Motors and generators are classified under a generic term “rotating machines” in as much as they are the same devices to produce electromagnetic or electromechanical conversion.
The subject of electric motors and generators are already well known to all electrical engineering students and practitioner. For this reason, this page will dwell only on the various types of motors and generators used in industries, including their operating characteristics and reactance essential to the calculation of short circuit current. The latest motor development, which is the permanent magnet synchronous motor (PMSG), where the excitation field is provided by a permanent magnet in lieu of a coil, will also be briefly discussed.
DC motors or “dynamos” are still in use today for electronic devices, small generators for bicycles, rechargeable battery operated home improvement tools, and variable speed applications. However, the use of variable speed DC motors have now been replaced by induction motors as cost of electronic variable speed drives dropped considerably over the last few years. The advantage of a DC motor is its simple construction; but the periodic replacement and maintenance of brushes and collector makes the DC motor disadvantageous to industries that allow little downtime in the upkeep of their machineries. The largest DC motors still in use today are in light railway transit propulsion system (LRT), where the AC supply is converted to DC by rectifier diodes. DC propelled LRT system is now being slowly replaced by AC induction motor with state of the art variable frequency drives (VFD) The advantages of induction motor in rail transit propulsion system include direct connection to AC power supply, smoother acceleration, improved climbing capability and many more, making AC train propulsion system far exceed than that offered by conventional light rail DC propulsion systems.
The most commonly used AC motor for industrial application is the induction motor where the rotor current that produces the torque is electromagnetically induced by the magnetic field of the stator winding. Induction motor is also called a “rotating transformer“ with the stator as its primary circuit, and the rotor its secondary. The difference is a transformer induces current to its secondary by periodically reversing magnetic flux in the primary side; while induction motor stator (primary), induces current to the rotor (secondary) by rotating magnetic field. Induction motors are called asynchronous motors, since they rotate slightly below synchronous speed (ns = 120f / P) and the difference in speed is called “slip” as shown by the formula below:
%slip = [ (ns– n) / ns ]100%; where n = motor speed in revolutions per minute and ns = synchronous speed in revolutions per minute.
Typical hydro-electric turbine generator operation” Take note of the man’s size at the left side – Wikimedia Commons
Induction motors do not need “commutation” nor internal or external excitation circuit found in DC and synchronous motors. Their rotors are either wound type or squirrel-cage type design (no windings). The induction motor’s rotational speed is based on the AC supply frequency and the motor’s number of poles as shown:
ns = 120f / P; where ns = synchronous speed in revolutions per minute; f = AC power supply frequency in hertz and P = the motor’s number of poles (pole pairs).
As an example, a 4 pole induction motor synchronous speed ns = 120 X 60/4 = 1800 RPM when operated at 60hz power supply frequency. Three phase induction motors have fixed speed unless a variable frequency drive is used to adopt them to variable speed service used in several industries. Improvement in the design and physical size of variable frequency drives was made possible by the use of thyristors, a two to four leaded semiconductor device that can function as low or high-speed bistable (flip-flop) switch. A good example is the common three lead Silicon Controlled Rectifier (SCR)
1. Squirrel Cage– These are constructed from cast aluminum or copper bars that are connected in common by a ring shaped conductors at each end.
2. Wound Rotor – Three phase wound rotor induction motors have rotor windings that are connected to slip rings where they may be connected through external resistances to vary the speed. Modern method of varying the speed today, however, utilizes the Variable Frequency Drives (VFDs), a power electronic circuitry designed to vary the speed of wound rotor induction motor by connecting it to the motor in same manner as the old method of connecting a resistance to the motor. VFD application includes the speed control of large windmill turbine generators that are connected to a fixed frequency grid. The speed of a turbine generator varies, since the wind that turns the windmill blades also varies. Other VFD applications include induction motor compressors, pump drives, etc.
“Flux switching alternator” – Image Credit: Wikimedia Commons Author: User: Andy Dingley August 20, 2011
A synchronous motor rotates at a constant speed and has a stator that is similar to the wound rotor induction motor. A large synchronous motor rotor has a field winding that is fed by external DC source via the slip ring or rotating diode rectifiers supplied by three phase excitation generators fitted to a synchronous generator shaft. The field winding and the external DC excitation of a synchronous motor builds the magnetic field in the rotor that can make the motor run at either leading or lagging power factor making them suitable for power generation or power factor correction.
When used to improve power factor, a synchronous motor is called synchronous capacitors or synchronous compensator. The motor does not have a mechanical load connected to its shaft and spins freely. Its field is connected to a voltage regulator to either provide or receive reactive power as may be required to adjust or improve the grid or an industrial plant’s power factor. The synchronous motor’s advantage over capacitor banks is its capability to continuously adjust to achieve appropriate power factor at lower or higher grid voltage level; reactive power from capacitor banks, on the other hand, decreases as the system voltage decreases. The synchronous motor disadvantage is it consumes more energy than a static capacitor bank.
Synchronous motors used for electricity generation have either 4 poles or 2 poles depending on where they are used. Large hydro generators of up to 1000MVA have 2 poles while the 4 pole turbo machines can be as high as 1800MVA.
“The rotating magnetic field is formed from the sum of the magnetic field vectors of the three phases of the stator windings” – Wikimedia Commons Author: User: Mtodorov_69, 2006-03-14
Synchronous motor assemblies come in two classes: the high speed and the low-speed type. The high-speed type has a cylindrical rotor and is used for generators rotating at 1800 RPM or higher. The low-speed type has either salient pole construction or cylindrical rotors that are manufactured as one piece. The slow speed types are suitable for hydropower plants with a typical speed of 400RPM or less.
The magnetic field in Permanent Magnet Synchronous Motors (PMSGs) is produced by permanent magnets that require no external DC excitation circuit, slip rings and brushes. The PMSG converts electric energy to mechanical energy and converts mechanical energy to electrical energy when the rotor is rotated. They are also called synchronous motors since the frequency of the induced voltage in the stator is directly proportional to the rotational speed of the rotor, with its synchronous speed also found from the same formula, ns = 120f / P.
In the earlier days, permanent magnet generators used in car alternator systems have safety issues during servicing or repairs: where the rotor must be taken out of the stator. Today, however, the task is carried out by qualified service shops equipped with appropriate tools and riggings and especially trained personnel to service the permanent magnet car alternator.
PMSMs are fast gaining ground for the design of wind turbine generators because it is smaller, stable, more efficient and requires lesser maintenance due to fewer parts. Generating capacities of 20kW to 1,500kW with rotational speeds range of 100 to 700 RPM are now available for wind turbine and mini-hydro applications. Wind turbines of 2.5MW generating capacity have blades that can be as large as 45meters.
Synchronous Motor Rotor
Disassembled rotor of a large synchronous motor showing its inner components. The slip rings is seen beneath the rotor drum – Courtesy of Wikimedia Commons Author: Les Chatfield, Brighton, England”
Fault Current Contribution of Rotating Machines
Rotating Machine Reactance
The expression of multi-term formula for changing values of a rotating machine’s reactance with time as an independent variable is quite involved. For simplicity, three values of reactance for motors and generators are given below which will yield a satisfactory result for the calculation of fault currents at a specific time:
Subtransient reactance X”d is the stator winding apparent reactance at the instance of fault. It is the factor that determines the fault current flow during the first few cycles after a short circuit.
Transient reactance X’d is apparent initial reactance of the stator
winding when only the field winding is considered. The transient reactance determines the fault magnitude 0.5sec to 2secs depending upon the machine’s design.
Synchronous reactance Xd determines the current flow when a steady state condition is reached and becomes effective only several seconds after the fault. It is generally not used in short-circuit calculations.
Disassembled Synchronous Motor Stator
The difference between the induction motor and the synchronous motor is that the synchronous motor rotates in exact synchronism with the line frequency. The picture is courtesy of Wikimedia Commons Author: Les Chatfield, Brighton, England
Short Circuit Current of a Motor or Generator:
Isc = Full load amperes X 100 / %X”d
Locked Rotor Current or Locked Rotor Ampere
Locked Rotor Current or Locked Rotor Amperes is the inrush current when a motor is locked and prevented from rotating. Motors with fixed loads attached have high LRA approaching that of 12X normal motor current that decreases with time as the motor starts to rotate. Reduced voltage starters like wye-delta or series resistance type starters are the starters used for large motors with a heavy initial load. The typical starting value for wye-delta starters is about 30% normal running current; with starting torque of about 30% normal value. The formulas for computing Locked Rotor kVA and Locked Rotor Current are shown below:
Locked Rotor kVA = Motor hp X Maximum NEC code letter value;
Locked Rotor Current = Locked rotor kVA / √3 X kV
During a fault where the supply voltage falls to either zero (bolted fault), or to some other voltage level that is dictated by a fault resistance, the motor or generator continues to rotate because of inertia. Motors act as generators during this period, and try to give the energy back to the power supply. Fault contribution of large induction motor rated 250hp or higher figures significantly in short circuit calculations. For smaller motors of less than 50hp, however, the fault contribution are grouped together. Induction motors, compared to synchronous motors, do not have a field coil, but the rotor bars behave like the “amortisseur winding*” in a synchronous motor. It is for this reason that induction motors have no transient reactance X’d, but only subtraction reactance X”d. Wound rotor and squirrel cage induction motor contribute their fault currents in same manner. Induction motors have relatively brief short circuit contribution (about 1 to 3 cycles) and are expressed in symmetrical amperes. Their fault contribution is not considered when the protective device used is a time-overcurrent relay (Device #51) that operates at 3 or more cycles; but considered when combined with instantaneous overcurrent relay (Device #50). Protection specialists rarely use device #51 alone, but combine them with the instantaneous overcurrent relay #50 for feeder protection.
The subtransient reactance of an induction motor is about:
X”d = Full load amperes/Locked rotor amperes
*An amortisseur winding is a squirrel cage winding positioned near the surface of a synchronous motor and acts as a damper for sudden changes in motor speed due to varying load.
A synchronous motor has the same kind of reactance as a generator. During a fault, a generator continues to rotate due to inertia contributing fault current of varying waveforms that is directly proportional to its changing reactance before it is interrupted by its circuit breaker. Generator circuit breakers have “rated permissible tripping delay” of about 0.25secs (about 15cycles) per ANSI Standards C37.04-1999, C37.06-2000, and C37.09-1999. It is during this few cycles before the breaker trips that motor excitation and speed is considered constant. The varying reactance is expressed using appropriate equations against time that is treated as an independent
variable (see Rotating Machine Reactance above).
It is worth discussing the small portable inverter generator here since they operate differently from conventional portable electrical generators. The line frequency of conventional generators, no matter how small or how big, depends on the number of poles by the equation: f= ns x p/120. The 120 is fixed and derived from the electric relationship of electrical motors with fixed poles. As an example, a 4 pole conventional generator must rotate at 1800rpm in order to produce the frequency of 60hz; while a two pole generator will have to rotate at 3600rpm. Inverter generators defy this principle since the line frequency runs at fix frequency of 50hz or 60hz – independently – of its generator rotational speed. Thus, an inverter generator produces a fix 60hz power supply making it suitable for powering critical electronic equipment or devices. Inverter generators, too, have better frequency control stability during power surges, such as during motor starting, where the starting current is usually 2 to 3 times full load current. This factor maintains the output voltage at an almost constant value.
Inverter generators are not relatively new; they have been available for some time. The much higher cost
of these units compared to standard generators, probably, is the factor that discourages homeowners from buying it: if noise level, frequency of use, weight and dimensions of the unit is not a factor. The cost of an inverter generator is understandably higher, because of the advanced electronic circuitry and the
special high-tech magnets used. Inverter generators, however, have limited capacity as of now, and are available only from 800watts or lower to 7000watts.
How it Works
An Inverter generator is a 3-ph generator; the 3-ph current produced is converted to DC via full wave rectifier circuitries and then inverted back to 1-ph AC. Inverter generators are about 35% to 40% lighter than the standard portable generator, thus they are more compact. They are fuel efficient since they need not operate at fixed rpm of 3600 or 1800rpm in order to fix the line frequency at 50hz or 60hz, 1-ph, 120 or 230VAC output. This special characteristic makes the Inverter generator “super silent” at only 49dBA to 65dBA, which is equivalent to a sound level of normal conversation at 3ft.
One caveat, be sure that the line frequency (50hz or 60hz) of the generator, inverter type or standard, matches that of your country’s. Some seller specifies their inverter generator is 50hz/60hz, which I find odd since there is no selector switch to operate it at either. A 50hz/60hz genset is ok if there are nomotor loads.
Compute for the 50hz horsepower, 50hz speed and the 50hz voltage when a 3hp, 1800rpm, 240V, 60hz motor is operated on a 50hz mains.
Ratio = Voltage/Frequency = 240/60 = 4;
V50hz = R x F =4 X 50 = 200V. This is the voltage that 3hp, 60hz motor should be operated on a 50hz mains.
Or simply use ratio and proportion:
240/60 = V50hz/50hz,
V50hz = 200Volts
The 50hz speed will be: 1800 x 50hz/60hz = 1500 rpm
The 50hz hp will be: 3 x 50hz/60hz = 2.5hp
Take note that 50hz/60hz is for 60hz motors to be operated on a 50hz mains; for 50hz to 60hz, the ratio is reversed to 60hz/50hz. Failure to observe voltage/frequency ratio for induction motors will result to heat buildup and ultimately failure of the motor.
Universal electric motors are not affected by the line frequency 50hz or 60 hz, and they can even run on DC mains at the same voltage level. Typical universal motor applications are power tools, household appliances like vacuum cleaners and some kitchen tools and appliances. Universal motors are identified by the presence of carbon brushes and give off loud whirring sound when running.
– Electrical Motors/Generator Technology by: Johan Driesen, Ronnie Belmans
K.U.Leuven, Department Electrical Engineering (ESAT) Leuven Belgium
– Electric Power System Protection and Coordination by: Michael Anthony – University of Michigan General Electric Short circuit Calculations
– Permanent Magnet Technology – A solution to mini and small hydropower Generator circuit breakers have special requirements for generator protection – Eaton