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Categories: Wooden Work Bench | Author: admin 04.12.2013

Modern solid state electronic circuits drive brushless DC motors with AC waveforms generated from a DC source.
Cruise ships and other large vessels replace reduction geared drive shafts with large multi-megawatt generators and motors.
At the system level, (Figure above) a motor takes in electrical energy in terms of a potential difference and a current flow, converting it to mechanical work. Early designers of AC motors encountered problems traced to losses unique to alternating current magnetics.
Once Steinmetz's Laws of hysteresis could predict iron core losses, it was possible to design AC motors which performed as designed.
Single phase synchronous motors are available in small sizes for applications requiring precise timing such as time keeping, (clocks) and tape players. Above 10 Horsepower (10 kW) the higher efficiency and leading powerfactor make large synchronous motors useful in industry. Since motors and generators are similar in construction, it should be possible to use a generator as a motor, conversely, use a motor as a generator. In the case of a small synchronous motor in place of the alternator Figure above right, it is not necessary to go through the elaborate synchronization procedure for alternators. Assuming that the motor is up to synchronous speed, as the sine wave changes to positive in Figure above (1), the lower north coil pushes the north rotor pole, while the upper south coil attracts that rotor north pole.
The current in the coils of a single phase synchronous motor pulsates while alternating polarity.
Rather than wind 12-coils for a 12-pole motor, wind a single coil with twelve interdigitated steel poles pieces as shown in Figure above.
The rotor (Figure above) consists of a permanent magnet bar and a steel induction motor cup. A 3-phase synchronous motor as shown in Figure below generates an electrically rotating field in the stator. The 3-phase 4-pole (per phase) synchronous motor (Figure below) will rotate at 1800 rpm with 60 Hz power or 1500 rpm with 50 Hz power. Small multi-phase synchronous motors (Figure above) may be started by ramping the drive frequency from zero to the final running frequency. The block diagram (Figure above) shows the drive electronics associated with a low voltage (12 VDC) synchronous motor. If constant and accurate speed of rotation is required, (as for a disk drive) a tachometer and phase locked loop may be included. A motor driven by square waves of current, as provided by simple hall effect sensors, is known as a brushless DC motor. If a motor is driven by sinewaves of current synchronous with the motor back emf, it is classified as a synchronous AC motor, regardless of whether the drive waveforms are generated by electronic means. The generation and synchronization of the drive waveform requires a more precise rotor position indication than provided by the hall effect sensors used in brushless DC motors. If the rotor and stator of a conventional rotary synchronous motor are unrolled, a synchronous linear motor results. This leading power factor can be exaggerated by removing the mechanical load and over exciting the field of the synchronous motor.
The variable reluctance motor is based on the principle that an unrestrained piece of iron will move to complete a magnetic flux path with minimum reluctance, the magnetic analog of electrical resistance. If the rotating field of a large synchronous motor with salient poles is de-energized, it will still develop 10 or 15% of synchronous torque. If slots are cut into the conductorless rotor of an induction motor, corresponding to the stator slots, a synchronous reluctance motor results. If an iron rotor with poles, but without any conductors, is fitted to a multi-phase stator, a switched reluctance motor, capable of synchronizing with the stator field results. Sequential switching (Figure below) of the stator phases moves the rotor from one position to the next.
If one end of each 3-phase winding of the switched reluctance motor is brought out via a common lead wire, we can explain operation as if it were a stepper motor. Variable reluctance motor drive waveforms: (a) unipolar wave drive, (b) bipolar full step (c) sinewave (d) bipolar 6-step. An electronic driven variable reluctance motor (Figure below) resembles a brushless DC motor without a permanent magnet rotor. While the variable reluctance motor is simple, even more so than an induction motor, it is difficult to control. Slo-syn synchronous motors can run from AC line voltage like a single-phase permanent-capacitor induction motor. Stepper motors are rugged and inexpensive because the rotor contains no winding slip rings, or commutator. Since stepper motors do not necessarily rotate continuously, there is no horsepower rating.
Stepper motors move one step at a time, the step angle, when the drive waveforms are changed. There are three types of stepper motors in order of increasing complexity: variable reluctance, permanent magnet, and hybrid. A variable reluctance stepper motor relies upon magnetic flux seeking the lowest reluctance path through a magnetic circuit. Variable reluctance stepper motors are applied when only a moderate level of torque is required and a coarse step angle is adequate. The only type stepper with no detent torque in hand rotation of a de-energized motor shaft. Permanent magnet stepper motors require phased alternating currents applied to the two (or more) windings.
The contruction of a permanent magnet stepper motor is considerably different from the drawings above. The permanent magnet stepper (Figure above) only has two windings, yet has 24-poles in each of two phases.
The hybrid stepper motor combines features of both the variable reluctance stepper and the permanent magnet stepper to produce a smaller step angle.
An un-powered stepper motor with detent torque is either a permanent magnet stepper or a hybrid stepper. Brushless DC motors were developed from conventional brushed DC motors with the availability of solid state power semiconductors. High torque pancake motors may have stator coils on both sides of the rotor (Figure above-b). Lower torque applications like floppy disk drive motors suffice with a stator coil on one side of the rotor, (Figure above-a).
The simple cylindrical 3-φ motor Figure above is commutated by a Hall effect device for each of the three stator phases. The above cylindrical motor could drive a harddrive if it were equipped with a phased locked loop (PLL) to maintain constant speed. If the three stator phases were successively energized, a rotating magnetic field would be generated.

You may recall that conventional DC motors cannot have an even number of armature poles (2,4, etc) if they are to be self-starting, 3,5,7 being common. Nikola Tesla conceived the basic principals of the polyphase induction motor in 1883, and had a half horsepower (400 watt) model by 1888.
An induction motor is composed of a rotor, known as an armature, and a stator containing windings connected to a poly-phase energy source as shown in Figure below. The stator in Figure above is wound with pairs of coils corresponding to the phases of electrical energy available. The stator in Figure above has salient, obvious protruding poles, as used on Tesla's early induction motor.
However, for larger motors less torque pulsation and higher efficiency results if the coils are embedded into slots cut into the stator laminations. In Figure above, the windings for both a two-phase motor and a three-phase motor have been installed in the stator slots.
The key to the popularity of the AC induction motor is simplicity as evidenced by the simple rotor (Figure below). A rotating magnetic field is created by two coils placed at right angles to each other, driven by currents which are 90o out of phase.
In Figure above, a circular Lissajous is produced by driving the horizontal and vertical oscilloscope inputs with 90o out of phase sine waves.
Figure above shows the two 90o phase shifted sine waves applied to oscilloscope deflection plates which are at right angles in space.
If a pair of 90o out of phase sine waves produces a circular Lissajous, a similar pair of currents should be able to produce a circular rotating magnetic field. The rotation rate of a stator rotating magnetic field is related to the number of pole pairs per stator phase. The short explanation of the induction motor is that the rotating magnetic field produced by the stator drags the rotor around with it. When power is first applied to the motor, the rotor is at rest, while the stator magnetic field rotates at the synchronous speed Ns.
The frequency of the current induced into the rotor conductors is only as high as the line frequency at motor start, decreasing as the rotor approaches synchronous speed. There are several basic induction motor designs (Figure below) showing consideable variation from the torque curve above. Various standard classes (or designs) for motors, corresponding to the torque curves (Figure below) have been developed to better drive various type loads. The brushless DC motor, actually an AC motor, is replacing the conventional brushed DC motor in many applications. Though few AC motors today bear any resemblance to DC motors, these problems had to be solved before AC motors of any type could be properly designed before they were built. Large synchronous motors are a few percent more efficient than the more common induction motors. However, the synchronous motor is not self starting and must still be brought up to the approximate alternator electrical speed before it will lock (synchronize) to the generator rotational rate. Such motors are not self starting if started from a fixed frequency power source such as 50 or 60 Hz as found in an industrial setting. These motors have a position sensor integrated within the motor, which provides a low level signal with a frequency proportional to the speed of rotation of the motor. This type of motor has higher ripple torque torque variation through a shaft revolution than a sine wave driven motor. The synchronous motor can also be smaller, especially if high energy permanent magnets are used in the rotor.
As a result the industrial grade electronic speed control used with induction motors is also applicable to large industrial synchronous motors.
This is often usefull in cancelling out the more commonly encountered lagging power factor caused by induction motors and other inductive loads.
However, microprocessors and solid state power drive makes this motor an economical high performance solution in some high volume applications.
Electronic control solves this problem and makes it practical to drive the motor well above and below the power line frequency.
Rather than a stepper, a variable reluctance motor is optimized for continuous high speed rotation with minimum ripple torque. The rotor moves in discrete steps as commanded, rather than rotating continuously like a conventional motor. A considerable design effort is required to optimize the servo amplifier gain vs phase response to the mechanical components. The step angle is related to motor construction details: number of coils, number of poles, number of teeth.
This torque load on the stepper is due to frictional (brake) and inertial (flywheel) loads on the motor shaft. The direction, step rate, and number of steps are controlled by a stepper motor controller feeding a driver or amplifier. Otherwise, we show the internal construction of a variable reluctance stepper motor in Figure above. The 6-wire motor, the most common arrangement, is intended for unipolar drive because of the center taps.
True synchronous motors are considered to be single speed, a submultiple of the powerline frequency.
Thus, it is possible for a hypothetical 4-pole motor to come to rest at a torque minima, where it cannot be started from rest. By poly-phase, we mean that the stator contains multiple distinct windings per motor pole, driven by corresponding time shifted sine waves.
The 2-phase induction motor stator above has 2-pairs of coils, one pair for each of the two phases of AC. The National Electrical Manufacturers Association (NEMA) has specified motor classes A, B, C, and D to meet these drive requirements. Nikola Tesla envisioned an entirely new type of motor when he visualized a spinning turbine, not spun by water or steam, but by a rotating magnetic field. And, the stepper motor, a digital version of motor, is driven by alternating current square waves, again, generated by solid state circuitry Figure above shows the family tree of the AC motors described in this chapter. Some of the electric energy is lost to heat, another form of energy, due to I2R losses in the motor windings. This knowledge of eddy current and hysteresis was first applied to building AC commutator motors similar to their DC counterparts.
If a load such as a brake is applied to one of the above units, the angle of the rotor will lag the stator field as at (3), extracting energy from the AC line, like a motor. Once up to speed, the synchronous motor will maintain synchronism with the AC power source and develop torque. This angle increases with loading until the maximum motor torque is reached at α=90o electrical. Since the coil field pulsates and does not rotate, it is necessary to bring the permanent magnet rotor up to speed with an auxiliary motor.

The induction motor cup outside of the bar magnet fits outside and over the tabs for self starting. Furthermore, the rotor is not a permanent magnet as shown below for the multi-horsepower (multi-kilowatt) motors used in industry, but an electromagnet.
The advent of modern solid state electronics makes it possible to drive these motors at variable speed. Originally, large industrial synchronous motors came into wide use because of this ability to correct the lagging power factor of induction motors. However, this is offset by the cost of the electronic control, which is not nearly as simple as that for a brushless DC motor. A variable reluctance motor driven by a servo, an electronic feedback system, controls torque and speed, minimizing ripple torque.
When stopped but energized, a stepper (short for stepper motor) holds its load steady with a holding torque. Once the motor is up to speed, pull-out torque is the maximum sustainable torque without losing steps.
Sequentially switching the stator phases produces a rotating magnetic field which the rotor follows.
Brushless DC motors tend to be small– a few watts to tens of watts, with permanent magnet rotors.
If the moving magnetic lines of flux cut a conductive disk, it will follow the motion of the magnet. The combination of 90o phased sine waves and right angle deflection, results in a two dimensional pattern– a circle. By analogy three windings placed 120o apart in space, and fed with corresponding 120o phased currents will also produce a rotating magnetic field.
This figure below could either be two paralleled and synchronized alternators driven by a mechanical energy sources, or an alternator driving a synchronous motor. This makes it possible to nearly cancel an arbitrary lagging power factor to unity by paralleling the lagging load with a synchronous motor. Wide spread acceptance of the stepper motor within the last two decades was driven by the ascendancy of digital electronics.
The 5-wire motor can only be driven by unipolar waves, as the common center tap interferes if both windings are energized simultaneously. For example, half stepping the motor moving the print head across the paper of an inkjet printer would double the dot density. The result is that the rotor moves in steps of a quarter of a tooth when the phases are alternately energized. In the next two figures, we attempt to illustrate the quarter tooth rotation produced by the two stator phases offset by a quarter tooth, and the rotor half tooth offset.
The major difference is that synchronous motors develop a sinusoidal back EMF, as compared to a rectangular, or trapezoidal, back EMF for brushless DC motors. The speed of a brushless DC motor is not fixed unless driven by a phased locked loop slaved to a reference frequency. That is, one coil corresponds to a N-pole, the other to a S-pole until the phase of AC changes polarity. However, a small synchronous motor, which mounts inside a drive wheel, makes it attractive for such applications.
A synchronous condenser is operated in a borderline condition between a motor and a generator with no mechanical load to fulfill this function.
Low power factor, low pull-out torque, and low efficiency are characteristics of the direct power line driven variable reluctance motor. Stepper motor coils are wound within a laminated stator, except for can stack construction. While we include numerous illustrations of two-phase motors for simplicity, we must emphasize that nearly all poly-phase motors are three-phase. This current flow creates an electromagnet whose polarity opposes the motion of the permanent magnet– Lenz's Law.
Numerous problems were encountered due to changing magnetic fields, as compared to the static fields in DC motor motor field coils. Yet small brushed AC motors, similar to the DC variety, persist in small appliances along with small Tesla induction motors. The point is that in either case the rotors must run at the same nominal frequency, and be in phase with each other. The rotor with multiple magnet poles (below right) is used in any efficient motor driving a substantial load. Yes, we do not want the motor to alternately speed and slow as it moves audio tape past a tape playback head. The high temperature superconducting version of this motor is one fifth to one third the weight of a copper wound motor.[1] The largest experimental superconducting synchronous motor is capable of driving a naval destroyer class ship. Such was the status of the variable reluctance motor for a century before the development of semiconductor power control. Waveforms (a & b) are applicable to the stepper motor version of the variable reluctance motor. As long as the load torque does not exceed the motor torque, the controller will know the carriage position.
If there were no mechanical motor torque load, no bearing, windage, or other losses, the rotor would rotate at the synchronous speed.
Higher efficiency higher torque multi-pole stator synchronous motors actually have multiple poles in the rotor. The strict drive requirements make this motor only practical for high volume applications like energy efficient vacuum cleaner motors, fan motors, or pump motors. The torque available is a function of motor speed, load inertia, load torque, and drive electronics as illustrated on the speed vs torque curve.
Large industrial synchronous motors are self started by embedded squirrel cage conductors in the armature, acting like an induction motor. It is implemented with a fast microprocessor driving a pulse width modulator for the stator phases.
One such vacuum cleaner uses a compact high efficiency electronic driven 100,000 rpm fan motor. Thus, a 4-pole stepper motor may have two phases composed of in-line pairs of poles spaced 90o apart. No brushes, no commutator, no rotor windings, no permanent magnets, simplifies motor manufacture. This speed is frequently not attainable due to mechanical resonance of the motor load combination.

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