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THREE-PHASE ALTERNATING CURRENT MOTORS

THREE-PHASE ALTERNATING CURRENT MOTORS - Free Online Tutorial Part 2


MOTOR CHARACTERISTICS

The resistance of the stator windings is very low. The less resistance a component has, the  greater the current from the generator. Motor current requirements can be, among others,  attributed simply to size. The larger the stator winding diameter is, the larger the motor itself  is constructed. A motor, with its low resistance stator windings, initially reacts as a short  circuit. It is not until the expanding and contracting magnetic fields cut the many turns of wire  adjacent to each conductor in the stator winding that the current is further reduced. This  momentary inrush of current, combined with the transformer-like action, described in  Short-Circuit Rotor Bars, accounts for the overall current needed for a motor.

When the vessel is initially started, a ship's electrical distribution system may have only  lights in operation. There is very little current registering on the switchboard ammeters. This  is because the resistance in the light bulbs is so high. The high resistance keeps current down.

As soon as a motor is connected to the line, the current draw becomes excessive. The ammeter will  register more than six times the normal operating current of the motor. This is what happens: The  motor's internal wiring is of negligible resistance. Since all electrical components are  connected in parallel in the distribution system, the parallel circuit rules apply. Resistance in  a parallel circuit is always less than the smallest resistor. (This is why the largest idle motor  is of considerable concern when designing a ship's distribution system.) The motor wire  resistance is now the only determining factor for the generator's current output. The current  immediately supplied by the generator is called inrush current. If the rotor is mechanically  prevented from moving, the current is then called locked rotor current.

Westinghouse developed a program to investigate motor circuit protection. A power source and  cabling system was designed to handle LRC levels far in excess of that normally found on Army  watercraft. The objectives of the test was to determine how much the fault current would exceed  the normal full-load current if a rotor was mechanically prevented from rotating. Results show  that lock rotor current progresses in steps. Approximately 44 cycles after the initial LRC, LRC  almost doubled in value. This double LRC was maintained for an additional 42 cycles until the LRC  increased again. This time the LRC was stepped up to three times initial LRC. The LRC continued  to increase in steps of similar values with fewer cycles between steps. Test results hold little  consolation in the knowledge that at no time did the fault current exceed 50 times the FLC. The  test established that motor failures start at relatively low values (6 x FLC) and cascade quickly  in mere seconds. A current draw of the observed magnitude would devastate the current-producing  capacity of the generating system and effectively terminate the operation of the distribution  system if not interrupted rapidly. Remember, all improperly protected circuits are fire hazards!

The induction motor poses many problems for the electrical system environment. The motor's great  current draw can tax the electrical system to the extent that the generated voltage will drop.  (There is internal resistance in the generator, too. The greater the current through the  generator's conductors, the greater the voltage dropped in the entire electrical system, E = IR).  When this generated voltage drops below a certain point, relays, contractors, and other  electrical holding coils become de-energized, and their associated equipment stops operating.

A complete understanding of motor operating characteristics is necessary to understand the  effects of the motor on the electrical system and the requirements for protecting a motor against  overload conditions. The two most prominent effects from the motor are --

- Inductive reactance.
- High rotor EMF.


Inductive Reactance

The discussion on transformers explained the properties of induction on a coil of wire. Except  for the minimal resistance of the wire itself, there appears to be nothing to prevent a power  source from restricting the majority of its current. As it turns out, induction opposes a change  in current. A back voltage or counter EMF (CEMF) is developed and pushes back on the power  supply. In the DC system, the CEMF restricts current flow. In AC, the CEMF impedes current flow  change. The AC system with its various amplitudes and current directions creates a generator out  of any inductor. This shuttle power is inductor-generated and must be overcome by the generator.  When the inductive reactance (shuttle power), the motor's load, and assorted losses are overcome,  the generator supplies only enough additional current to keep the motor rotor turning. The only  problem exists with the inductive reactance. This generated CEMF and its resulting current are  there to be overcome. Inductive reactance, therefore, is not consumed.

Whenever inductance is involved in the electrical system, a lagging power factor results. The  power factor is extremely poor when the motor is first started. The lower the power factor, the  greater the increase in current needed to operate the motor. A power factor of .5 can be expected  when a motor is first started. At the motor's rated speed, a power factor of .8 is normal. Unity  or 1.0 is the best use of power. Not only does the generator have to supply current for  overcoming the wire resistance, but it must overcome the inductive reactance from the motor  itself.

Never select a motor that is overrated for its application. Contrary to popular belief, when a  motor is not operated at its rated capacity, the electrical system efficiency is decreased. The  power factor is decreased, goes further away from unity, and more power is required to operate  the motor than would have normally been required for a motor operating at the designated rated  capacity.

Never operate a motor above its rated capacity. It will not operate long. Motors and generators  can easily operate at many times their normal current ratings for a short period of time. Even  so, excess heat is generated. If this heat is not permitted to dissipate rapidly, insulation  damage will result.


High Rotor EMF

Inductive reactance is always an important consideration when choosing motors for the electrical  system. But the induction motor has another characteristic that influences the electrical  environment even more. This is called the rotor EMF.

The motor acts much like a transformer. The stator winding becomes the primary winding, and the  rotor becomes the secondary winding. If the secondary winding of a transformer becomes shorted  out, the primary winding effectively becomes the generating source. The primary winding, an  extension of the generator, provides as much current as possible according to the Maximum Power  Transfer Theorem.

At the instant when the rotor has not yet begun to move and current is applied to the stator,  there is a maximum slip. There is maximum relative motion between the stator and the rotor and a  maximum induced voltage into the low-resistance rotor bars. These rotor bars act like a short  circuit drawing very large currents from the source because there is negligible resistance to  restrict the current flow.

The stator windings have extremely large currents because of the large induced rotor EMF. Both  the rotor and the stator develop maximum magnetic fields from maximum current flows.

The rotor's magnetic field, from induction, is of the opposite polarity of the stator's magnetic  field. The rotor starts to move. As the rotor speed increases, the relative motion between the  two windings decreases. The decreasing relative motion decreases the EMF and the resulting  current flow in the rotor bars. The power source demand decreases as does the current flow to the  stator.

This phenomenon is readily observable by using an induction ammeter and an AC motor. Simply place  the jaws of the ammeter around one insulated conduct or (not all). Start the motor and observe  the meter readings. The current will start very high and then taper off quite rapidly as the  motor increases in speed.


Load Changes

Counter electromotive force developed in the stator windings could restrict current flow to  moderation, except for the overwhelming EMF induced in the rotor. Many other factors affect the  operation of the motor, such as impedance, changes in torque, and the angle in degrees separating  the stator and rotor magnetic fields. The table below is a simple reference to the factors affecting a  motor and the electrical environment under three motor operations.
















The following is a brief outline on the motor-operating characteristics under several conditions:

 
- When the motor is operating at no-load conditions, the rotor speed gets very close to  synchronous speed. Very little EMF is induced in the rotor bars, just enough to overcome  mechanical losses. Current draw is low.

- As the motor becomes increasingly more loaded, the slip increases, and relative motion  increases. Induced rotor EMF increases and with it a resulting increase in current flow in both  the rotorand stator windings. The increased magnetic fields increase torque and the ability of  the motor to return to its proper speed. Current automatically increases as the rotor slows down.

- During an overload condition, the rotor is slowed excessively. The EMF induced in the rotor and  its subsequent current flow in both the rotor and stator can burn up the insulation windings and  destroy the motor. Current becomes destructive.


MOTOR PROTECTION

Motor requirements for current vary widely with the load. In addition, the current actually  exceeds the normal operating range when the motor is first started. How then can the motor be  protected against the excessive currents outside the normal parameters of operation and still be  protected from small prolonged current increases?

Fuse Protection

Fuses have several disadvantages in protecting the motor. If a fuse is used to protect the motor  for its full-load current rating, then the fuse would open during the initial inrush of current.  A fuse designed to pass inrush current would not protect the motor against currents less than the  inrush but greater than the normal full-load current. For every 1C rise over normal ambient  temperature ratings for insulation, it has been estimated that the life expectancy of a motor can  be reduced almost a year. Current generates heat in a motor. Heat destroys the motor insulation.

Time-delay fuses have been used for motor protection in the past. However, another problem  develops when using three fuses for the protection of the three-phase motor. Should only one of  the three fuses open when the motor was operating, the motor would not stop immediately. It would  continue to operate. The operation of three-phase motors on only two lines constitutes a  single-phase condition. The three-phase motor cannot operate single phasing for long without  internal damage. This would not become apparent until enough damage was incurred that the motor  would be irreparable. The fuse was not the answer for protecting three-phase motors.


Magnetic Motor Starters

The magnetic motor starter is a magnetic contactor with an overload protection device. Unlike the  fuse, the magnetic motor starter does not have to be replaced. It can be reset repeatedly.


















The Motor Circuits

Larger current-demanding motors use two circuits for operation. One circuit is the three-phase  power circuit supplied from the distribution power panel. The other electrical circuit is the  control circuit.

The figure shows the magnetic motor starter and the power circuit from the distribution power  panel. The heavy, dark lines provide the three-phase, high current-carrying power to the motor.

Inside the magnetic motor starter, directly under the coil, are three large main contact sets.  These contacts are in series with the power panel A, B, and C phase terminals and the T1, T2, and  T3 motor terminals. As long as these contacts are closed, current from the power distribution  panel can operate the motor. This is one circuit.

The other circuit controls the three large contact sets explained above. The coil in Figure 16-9  actually moves the contacts. The figure shows the control circuit that the coil is actually in.  M represents the coil in the figure.






























The M coil is supplied single-phase power from the magnetic motor starters A and B phase  terminals (also known as L1 and L2 terminals). The figure above shows two M coils: one in its true  physical position in the magnetic motor starter and the other in the line diagram to explain its  function electrically. There is actually only one M coil. The same applies to the NC overload  contacts.

When the START button is pressed, a complete circuit from A phase through the M coil, through the  NC overload contacts, to the B phase is completed in the control circuit. The M coil energizes  and moves a bar, known as an armature, that is in physical contact with the three large power  contacts in the motor's three-phase power circuit. The figure below illustrates this action.



































The main power circuit contacts for the motor are held open by spring tension (see Figure view  A). When the coil becomes energized, the magnetic attraction between the armature and the magnet  overcomes spring tension, and the main contacts for the motor close (Figure view B). The  motor now operates.

When the current to the motor is too great, the overload heaters get hot. The heaters are in  series with the motor terminals and the main contacts for the motor. The heaters directly control  what happens to the NC overload contacts in the control circuit. When the heaters get hot enough,  the overload contacts open, and the M coil de-energizes. The loss of the magnetic field allows  spring pressure to open the three main contacts in series with the motor, and the motor stops  operating. By de-energizing the one coil (M), all three sets of main contacts open. Detrimental  single phasing is avoided.

A minor disadvantage of the thermal overload device is its need to cool off before being reset.  The Figure shows a magnetic motor starter and the overload heater and NC overload contact section  separately.




Thermal Heater and NC Overload Operation

The common thermal overload uses heater coils in the main power line in series with the main  contractors and the motor stator windings. The current going to the motor must go through the  overload heaters first. These heater coils surround a eutectic alloy solder pot. Eutectic means it has a very low melting point. Characteristically, a eutectic solder goes from  solid to liquid and back again without developing a mushy condition.



















The solidified solder holds a ratchet wheel and pin assembly firmly in place (Figure 16-14). The  ratchet wheel is under tension and holds a set of contacts closed. These contacts have the  ability to interrupt the magnetic coil circuit that opens and closes the main contacts. When the  magnetic coil is de-energized, the main contacts open. The main contacts no longer supply power  to the motor, and the motor stops.

The thermal overloads effectively monitor motor current by developing a comparative heat in the  heater coils. The more current that flows though the heaters, the faster the heaters become hot.  When the motor is first started, the heat from the momentary high inrush current is dissipated  rapidly by the heater coils. The operation of the motor is not interrupted. If, however, the high  current should last but another moment longer, the contacts would open, and the motor would stop.  If a small overcurrent condition exists, the heaters will still get hot enough to melt the  eutectic alloy, but it will take longer. Once enough heat is generated in the heaters and the  eutectic alloy melts, the ratchet wheel and pin assembly move under spring pressure. As a result,  the contacts in the control circuit of the magnetic motor starter open. This de-energizes the  coil in the magnetic motor starter and opens the main contacts, disconnecting the motor from the  line. Notice in the figure below that the overload contacts are not in the motor power supply line.  They are in the control circuit that operates the main contactors. The main contractors and the  overload heaters are in the motor's main three-phase supply line.



























The protection afforded by the overload device is determined by the heater coil selection. By  using different heater coils, a variety of overcurrent protection can be selected. This must be  based on the full-load current rating of the motor. The temperature surrounding the motor and the  magnetic motor starter must also be considered. Heat and current have the same destructive nature  toward electrical equipment. Electrical components in engine compartments are exposed to greater  heat than those in the ward room. Likewise, the controller, which houses the magnetic motor  starter, must be in the same area as the motor it protects. Only in this manner will the heater  be affected by the same ambient temperature as the motor windings.

Proper motor protection is required in the motor control centers in the engine room. The MCC is  air conditioned, and the motors in the engine compartment are not. If adequate motor protection  selection is not provided, additional investigation is necessary.

Every motor starter manufacturer has specific overload guidelines supplied with the equipment.  Magnetic motor starters are provided with heater selection charts because magnetic motor starters  do not come with overload heaters. Each heater must be identified for the specific motor  application, full-load current, and ambient temperatures. The manufacturer guides are  self-explanatory. Additional information is available in the Code of Federal Regulations, Title  46, Subpart 111.70, and the National Electrical Code (NEC), Article 430.

A less common protective device is the magnetic overload relay. This device uses a current coil  that creates a magnetic field in proportion to the current carried in it. Once the magnetic field  is strong enough, the contacts are opened, and the circuit is de-energized. The main benefit to  this type of overload device is its ability to be reset immediately.
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