Saturday, March 23, 2013

How Electric Motors Work


     Magnetic Fields

Magnetic fields exert a force on ferrous metals (like iron) and magnets as well as on electric currents without any physical contact. Lines of force or flux were invented to help us visualize the magnetic field. Stronger magnetic fields are shown with more lines of flux. Magnetic flux density is proportional to the number of flux lines per unit area. See Figure 1.

DC Motor Action

An electric current produces a magnetic field. The flux lines of a staight, current carrying conductor are concentric rings around the conductor. See Figure 2. The direction of the magnetic field lines are determined by the direction of the current. Your right hand can be used to show this relationship. Your thumb points in the direction of current and your fingers curl in the direction of magnetic field.
Current flowing through a conductor in a magnetic field exerts a sideways force on the conductor. In Figure 3, the permanent magnetic field and the induced magnetic field oppose each other in the region above the wire, reducing the total flux. Below the wire, the two fields are in the same direction and the total flux is increased. The resulting magnetic force causes the conductor to move upwards into the area of the weaker magnetic field.
If an armature loop is placed in a magnetic field, the field around each conductor is distorted. See Figure 4.
These repulsion forces are proportional to the flux density and the current in the armature loop. The repulsion forces push the armature upwards on the left and downwards on the right. These forces are equal in magnitude and opposite in direction and produce a torque which causes the armature to rotate clock-wise.

Commutation

The magnitude of this torque is equal to the force multiplied by the perpendicular distance between the two forces. It is maximum when the conductors are moving perpendicular to the magnetic field. When the loop is in any other position, the torque decreases. When the plane of the loop is perpendicular to the magnetic flux (we call this the neutral plane), the torque equals zero. As soon as the armature passes this point, it experiences a force pushing it in the opposite direction and is eventually magnetically held at the neutral position. In order to maintain the motion of the armature, the battery connections to the armature loop must be reversed as the loop rotates past the neutral plane. This is the basic principle behind a DC electric motor. Electrical energy (current) supplied to the armature is transformed into mechanical motion (the loop rotates).
With the type of motor described above, the torque varies from zero to its maximum twice in each revolution. This variation in torque can cause vibration in the motor and the equipment it drives. Also, a motor stopped with thearmature in the neutral plane is very difficult to start. Additional armature coils solve both of these problems.
Figure 5 shows a motors with one coil, two coils, and 16 coils. The more coils that an armature has (each with two commutator segments), the smoother the torque output. Torque never drops to zero when there are two or more coils.

Back EMF

Whenever a conductor moves through magnetic lines of flux, voltage (emf) is induced in the conductor which is opposite to the voltage you applied to the motor to make it spin. The magnitude of this emf depends on the speed of rotation. It is called the back emf or contervoltage. The difference between the applied voltage and the back emf determines the current in the motor circuit. So, the back emf helps to limit the current flowing in the armature.

DC Motor Types — Permanent Magnet Motors

Permanent magnet (PM) motors are comparably small, light, efficient motors. Their high efficiency and small size are due to the use of permanent magnets to produce the magnetic field. They do not have the added bulk and electrical losses of the field windings normally required to produce the magnetic field.
Permanent magnets are produced by ferromagnetic materials that have been magnetized by an external magnetic field. Ferromagnetic materials can produce magnetic fields several times greater than the external field and will remain magnetized even after the applied magnetic field is removed.

Speed Regulation

Speed regulation is easily accomplished in a PM motor because the speed is linearly related to the voltage. The speed can be increased simply by increasing the voltage. The speed is inversely proportional to the torque. This means that the torque increases as the motor slows down for heavy loads.
See Figure 6. The torque a motor can apply at start up (starting torque) and the torque which causes the motor to breakdown (breakdown torque) are the same
for these motors. PM motors have a high starting torque for starting large loads. This torque results from a high starting current, 10 to 15 times normal running current. PM motors cannot be continuously operated at these currents, though, since overheating can occur. Runaway in a motor occurs when the motor builds up speed under no load until its bearings or brushes are destroyed. Runaway is unlikely in PM motors.

Dynamic Braking

Sometimes it’s necessary for a motor to stop rotating quickly after power is disconnected from the motor. This can be achieved by mechanical braking (friction) or electrical braking (dynamic braking). Dynamic breaking is accomplished in a PM motor by shorting the armature connections and converting the motor into a generator. The rotational mechanical energy is converted to electrical energy and then to heat. PM motors can be braked very quickly using this method without the use of brake shoes which wear out. PM motors are also be easily reversible when the motor is running or stopped.
The most serious disadvantage of PM motors is that the PM fields can be demagnetized by the high armature currents that result from stalling or “locked rotor operation.” This problem becomes more of a concern at temperatures below 0°C. Also, permanent magnet motors are normally small motors because permanent magnets can’t supply enough magnetic field to produce large PM motors.
PM motors can be used for applications requiring small, efficient motors which have high starting torques and low running torques (inertial loads). They are commonly used in well pumps and appliances in RV systems. Jim Forgette of Wattevr Works uses PM motors in his washing machine retrofit kits.

Shunt Motors

In shunt motors, the magnetic field is supplied by an electromagnet which is connected in parallel with the armature loop. The primary advantage of shunt motors is good speed regulation.
Variations in torque by the load do not have a big effect on the speed of the motor unless it is overloaded. Shunt motors have lower starting torques and lower starting currents (three times running currents) than other motors of same horse power. See Figure 7.
Figure 7
The National Electrical Manufacturer’s Assn has agreed on four standard speeds for shunt motors: 1140, 1725, 2500, and 3450 rpm. The speed is normally controlled by varying the armature supply voltage. Speed varies linearly with armature supply voltage and torque is unaffected.
Shunt motors are typically used for loads which require good speed regulation and fair starting torque. If very heavy loads are to be started, a starting circuit may be required. Starting circuits connect progressively smaller resistances in series with the armature. Runaway can occur in shunt motors if the field current is interrupted when the motor is turning but not loaded. Dynamic braking and reversibility are both options with shunt motors.

Series Motors

In series motors, the field coil is connected in series with the armature loop. The field coil has a large current (the full armature current). Heavier copper is used for the field coil but not many turns are needed. Series motors are usually less expensive and smaller in size than other motors of the same horsepower because less copper is used.
Due to the small number of turns and the resulting low inductance, series motors can operate on both ac and DC power. For this reason, series motors are often called universal motors. Power to both the field and armature loops reverses at the same time when operated on ac power and so the resulting magnetic force remains the same. Series motors may perform differently on ac than DC because of the difference in impedance of the windings. One shouldn’t assume all series motors are universal. Some may be optimized for a particular power supply and perform poorly or fail prematurely if not operated on the correct supply.
As the motor’s speed is decreased by heavy loads, the motor supplies high torque to drive the load. This helps prevent stalling and provides high starting torque. Starting currents are also high but are not usually a problem because series motors are normally small motors. See Figure 8. The speed of series motors can be adjusted by varying the supply voltage with a rheostat, variable transformer or electronic controls. Series motors are not normally used if constant speed over a range of loads is required.
Series motors are very common motors in household appliances and power tools. They are used in blenders, juicers, food processors, and hand power tools such as drills. They are very versatile and have the highest horsepower per pound and per dollar of any motor that operates on standard single phase ac power. They deliver high motor speed, high starting torque and wide speed capability. Series motors are usually operated at speeds over 7000 rpm or more. In routers, small grinders and sanders, speeds of 25,000 rpm are not uncommon. Series motors are often connected to a built-in gear train to reduce shaft speed and/or provide more torque. Gear trains also provide loading which prevents runaway.
Series motors have comparatively high maintenance. Brushes and bearings need to be regularly replaced. They are the only motors that are usually given an intermittent duty rating. Other disadvantages of series motors are that they are not usually designed for dynamic braking and reversibility. They should not be run without a load as runaway can occur.
Series motors have a moderately low power factors — normally between 0.5 and 0.7. Resistors have a power factor of one. The more reactive a component, the lower its power factor. Low power factors can be a problem for modified sine wave inverters. Appliances with low power factors may run three quarter speed. Sine wave inverters do not have trouble with power factors less than one. Series motors are typically small motors and so their high starting currents are not usually a problem for inverters.

Compound Motors

A compound motor provides a mixture of the characteristics of both shunt and series motors. Its field coil is split into a series field which is connected in series
with the armature and a shunt field which is connected in parallel with the armature. The magnetic fields can either aid (cumulative compound) or oppose each other (differential compound).
Cumulative and differential compound motors have different speed/torque characteristics. Cumulative compound motors provide more torque than shunt wound motors and better speed regulation than series wound motors. Differential compound motors have almost perfect speed regulation but lower starting torque. See Figure 9.
Compound motors were often used in the past. Inexpensive electronic controls has made it possible to replace them in many cases with lower cost series and shunt motors. They are still used sometimes in large DC equipment which require high torque and good speed regulation.

Brushless DC Motors

Brushless DC motors are actually not DC motors at all. They are ac motors with built-in micro inverters to change the DC supplied to the motor into ac to be fed to the field windings. A logic circuit senses the position of the permanent magnet rotor and controls the distribution of current to the field windings. Field windings are energized in sequence to produce a revolving magnetic field.
The greatest advantage of brushless DC motors is the replacement of carbon graphite brushes and commutators with long life solid state circuitry. They provide low maintenance, low electrical noise motors with good speed control and constant torque. They cannot, however, be easily reversed and are not easily adaptable to dynamic braking. They are also more expensive than conventional DC motors. They are used frequently in audio-visual equipment and “muffin” cooling fans, such as the ones found in inverters, charge controllers, and computer equipment. They are also used in Sun Frost refrigerators.

AC Motors — Induction Motors

The majority of motors in service today are ac motors. Many of these are universal motors. Induction motors,
though more expensive, are also very common due to their high reliability. Polyphase induction motors are cheaper, more efficient, more reliable, and have a higher starting torque than single phase induction motors. We are only discussing single phase induction motors here though because only single phase power is available to most homes.
Induction motors use a squirrel cage rotor construction. This means that the rotor is made of thick aluminum or copper that is one turn only and is joined at each end by an aluminum or copper ring. This frame is then filled in with laminated iron to provide a low reluctance magnetic path. The bars of the rotor are angled with respect to the shaft to provide a smoother output torque and more uniform starting performance.
Voltage is induced in the rotor when it is placed in a rotating magnetic field. The induced voltage produces a high current because of the rotor’s very low resistance. This high current flowing in the rotor produces its own magnetic field. The magnetic interaction of the rotor and the rotating stator field exerts a torque on the rotor, making it follow the magnetic field. Thus an induction motor produces a torque on the rotor without any electrical connections to the rotor. This eliminates the use of brushes and bearings and is the reason for the induction motor’s high reliability.
Normally, the rotating magnetic field in induction motors is produced with three-phase power. A magnetic field established with single phase power will pulse with intensity but will not rotate. A squirrel cage rotor placed between the poles of a single phase motor will therefore not rotate either. Once the rotor begins rotating, however, it will continue to rotate. Thus some means must be employed to create a rotating magnetic field to start the rotor moving. This method determines the type of single phase ac induction motor.

Split-phase Motors

In split-phase motors, a rotating magnetic field is produced with a start winding and a run winding. The start winding is made of smaller gauge wire. The resulting higher resistance and lower reactance produces an approximately 60° phase difference between the currents in the two windings.
This phase difference produces a rotating magnetic field which causes the rotor to start rotating. See Figure 10 below.
DC Motor Characteristics
Motor Type Starting Torque Starting Current Revers-ibility Speed Dynamic Braking Size/
Weight
Cost Horsepower Range
PM high high easy varying yes smallest low under 1
Shunt low low easy constant yes normal moderate any
Series high very high not usually high & varying no small low under 2
Compound (Dif) low low easy very constant yes large high any
Compound (Cum) high high easy fairly constant yes large high any
Brushless high high difficult constant no small high low
 
The start winding is disconnected from the circuit when the motor reaches 70% of operating speed.
The start winding will overheat if it conducts current continuously. Once the rotor begins turning, the distortion of the stator magnetic field by the rotor’s magnetic field produces enough magnetic field rotation to keep the rotor turning.
Split-phase motors are very common and not very expensive. Oxidation of centrifugal switches was once the most common type of failure. Solid state devices have improved the motor’s reliability. They have a moderate starting torque and a high starting current (8–10 times running current).
They are a good choice for easy to start application such as large fans, blowers, washing machines and some power tools, including bench grinders and large table saws. Overheating can occur if the motor is heavily loaded and the speed kept too low for the switch to open. Heat builds up with the high starting current and the high start winding resistance. Overheating can also result from frequent starting and stopping.
Split-phase motors operate at practically constant speed and come up to rated speed very quickly. The motor’s speed varies from 1780 rpm at no load to 1725–1700 rpm at full load for a 4 pole 60 Hz motor. Split-phase motors can be reversed while at rest but not during operation. Dynamic braking can be accomplished by supplying DC power to the field coils via either an external DC supply or a rectifier, resistor and charging capacitor.
Split-phase motors can cause problems on inverters because of their very high starting currents. Richard learned a trick after damaging many inverters trying to start his bench grinder. If you start the wheel turning with your finger, you can get the grinder started with a lower current. Be sure to get your finger out of the way before you turn the switch on.

Capacitor-Start Motors

Capacitor-start motors have a higher starting torque and lower starting current than split-phase motors. They do this by connecting a capacitor in series with the start winding which increases the phase difference between the start and run fields. Low cost ac electrolytic capacitors are normally used since they are only used for a few seconds when starting.
Capacitor-start motors are used to start very heavy loads such as refrigerators, pumps, washing machines and air compressors. The starting currents can be quite high when the motor is operated with large loads. This much current is hard on centrifugal switch contacts and so many capacitor-start motors use a current or potential relay instead of a centrifugal switch.
Capacitor-start motors often have problems on modified sine wave inverters. The field coils and the capacitor make up a tuned circuit which requires 60 Hz frequency for proper operation. Although modified sine wave inverters have an average 60 Hz frequency, the instantaneous frequency is sometimes much, much higher. Richard’s found in his experience that substituting the capacitor for a higher or lower value may solve the problem. It’s a matter of testing different values. Sine wave inverters do not have any problems starting capacitor-start motors.

Permanent-Split-Capacitor (PSC) Motors

Centrifugal switches and relays are the most likely part of the capacitor-start motor to fail. They can be removed if slightly larger wire is used for the start windings so that they can be left connected without overheating. A higher capacitor value is required to compensate for the higher inductance of the larger windings. Oil-bath type capacitors are usually used because the capacitor is now used during start and run operation.
AC Motor Characteristics
Motor Type Starting Torque Starting Current Reversibility Speed Dynamic Braking Cost Horsepower Range
Split-phase moderate high easy, at rest relatively constant yes normal up to 2
Capacitor-start high medium easy, at rest relatively constant yes high-normal up to 5
PSC mod. high med. low easy relatively constant yes high-normal up to 5
Two-capacitor high medium easy, at rest relatively constant yes high-normal up to 5
Shaded-pole low low not reversible relatively constant yes low up to 1/2
PSC motors operate in much the same way as a two phase ac motor. The capacitor ensures that the capacitor winding is out of phase with the main winding. There is now a rotating magnetic field during start and run operation. This gives the motor greater efficiency and quieter and smoother operation than ac induction motors that only have a rotating magnetic field during start operation. The capacitor value is a compromise between the optimum value for starting and running. This results in a lower starting torque than the capacitor-start motor.
PSC motors are used in applications where frequent starts and stops and quiet smooth operation is required. Examples are instrumentation and low noise equipment fans.

Two-Capacitor Motors

Two capacitor start, one capacitor run motors use an electrolytic capacitor for starting and an oil-type capacitor for starting and running.
The two capacitors are connected in parallel. This motor type preserves the efficiency and smooth, quiet operation of PSC motors while running and provides the high starting torque characteristic of the capacitor-start motors. Optimum starting and running characteristics are obtained at the expense of using some sort of switch again.

Shaded-Pole Motors

Shaded-pole motors’ magnetic fields are made to rotate by the inductive effect of two or more one-turn coils next to the main windings in the stator. The time varying magnetic field set up by the alternating current in the main winding induces current in the shading coils. The induced current in turn establishes a magnetic field in the shading coils which lags behind the main field by about 50°. This sets up a rotating magnetic field in the stator.
Shaded-pole motors are simple in design and construction. They have no internal switches, brushes, or special parts. These motors offer substantial cost savings in applications which require constant speed and low power output.
Shaded-pole motors are inefficient, have low starting torque and can have unsmooth running torque. They are nonetheless cheap and reliable and are used in countless consumer applications ranging from inexpensive blowers to room air conditioner fans. Shaded-pole motors run without problems on sine wave inverters but may run slow on modified sine wave inverters.

Speed Control of ac Motors

Speed control of ac series motors can be accomplished by using SCR’s and triacs to turn ac power on for only part of each cycle, reducing the average voltage to the motor without dissipating large amounts of power.
Induction motors are usually designed to run at a single speed controlled by the frequency of the ac power supply driving them (which is usually a constant 60 Hz). At a higher cost, they are sometimes specially designed to provide speed variations. This is usually accomplished by changing the number of poles. A motor with two coils per phase will run half as fast as a motor with one coil per phase. Thus a motor can be made with two or three coils per phase and the number of coils can be switch selected.

Energy Efficient Electric Motors

Split-phase, capacitor-start, PSC and two-capacitor motors are all available in energy efficient models. Improvements in efficiency are mainly due to increased conductor and rotor areas, improved grade of steel and improved ventilation. These motors are begining to be found in larger home applliances and may make these appliances an option for RE systems.    

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