬립 링은 정기적인 유지 보수, 브러시 교체, 청소가 필요하고 촉발될 수 있으므로 주요 신뢰성 단점이 있습니다. 이로 인해 대부분의 DC 기계를 AC 기계로 교체하고 나머지 DC 기계에는 장난감및 간단한 저전동 공구와 같은 영구 자석 필드 여기가 있습니다. 브러시리스 DC 기계(또는 BLDC)라고 불리는 AC 기계는 DC 소스 및 전원 전자 인버터를 사용하여 인버터에서 AC 전압을 제거하는 AC 기계입니다.
이 실험의 목적은 션트와 시리즈라는 두 가지 주요 DC 컴퓨터 구성을 테스트하는 것입니다. 테스트는 기계의 잔류 플럭스를 추정하고 다양한 구성의 부하 및 적재 특성을 연구하기 위한 것입니다.
Series windings typically carry high current rated at the machine's rated armature current, since both series and armature windings are in series. Therefore, series windings are expected to be on the order of a mΩ to a few Ω. Shunt windings on the other hand should draw minimum current from the source which power them along with the machine's armature, and therefore, have large resistance values of tens to hundreds or even thousands of Ω.
The residual λR can be estimated by measuring the armature voltage at no load. Since this a no-load condition, the back e.m.f. and armature voltage are the same, and the back e.m.f. (EA) is a function of λR such that EA=If λRωm where If is the field current and ωm is the mechanical speed.
Each type of machine has its own voltage-current or torque-speed curve. The advantage of shunt generators is that they can provide voltage without having any load up to full load, while series generators are characterized by not being able to provide any voltage unless there is some load.
DC machines are significantly less common than they used to be before the invention of AC induction and synchronous machines. They remain common in simple low power applications such as toys, small robots, and legacy equipment. Permanent magnet DC machines, which use abundant non-rare-earth magnets, are more common than their shunt and series counter parts due to simpler excitation, especially in low cost and low complexity applications.
DC Motors, drive equipment, ranging from small toys and rechargeable power tools, to electric vehicles. These electromechanical machines consist of an inner conductive coil, called the armature, and an outer magnet, called the stator. A DC source provides current to the armature through a commutator slippering. Inducing electromagnetic force and allowing rotation of the loop. The magnitude of the electromagnetic force depends on the angle between the magnetic field and the coil, creating fluctuations in torque with rotation. Multiple windings, spaced around the armature, minimize torque fluctuations, and prevent the commutator form shorting out the power supply. The commutator slippering periodically switches the direction of current through the coil, further preventing alignment of magnetic fields. This video introduces DC motor configurations, and demonstrates the measurement of DC motor performance characteristics, such as speed, current, and voltage with varying load.
Permanent magnet staters, in DC machines are the most common, however, when the staters magnetic field is produced through conductor windings, performance characteristics, such as speed and torque output, can be modified through electric field design. For example, speed is related to the voltage developed by the motor, called the electro motor force, or EMF. Similarly, torque is proportional to current. These characteristics vary depending on the design of the motor, and influence the motor design selected for certain applications. The four basic electronic configurations of DC machines are separately excited, shunt, series, and compound. Separately excited motors use separate power supplies for the field and armature, allowing for independent control to support varying loads. In shunt design, the most common configuration, field windings are connected parallel to the armature load, with a common DC supply. This provides adjustable speed with varying load, which is useful in machine tools and centrifical pumps. In series configuration, a DC supply powers the field and armature in series. This delivers higher starting torque for overcoming intertial loads in equipment, such as trains, elevators, or hoists. Compound design motors use both shunt and series circuits for both high starting torque and speed regulation. The shunt field may be loading before or after the series field. Now that the configurations of DC motors have been outlined, the analysis of current, voltage, and load relationships in shunt DC motors will be demonstrated.
The data collected in the DC tests can be used to build equivalent circuit models if needed. Before measuring the electrical characteristics of the DC motor, set the low power DC supply to 0.8 amps, and connect the supply terminals to the machine armature. Then, record the supplies voltage and current. Next, use a multimeter to measure voltage and current across the armature, winding the shunt field and the series field. Use the data to estimate the resistance in each component. After measuring the basic characteristics of the DC motor generator, set the built in field rheostat to the maximum settings, and measure its resistance. Finally, set the external series field rheostat to its upper limit, and measure its resistance.
Following the DC motor tests, a synchronous machine is used to rotate the DC machine’s armature. Thus, the DC machine is run as a generator, without field excitation, then with no load. Under these conditions, the terminal voltage equals EMF. The rotational speed of the generator is measured, and used to calculate the magnetism retained by the armature in the absence of coil excitation, called residual magnetism. First, check that the three phase disconnect, synchronous motor, and DC motor are all switched off. Then, attach a small piece of tape to the DC motor external rotor. After checking that the variac is set to zero percent, wire the variac to the three phase outlet. Next, connect the setup as shown. Then, check that the start run switch is in the start position. Following the adjustments to the variac, confirm that all connections are clear from the supply terminals. Only then, turn on the three phase disconnect switch. Next, turn on the high voltage DC power supply, press the VI display button to display the operating end current, and adjust the voltage knob to 125 volts. Do not press the start button before adjusting the voltage knob. Press the start button the DC power supply panel, and switch on the equipment. Next, slowly increase the variac output until the terminal voltage reads 120 volts. When the synchronous motor reaches a steady state rotational speed, flip the start run switch to run. Pay attention to machine sound changes. The machine sound becomes monotonic at steady state. Use the strobe light to freeze the motion of the motor by synchronizing the strobe rate to the motor rotation speed. The tape attached to the rotor will appear stationary when the strobe light is synchronized. Confirm that this rate is the motor speed by slowly increasing the strobe rate to synchronize the fan at the next highest rate. If correct, this will be double the first observed strobe synchronization rate. This start up sequence will be repeated before each subsequent test run. After startup, record the rotational speed of the motor and the armature voltage. Then use this data to calculate the residual magnetic field strength.
DC machines are used in a variety of applications. Once operating parameters of different machines are characterized, they can be chosen based on design specifications for a particular device. The DC generator can be characterized in various configurations, such as the shunt configuration. With switch S1 open, for no load testing, the field end load resisters are adjusted to the maximum. Then, the shaft speed and terminal voltage are recorded as described previously. The shunt resistance is reduced in five steps until the minimum resistance is reached. And the terminal voltage and current across the shunt resistor measured. The motor can be measured with simulated loads using load resistors, following the same protocol. Each type of DC generator has its own voltage current output. Shunt generators can provide voltage for a wide range of current load, while series generators provide increasing voltage with current load. In a variety of applications, where a wireless power source is preferred, such as motorized prosthetics, DC motors are the actuator of choice. In neurally controlled lower limb prosthetics, either surface or transdermal sensors are used to send signals to motorized joints in the replacement limb, much as in an intact leg. Gate and foot flection are controlled more naturally and intuitively than would be possible using a rigid limb replacement.
You’ve just watched Jove’s introduction to DC motors. You should now understand how a DC motor works and how to characterize its parameters. Thanks for watching.
