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INDUSTRIAL CONTROL HANDBOOK - 2.9 CONTROL OF MOTORS

Motors must be started and stopped efficiently. In automated processes, the angular position, speed, and/or acceleration of a motor also must be controlled, despite variations in the loading of the motor. In most of these applications the main controller is a digital computer, which provides control signals to a motor controller, which in turn provides power to the motor.

 

2.9.1 DC Motors for Control Applications

The permanent magnet motor can have DC power applied to the armature (via the commutator), but the strength of the permanent magnetic field can't be varied. This type of DC motor, which is the cheapest and the most common in small sizes, can be controlled only by controlling the current in the armature. We will see that this method of control will allow only speed to be decreased from the "nominal" value listed on the motor's nameplate. The direction that this motor rotates can be changed by changing the polarity of the applied DC.

Torque output of a standard permanent magnet motor is limited by the amount of current that the armature windings can carry without burning out. Lots of current carrying capacity means heavier armatures. Figure 2-19 demonstrates the speed versus torque relationship for a permanent magnet motor at a given supply voltage. Printed circuit and moving coil motors are permanent magnet motors that include no iron in their armatures, thus reducing armature weight (and therefore inertia). Printed circuit motors have "windings" that are little more than circuit board tracks. Moving coil motor armatures consist of woven copper wire windings, set in epoxy to hold their shape. These motors usually rotate very rapidly, and external gearing trades away speed for increased torque.

There are three types of wound field DC motors. The first, with speed/torque characteristics as shown in Figure 2-19, is the series wound. In this type of motor, the field windings and armature windings are wired in series. Current passing through the field windings must also pass through the commutator to the armature windings. Reducing the DC current to the field also reduces armature current. Since reducing armature current reduces speed, while reducing field strength increases speed, control of this type of motor is difficult. In fact, if the motor is allowed to run without a frictional load, it can accelerate all by itself until it self-destructs. It is also an interesting fact that the direction of rotation of a series wound DC motor cannot be changed by changing the polarity of the DC supply.

When armature current direction is changed by reversing DC polarity, the magnetic field polarity also reverses, so the induced torque direction remains the same.

Figure 2-18 Motor classifications

Figure 2-19 Speed/torque output relationships for DC motors. Voltage is constant in each case.

Another type of wound field DC motor is the shunt wound motor. In the shunt wound motor, the field windings and the armature windings are brought out of the motor casing separately, and the user connects them to separate supplies so that the field strength and the armature current can be controlled independently. This type of motor can, depending on the type of control selected, have its speed reduced or increased from the nominal values. The direction of rotation of this type of motor can be changed by changing the polarity of either, but not both, of the supplies. The speed/torque relationship for this type of motor at a given voltage supply is shown in Figure 2-19.

 

2.9.1.1 Speed Control for DC Motors

Two simple formulas are important in understanding the response of DC motors to changes to supplied power. The first,

Speed Control for DC Motors

where:

RPM = Motor rotational speed

Va = Voltage across the armature

la = Current in the the armature

Ra = Resistance in the armature

F = Field strength

shows the relationship of the motor speed to applied armature voltage (decreased Va will decrease RPM), to armature resistance (adding resistance to Ra will reduce RPM), and to the field strength (a decrease in F will increase speed). Armature current is slightly harder to control because of the variation in CEMF as speed increases, but control of speed through current control is often the method of choice. It allows maximizing torque without exceeding the maximum current range of the motor.

The second equation,

T = K* F* la

where:

T = motor output torque K = a constant for the motor shows the relationship between torque, field strength, and armature current. Note that a decrease in field strength, which we said earlier would cause an increase in speed, will also cause a reduction in the available torque unless armature current is increased. In fact, reducing field strength reduces CEMF, so armature current does increase.

Figure 2-20 A motor with its speed controlled by a servosystem

Figure 2-20 A motor with its speed controlled by a servosystem 2.9.1.2 Stopping of DC Motors

2.9.1.2 Stopping of DC Motors

Stopping of a motor is a form of speed control. Methods used to stop DC motors are similar to speed control techniques. To stop a motor, you must accelerate it in the direction opposite to that in which it is moving. Mechanical brakes can be used, but will not be discussed here.

There are two methods of electrically stopping a DC motor. The most common is the dynamic braking method. To stop a DC motor using this method, the magnetic field remains in place, but the armature voltage supply is replaced by a resistor. The motor thus becomes a generator, and its kinetic energy is converted to electric current that bums off in the resistor. Small motors can be stopped in milliseconds using this technique.

The other electrical method to stop a DC motor is called plugging. As in dynamic braking, the magnetic field must be retained until the motor comes to a stop. Unlike dynamic braking, the armature is connected to a DC supply of opposite polarity. This results in a dramatic acceleration in the opposite direction. Armature current due to the (reversed) armature DC supply is further increased by the current due to the existing CEMF! Braking is rapid, but the high current is hard on the armature. This type of braking is therefore used only for emergency braking or for the braking of motors specially built for this type of extra armature current.

Plugging carries another potential problem. Once plugging stops a motor, it will then accelerate the motor in the opposite direction. Motors that are stopped by plugging need to have a zero speed switch mounted on the motor shaft or on the load. A zero speed switch contains an inertial switch that disconnects the armature voltage supply when motor speed reduces to a near stop.

These electrical braking techniques will not hold a motor in the stopped position, so if positive braking is required to hold a load steady at the stopped position, then:

  • A mechanical brake can be used to very rigidly hold a load wherever it happens to stop, or
  • A positional servosystem can cause the motor to stop at a given position.

In Figure 2-18, we divided AC motors into three types:

  • Universal
  • Synchronous
  • induction

As engineers become more comfortable with our new ability to control AC frequency, AC motors are becoming more commonly used in control applications.

The most often used AC motor, when control of torque, speed, or position is required, is the induction motor. This motor is called an "induction" motor because current is induced into the rotors conductors by a magnetic field that moves past them. This current, which moves through the magnetic field, causes torque, so the rotor turns. Let’s look at this more closely.

The induction motor's magnetic field is caused by AC power in the field windings. The magnetic field rotates around the rotor. Figure 2-21 shows how three phase AC supplied to the field windings of an AC motor causes the field to rotate. Since the field is initially rotating around a stationary rotor, then the rotor windings are moving relative to the field. Current is induced into the conductors (remember the right hand rule).

The conductors may be windings of copper wire in a wound field induction motor. They may be copper bars, parallel to each other and connected to a common copper ring at both ends. This copper bar and connecting ring arrangement, when seen without the iron core of which it is part, looks like the exercise ring used by pet mice or (presumably) pet squirrels. Hence the name for this type of motor: the squirrel cage motor.

Note that current is induced into the rotor and no commutator or slip rings are necessary. This motor is therefore brushless. If DC power is electronically switched around the field coils instead of having an AC supply drive the motor, then the motor could be called a brushless DC motor.

Figure 2-21 Rotation of the field in an AC motor supplied with three phase AC

Figure 2-21 Rotation of the field in an AC motor supplied with three phase AC

The more the rotational speed mismatch between the moving magnetic field and the rotor, the more current is induced in the rotor. When the rotor is stationary, therefore, there is a large current induced. This current, moving in the windings or squirrel cage, moves across the lines of magnetic flux. The left hand rule tells us that current moving in a field will result in a force on the conductor, and therefore torque to accelerate the rotor. The more current, the more torque. The torque will accelerate the rotor in the same direction that the field rotates.

In Figure 2-21, we saw that three phase AC, if each phase is wired to a separate set of poles in an AC motor's field, will cause a rotating field. If we tried the same type of simple connections using single phase AC, Figure 2-23 shows that the field simply alternates; it does not rotate. We need some way of defining the direction of the rotation, so that a single phase AC motor can operate.

Figure 2-22 Speed/torque relationship for an AC induction motor

Figure 2-22 Speed/torque relationship for an AC induction motor

The shaded pole, single phase, AC induction motor uses a cheap but energy wasteful method to define the direction of rotation of the field. This type of construction is common in small motors where energy efficiency is somewhat unimportant. Figure 2-24 shows a copper ring around the smaller of two sections of the same pole in a single phase induction motor. When the AC tries to change the polarity of the whole upper pole to a north, the change in the field induces a current in the copper ring. Current in the ring opposes the setting up of the magnetic field, effectively delaying the rate at which this portion of the pole changes to a north pole. The result is that the change in magnetic polarity sweeps across the face of this single pole, defining a direction of rotation for the field. This method is energy-wasteful because it continues to spend energy defining the direction of rotation after the rotor has started. The direction of rotation of a shaded pole motor cannot be changed.

Another type of AC induction motor that can be used with single phase AC power is the split phase motor. In this motor, each motor pole has another starting-up pole wired in parallel. The parallel circuits usually have an inertial switch in series with the starting pole. Figure 2-25 shows some motors of this type. In all three cases, the purpose of the inertial switch is to disconnect the "start" winding once the motor has started, to reduce energy waste. Before the start windings disconnect, however, the characteristics of the circuits are such that the magnetic polarity of the start windings either leads or lags the changing polarity of the "run" windings. If the wires for the start winding and the wires for the run winding are brought out of a split-phase motor separately, then the direction of rotation can be changed by reversing the connections of either (not both) windings to the AC supply.

In Figure 2-25(a), the start winding of the resistance start, split phase, AC induction motor has fewer windings than the run winding. The start winding therefore has less inductance, so the magnetic polarity of this pole changes earlier than the run winding. The direction of rotation of the field is thus from the start to the run windings.

Figure 2-23 Single phase AC showing alternating magnetic field

Figure 2-23 Single phase AC showing alternating magnetic field

Figure 2-24 A shaded pole motor showing the direction of rotation of the magnetic field

Figure 2-24 A shaded pole motor showing the direction of rotation of the magnetic field

Figure 2-25(b) shows a reactor start motor. The added inductor in the run winding circuit causes the change in magnetic polarity of the run winding to lag the polarity change of the start winding. The effective direction of rotation of the field is from the start to the run winding.

In Figure 2-25(c), the capacitor start motor has a capacitor in series with the start winding. This combination causes the magnetic polarity of the start winding to lead the run winding. The effective direction of rotation of the field is thus from the start winding to the run winding. It should be noted here that in some motors of this type, the capacitor and start winding are not disconnected even after the motor reaches speed. These motors are called capacitor run motors, or sometimes permanent capacitor motors.

The synchronous AC motor is called synchronous because the rotor follows the magnetic field exactly as it rotates around in the housing. Figure 2-21 showed how three phase AC supplied to the field windings causes the magnetic field to rotate. The synchronous motor's rotor will "lock onto" the magnetic poles of the field because the rotor has magnets as well. The magnets in the rotor may be permanent magnets.

Note that the field we examined in Figure 2-21 inherently rotated at 3600 RPM (with 60 Hz AC). There are three ways a stationary permanent magnet rotor can be accelerated to the speed at which it can catch and lock onto the rotating field:

Some synchronous motors require small auxiliary motors to accelerate them to the speed at which they can "lock on."

Others are caused to accelerate by the inclusion of squirrel cages in their rotors so that they start as if they were induction motors.

With control of the frequency of the AC supply, it is possible to control the speed of field rotation so that it accelerates slowly enough that the rotor can stay locked on.

Fractional hp synchronous motors can be made without magnets in the rotor. There are two types of these low-torque motors: hysteresis and reluctance.

The hysteresis motor has a rotor of cobalt steel that becomes magnetized in the presence of another magnetic field, but which has a high magnetic hysteresis (does not change its magnetic polarity easily). Once these rotors have been accelerated to the speed at which they can lock onto the rotating field, they will follow the rotating magnetic fields as if they had permanent magnets in their rotors.

Reluctance motors have high spots on their iron rotors, and these high spots tend to align in the direction of the magnetic field. As the magnetic field rotates, so does the rotor. Although this type of motor is classified as a fractional hp motor because its torque at synchronous speed is low, some are available with squirrel cages so that they can produce as much as 100 hp at lower speed.

Excited coil synchronous motors have windings in their rotors that can be provided with DC so that the rotors then have electro-magnetic poles. DC is supplied to these windings only after the rotor has been accelerated to locking on speed. Below locking-on speed, the unpowered windings act as a squirrel cage to accelerate the rotor. These motors need special speed-sensing circuitry to ensure that the DC is turned on to the rotor at the right speed and the right time.

The universal motor is similar in construction to a series wound DC motor. It will run on either DC or AC power. The universal motor is the only AC motor with a commutator, through which the armature is connected in series to the field windings. When AC power is supplied, the magnetic polarity of the field windings changes as the direction of the alternating current changes. When the magnetic field polarity changes, the direction of current flow in the armature also changes, so that torque is still generated in a consistent direction. As with the series wound DC motor, this motor will always run in the same direction (unless the wiring from the field windings to the armature is reversed).

This type of motor cannot be easily controlled, either by controlling AC amplitude or frequency but their speed can be controlled if the amplitude of the AC supply is adjusted. Portable handtools with variable speed controls are usually universal AC motors.

Figure 2-25 Split phase AC induction motors: (a) resistance start motor; (b) reactor start motor; (c) capacitor start motor.

Figure 2-25 Split phase AC induction motors: (a) resistance start motor; (b) reactor start motor; (c) capacitor start motor.

 

2.9.1.3 Stepper Motors

In terms of construction, stepper motors have much in common with AC synchronous motors. They come in three types:

The permanent magnet rotor type, as you might guess, has a rotor with permanent magnets that follow the magnetic field as it rotates around in the field windings. An advantage of this type is that they have a "detent torque," or a tendency to remain magnetically locked in position even when power to the motor is off ". The variable reluctance type has a toothed rotor, so that the high spots on the rotor tend to follow the rotating field.

The hybrid type has both permanent magnets and a toothed rotor.

The electromagnetic poles in a stepper motor's housing are designed such that they can be individually switched to DC. The rotor is supposed to "step" from one activated pole to the next as the poles sequentially turn on and off. A separately-supplied stepper motor controller controls in which order the windings receive DC. Stepper motor controllers "ramp" motors from one speed to the next by gradually changing the rate at which steps are supplied. They also allow motors to be operated in "slewing" mode, during which the rotor rotates without stopping after each step.

Basic stepper motors have an inherent step angle, or angle between possible step positions. Several techniques are now used to energize combinations of poles, and to control the DC level supplied to individual poles, so that the effective location of the magnetic field is between poles. As a result, "micro-steps" of fractions of a degree can be obtained.

Stepper motors are most commonly used in "open loop" position control. No position sensors are used, but as long as the rotor does not miss any steps (i.e., available motor torque is adequate), the position of the rotor is known. Stepper motors have also been used in systems with position sensors.

 

2.9.1.4 Brushless DC Servomotors

Brushless DC servomotors are like SCR motors in that they are really DC motors (permanent magnet DC motors), and in that their controllers convert AC to DC and modify the effective DC level in response to a command signal.

Figure 2-26 DC control and switching in an electronically commutated timing motor

Figure 2-26 DC control and switching in an electronically commutated timing motor

A servomotor should have a high torque-to-inertia ratio, so that it can accelerate quickly. A standard permanent magnet DC motor can be built for greater torque by increasing the amount of conductor in the armature so that more current can be moved across the magnetic field. Unfortunately, this also increases the inertia of the armature, so more torque is required to accelerate the rotor!

The previously-discussed printed circuit and basket wound DC motors are often used as servomotors. Their armatures contain nothing but conductors, yet these motors still have an unsatisfactory torque/inertia ratio for many applications. Robots built with these types of motors usually also have high ratio gear drives (often harmonic drives) to trade away speed to gain torque.

There are two brushless DC servomotor solutions to the torque/inertia dilemma. Both solutions also eliminate the commutator and brush.

In the type of brushless DC servomotor shown in Figure 2-27 the armature is held stationary and the permanent magnetic housing is allowed to rotate about it! Since the inertia in the armature is now unimportant to the motor's acceleration, it can be built heavy to carry very high currents, so tremendous torques can be generated. Electronic commutation to switch the DC from one stationary winding to the next makes this construction possible. Hall-effect sensors detect the position of the magnets in the rotating field and ensure DC is switched to the correct stationary armature winding. Recent innovations ill the production of lightweight magnets, used in the rotating housing, have helped as well. These new brushless DC motors, used without gearing, are finding applications in many modern direct drive servosystems.

A further improvement in the design of brushless DC servomotors, shown in Figure 2-28, has the DC motor turned inside out. The armature windings have been moved into the stationary housing, surrounding the rotor. The permanent magnets are mounted in the rotor. Since this rotor can have a smaller diameter than that in Figure 2-27, it has even less rotational inertia.

Figure 2-27 Brushless DC servomotor with stationary armature

Figure 2-27 Brushless DC servomotor with stationary armature

Figure 2-27 Brushless DC servomotor with stationary armature

Figure 2-28 Brushless DC servomotor turned inside out

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