E3106/10/23
ELECTRICAL MACHINERY & CONTROL
UNIT 10
OBJECTIVES
General Objective
To know and understand the basic of motor controller
Specific Objectives
By the end of this unit, you would be able to:
- state the ways on how electronics can be used to control motor speed and torque.
- identify the components of motor controller and the symbols.
INPUT
10.0 INTRODUCTION TO ELECTRIC MOTOR CONTROLLER
ust about every area of machine control has been invaded by electronics technology. The use of electronics to control the motors will be explained in this chapter. A small amount of reviewed material on the components such as diod, transistor and thermostat used in motor control circuitry is also given.
…KEY WORDS…
- Speed
- Torque
- Armature current
- Field current
The emphasis in this chapter is on the control of motor
speed and torque. These two factors do not always go
together. For example, an increase in torque is not always
accompanied by an increase in speed.
The most common ways to control motor speed and torque
are to control either the armature current or the field
current (or, in some cases, control of both).
As a general rule, the speed of a DC motor is controlled by controlling the armature current whenever the speed is below the motor-rated speed, and the field current is controlled when the motor is run above its rated speed.
10.1 DEFINITION OF MOTOR CONTROLLER
The control of motor speed is accomplished by controlling the current or voltage to the motor. Actually, that means controlling the power delivered to the motor. Remember that power = volts ´ amperes. So if you decrease either the voltage or current, the power will also naturally decrease. A study of motor control, then, involves the study of methods to control power delivered to the motor.
10.1.1 Starting a shunt motor
If we apply full voltage to a stationary shunt motor, the starting current in the armature will be very high and we run the risk of
- burning out the armature;
- damaging the commutator and brushers, due to heavy sparking;
- overloading the feeder;
- snapping off the shaft due to mechanical shock;
- damaging the driven equipment of the sudden mechanical hammer blow.
All DC motors must, therefore, be provided with means to limit the starting current reasonable values, usually between 105 and twice full-load current. One solution is to connect a rheostat in series with armature. The resistance is gradually reduced as the motor accelerates and is eventually eliminated entirely, when the machine has attained full speed.
Today, electronics methods are often used to limit starting current and to provide speed control.
(a) Face-plate starter
Figure 10.1: Manual face-plate stater for a motor
Figure 10.1 shows the schematic diagram of a manual face-plate starter for a shunt motor. Bare copper contacts are connected to current-limiting resistors R1, R2, R3 and R4. Conducting arm 1 sweeps across the contacts when it is pulled to the right by means of insulated handle 2. In the position shown, the arm touches dead copper contact M and the motor circuit is opened. As we draw the handle to the right, the conducting arm first touches fixed contact N.The supply voltage Es immediately
causes full field Ix to flow, but the
armature current I is limited by the
four resistors in the starter box.
The motor begins to turn and, as
the back EMF E0 builds up, the
armature current gradually falls.
When the motor speed ceases to rise
any more, the arm is pulled to
the next contact, thereby removing
resistor R1 from the armature circuit. The immediate jumps to a higher value and the motor quickly accelerates to the next higher speed. When the speed again levels off, we move to the next contact, and so forth, until the arm finally touches the last contact. The arm is magnetically held in this position by a small electromagnet 4, which is in series with the shunt field.
If the supply voltage is suddenly is interrupted, or if the field excitation should accidentally be cut, the electromagnet releases the arm, allowing it to return to its dead position, under the pull of spring 3. This safety feature prevents the motor from restarting un-expectedly when the supply voltage is reestablished.
- Stopping a motor
One is inclined to believe that stopping a DC motor is a simple, almost trivial, operation. Unfortunately, this is not always true. When a large DC motor is coupled to a heavy inertia load, it may take an hour or more for the system to come to a halt. For many reasons such a lengthy deceleration time is often unacceptable and, under this circumstances, we must apply a braking torque it ensure a rapid stop. One way to brake the motor is by simple mechanical friction, in the same way we stop a car. A more elegant method consists of circulating a reverse current in the armature, so as to brake the motor electrically. Two methods are employed to create such an electromechanical brake: (a) dynamic braking and (b) plugging.
Figure 10.2: Armature to a DC source Es
(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)
(a) Dynamic braking
Consider a shunt motor whose field is directly
connected to a source Es and whose armature is
connected to the some source by means of a
double-throw switch. The switch connects the
armature to either the line or to an external resistor
Figure 10.3: Armature on open circuit
generating a voltage Eo
(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)
R (Figure10.2). When the motor is running normally, the direction of the armature current I1 and the polarity of the back EMF E0 are shown in Figure 10.2. Neglecting the armature IR drop, E0 is equal to ES.
If we suddenly open switch (Figure. 10.3), the motor continues to turn, but its speed will gradually drop due to friction and wind age losses. On the other hand, because the shunt field is still exited, induced voltage E0 continues to exist, falling at the same rate as speed. In essence, the motor is now a generator whose armature is on open-circuit.
Let us close the switch on the second set contacts so
that the armature is suddenly connected to the external resistor (Figure 10.4). Voltage E0 will immediately produce an armature current I2. However, this current flows in the opposite
Figure 10.4: Dynamic breaking
(Source:Electrical Machines, Drives and Power System 5th edition; Wildi Theodor)e
direction to the original current I1. It follows that a reverse torque is developed whose magnitude depends upon I2. The reverse torque brings the machine to a rapid, but very smooth stop.
In practice, resistor is chosen so that the initial braking current is about twice the rated current. The initial braking torque is then twice the normal torque of the motor.
As the motor slows down, gradual decrease in E0 produces a corresponding decrease in I2. Consequently, the braking torque becomes smaller and smaller, finally becoming zero when the armature ceases to turn. The speed drops quickly at first and then more slowly, as the armature comes to a halt. The speed decreases exponentially, somewhat like the voltage across a discharging capacitor. Consequently, the speed decrease by half in equal intervals of times T0. To illustrate the usefulness of dynamic braking, Figure 10.5 compares the speed-time curves for a motor equipped with dynamic braking and one that simply causes to a stop.
Figure 10.5: Speed versus time curves for various braking methods
(Source: Electrical Machines, Drives and Power System 5th edition; Wildi Theodore)
Fig 10.3
(b) Plugging
Figure 10.6: Armature connected to DC source Es |
Figure 10.7: Plugging |
We can stop the motor even more rapidly by using a method called plugging. It consists of suddenly reversing the armature current by reversing the terminals of the source (Figure 10.6). Under normal motor conditions, armature current I1 is given by I1 = (ES - E0) / R0 where R0 is the armature resistance. If we suddenly
reverse the terminals of the source, the net voltage acting
on the armature circuits becomes (E0 + ES). The so-called
counter-EMF E0 of the armature is no longer counter to
anything but actually adds to the supply voltage ES. This
net voltage would produce an enormous reverse current,
perhaps 50 times greater than full-load armature the
commutator, destroying segments, brushes, and supports,
even before the line circuits breakers could open.
To prevent such a catastrophe, we must limit the reverse
current by introducing a resistor R in series with the
reversing circuits (Figure 10.7). As in dynamic braking,
the resistor is designed to limit the initial braking current
I2 to about twice full-load current. With this plugging
circuit, a reverse torque is developed even when the armature has comes to a stop. In effect, zero speed E0 = 0, but I2 = ES / R, which is about one-half its initial value. As soon as the motor stops, we must immediately open the armature circuit, otherwise it will begin to run in reverse. Circuit interruption is usually controlled by an automatic null-speed device mounted on the motor shaft.
The curves on Figure 10.5 enable us to compare plugging and dynamic braking for same initial braking current. Note that plugging stops the motor completely after an interval 2TO. On the other hand, if dynamic braking is used, the speed is still 25 percents of its original value at this time. Nevertheless, the comparative simplicity of dynamic braking renders it more popular in most applications.
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 10 A
- The most common way to control motor speed and torque are to control either the ___________ or the ______________.
- Whenever the operating speed is below the rated speed, the general rule is to control speed by controlling the _______________.
- The equation for DC power is P = _______________.
FEEDBACK TO ACTIVITY 10 A
- armature current ; field current
- armature current
- VOLTS ´ AMPS
INPUT
10.2 COMPONENTS OF MOTOR CONTROLLER
Figure 10.8: Motor control with a rheostat
Figure 10.8 shows one methods of controlling power to the motor (which is locked in this case). The load in this circuit can be either the armature or the field of the motor being controlled. The rheostat is used to control the circuit current. If the arm of rheostat is moved toward point a, the circuits will be increased.
Although the circuits of Figure 10.9 can be used for
controlling small motors, it is very inefficient for two
reasons. Remember that any time current flows
through a resistance there are always two effects; (1)
there is always a voltage drop across the resistor; and (2) there is always heat generated.
Figure 10.9: Motor control with a
potentiometer
The heat generated by the rheostat represents lost power. Power lost in the form of heat can never be recovered. The lost power cannot be delivered to the motor. Figure 10.9 shows another method of controlling the power delivered to a motor. In this case a potentiometer is used for control. It is used
divide the applied voltage V. Moving the variable resistor toward a up will cause a larger voltage to be developed across the load.
The potentiometer control circuit of Figure 10.8 and Figure 10.9 are shown to be operated from a battery. In a few cases where motor control is necessary in installation, the use of AC power delivered by the power company is more efficient and certainly less costly. For that reasons, rectifier circuits are used to convert AC power into a DC for much of the motor operation.
10.2.1 Typical Components Found in Motor Control
| Symbol | Component |
(i) | Push button; push to close | |
(ii) | Push button; push to open | |
(iii) | Fuse | |
(iv) | Relay coil; contact change state when the coil energizes | |
(v) |
| Contact open when coil de-energized |
(vi) |
| Contact shut when coil de-energized |
(vii) | Overload heater | |
(viii) | Overload contact; open when the heater gets too warm |
Figure 10.10: Components in motor controller
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 10 B
- A variable resistor can be used to control DC power if it is connected as a ______________ or as a ______________.
- The first reason why a variable resistor control for motors is not the best way is because there is always a voltage drop across the controlling resistor. That reduces the amount of voltage available for the motor. The second reason is the fact that the resistor dissipates heat and that represents lost ____________________ to the motor.
- Name two effects that always occur when current flows through a resistor.
a. ______________________
b. ______________________
FEEDBACK TO ACTIVITY 10 B
10.4 Rheostat ;Potentiometer
10.5 power
10.6 Heat ; voltage drop
INPUT
10.3 THE MAIN CIRCUIT AND CONTROLLER BLOK DIAGRAM
Before discussing a few specific examples of motor control circuits it is important to keep a few precautions in mind. Some motors have internal fans connected to their shaft to cool the windings. The motor specifications are based on the ability of this internal fan to keep the motor cool. If the motor run at a low speed there will be insufficient motion of the air, the windings will become overheated, and the motor will not be able to operate within its specified conditions. As a matter of fact this can result in a complete motor burnout.
Another thing to keep in mind is that the motor is usually constructed to operate at a specific RPM. This is usually indicated by the fact that the specifications are given for that particular motor speed. If you increase the speed of a motor too much, internal stresses can be produce. This could ultimately destroy the motor.
10.3.1 Control of DC Motor Speed by Pulse Width Modulation
Pulse width can be used to accomplish the same thing as SCR control. The basic principle is illustrated in Figure 10.10. In Figure 10.10a, the pulse sizes are such that a square wave is being generated. If the pulse width is increased but frequency remained the same as shown in Figure 10.10b, the average and RMS values of the waveform are increased.
On the other hand, if the pulse width is decreased from the square wave, as shown in Figure 10.10c, the average and RMS values are reduced. The speed of DC motors can be controlled simply by using a circuit that makes possible to control the pulse width and therefore the DC power to the armature or field.
(c)
(b)
(a)
A simple example is shown in Figure 10.11a.
Figure 10.11b shows a block diagram of the pulse width
modulation control. This particular circuit uses a
multivibrator, which is a two-transistor oscillator
(a)
that produces a square wave output. The width ofthe output pulses is controlled by the variable
resistor. The output is delivered to a power
amplifier that amplifies the pulses delivered to
the DC motor.
(b)
Figure 10.11: Circuit for controlling pulse width: (a) a multivibratorand power amplifier combination; (b) Block diagram in the circuit (a)
Instead of controlling pulse width, the pulse frequency can be controlled. Changing the input frequency to some motors like the synchronous motor-will change its speed. The output frequency of the VCO in a phase loop can be used for AC motor operation. A high-power driver must also be used. Changing the VCO frequency by changing the code to the divide by N circuit will change the synchronous motor speed.
10.3.2 Reduced-voltage starting
Some industrial loads have to be started very gradually. Examples are coil winders, printing presses, conveyor belts, and machines that process fragile products. In other industrial applications, a motor cannot be directly connected to the line because the starting current is too high. In all these cases we have to reduce the voltage applied to the motor either by connecting resistors (or reactors) in series with line or by employing an autotransformer. In reducing the voltage, we recall the following:
- The locked-rotor current is proportional to the voltage: reducing the voltage by half which reduces the current by half.
- The locked-rotor torque is proportional to the square of the voltage: reducing the voltage by half and reduces the torque by a factor by four.
10.3.3 Primary resistance starting
Primary resistance starting consists of placing three resistors in series with the motor during the start-up period (Figure 10.12). Contactor A closes first and when the motor has nearly reached synchronous speed, a second contact B short-circuits the resistors. This method gives a very smooth start with complete absence of mechanical shock. The voltage drop across the resistors is high at first, but gradually diminishes as the motor pick up speed and the current falls. Consequently, the voltage across the motor terminals increases with speed, and so that electrical and mechanical shock is negligible when full voltage is finally applied (closure of contractor B). The resistors are short-circuited after delay that upon the setting of a time-delay relay.
The schematic control diagram (Figure 10.13) reveals the following circuit elements:
A, B : Magnetic contractor relay coils
Ax : auxiliary contact associated with A
RT : time-delay relay that closes the circuit of boil B after preset interval of time.
Figure 10.12: Simplified schematic diagram of the power section of a reduced-voltage primary resistor stator
|
|
Figure 10.13: Control circuit of Figure 10.12 | Figure 10.14: Control circuit of Figure 10.12 using an auxiliary relay RA |
.
As soon as the start pushbutton is depressed, relay coils A and RT are exited. This causes the contacts A and Ax to close immediately. However, the contact RT only closes after a certain time delay and so the relay coil of contractor B is only excited a few second later.
If the magnetic contactors A, B are particularly large, the inrush exciting currents could damage the start pushbutton contacts if they are connected as shown in Figure10.13. In such cases, it is better to add auxiliary relay having more robust contacts. Thus in Figure10.14, the purpose of auxiliary relay RA is to carry the exciting currents of relay coils A and B. Note that the start pushbutton contacts carry only the exciting current of relay coils RA and RT. Other circuit components are straightforward, and the reader should have no difficulty in analyzing the operation of the circuit.
How are the starting characteristics affected when resistors are in inserted in series with the stator? Figure10.15 shows the torque-speed curve 1 when full voltage is applied to a typical 3-phase, 1800 r/min induction motor. Corresponding curve 2 shows what happens when resistors are inserted in series with the line. The resistors are chosen so that the locked-rotor voltage across the stator is 0.65pu. The locked-rotor torque is, therefore, (0.65)2 = 042 pu or only 42 percent of full-load torque. This means that the motor must be started at light load.
Figure10.16 shows the current versus speed curve 1 when full voltage is applied to the stator. Curve 2 shows the current when the resistors are in the circuit. When the speed reaches about 1700 r/min, the resistors are short-circuited. The current jumps about 1.8 pu to 2.5 pu, which is a very moderate jump.
Primary resistance starting with voltage reduced to 0.65pu Full-voltage starting
| Primary resistance starting with voltage reduced to 0.65pu Full-voltage starting |
Figure 10.15: Typical torque-speed curves of a 3 phase squirrel-cage induction motor | Figure 10.16: Typical current-speed curves of a 3 phase squirrel-cage induction motor |
Test your UNDERSTANDING before you continue to the next input
ACTIVITY 10 C
- What is the danger of running a motor below its rated speed?
______________________________
- What is the danger of running a motor above its rated speed?
______________________________
- A _____________ is controlling the width of the output pulse.
FEEDBACK TO ACTIVITY 10 C
10.7 it may overheat;
10.8 mechanical damage
10.9 variable resistor
SELF-ASSESMENT
If you face any problem, discuss it with your lecturer
You are approaching success. TRY all the questions ini this self-assessment section and check your answers with those given in the feedback on Self-Assessment given on the next page.
Question 10-1
Why can’t we apply full voltage to a stationary shunt motor?
- ________________________
- ________________________
- ________________________
- ________________________
Question 10-2
Explain how the plugging can stop the motor.
Question 10-3
List down the component in motor controller and symbols.
Question 10-4
Suitable the word in the text A with the word in the text B
Text A Text B
Electromechanical brake
| Locked-rotor current |
Motor controller
| Electromagnet release the arm, return to its dead position
|
Control pulse width
| Dynamic braking |
Reducing the voltage
| Armature current, field current |
Control DC motor
|
Control pulse frequency |
Voltage suddenly interrupted
|
Rheostat and potentiometer |
FEEDBACK TO SELF-ASSESMENT
Question 10-1
- starting current in the armature is very high.
- Overloading the feeder
- Burning out the armature
- Damaging the commutator and brushes
Question 10-2
Refer from the note 10.1
Question 10-3
Refer from the note 10.2
Question 10-4
Text A Text B
Electromechanical brake
|
|
Locked-rotor current |
Motor controller |
| Electromagnet realease the arm, return to its deads position
|
Control pulse width |
| Dynamic braking |
Reducing the voltage
|
| Armature current, field current |
Control DC motor
|
|
Control pulse frequency |
Voltage suddenly interrupted |
|
Rheostat and potentiometer |
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