DC Motors

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.

Four main configurations of DC machines exist: separately excited, shunt, series, and compound. These configurations are classified based on the location of the field excitation, where the field is one of the magnetic fields necessary to operate the machine as a motor or generator. Since the field winding is powered by a DC source, that source can be the same as the one powering the DC motor's armature, or can be separate. When separate, the machine is termed "separately excited," and when not, the location of the field winding in the motor's circuit determines what type of configuration it is. If the field winding is placed in parallel with the armature winding to see the same voltage source powering the armature, the machine is in the parallel or shunt configuration.

If the field winding is in series with the armature winding so they have the same current flow, the machine is in the series configuration. If both windings are available, i.e. shunt and series windings are used, then the machine is in the compound configuration. The separately excited configuration is independent from the armature and can be regulated to support various load through automatic control. However, shunt, series, and compound configurations draw current from the same armature source and are therefore affected by the load and armature voltage variations.

With no field excitation, residual magnetism due to the residual magnetic field (λ_R) in the machine acts as a source for minor field excitation. This can be expressed as an additional term in the back e.m.f. (E_A) equation "λ_Rω" which is added to "KI_Fω" where ω is the mechanical speed of the machine. For a compound DC machine, E_A is thus,

E_A= K_shI_Fshω+ K_seI_Fseω+ λ_Rω, (1)

where "se" stands for series, "sh" stands for shunt, and K terms are field constants that relate field current and mechanical speed to the back e.m.f. Remember that K values are constant until a saturation limit is reached, after which E_A saturates to a certain value.

Ideally, λ_R is assumed to be zero, but this is not realistic. In order to determine λ_R, a DC machine is run as a generator without shunt or series excitation and at no load. Thus, the terminal voltage measured V_A=E_A. If ω is measured, λ_R can be determined. E_A is a characteristic voltage of DC machines, a voltage that counters the armature voltage to limit the current into the machine. In motor operation, the E_A is less than the armature voltage, and the higher E_A leads to less armature current draw. It is dependent on the shaft speed as shown in Equation 1, and therefore having a higher E_A causes higher speed operation. In generator applications, E_A is the induced voltage from rotating one magnetic field on the armature vs. the field.

For a shunt machine, Equation 1 still holds, but I_Fse is set to zero; for a series machine, Equation 1 still holds, but I_Fsh is set to zero. Compound machines have both shunt and series connected and can be in long- or short-form. When both fields exist, their effect can add up or oppose each other as seen by the armature, and these configurations are termed cumulative or differential. These configurations can be achieved by varying the location of the shunt field before or after the series field, and by having the field currents enter or leave their respective dots. Fig. 1-4 show all four configurations.

Figure 1: A schematic of a cumulative long compound configuration.

Figure 2: A schematic of a cumulative short compound configuration.

Figure 3: A schematic of a differential long compound configuration.

Figure 4: A schematic of a differential short compound configuration.

The goal of this experiment is to compare current, voltage and load relationships in series and shunt configured DC motors. Since only one high power DC power supply is available in this demonstration, separately excited operation is not covered. For shunt and series configurations, the prime mover of the DC generator is a synchronous motor that regulates its speed to 1800 RPM. Any time a DC current measurement is needed, such as I_A or I_Fsh, use the digital multi-meter in current mode (make sure the terminals on the multi-meter are in the current configuration).

1. DC Tests

1. With the low-power DC power supply limited to 0.8 A, connect the supply terminals to the DC machine armature.
2. Record the supply's DC voltage and current readings.
3. Estimate the resistance of each winding.
4. Repeat for the other windings, shunt field and series field, one at a time.
5. Turn off and disconnect the low-power DC power supply.
6. Set the built-in field rheostat to maximum resistance and measure its resistance.
7. Set the series field rheostat (external) to the maximum resistance and measure its resistance.

2. Prime-Mover Setup and Residual Magnetism

The prime-mover in this experiment is the synchronous machine, which operates as a motor that spins the DC generator rotor (armature).

1. Make sure the three-phase disconnect switch, synchronous motor switch, and DC motor switch are all off.
2. Check that the VARIAC is at 0%.
3. Wire the VARIAC to the three-phase outlet, and connect the setup shown in Fig. 5.
4. Check that the "Start/Run" switch is in the "Start" position.
5. Turn on the three-phase disconnect switch.
6. Turn on the high-voltage DC power supply.
7. Make sure all connections are clear from the supply terminals.
8. Press the "V/I DIS" button on the supply to display the voltage and current operating points. Adjust the voltage knob to 125 V.
   1. Do not press the start button.
9. Press the "Start" button on the DC power supply panel.
10. Slowly increase the VARIAC output until V_AC1 reads 120 V.
11. When the synchronous motor reaches a steady-state speed, flip the Start/Run switch into the Run position.
12. Measure and record the rotational speed using the strobe light and record V_A.
13. Turn off the DC power supply and return the VARIAC to 0%.
14. Reset the "Start/Run" switch to "Start."
15. Turn off the three-phase disconnect switch.

Figure 5: A schematic of how to setup the prime-mover. Please click here to view a larger version of this figure.

3. DC Shunt Generator Characterization

1. On the DC generator side, connect the shunt field in parallel with the armature field as shown in Fig. 6.
2. Use the built-in rheostat for R_Fsh(ext), and use the multi-meter as an ammeter to measure I_Fsh.
3. Keep "S₁" open for a no-load test.
4. Keep "R_Fsh(ext)" at maximum resistance.
5. Turn on the three-phase disconnect switch.
6. Press the "Start" button on the DC power supply panel.
7. Slowly increase the VARIAC output until V_AC1 reads 120 V.
8. When the synchronous motor reaches a steady-state speed, flip the "Start/Run" switch into the "Run" position.
9. Measure the shaft speed using the strobe-light technique described elsewhere.
10. Record V_A at this no-load condition on the DC generator side.
11. Reduce R_Fsh(ext) until the voltage generated at V_A is around 150 V.
12. After that point, reduce "R_Fsh(ext)" in five almost-equal steps until the minimum resistance is reached.
    1. For each step, measure V_A and I_Fsh.
13. Leave R_Fsh(ext) at its minimum value.
14. Turn off the DC power supply.
15. Reduce the VARIAC output to 0%.
16. Move the ammeter from measuring I_Fsh to measure I_A.
17. Restart the setup as described earlier.
18. Set R_L to 300 Ω, and turn on "S₁". Measure V_A and I_A.
19. Turn off "S₁," set R_L to 200 Ω, then turn on "S₁." Measure V_A, and I_A.
20. Turn off "S₁," set R_L to 100 Ω, then turn on "S₁." Measure V_A, and I_A.
21. Turn off the DC power supply and set the VARIAC output to 0%.
22. Keep the synchronous generator side of the setup intact.
23. Disconnect the DC generator connections.
24. Reset the "Start/Run" switch to "Start."
25. Turn off the three-phase disconnect switch.

Figure 6: A schematic of the shunt DC generator setup. Please click here to view a larger version of this figure.

4. DC Series Generator Characterization

1. On the DC generator side, connect the series field in series with the armature field as shown in Fig 7.
   1. Use the external rheostat for R_Fse(ext).
   2. Use the built-in rheostat as R_L and have it at maximum resistance.
   3. Keep "S₁" open for a no-load test.
   4. Keep R_Fse(ext) at maximum resistance.
2. Turn on the three-phase disconnect switch.
3. Press the "Start" button on the DC power supply panel.
4. Slowly increase the VARIAC output until V_AC1 reads 120 V.
5. When the synchronous motor reaches a steady-state speed, flip the "Start/Run" switch into the "Run" position.
   1. Measure V_A at this no-load condition on the DC generator side.
6. Turn on "S₁" and reduce R_Fse(ext) as needed to see non-zero V_A.
7. Vary R_L in five almost-equal steps until its 50% setting is reached, set to 300 Ω, and turn on "S₁." Measure the speed, V_A, and I_A.
   1. Turn off "S₁," set R_L to 200 Ω, then turn on "S₁." Measure the speed, V_A, and I_A.
   2. Turn off "S₁," set R_L to 100 Ω, then turn on "S₁." Measure the speed, V_A, and I_A.
8. Turn off the DC power supply.
   1. Set the VARIAC output to 0%.
   2. Keep the synchronous generator side of the setup intact.
   3. Disconnect the DC generator connections.
   4. Reset the "Start/Run" switch to "Start."
9. Turn off the three-phase disconnect switch.
10. Disassemble all wires and meters.

Figure 7: A schematic of the series DC generator setup. Please click here to view a larger version of this figure.

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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 λ_R 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. (E_A) is a function of λ_R such that E_A=I_f λ_Rω_m where I_f  is the field current and ω_m 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.