UNIVERSAL MOTOR

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Generally, the electric motors are operated either in DC Power or AC Power. But for some specific applications, it is desirable to introduce a motor that operates on either ac or dc supply. The word ‘Universal’ signifies that something which is compatible with versatile inputs. We have build small series motors up to ½ KW rating which operates on single phase ac supply as well as on dc . Such motors are called universal motors . A universal motor is a specifically designed series wound motor, that operates at approximately the same speed and output on either ac or dc voltage. In case of universal motor, the speed of rotation is slightly lesser when operating in AC. Because, the reactance voltage drop is present on ac but not on dc. So, the motor speed is somewhat lower for same load in ac operation than dc . This takes place especially at high loads . Most universal motors are designed to operate at speeds exceeding 3500 rpm . We will explain discuss the construction of this type of motor

The universal motor is basically a series DC motor which is specially designed to operate on AC as well as on DC. A standard DC series motor has very poor characteristics when operated on AC, mainly due to two reasons:

a) The high reactance of both the armature and field windings limits AC current to a much lower value than DC current (for the same line voltage).

b) If solid steel is used for the stator frame, AC flux will produce large eddy currents in the frame with consequent heating. 
To insure satisfactory operation of the universal motor from an AC power source, some modifications are necessary. The reactance of the series field and armature windings must be reduced as much as practicable. The reactance of the series field winding can be somewhat reduced by using fewer turns of heavier wire. However, it would not be practical to eliminate the reactance voltage drop due to the series field since that would also eliminate the magnetic field. The reactance voltage drop due to the armature winding can be practically eliminated by use of a compensating winding. The compensating winding is connected in series with the armature winding (conductive compensation) and arranged such that the ampere-turns of the compensating winding oppose and neutralize the ampere-turns of the armature. To realize this compensation, the compensating winding is displaced by 90 electrical degrees from the field winding. Since the motor used in this experiment us a 4-pole motor, the mechanical displacement is 450. The compensating winding also improves commutation considerably. This is a great adventure since the field of a universal motor commutation considerably. is weakened by lowering the reactance of the series field winding. If the compensating winding is short circuited (inductive compensation), the alternating currents in the armature are induced by transformer action into the shorted compensating winding, thus, effectively cancelling the reactive armature currents.

To reduce losses due to hysteresis and eddy currents, the field structure is laminated. Few universal motors operate at the same speed on AC as on DC. Whether it runs faster on AC or DC is a matter of design. The reactance of the armature winding can be lowered by placing a compensating winding on the stator so that the fluxes oppose or "cancel" each other. This same compensating winding can be connected in series with the armature winding. In this case, the motor is said to be conductively compensated. Under these conditions, the universal motor will have similar operating characteristics whether on AC or DC power

The compensating winding may be simply shorted upon itself, so that it behaves like a short circuited secondary of a transformer (the armature winding acting as the primary). The induced AC current in the compensating winding again opposes or "bucks" the armature current and the motor is said to be inductively compensated. The reactance of the field winding can be kept low by limiting the number of turns

The starting torque of a universal motor is determined by the current that flows through the armature and field windings. Due to the inductive reactance of these windings the AC starting current will always be less than the DC starting current (with equal supply voltages). Consequently, the starting torque on AC power will be lower than the starting torque on DC power.

The compensating winding has the important role of reducing the overall reactance of the motor. However, it also has an equally important part in opposing armature reaction, thereby improving commutation. An uncompensated universal #motor will lose most of its power. Sparking at the brushes will also be markedly worse

Speed control of universal motor

Speed control of universal motor is best obtained by solid-state devices. Since the speed of these motors is not limited by the supply frequency and may be as high as 20,000 rpm , they are most suitable for applications requiring high speeds . The factors that determine the speed for any dc motor are the same as those for ac series or universal motors i.e flux and generated voltage . Generated voltage change is rarely employed in speed control method. Instead line voltage is varied .This has been accomplished by means of tapped resistor , rheostat in series with the line. Another method is by using a tapped field , thereby reducing the flux and hence raising the speed . This can be achieved by any one of the methods that follow : By using field poles wound in various sections with wires of different size and bringing out the tapsfrom each section By using tapped nichrome wires coils wound over a single field pole. In this method torque decreases with increase in speed 

STEPPER MOTOR

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STEPPER MOTOR 

Stepper motors provide a means for precise positioning and speed control without the use of feedback sensors. The basic operation of a stepper motor allows the shaft to move a precise number f degrees each time a pulse of electricity is sent to the motor. Since the shaft of the motor o moves only the number of degrees that it was designed for when each pulse is delivered, you can control the pulses that are sent and control the positioning and speed. The rotor ofthe motor produces torque from the interaction between the magnetic field in the stator and rotor. The strength of the magnetic fields is proportional to the amount of current sent to the stator and the number of turns in the windings

The stepper motor uses the theory of operation for magnets to make the motor shaft turn a precise distance when a pulse of electricity is provided. You learned previously that like poles of a magnet repel and unlike poles attract.  typical cross-sectional view of the rotor and stator of a stepper motor. From this diagram you can see that the stator (stationary winding) has eight poles, and the rotor has six poles(three complete magnets). The rotor will require 24 pulses of electricity to move the 24 steps to make one complete revolution. Another way to say this is that the rotor will move precisely 15° for each pulse of electricity that the motor receives. The number of degrees the rotor will turn when a pulse of electricity is delivered to the motor can be calculated by dividing the number of degrees in one revolution of the shaft(360°) by the number of poles (north and south) in the rotor. In this stepper motor 360° is divided by 24 to get15°.When no power is applied to the motor, the residual magnetism in the rotor magnets will cause the rotor to detent or align one set of its magnetic poles with the magnetic poles of one of the stator magnets. This means that the rotor will have 24 possible detent positions. When the rotor is in a detent position, it will have enough magnetic force to keep the shaft from moving to the next position. This is what makes the rotor feel like it is clicking from one position to the next as you rotate the rotor by hand with no power applied

When power is applied, it is directed to only one of the stator pairs of windings, which will cause that winding pair to become a magnet. One of the coils for the pair will become the North Pole, and the other will become the South Pole. When this occurs, the stator coil that is the North Pole will attract the closest rotor tooth that has the opposite polarity, and the stator coil that is the South Pole will attract the closest rotor tooth that has the opposite polarity. When current is flowing through these poles, the rotor will now have a much stronger attraction to the stator winding, and the increased torque is called holding torque. 

By changing the current flow to the next stator winding, the magnetic field will be changed 45°. The rotor will only move 15° before its magnetic fields will again align with the change in the stator field. The magnetic field in the stator is continually changed as the rotor moves through the 24 steps to move a total of 360°. the position of the rotor changing as the current supplied to the stator changes

 Stepper Motor Switching Sequence

The stepper motor can be operated in three different stepping modes, namely, full-step, half-step, and micro step.

Full-Step

The stepper motor uses a four-step switching sequence, which is called a full-step switching sequence which is already described above.

Half-Step

Another switching sequence for the stepper motor is called an eight-step or half-step sequence.   The main feature of this switching  sequence is that you can double the resolution of the stepper motor by causing the rotor to move half the  distance it does when the full-step switching sequence is used. This means that a 200-step motor, which has a resolution of 1.8°, will have a resolution of 400 steps and 0.9°. The half-step switching sequence requires a special stepper motor controller, but it can be used with a standard hybrid motor. The way the controller gets the  motor to reach the half-step is to energize both phases at the same time with equal current

In this sequence the first step has SW1  is on, and SW2,SW3 and SW4 are off. The sequence for the first step is the same as the full-step sequence. The second step has SW1 and SW2 are on and all of the remaining switches are off. This configuration of switches causes the rotor to move an additional half-step because it is acted upon by two equal magnetic forces and the rotor turns to the equilibrium position which is half a step angle. The third step has SW2 is on, and SW1, SW4 and SW3 are off, which is the same as  step 2 of the full step sequence. The sequence continues for eight steps and then repeats. The main difference between this sequence and the full-step sequence is that the energizing sequence for half step is A A BB BC CC DD  DA.

Micro Step Mode

The full-step and half-step motors tend to be slightly jerky in their operation as the motor moves from step to step. The amount of resolution is also limited by the number of physical poles that the rotor can have. The amount of resolution (number of steps) can be in-creased by manipulating the current that the controller sends to the motor during each step. The current can be adjusted so that it looks similar to a sine  wave.sent to each of the four sets of windings is timed so that there is always a phase difference with each other.The fact that the current to each individual phase increases and decreases like a sine wave and that is always out of time with the other phase will allow the rotor to reach hundreds of intermediate steps. In fact it is possible for the controller to reach as many as 500 micro steps for a full-step sequence, which will provide 100,000 steps for each revolution.

THREE PHASE TRANSFORMER

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It is the three phase system which has been adopted world over to generate, transmit and distribute electrical power. Therefore to change the level of voltages in the system three phase transformers should be used.
Three number of identical single phase transformers can be suitably connected for use in a three phase system and such a three phase transformer is called a bank of three phase transformer. Alternatively, a three phase transformer can be constructed as a single unit

In a single phase transformer, we have only two coils namely primary and secondary. Primary is energized with single phase supply and load is connected across the secondary. However, in a 3phase transformer there will be 3 numbers of primary coils and 3 numbers of secondary coils. So these 3 primary coils and the three secondary coils are to be properly connected so that the voltage level of a balanced 3-phase supply may be changed to another 3-phase balanced system of different voltage level.Suppose you take three identical transformers each of rating 10 kVA, 200 V / 100 V, 50 Hz and to distinguish them call them as A, B and C. For transformer-A, primary terminals are marked as A1A2 and the secondary terminals are marked as a1a2. The markings are done in such a way that A1 and a1 represent the dot (•) terminals. Similarly terminals for B and C transformers are marked

THREE PHASE TRANSFORMER WINDING DIAGRAM

It may be noted that individually each transformer will work following the rules of single phase transformer i.e, induced voltage in a1a2 will be in phase with applied voltage across A1A2 and the ratio of magnitude of voltages and currents will be as usual decided by a where a = N1/N2 = 2/1, the turns ratio. This will be true for transformer-B and transformer-C as well i.e., induced voltage in b1b2 will be in phase with applied voltage across BB1B2B and induced voltage in c1c2 will be in phase with applied voltage across C1C2. 

Now let us join the terminals A2, BB2 and C2 of the 3 primary coils of the transformers and no inter connections are made between the secondary coils of the transformers. Now to the free terminals A1, B1B and C1 a balanced 3-phase supply with phase sequence A-B-C is connected

. Primary is said to be connected in star.

If the line voltage of the supply is V =200*1.73 VLL, the magnitude of the voltage impressed across each of the primary coils will be 3 times less i.e., 200 V. However, the phasors 12AAV  12BBVand 12CCVwill be have a mutual phase difference of 120º Then from the fundamental principle of single phase transformer we know, secondary coil voltage 12aaVwill be parallel to 12AAV; 12bbVwill be parallel to 12BBVand 12ccVwill be parallel to 12CCV. Thus the secondary induced voltage phasors will have same magnitude i.e., 100 V but are displaced by 120º mutually. The secondary coil voltage phasors 12aaV, 12bbVand 12ccV are shown in figure 26.2. Since the secondary coils are not interconnected, the secondary voltage phasors too have been shown independent without any interconnections between them. In contrast, the terminals A2, B2 and C2 are physically joined forcing them to be equipotential which has been reflected in the primary coil voltage phasors as well where phasor points A2, B2 and C2 are also shown joined. Coming back to secondary, if a voltmeter is connected across any coil i.e., between a1 and a2 or between b1 and b2 or between c1 and c2 it will read 100 V. However, voltmeter will not read anything if connected between a1 and b1 or between b1 and c1 or between c1 and a1 as open circuit exist in the paths due to no physical connections between the coils

Imagine now the secondary coil terminals a2, b2 and c2 are joined together physically 

So the secondary coil phasors should not be shown isolated as a2, b2 and c2 become equipotential due to shorting of these terminals. Thus, the secondary coil voltage phasors should not only be parallel to the respective primary coil voltages but also a2, b2 and c2 should be equipotential. Therefore, shift and place the phasors 12aaV, 12bbVand 12ccVin such a way that they remain parallel to the respective primary coil voltages and the points a2, b2 and c2 are superposed.

Here obviously, if a voltmeter is connected between a1 and b1 or between b1 and c1 or  between c1 and a1 it will read corresponding phasor lengths a1b1 or b1 c1 or c1a1 which are all equal to 200   3V. Thus,             Va b11           ,           b c12V and    2c a1V are of same magnitude and displaced mutually by 120º to form a balanced 3-phase voltage system. Primary 3-phase line to line voltage of 200  3V is therefore stepped down to 3-phase, 100      3V line to line voltage at the secondary. The junction of A2, BB2 and C2 can be used as primary neutral and may be denoted by N. Similarly the junction of a2, b2 and c2 may be denoted by n for secondary neutral.

Star-delta connection 

To connect windings in delta, one should be careful enough to avoid dead short circuit. Suppose we want to carry out star / delta connection with the help of the above single phase transformers. HV windings are connected by shorting A2, BB2 and C2 together

As we know, in delta connection, coils are basically connected in series and from the junction points, connection is made to supply load. Suppose we connect quite arbitrarily (without paying much attention to terminal markings and polarity), a1 with b2 and b1 with c1. Should we now join a2 with c2 by closing the switch S, to complete the delta connection? As a rule, we should not join (i.e., put short circuit) between any two terminals if potential difference exists between the two. It is equivalent to put a short circuit across a voltage source resulting into very large circulating current. Therefore before closing S, we must calculate the voltage difference between a2 with c2. To do this, move the secondary voltage phasors such that a1 and b2 superpose as well as b1 with c1 superpose - this is because a1 and b2 are physically joined to make them equipotential; similarly b1 and c1 are physically joined so as to make them equipotential. The phasor diagram is

. If a voltmeter is connected across S (i.e., between a2 and c2), it is going to read the length of the phasorV. By referring to phasor 2diagram of figure 26.9, it can be easily shown that the voltage across the switch S, under this condition isV= a c o 100+ 2cos60 100 = 200V . So this connection is not proper and the switch S should not be closed.

VARIABLE FREQUENCY DRIVE (VFD)

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Variable frequency drive (VFD) usage has increased dramatically in HVAC applications. The VFDs are now commonly applied to air handlers, pumps, chillers and tower fans. A better understanding of VFDs will lead to improved application and selection of both equipment and HVAC systems. This paper is intended to provide a basic understanding of common VFD terms, VFD operation, and VFD benefits. In addition this paper will discuss some basic application guidelines regarding harmonic distortion with respect to industry standards

Understanding the basic principles behind VFD operation requires understanding the three basic  sections of the VFD: the rectifier, dc bus, and inverter. The voltage on an alternating current (ac) power supply rises and falls in the pattern of a sine wave

When the voltage is positive, current flows in one direction; when the voltage is negative, the current flows in the opposite direction. This type of power system enables large amounts of energy to be efficiently transmitted over great distances. The rectifier in a VFD is used to convert incoming ac power into direct current (dc) power. One rectifier will allow power to pass through only when the  voltage is positive. A second rectifier will allow power to pass through only when the voltage is negative. Two rectifiers are required for each phase of power. Since most large power supplies are three phase, there will be a minimum of 6 rectifiers used Appropriately, the term “6 pulse” is used to describe a drive with 6 rectifiers. A VFD may have multiple rectifier sections, with 6 rectifiers per section, enabling a VFD to be “12 pulse,” “18 pulse,” or “24 pulse.” The benefit of “multipulse” VFDs will be described later in the harmonics section. Rectifiers may utilize diodes, silicon controlled rectifiers 

microprocessor to control when the power may begin to flow, making this type of rectifier useful for solid-state starters as well. Transistors include a gate circuit that enables a microprocessor to open or close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most close at any time, making the transistor the most useful device of the three. A VFD using transistors in the rectifier section is said to have an “active front end.” After the power flows through the rectifiers it is stored on a dc bus. The dc bus contains capacitors to accept power from the rectifier, store it, and later deliver that power through the inverter section. The dc bus may also contain inductors, dc links, chokes, or similar items that add inductance, thereby smoothing the incoming power supply to the dc bus. smoothing the incoming power supply to the dc bus. The final section of the VFD is referred to as an “inverter.” 

The inverter contains transistors that deliver power to the motor. The “Insulated Gate Bipolar Transistor” (IGBT) is a common choice in modern VFDs. The IGBT can switch on and off several thousand times per second and precisely control the power delivered to the motor. The IGBT uses a method named “pulse width modulation” (PWM) to simulate a current sine wave at the desired frequency to the motor. Motor speed (rpm) is dependent upon frequency. Varying the frequency output of the VFD controls

motor speed: Speed (rpm) = frequency (hertz) x 120 / no. of poles