CONTROL OF DC MOTORS USING (Electric Motor)

3.6

DC/DC CONVERTERS

Using DC/DC converters to control DC motors is very effective. The speed of the motor is controlled by the on and off time of the DC voltage. This is done through different switching schemes. The switching schemes are varied to produce the control needed.
To understand the switching schemes, one must first understand some definitions. Figure 3.15 shows a typical voltage graph produced by a DC/DC converter. It shows a series of pulses. On the graph, the on time ton and off time toff are defined. From these quantities, the period T and duty cycle d are defined as follows.
tmpBC-22_thumb[2]
Using these definitions, different switching schemes can be defined. There are two main switching schemes. The first is pulse width modulation (PWM). In PWM, the period is constant and the duty cycle is variable. However, the duty cycle is limited between zero and one; therefore, the on time is less than the period. The second switching scheme is frequency modulation (FM). Within FM, there are two basic conditions. One is where ton is constant and the period is varied. Another is when toff is constant and the period is varied. Figure 3.16 shows the different switching schemes. Comparing the two different switching schemes, PWM has fewer harmonics and produces less noise.
Figure 3.17 shows a DC separately excited motor controlled by a DC/DC converter. To simplify the analysis, the switch Q will be assumed to be ideal. Another assumption is that Ea will be considered constant. This is a reasonable assumption because IF will be
Typical voltage graph.
FIGURE 3.15 Typical voltage graph.
Switching schemes for DC/DC converter.
FIGURE 3.16 Switching schemes for DC/DC converter.
DC separately excited motor controlled by a DC/DC converter.
FIGURE 3.17 DC separately excited motor controlled by a DC/DC converter.
constant and we can assume o is constant. Figure 3.18 shows the output currents produced by the DC/DC converter controlling a DC separately excited motor in continuous conduction mode (CCM).
To find the maximum current, shown in Fig. 3.18, the circuit in Fig. 3.19 will be analyzed. During this time 0 < t < dT, Va is equal to Vin – Ea, and ia(t = 0) = Ia,min. Using simple circuit analysis on the circuit in Fig. 3.19, we find that ia(t) is
tmpBC-26_thumb[2]Current graphs in CCM.
FIGURE 3.18 Current graphs in CCM.
Continuous conduction mode when 0 < t < dT.
FIGURE 3.19 Continuous conduction mode when 0 < t < dT.
tmpBC-29_thumb[4]
Figure 3.20 shows the circuit during dT < t < T. During this time interval, Q is off and D is conducting.
Using the fact that Va is equal to zero and ia(t = dT) = Iajmax, we can find the current. From Fig. 3.20, the current is
tmpBC-30_thumb[2]Continuous conduction mode when dT < t < T.
FIGURE 3.20 Continuous conduction mode when dT < t < T.
From these equations, the relations for Ia,min and Ia,max are found as follows:
tmpBC-32_thumb[2]
In order to find the relationship between the duty cycle and voltage, the average Va must be found:
tmpBC-33_thumb[2]
The speed of the DC separately excited motor can be changed by changing the duty cycle. If the inductance of the motor is too small, the current will go into discontinuous conduction mode. Figure 3.21 shows the voltage and current waveforms for DCM.
DC separately excited motor in DCM.
FIGURE 3.21 DC separately excited motor in DCM.
tmpBC-35_thumb[1]Discontinuous conduction mode during 0 < t < dT.
FIGURE 3.22 Discontinuous conduction mode during 0 < t < dT.
tmpBC-37_thumb[1]
In conclusion, electric motor drives have advanced since the old mechanically linked systems. The new drives are more accurate and consume less power. For example, slowing a pump or fan by using an electric drive reduces energy consumption more effectively than allowing the motor to run at constant speed and then restricting or bypassing the flow with a valve or damper.
Discontinuous conduction mode when dT < t < dxT.
FIGURE 3.23 Discontinuous conduction mode when dT < t < dxT.
Discontinuous conduction mode when dx T < t < T.
FIGURE 3.24 Discontinuous conduction mode when dx T < t < T.
In addition, electric motor drives can provide benefits when starting a motor. They can be used to slowly start the motor. The slow start reduces the mechanical stress on the motor and the load equipment. By slowly starting a motor, the voltage sag is reduced. Voltage sag causes lights to dim and other equipment like computers to shut down.
The future of electric motor drives is to have a simple motor and a complex power electronic converter. The power electronic converter is software based, while the electric machine is hardware based. Software is easier to manipulate than hardware. Furthermore, from a manufacturing standpoint, it is less expensive to have a software intensive drive compared to a hardware intensive drive. With the majority of electric power consumed by electric motors, the future of electric motor drives is good.


Next post:

Previous post: