DC Motors and Measurement of Residual Magnetism

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.