DC Motor Speed Control

The basis of all methods of DC motor speed control is derived from the equations:

the terms having their usual meanings. If the IaRa drop is small, the equations approximate to    or  。Thus, control of armature voltage and field flux influences the motor speed. To reduce the speed to zero, either U=0 orΦ=∞.The latter is inadmissible; hence control at low speed is by armature voltage variation. To increase the speed to a high value, either U is made very large or Φis reduced. The latter is the most practical way and is known as field weakening. Combinations of the two are used where a wide range of speed is required.

A Single-Quadrant Speed Control System Using Thyristors

A single-quadrant thyristor converter system is shown in Fig.1.For the moment the reader should ignore the rectifier BR2 and its associated circuitry (including resistor R in the AC circuit), since this is needed only as a protective feature and is described in next section.

Fig.1 Thyristor speed control system with current limitation on the AC side

Since the circuit is a single-quadrant converter, the speed of the motor shaft (which is the output from the system) can be controlled in one direction of rotation only. Moreover, regenerative braking cannot be applied to the motor; in this type of system, the motor armature can suddenly be brought to rest by dynamic braking (i.e. when the thyristor gate pulses are phased back to 180o, a resister can be connected across the armature by a relay or some other means).

Rectifier BR1 provides a constant voltage across the shunt field winding, giving a constant field flux. The armature current is controlled by a thyristor which is, in turn, controlled by the pulses applied to its gate. The armature speed increases as the pulses are phased forward (which reduces the delay angle of firing), and the armature speed reduces as the gate pulses are phased back.

The speed reference signal is derived from a manually operated potentiometer (shown at the right-hand side of Fig.23.1), and the feedback signal or output speed signal is derived from the resistor chain R1 R2, which is connected across the armature. (Strictly speaking, the feedback signal in the system in Fig.23.1 is proportional to the armature voltage, which is proportional to the shaft speed only if the armature resistance drop, IaRa, is small. Methods used to compensate for the IaRa drop are discussed in Reading Material.)Since the armature voltage is obtained from a thyristor, the voltage consists of a series of pulses; these pulses are smoothed by capacitor C. The speed reference signal is of the opposite polarity to the armature voltage signal to ensure that overall negative feedback is applied.

A feature of DC motor drives is that the load presented to the supply is a mixture of resistance, inductance, and back EMF Diode D in Fig.1 ensures that the thyristor current commutates to zero when its anode potential falls below the potential of the upper armature connection, in the manner outlined before. In the drive shown, the potential of the thyristor cathode is equal to the back EMF of the motor while it is in a blocking state. Conduction can only take place during the time interval when the instantaneous supply voltage is greater than the back EMF.Inspection of Fig.2 shows that when the motor is running, the peak inverse voltage applied to the thyristor is mush greater than the peak forward voltage. By connecting a diode in series with the thyristor, as shown, the reverse blocking capability of the circuit is increased to allow low-voltage thyristor to be used.

References:

Fig.2 Illustrating the effect of motor back EMF on the

Peak inverse voltage applied to the thyristor

Fig.3 Armature voltage waveforms

The waveforms shown in Fig.2 are idealized waveforms as much as they ignore the effects of armature inductance,commutator ripple,etc.Typical armature voltage waveforms are shown in Fig.3.In this waveform the thyristor is triggered at point A, and conduction continues to point B when the supply voltage falls below the armature back EMF.The effect of armature inductance is to force the thyristor to continue to conduct until point C,when the fly-wheel diode prevents the armature voltage from reversing. When the inductive energy has dissipated (point D), the armature current is zero and the voltage returns to its normal level, the transients having settled out by point E.The undulations on the waveform between E and F are due to commentator ripple.