Electrical Motor

Electrical Motor

Introductionan electro-mechanical device that converts electrical energy to mechanical energy

In simple words, a device that produces rotational force

The very basic principal of functioning of an electrical motor lies on the fact that force is experienced in the direction perpendicular to magnetic field and the current, when field and current are made to interact with each other.

DC MotorStator The static part that houses the field windings and receives the supplyRotor The rotating part that brings about the mechanical rotationsField Winding of DC Motor - basically form an electromagnet, that produces field flux within which the rotor armature of the dc motor rotates, and results in the effective flux cutting.Armature Winding Commutator of DC Motor -cylindrical structure made up of copper segments stacked together, but insulated from each other by mica - main function to commute or relay the supply current from the mains to the armature winding housed over a rotating structure through the brushes of dc motor.Brushes - made with carbon or graphite structures, making sliding contact over the rotating commutator -are used to relay the current from external circuit to the rotating commutator form where it flows into the armature windingThe commutator and brush unit of the dc motor is concerned with transmitting the power from the static electrical circuit to the mechanically rotating region or the rotor.

Operating Principle of DC MotorFlemings left hand rule:if we extend the index finger, middle finger and thumb of our left hand in such a way that the current carrying conductor is placed in a magnetic field (represented by the index finger) is perpendicular to the direction of current (represented by the middle finger), then the conductor experiences a force in the direction (represented by the thumb) mutually perpendicular to both the direction of field and the current in the conductor.

the supply voltage E and current I is given to the electrical port or the input port and we derive the mechanical output i.e. torque T and speed from the mechanical port or output port.The input and output port variables of the direct current motor are related by the parameter K.

The direct current motor is represented by the circle in the center, on which is mounted the brushes, where we connect the external terminals, from where supply voltage is given. On the mechanical terminal we have a shaft coming out of the Motor, and connected to the armature, and the armature-shaft is coupled to the mechanical load. On the supply terminals we represent the armature resistance Ra in series. Now, let the input voltage E, is applied across the brushes- Electric current which flows through the rotor armature via brushes, in presence of the magnetic field, produces a torque Tg - Due to this torque Tg the dc motor armature rotates Back emf: As the armature conductors are carrying currents and the armature rotates inside the stator magnetic field, it also produces an emf Eb in the manner very similar to that of a generator.The generated Emf Eb is directed opposite to the supplied voltageEb is proportional to speed N. That is whenever a direct current motor rotates, it results in the generation of back Emf. Now lets represent the rotor speed by in rad/sec. So Eb is proportional to .when the speed of the motor is reduced by the application of load, Eb decreases. Thus the voltage difference between supply voltage and back emf increases that means E Eb increases. Due to this increased voltage difference, armature current will increase and therefore torque and hence speed increases.

Synchronous MotorWhen a 3 phase electric conductors are placed in a certain geometrical positions (In certain angle from one another) there is an electrical field generate - the rotating magnetic field rotates at a certain speed called synchronous speed.

if an electromagnet is present in this rotating magnetic field, the electromagnet is magnetically locked with this rotating magnetic field and rotates with same speed of rotating field. Synchronous motors is called so because the speed of the rotor of this motor is same as the rotating magnetic field. It is basically a fixed speed motor because it has only one speed, which is synchronous speed and therefore no intermediate speed is there or in other words its in synchronism with the supply frequencyThis motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.is a doubly excited machine it is not self startingTo overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed. After some time magnetic locking occurs and the synchronous motor rotates in synchronism with the frequency.

Application of Synchronous MotorSynchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive.Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc.

Three Phase Induction MotorConstructionStator: is made up of numbers of slots to construct a 3 phase winding circuit which is connected to 3 phase AC source. The three phase winding are arranged in such a manner in the slots that they produce a rotating magnetic field after AC is given to them.Rotor: consists of cylindrical laminated core with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars which fits in each slots & they are short circuited by the end rings. The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed because this arrangement reduces magnetic humming noise & can avoid stalling of motorProduction of Rotating Magnetic FieldThe stator of the motor consists of overlapping winding offset by an electrical angle of 120. When the primary winding or the stator is connected to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed. According to Faradays law an emf induced in any circuit is due to the rate of change of magnetic flux linkage through the circuit. As the rotor winding in an induction motor are either closed through an external resistance or directly shorted by end ring, and cut the stator rotating magnetic field, an emf is induced in the rotor copper bar and due to this emf a current flows through the rotor conductor.This motor is also called as asynchronous motor because it runs at a speed less than synchronous speed. the relative velocity between the rotating flux and static rotor conductor is the cause of current generation; hence as per Lenz's law the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity.Three Phase Induction Motor is Self StartingIn three phase system, there are three single phase line with 120 phase difference. So the rotating magnetic field is having the same phase difference which will make the rotor to move. If we consider three phases a, b and c, when phase a is magnetized, the rotor will move towards the phase a winding, in the next moment phase b will get magnetized and it will attract the rotor and than phase c. So the rotor will continue to rotate.The speed of induction motor is given by,

Where N is the speed of rotor of induction motor, Ns is the synchronous speed, S is the slip. Types Squirrel cage induction motorSlip ring induction motor

Two important attributes relating to efficiency of electricity use by A.C. Induction motors areefficiency (), defined as the ratio of the mechanical energy delivered at the rotating shaft to the electrical energy input at its terminals, power factor (PF)a high value for and a PF close to unity are desired for efficient overall operation in a plant.Squirrel cage motors are normally more efficient than slip-ring motorsHigher-speed motors are normally more efficient than lower-speed motors.Efficiency is also a function of motor temperature. Totally-enclosed, fan-cooled (TEFC) motors are more efficient than screen protected, drip-proof (SPDP) motors.With most equipment, motor efficiency increases with the rated capacity.Motor efficiencyThe efficiency of a motor is determined by intrinsic losses that can be reduced only by changes in motor design. Intrinsic losses are of two types: fixed losses - independent of motor load, and variable losses - dependent on load.Fixed losses consist of magnetic core losses and friction and windage losses. Magnetic core losses (sometimes called iron losses) consist of eddy current and hysteresis losses in the stator vary with the core material and geometry and with input voltage.Friction and windage losses are caused by friction in the bearings of the motor and aerodynamic losses associated with the ventilation fan and other rotating parts.Variable losses consist of resistance losses in the stator and in the rotor and miscellaneous stray losses. Resistance to current flow in the stator and rotor result in heat generation that is proportional to the resistance of the material and the square of the current (I2R). Stray losses arise from a variety of sources and are difficult to either measure directly or to calculate, but are generally proportional to the square of the rotor current.

Let, Pin = electrical power supplied to the stator of three phase induction motor,VL = line voltage supplied to the stator of three phase induction motor,IL = line current,Cos = power factor of the three phase induction motor.Electrical power input to the stator, Pin = 3VLILcosA part of this power input is used to supply stator losses which are stator iron loss and stator copper loss. The remaining power i.e ( input electrical power stator losses ) are supplied to rotor as rotor input.So, rotor input P2 = Pin stator losses (stator copper loss and stator iron loss).

Now, the rotor has to convert this rotor input into mechanical energy but this complete input cannot be converted into mechanical output as it has to supply rotor losses. As explained earlier the rotor losses are of two types rotor iron loss and rotor copper loss. Since the iron loss depends upon the rotor frequency, which is very small when the rotor rotates, so it is usually neglected. So, the rotor has only rotor copper loss. Therefore the rotor input has to supply these rotor copper losses. After supplying the rotor copper losses, the remaining part of Rotor input, P2 is converted into mechanical power, Pm.

Let Pc be the rotor copper loss,I2 be the rotor current under running condition,R2 is the rotor resistance,Pm is the gross mechanical power developed.Pc = 3I22R2Pm = P2 Pc

Now this mechanical power developed is given to the load by the shaft but there occur some mechanical losses like friction and windage losses. So, the gross mechanical power developed has to supplied these losses. Therefore the net output power developed at the shaft, which is finally given to the load is Pout.Pout = Pm Mechanical losses (friction and windage losses).

Efficiency is defined as the ratio of the output to that of input,

Rotor efficiency of the three phase induction motor

= gross mechanical power developed / rotor input

Three phase induction motor efficiency,

Field Tests for Determining EfficiencyNo Load Test:The motor is run at rated voltage and frequency without any shaft load. Input power, current, frequency and voltage are noted.From the input power, stator I2R losses under no load are subtracted to give the sum of Friction and Windage (F&W) and core losses.To separate core and F & W losses, test is repeated at variable voltages.F&W and core losses = No load power (watts) - (No load current)2 Stator resistanceStator and Rotor I2R Losses:The resistance must be corrected to the operating temperature.For modern motors, the operating temperature is likely to be in the range of 100C to 120C and necessary correction should be made.The correction factor is

The rotor resistance can be determined from locked rotor test at reduced frequency, but rotor I2R losses are measured from measurement of rotor slip.

Rotor I2R losses = Slip (Stator Input Stator I2R Losses Core Loss)

Accurate measurement of slip is possible by stroboscope or non-contact type tachometer.Slip also must be corrected to operating temperature.

Stray Load Losses:These losses are difficult to measure with any accuracy. IEEE Standard 112 gives a complicated method, which is rarely used on shop floor.IS and IEC standards take a fixed value as 0.5 % of input. The actual value of stray losses is likely to be more. IEEE 112 specifies values from 0.9 % to 1.8 %

Estimation of efficiency in the field can be done as follows:Measure stator resistance and correct to operating temperature.From rated current value , I2R losses are calculated.From rated speed and output, rotor I2R losses are calculatedFrom no load test, core and F & W losses are determined for stray loss