|Home | Articles | Forum | Glossary | Books|
The mechanical and hydraulic arrangements described in the preceding sections transform mechanical energy. They don't generate it. We still need some source of mechanical motion to produce the mechanical energy. The usual source is an electric motor, or, when only a small amount of motion is required, a solenoid. Selecting a motor for a particular application usually isn’t an easy job for the electronics technician; not because it's difficult, but simply because as a general rule, the technician isn’t very familiar with motors.
There are many different types and sizes of motors available and selection of the proper motor type for a particular job requires some knowledge of how motors work and what the various specifications mean.
ill. 5-1 shows a functional sketch of an elementary dc motor. It consists of a moving coil called the armature winding and a stationary coil called the field winding. Each of these windings carries a current and therefore produces a magnetic field. Because the armature of the motor rotates, it produces a rotating magnetic field. The field winding being stationary produces a stationary field. It is the interaction of these fields that causes the motor to rotate. In some motors the field winding may be replaced by a permanent magnetic.
The operation of the motor is shown in ill. 5-2. At A, the armature is in such a position that it produces a magnetic field with a north pole at the top and a south pole at the bottom. Because unlike poles attract and like poles repel, the interaction of the two fields will cause the armature to rotate in a counterclockwise direction.
In ill. 5-2B we have reached a dilemma, the unlike magnetic poles of the armature and the field have lined up. There is no longer any torque to cause the armature to rotate. Unless some other provision was made this is as far as our motor would turn. Fortunately something has been done. The current to the armature is fed through brushes and a commutator. The brush and commutator arrangement reverses the connections to the armature winding at the point in its rotation where the torque would go to zero. This, in turn, reverses the polarity of the field and the armature continues to rotate in a counterclockwise direction.
Our motor of ill. 5-2 has been simplified to illustrate the basic principle. If a real motor had only one armature winding, the motion would be rather jerky because the torque would be maximum when the unlike poles were coming close together. In a practical motor, there are several armature windings, each with its own pair of commutator segments. This produces a practically constant torque. ill 5-3 shows a sketch of a commutator with several armature windings. As each armature winding is switched into the circuit it will produce a magnetic field that interacts with the stationary field to produce torque.
ill. 5-3. Practical dc motor armature.
Our simplified motor has only two magnetic poles from the field, so it's called a two-pole motor. Practical motors may have two, four, or more, poles.
To cause a dc motor to run, a current must flow in both the field and armature windings. The way in which the windings are connected together will have a pronounced influence on how the motor behaves. Before we discuss this aspect of motors, we should get a good understanding of the relationship between voltage, current, torque, and speed in a motor.
A very useful concept in understanding motor operation is that of counter electromotive force or counter emi. This concept is easy to understand when you realize that a motor and a generator are nearly identical. In fact, when a motor is turning, it will generate a voltage in the same way that a generator will. It really doesn’t matter whether we turn the motor by rotating its shaft or whether we cause it to rotate by applying a voltage. In either case, a voltage will be generated across the armature winding. This voltage op poses the applied voltage. That’s why it’s called a counter emf.
We can now draw a rough equivalent circuit of the armature portion of our motor. For convenience, we won’t bother with the commutator. We know that the windings will have some resistance, so we show a resistor in series in the circuit of ill. 5-4. We also know that the motor generates a counter emi, so we will also show a generator in series. For the time being that’s all we need to explain the basic operation of the motor.
Now, let’s suppose that the motor of ill. 5-4 is at rest. Since it's not rotating, there will be no counter emf. When we close the switch, current will be limited only by the resistance of the armature windings which is quite low. Thus there will be a large surge of current flowing into the motor. This tells us that when we apply power to a motor we can expect a large starting current, until the motor gets going.
Once the motor starts to spin, the counter emf will be developed. This counter emf opposes the applied voltage so that the current flowing through our equivalent circuit will be reduced. The faster the motor turns, the higher the counter emf, and hence the lower the current.
The counter emf also depends upon the strength of the magnetic field from the field winding. The greater the field strength the higher the counter emf for a given speed of rotation. The applied voltage must always be higher than the counter emf because we must have some current flowing through the motor to keep it from turning. If we increase the strength of the field, the motor won’t have to turn as fast to generate the same counter emf. This means that increasing the field strength will decrease the speed of the motor. Decreasing the field strength will increase the speed of rotation of the motor. At first, this sounds backward—like we are getting something for nothing, but there is more to consider. What we really want to get out of the motor isn’t speed, but torque or rotational force. The torque increases with both the current through the armature and the strength of the field.
So far we haven’t considered the affect of a mechanical load on a motor. Suppose that a motor represented by our equivalent circuit of ill. 5-4 is running at full speed with no load other than the friction of its bearings. Intuitively, we feel that the motor wouldn’t be drawing much current because it'sn’t delivering much power. This is true, and the reason is that the counter emf is nearly equal to the applied voltage.
If we attach a mechanical load to the shaft of the motor, the motor will tend to slow down. This slowing down will reduce the counter emf allowing the current to increase. The increase in current will cause the torque to increase so that the motor can drive the load. Of course, if we apply too much of a load to the motor we may stall it. In the stalled condition, there will be no counter emf with the result that the current will be excessive and will probably burnout the armature winding.
There are three basic types of dc motors, they differ primarily in the way that the armature and field windings are connected together. When we select a type of motor for a particular application, we are usually interested in how its speed and torque vary with different load conditions.
THE SERIES MOTOR
ill 5-5 shows a diagram of a motor with the armature and field windings connected in series. Not surprisingly, it's called a series motor. In this arrangement, all of the armature current also flows through the field winding. For this reason the field winding consists of comparatively few turns of heavy wire.
From an earlier discussion of motors, we can see how the series motor will behave with varying load conditions. First let’s look at what happens with no load. When the motor is at rest, there will be no counter emf. Thus when the switch in ill. 5-5 is closed there will be a large current through the armature and field windings which are connected in series. Since the torque increases with both armature current and field strength, there will be a very high starting torque. As the motor turns faster and faster, a high counter emf will be developed, but at the same time it will weaken the field, tending to make the motor turn even faster. With no load a series motor will develop damaging high speeds. They should never be run unloaded.
A mechanical load applied to the shaft of a series motor will slow it down. This will reduce the counter emf so that current and hence the torque will increase. A series motor therefore has a very high starting torque, and a speed that's determined primarily by the mechanical load. It is ideal for applications where a high torque is required for a short period of time. This includes many control applications.
ill 5-6 shows the characteristics of a series motor in graph form. Of course, this particular motor is much larger than any motor that would find application in a control system, but the shape of the curves will be the same for just about any series dc motor. The curves show that as the load on the door increases, the speed will decrease, and the torque, the current, and the efficiency will all increase.
THE SHUNT MOTOR
ill 5-7 shows a motor with the field and armature windings connected in parallel across the power source. Because of this parallel connection it's called a shunt motor. The field winding usually has many turns of fine wire. Compared to the series motor, the field current is small but the field strength can be made as large as necessary by using many turns of wire. With this arrangement, the field current, and hence the field strength, is nearly constant.
When power is applied to the motor of ill. 5-7, the armature current will be high, causing a high torque. This starting torque won't be as high as in the series motor because the surge of starting current doesn’t flow through the field winding. As the motor comes up to speed, the counter emf will increase until it nearly reaches the applied voltage. At this point, the speed will stop increasing and will remain nearly constant.
As a mechanical load is applied, the motor will tend to slow down, but this will reduce the counter emf which will, in turn, increase the torque. This will bring the speed back to nearly its “no load” value. The shunt motor thus has a reasonable starting torque and nearly constant speed under varying load conditions. It is used in applications where constant speed is required.
ill 5-8 shows the characteristics of a shunt motor. Again, the particular motor illustrated is much larger than the motor of a control system. It will, however, illustrate the properties of the shunt motor. The comparative independence of the speed with varying load conditions is illustrated by the top curve on the graph. Note that the speed of the motor will remain nearly constant as the load is increased. At a load somewhat less than the rated load of the motor, the speed will start to drop slightly, but for all practical purposes, we can consider the shunt motor to be a constant speed device. Both the current and the torque increase linearly with load, and the efficiency is maximum at some load less than the rated load of the motor.
THE COMPOUND MOTOR
It is possible to use two field windings on a motor—one in series with the armature, and one in parallel with it as shown in ill. 5-9. This arrangement is called a compound motor. There are two possible ways to connect the two field windings of a compound motor. If the two field windings are connected so that they will aid each other the motor is called a cumulative compound motor. If they oppose each other it's called a differential compound motor.
The cumulative compound motor behaves a lot like a series motor. There will be a high starting torque because one of the field windings is in series with the armature. As in a series motor, the speed will increase as the mechanical load decreases. It won't increase without limit, however, because the parallel field has a nearly constant field strength. This motor can be used in applications where a high torque is required and the load may vary over a wide range. The differential compound motor is much like a shunt motor, but it has a more nearly constant speed. It has little advantage over the shunt motor, and therefore it's rarely used.
THE UNIVERSAL MOTOR
A widely used variation of the series motor is called a universal motor because it can work on either ac or dc.
The universal motor uses laminated iron parts just as a transformer does. This eliminates the adverse effects of eddy currents in the magnetic parts of the circuit. Usually fewer turns are used in both the armature and field windings so that the reactance will be low enough to allow an adequate current to flow.
Usually a universal motor will run somewhat faster on dc than it will on ac. This is because on dc we are concerned with the winding resistance whereas with ac we are concerned with both the resistance and reactance of the windings.
The universal motor has many of the characteristics of the series motor. It has a high starting torque and is capable of handling large loads over a short period of time. Universal motors are used in small appliances such as electric drills vacuum cleaners, and food blenders.
Some motors can be driven directly by a power source in series across the switching contacts of a relay. In some cases, however, better performance can be achieved with a transistor switch between the relay and the motor. A typical circuit's illustrated in ill. 5-10. The battery is the power source for both the transistor switch and the motor. Of course, any type of power supply could be substituted for the battery shown here.
A variation on this basic circuit's shown in ill. 5-11.
In a great many control systems, it's necessary to have control over which direction a motor runs. E.g., if a motor is used to open a door, it will probably also be used to close the door. It will be necessary for the motor to run in one direction to open the door and in the reverse direction to close the door.
The direction of rotation for all the motors described in this section can be reversed simply by reversing the direction of current flow in either the armature or the field windings, but not both. This is true of most dc motors. Of course, this won’t work in an ac motor where the current is constantly reversing its flow anyway. Fortunately, dc motors tend to be less expensive, less bulky, and easier to work with than their ac counterparts. There would be little to be gained by using an ac motor in a home control system.
ill 5-12 shows how a DPDT relay may be used to control the direction of the current flow through the motor’s field windings. The relay has a low voltage coil (6-24 volts) so that it can be used with any of the control arrangements described earlier. When the relay isn't energized, the motor will run in one direction, and when the relay is energized, the motor will run in the opposite direction.
One problem with this simple arrangement is that the motor will be running constantly. The relay only changes the direction, it does not turn the motor on and off. This would have to be done with a second relay.
Another approach to bidirectional motor control is shown in ill. 5-13. Two separate relays, each powered by its own independent control signal, are used. If both relays are deenergized (switch contacts open), the motor is turned off. A signal passing through the coil of relay 1 will turn on switch transistor Q1, causing the motor to rotate in one direction, say clockwise. Tithe control signal is applied to relay 2 rather than relay 1, Q2 will be switched on. Since the power supply controlled by this transistor has the polarity reversed compared to that of Q1, the motor will rotate in the opposite direction.
ill. 5-13. This two relay/two transistor circuit can provide bidirectional motor control.
REVERSING MOTOR DIRECTION
There is a potential problem with this circuit. Care must be taken to avoid allowing both of the relays to be energized at any one time. The conflicting control signals could damage the motor and one or both transistors.
As in the earlier motor control circuits, the batteries may be replaced with any suitable power supply.
An alternative arrangement of this circuit's shown in ill. 5-14. It operates in basically the same way as the circuit of ill. 5-13, but in this arrangement only a single battery/power supply is needed. The switching transistors still reverse the apparent polarity to the motor.
There are many applications where we want to limit the amount of travel of a motor controlled device. Usually this isn’t important in cases where we can either see what is being moved or have an indication of its state. It can be important, however, when we don’t have a good idea of what is going on physically.
E.g., suppose we are using a small motor to turn an attenuator that controls the volume of a stereo system. We push a button that makes the motor run forward to increase the volume, and push another button that makes the motor run in reverse to decrease the volume. The human ear isn't a good judge of audio volume, so it's possible that we might try to continue to increase the volume after the attenuator has reached the limit of its rotation. The motor used in the system might well be strong enough to completely ruin the attenuator.
One approach to the problem is to make the mechanical coup ling between the motor and its load rather loose so that the shaft will slip if the motor drives the load to its limit. In ill. 5-15, a motor shaft is coupled to an attenuator through a coupling which grips the two shafts by means of two set screws. If one of the set screws is tightened just enough so that the motor will turn the shaft of the attenuator, the coupling will slip when the attenuator reaches the limit of its rotation. The principle disadvantage of this arrangement is that after it has slipped a few times, it may become erratic and not turn the attenuator at all.
Another approach to the problem is to reduce the voltage applied to the motor by means of a series resistor as shown in ill. 5-16. The resistor is adjusted so that the motor will turn the shaft of the attenuator but will stall when the limit of rotation is reached. With a small motor and a series resistor, no harm will be done if the motor is stalled for a few seconds.
A more elaborate arrangement for limiting the amount of travel of a motor driven is shown in ill. 5-17. Here relay K1 is a DPDT relay that reserves the direction of rotation of the motor, and switches S1 and S2 are normally closed, snap-action limit switches. If this circuit were used with our motor driven attenuator, one switch, S1, would open the attenuator was at the highest volume setting and the other switch, S2, would open at the lowest volume setting.
When power is applied to the circuit of ill. 5-17, the motor will start to run in the forward direction. As soon as the attenuator reaches the high-volume limit of its rotation, switch Si will open. This will allow gate current to flow into the SCR. The SCR current will energize the reversing relay K1 so that the motor will immediately reverse direction before the attenuator is damaged.
Of course, switch Si will close again as soon as the motor reverses direction. This won’t affect anything, however, because the SCR will continue to conduct once it has fired.
Now suppose the motor is allowed to run until the attenuator reaches its low volume limit. Switch S2 will now open. This will shut off the SCR and de-energize the reversing relay so that the motor will run in the forward direction.
ill 5-18 shows a cam and two limit switches arranged to detect limits of rotation. The system isn’t restricted to this type of switching arrangement. The limit switches can be used in any way that they will detect limits of any type of motion.
There are some disadvantages to the circuit of ill. 5-17. The chief one is that the only way we can reverse the direction of the motor to reduce volume is to run the system up to volume where the reversing relay will be energized. This could be very annoying, particularly if the stereo were capable of producing a very large amount of power. One solution is to modify the circuit of ill. 5-17 so that when power is applied to the circuit, the motor will always start to turn in a direction as to lower, rather than raise, the volume. This would prevent large blasts of sound while adjusting the sound level.
Still another arrangement is to use the circuit of ill. 5-17 in connection with a reversing switch or reversing relay. With this arrangement, the direction of rotation is controlled by the position of the switch or relay. The limit switches are used only to change the direction if the motor should reach its limit of travel.
Most of the small motors used in control systems will probably be dc motors because it's so easy to reverse them and to control their speed. The principal limitation of the dc motor as far as home control systems is concerned is that it requires dc for operation and the power available in a home is ac. This means that the ac must be rectified for operating the motor. In a small motor, this isn’t a problem. Rectifier diodes of the types used in electronic power supplies can be used satisfactorily.
In applications where a lot of power is required, it's probably best to use an ac motor. The basic principles of ac motors are covered in the following paragraphs.
There are a great variety of ac motors that are available for building control systems. If a new motor is being purchased for a particular application, the manufacture or his agent can help in selecting a motor that's well suited for the application. This makes selecting the motor very simple. Unfortunately, builders of small control systems usually don't buy new motors. They use second hand motors that are purchased from a second hand outlet, or are salvaged from an appliance that has been discarded. In order to make proper use of such motors, we need to know something about the various types of ac motors, their characteristics, and how to recognize the different types.
In general, small ac motors can be grouped into the six different categories listed in Table 5-1. The biggest difference between all of these different kinds of motors is in the amount of starting torque that they will develop and in their starting current requirements. Table 5-2 lists many different types of ac motors and their pertinent characteristics.
The reason that the starting requirements of a single phase motor are so important is that a single-phase motor with an armature and field coil won't start by itself. Once it's started, a rotating magnetic field is produced and the motor keeps rotating. Most of the differences between the various types of motors involve the arrangement that's used to get the motor started in the desired direction.
Table 5-1. Types of Small Electric Motors.
1. Split-Phase (SP)
a. Capacitor Start (CS-1A) (Capacitor Start-Induction Run)
b. Two-Value Capacitor (CS-CR) (Capacitor Start-Capacitor Run)
c. Permanent-Split Capacitor (PSC) Wound Rotor
3. a. Repulsion-Start (RS)
b. Repulsion-Induction (RI)
c. Repulsion (A)
5. Universal or Series (UNIV)
Table 5-2. Characteristics of ac Motors.
ill 5-19 shows a diagram of a split-phase ac motor. It has two field windings. Once it has started to rotate, the main winding will generate a rotating field and will keep the motor turning. The auxiliary winding is used to generate the starting torque. When the motor is at rest, the centrifugal switch is closed switching the auxiliary winding into the circuit. Once the motor develops sufficient speed, centrifugal force causes the switch to open and the motor rims on the current flowing in the main winding. The direction of rotation can be changed by reversing the connections to the auxiliary winding.
The split-phase motor has a low starting torque as compared to other types. It is only suitable for driving loads that have a low starting torque such as fans. Because of the low starting torque and high starting current motors of this type are usually used where low cost is important.
There are several types of motors that use a capacitor in connection with a winding to obtain starting torque. ill 5-20 shows a capacitor-start, induction-run motor. This motor is quite similar to the split-phase motor with one very important difference. A capacitor is used in series with the auxiliary winding. The presence of this capacitor in the starting circuit will give the motor about twice as much starting torque as the split-phase type with only two thirds of the starting current. The direction of rotation in this motor can also be reversed by reversing the connections to the auxiliary winding. The principal limitation of the capacitor-start motor is that the capacitance of the electrolytic starting capacitor may be reduced at low temperatures. This, in turn, will reduce the starting torque.
ill 5-21 shows another capacitor type of motor where one capacitor is used for starting and another smaller capacitor is left in the circuit all the time. The running capacitor provides power factor correction and reduces the required operating current. It has a little more starting torque than the other types and can handle loads that are harder to start. The current requirement is about the same as for the motor of ill. 5-20. As with the other types, the direction of rotation can be changed by reversing the connections to the auxiliary winding.
ill 5-22 shows a different type of capacitor motor that doesn’t require a centrifugal switch. It is a permanent-split capacitor motor. The capacitor is left in the circuit at all times. Thus it's like the motor shown in ill. 5-21 except that the same value of capacitance is used for both starting and running. The price that we pay for this simplification is that the starting torque is much lower—about the same as that of the split-phase motor.
Because no starting mechanism is used in the motor of ill. 5-22, it can be run at variable speeds by reducing the supply voltage. The speed cannot be reduced below about 75 percent of the synchronous speed because the torque will drop rapidly and the motor may stall.
All of the motors we have discussed so far have no electric connections from the outside to the rotor. The rotor winding is usually a few turns of heavy wire, often in what is called a squirrel cage configuration.
There are several types of motors that use wound rotors. These motors are more expensive than the split-phase or capacitor types of motors and require more maintenance because the brushes and commutator will wear. Wound rotor motors can provide a very high starting torque at a comparatively low starting current.
The brushes in the repulsion motor are arranged in such a way that the magnetic field of the rotor is inclined with respect to the field from the stator. The result is that there is a strong torque even before the motor starts to rotate. ill 5-23 shows a repulsion- induction motor. Here the wound rotor is connected in such a way that the wound rotor is connected into the circuit for starting, and then shorted after the motor comes up to speed. Thus the motor starts as a repulsion motor and runs as an induction motor.
In a straight repulsion motor, the wound rotor is left in the circuit at all times. Sometimes the brush assembly can be rotated with respect to the stator so that the speed of the motor can be varied.
ill 5-24 shows the diagram of a shaded pole motor. In this motor, the starting torque is provided by two small short-circuited windings on the poles of the motor. Thus the motor will start with no other starting arrangement and will always run in the same direction. These motors have very little starting torque and are only used where a very low torque is required as in a phonograph or an electric clock. The motor runs in synchronism with the frequency of the power line so it can be used to drive timing mechanisms accurately.
One of the problems in using a second hand motor is the question of whether or not it will do the job. To some extent, particularly with a small motor, the best way to be sure of this is to try it in the application while carefully watching for signs of over loading or overheating.
The nameplate of the motor, which usually looks something like that shown in ill. 5-25, gives a great deal of information about the motor. The top line on this nameplate gives the specifications of the motor that have been standardized by the National Electrical Manufacturer’s Association (NEMA), as well as the manufacturer’s identification data. Most of the information on the top line has to do with the shaft size, amount of insulation, and similar factors. Probably the only item of interest is the insulation rating which has to do with the maximum temperature at which the motor can be operated. There are four classes valid to fractional horsepower motors.
Naturally a motor can be operated at below its rated temperature, but it shouldn’t be allowed to operate where temperature will exceed the rated value.
Most of the entries on the second line of the nameplate are self-explanatory except the Service Factor (S.F.). This is the amount of overload that the motor can tolerate on a continuous basis. The Design entry of the third line applies only to larger motors. The code letter specifies the ratio of the locked rotor to the horsepower rating. In small motors, the code is usually K, or L. K means that the locked rotor kva will be between 8 and 9 times the horsepower rating of the motor; L means that it will be between 9 and 10 times.
MOTOR SERVICING AND REPAIR
A new electric motor is a very reliable device and will normally require very little servicing, except for occasional lubrication. A second-hand motor, which is what will probably be used in the majority of control systems, may require servicing to bring it back to good operating condition.
The first thing to do with a salvaged motor is to clean it thoroughly. Dirt and dust should be removed from all of the air passages. Otherwise, the motor can overheat under normal operating conditions.
The bearings must be checked for wear. This is done by moving the rotor shaft sideways and back and forth. If the motor is used in intermittent service, a small amount of side play and end play can be tolerated, but if the movement is excessive, the motor will probably cause trouble—usually low torque, and excessive starting and running current. The motor should be lubricated, preferably in accordance with the manufacturer’s instructions if they can be obtained. If not, a small amount of machine oil in the bearings will be better than nothing. Be careful not to use too much oil. It will just lead to the accumulation of dust and dirt.
If the motor uses a centrifugal starting switch, the contacts should be cleaned. This should be done with very fine sandpaper. Don't use emery cloth because emery powder is an electrical conductor and can cause problems.
Table 5-3 lists some of the common problems encountered in ac motors together with their solutions.
Table 5-3. Common Motor Troubles and Repairs.
A solenoid is simply an electromagnet that's arranged to pull or push something when power is applied to it. ill 5-26 shows a simple solenoid. It consists of a winding of many turns on a hollow form. In the center of the form is an armature made of soft iron or of permanent magnet material. When the coil is energized its magnetic field will pull the armature into the coil. Often a spring is used to push the armature back out of the coil when power is removed.
Solenoids can be used in applications where only a small amount of motion is required. The solenoid valve described in the preceding section is a good example. The plunger of the valve doesn’t have to be moved very far to open or close the valve. Another very common application of the solenoid is in an electric door lock. Here the coil is used to move the tongue of the lock so that it will lock or unlock a door or window.
In an application where a mechanism is used to open and close outside doors, the solenoid electric lock is very useful. A conventional lock isn’t as useful because someone has to go to the door and unlock it before the control system can open it. With an electric lock, the door can be unlocked electrically whenever the control system tries to open it.
Construction of a solenoid device is usually quite a chore. It isn’t easy to tell in advance just how much current should be used, nor how many turns of wire should make up the coil. It is usually much better to take a solenoid that was designed for the purpose, or to convert an existing solenoid for use in a particular application.
If a permanent magnet is used for the armature, the direction of travel will depend on the polarity of the current applied to the coil. ill 5-27 shows a polarized solenoid that will provide a force in either direction. A short section of the armature is made of magnetic material such as soft iron. The rest of the armature is made of nonmagnetic material such as brass. The direction of travel of the armature depends on which coil is energized. When coil A is energized, the armature will move to the left in the figure; when coil B is energized, it will move to the right. If a solenoid of this type is used, the control system must be designed so that both coils won't be energized at the same time.
|Top of Page||PREV:||NEXT:||Electrical Control Devices||HOME|