Source: Ali Bazzi, Department of Electrical Engineering, University of Connecticut, Storrs, CT.
The DC machine operates with DC currents and voltages as opposed to an AC machine, which requires AC currents and voltages. DC machines were the first to be invented and utilize two magnetic fields that are controlled by DC currents. The same machine can be easily reconfigured to be a motor or generator if appropriate field excitation is available, since the DC machine has two fields termed field and armature. The field is usually on the stator side and the armature is on the rotor side (opposite or inside-out compared to AC machines). Field excitation can be provided by permanent magnets or a winding (coil). When current is applied to the armature or rotor coil, it passes from the DC source to the coil through brushes that are stationary and slip rings mounted on the rotating rotor touching the brushes. When the rotor armature coil is a current-carrying loop, and is exposed to an external field from the stator or field magnet, a force is exerted on the loop. Since the loop is “hanging” on both sides of the motor using bearings, the force produces a torque that will rotate the rotor’s shaft rather than move it in any other direction.
This rotation causes the magnetic fields to align but at the same time, slip rings switch sides on the brushes, or “commute,” and this is what is known as the commutation process. When this commutation occurs, current flow in the rotor coil is reversed and magnetic fields oppose each other again, causing further torque in the same direction of rotation. This process continues and the rotor shaft spins providing motor action. In generator operation, mechanical rotation is provided to the rotor shaft and current flows out of the rotor after it is induced due to a moving coil under a magnetic field.
The machines discussed in this experiment have a field winding rather than permanent magnets. A commutation process that is critical in DC machine operation uses slip rings and brushes to transfer energy from the rotor (armature) to the outside world since the rotor is spinning and having spinning wires would twist and break them. However, these brushes and slip rings have major reliability drawbacks as they require regular maintenance, brush replacement, cleaning, and may cause sparking. This has led to replacement of most DC machines by AC machines that do not have these issues, and remaining DC machines mostly have permanent magnet field excitation, such as in toys and simple low-power tools. AC machines termed brushless DC machines (or BLDCs) are AC machines that utilize a DC source and power electronic inverter to get AC voltages out of the inverter.
The objective of this experiment is to test two main DC machine configurations: shunt and series. Tests are intended to estimate the residual flux in the machine and to study the no-load and loading characteristics of different configurations.
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.
