11-5. Specifying a Motor Starter

Chapter 11 - Motor Controls

11-5. DC Motor Controller

Before the advent of modern solid state motor controllers, most motor applications that required variable speed required the use of dc motors. The reason for this is that the speed of a dc motor can be easily controlled by simply varying either the voltage applied to the armature or the current in the field winding. In contrast, the speed of an ac motor is determined, for the most part, by the frequency of the applied ac voltage. Since electricity from the power company is delivered at a fixed frequency, ac motors were relegated to constant speed applications (such as manufacturing machinery) while dc motors were used whenever variable speed was required (in cranes and elevators, for example). This situation changed radically when variable frequency solid state motor controllers were introduced, which now allow us to easily and inexpensively operate ac motors at variable speeds.

Although solid state controllers have allowed ac motors to make many inroads into applications traditionally done by dc motors, there are still many applications where a dc motor is the best choice for the job, mostly in battery powered applications. These are usually in the areas of small instrument motors used in robotics, remotely controlled vehicles, toys, battery operated electro-mechanical devices, automotive applications, and spacecraft. Additionally, the universal ac motor, which is used in ac operated power hand tools and kitchen appliances, is actually a spinoff of the series dc motor design (it is not an induction motor) and can be controlled in the same manner as a dc motor, i.e., by varying the applied voltage.

The voltage applied to the armature and the current through the field of a dc motor can be reduced by simply adding a resistance in series with the armature or field, which is a lossy and inefficient method. Under heavy mechanical loads, the high armature currents cause an exponentially high I2R power loss in any resistor in series with the armature. Solid state electronics has made improvements in the control of dc motors in much the same way that it has with ac motors. In this chapter we will examine two of these solid state control techniques. In the first, we will be controlling the speed of a dc motor that is powered from a dc source. In the second we will be using an ac power source. In both cases, we will control the motor speed by varying the average armature voltage. We will assume the field is held constant either by the use of permanent magnets or by a constant field current.

dc Motor Control with dc Power Source

Consider the circuit shown in Figure 11-5. In this case we have a dc voltage source

V, a resistor R, inductor L, diode D, and a semiconductor switch Q (shown here as an N-channel insulated gate MOSFET). The signal applied to the gate of the switch Q is a pulse train with constant frequency f (and constant period T), but with varying pulse width t. The amplitude of the signal applied to the gate will cause the switch to transition between

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Chapter 11 - Motor Controls

cutoff and saturation with very short rise and fall times. The relative values of R and L are selected such that the time constant J = L/R is at least 10 times the period T of the pulse train applied to the gate of Q. The long L/R time constant will have a low-pass filtering effect on the chopped output of the switch Q, and will effectively smooth the current into dc with very little ac component.

t

T

Figure 11-5 - Simple dc Switch Voltage Controller

For the switch Q, the ratio of the on-time t to the period T is defined as the duty cycle, and is represented as a percentage between zero and 100%. If we apply a pulse train with a 0% duty cycle to the gate of the switch, the switch will remain off all of the time and the voltage on the resistor R will obviously be zero. In a similar manner, if we apply a pulse train with a duty cycle of 100%, the switch will remain on all the time, the diode D will be reverse biased, and, after 5 time constants, the voltage on the resistor R will be V.

For any duty cycle between 0% and 100%, the average resistor voltage will be a corresponding percentage of the voltage V. For example, if we adjust the applied gate pulses so that the duty cycle is 35% (i.e., ON for 35% of the time, OFF for 65% of the time), then the voltage on the resistor R will be 35% of the input voltage V. This is because, during the time that the switch is ON, the inductor L will store energy; during the time the switch is OFF, the inductor will give up some of its stored energy keeping current flowing in the circuit through inductor L, resistor R, and the forward biased diode D (in this application, the diode is called a freewheeling diode or commutation diode). Although the operating frequency of the switch will affect the amplitude of the ac ripple voltage on

R, it will not affect the average voltage. The average voltage is controlled by the duty cycle of the switch. Whenever the duty cycle is changed, the voltage on the resistor will settle within five L/R time constants.

Although this seems to be a rather involved method of simply controlling the voltage on a load, there is one major advantage in using this method. Consider the power dissipation of the switch during the times when it is either OFF or ON. The power

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Chapter 11 - Motor Controls

Q

Figure 11-8 - Improved Pulse Width Modulator

dc Motor Speed Controller

dc Motor Control with ac Power Source

Obviously, in order to operate a dc motor from an ac source, the ac power must first be converted to dc. It should be noted that the dc used to operate a dc motor need not be a continuous non-varying voltage. It is permissible to provide dc power in the form of halfwave rectified or full-wave rectified power, or even pulses of power (as in the pulse width modulation scheme discussed earlier). Generally speaking, as long as the polarity of the voltage applied to the armature does not reverse, the waveshape is not critical. In its simplest form, a full wave rectifier circuit shown in Figure 11-9 will power a dc motor armature. However, in this circuit the average dc voltage applied to the armature will be dependent on the ac line voltage. If we desire to control the speed of the motor, we would need the ability to vary the ac line voltage.

Figure 11-9 - Full Wave Rectifier dc Motor Supply

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Chapter 11 - Motor Controls

To provide control over the average dc voltage applied to the motor without the need to vary the line voltage, we will replace two of the rectifier diodes in our bridge with silicon controlled rectifiers (SCRs) as shown in Figure 11-10.

Figure 11-10 - SCR dc Motor Control Circuit

Note in the circuit in Figure 11-10 that it is not necessary for all four rectifier diodes to be SCRs. The reason is that the remaining two diodes, D1 and D2, are simply steering diodes that route the return current from the armature back to the opposite side of the ac line. Therefore, we can completely control current flow in the circuit with the two SCRs shown. The phase angle firing control circuit monitors the sine wave of the ac line and produces a precisely timed pulse to the gate of each of the SCRs. As we will see, this will ultimately control the magnitude of the average dc voltage applied to the motor armature.

We will begin analyzing our circuit using the two extreme modes of operation, which are when the SCRs are fully OFF and fully ON. First, assume that the firing control circuit produces no signal to the gates of the SCRs. In this case, the SCRs will never fire and there will be zero current and zero voltage applied to the motor armature. In the other extreme, assume that the firing control circuit provides a short pulse to the gate of each

SCR at the instant that the anode to cathode voltage (VA-K) on the SCR becomes positive

(which occurs at zero degrees in the applied sine wave for SCR1 and 180 degrees for SCR2). When this occurs, the SCRs will alternately fire and remain fired for their entire respective half cycle of the sine wave. In this case, the two SCRs will act as if they are rectifier diodes and the circuit will perform the same as that in Figure 11-9 with the motor operating at full speed.

Now consider the condition in which the SCRs are fired at some delayed time after their respective anode to cathode voltages (VA-K) become positive. In this case, there will be a portion of the half wave rectified sine wave missing from the output waveform, which is the portion from the time the waveform starts at zero until the time the SCR is fired. When we fire the SCRs at some delayed time, we generally measure this time delay as a

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