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