INDUCTION MOTOR

Uses

Single phase IM- vacuum cleaner, hand drill, hand blower,mixer and grinder

3phase squirrel cage IM – Main sw pump, l.o pump,jcw pump

Double cage IM- Turning gear motor for high starting torque

Construction​

FRAME closely grained alloy CI
Stator core made up of thin laminations of silicon steel to reduce hysterisis and Eddy current loss. Insulated 3phase windings are placed in the evenly spaced slots provided in the core which are spaced 120degrees apart. Insulated 3phase windings usually has 3 sets of could with a 30% overlap.

Rotor core is built of silicon steel stampings individually insulated to eliminate Eddy currents.  Has skewed slots to give smooth start. Copper or aluminum bar is placed in each slot. Conductor bars may be insulated to cut stray losses.
AIR GAP abt 2-3mm. smaller the air gap, smaller thestarting surge current, higher the starting torque and higher the power factor.

Principle

When a 3phase supply is given to the 3phase stator winding, a rotating mag field s set up at synchronous speed due to the switching sequence of RYB.

The rotating field passes thru the air gap and cuts the rotor conductors. Due to the relative speed between the rotating field and stationary rotor, emf is induced in rotor conductors. Since the rotor circuit is short circuited, current starts flowing in the rotor conductors.

This induced current inturn produces a magnetic field which interacts with the stator rotating magnetic field and thus a rotor torque is produced in the conductor bars.

(A/c to lenz law, the direction of the rotor current will tend to oppose the cause producing them, which is the relative speed btw the rotating field and stationary rotor conductors. Hence the rotor rotates in the same direction as that of stator field to reduce this relative speed.)

  Note that when the load on the induction motor is increased the slip increases slightly which results in Greater relative speed between the rotating flux and the rotor conductors. consequently rotor current is increased producing a higher torque to meet the increased load. Thus the IM are termed constant speed motors.

Slip

Slip s the difference in rotor speed and synchronous speed.

The rotor never catches up the synchronous speed. If it does, there will be no relative speed btw rotating field and rotor conductors, no induced water currents and therefore no torque to drive the Rotor. The friction and windage would immediately cause the rotor to slow down. In IM, the change in slip in Induction Motors from no load to full load is 0.1-3%.

Power factor

Power factor depends on the active component of the total current that is Icos¢. At no load, an induction motor draws a large magnetizing current and a small active component to meet the no load losses. Therefore the induction motor takes a high no load current lagging the applied voltage by a large angle. Hence the power factor of an induction motor on no load is low.

Where an IM is loaded, the active component increases while the magnetizing current remain same . Consequently the power factor of the motor is increased. Due to the larger magnetizing current regardless of load, the power factor rarely exceeds 0.9 lagging.

Types of rotors

1. squirrel cage rotor... Cu bar in each slot of laminated cylindrical core. Then bars are shorted by end rings.

 Disadvantage cannot add external resistance as rotor is shorted permanently. Low starting torque.

The resistance cannot be varied in squirrel cage rotor as it is possible in slip ring induction motor. The fixed resistance of the rotor of the squirrel cage induction motor is very low. At the starting moment, the induced voltage in the rotor has same frequency as the frequency of the supply. Hence the starting inductive reactance gets higher value at stand still condition. The frequency of the rotor current gets same frequency as the supply frequency at standstill. Now the case is that the rotor induced current in spite of having higher value lags the induced voltage at a large angle. So this causes poor starting torque at the stand still condition. This torque is only 1.5 times of the full load torque though the induced current is 5 to 7 times of the full load current. Hence, this squirrel cage single bar single cage rotor is not being able to apply against high load. We should go for deep bar double cage induction motor to get higher starting torque.

2. wound rotor has laminated core and provided with 3phase, double layer, distributed winding in the rotor slots and are wound for the same no of poles as that of stator, i.e, the 3windings are starred internally and the other are brought to connect to the 3 insulated phosphor bronzeslip rings and brushes. the brushes are externally connected to a 3phase star connected variable rheostat.

At starting resistance is given to increase the torque and as motor picks up speed resistance is reduced, slip rings are shorted. And wound rotor runs like a squirrel cage rotor with rotor winding end shorted on both ends.

Starting torque

The rotor circuit has low resistance and high high inductance. At the starting, rotor frequency is equal to the stator frequency, the rotor reactance is large compared with the rotor resistance therefore a rotor current lags the Rotor e.m.f by larger angle which means power factor is low and consequently starting Torque is also low. (T prop to cos¢)

Therefore by adding resistance, rotor power factor improves which results in improved starting torque. (cos¢=R/Z)

This of course also increases the rotor impedance due to which rotor current is decreased.     (I=V/Z)

Methods of starting squirrel cage motors

1. DOL closing coil activates the main switch. Main disadvantages are high starting current and low starting torque and single speed operation.This torque is only 1.5 times of the full load torque though the induced current is 5 to 7 times of the full load current when motor is connected in delta. When connected in star, at the beginning, the starting current is reduced to 3 times the FLC.

2. Star delta... End connections are brought to a starter box where changeover contacts in starter enable star delta switching.

Starting current and torque are only third of DOL. Can only be used with low starting loads. Built in interlocks or double throw switches prevent closing together.

3. Stator resistance starting By using deep bar double cage IM

4. Auto transformer starting

Consists of switches and 3phase transformer of single winding type in btw main supply and motor. During starting, motor recieves voltage through the tapped transformer giving reduced voltage to the stator. Reduced voltage gives reduced starting current and also reduced torque.

5. Soft starting method which uses thyristors. These solid state controlled switches controls the current flow upto 1000amps. Each AC half sine wave, from the mains, can be passed thru thyristor in full or in part dictated by the early or late triggering.

Methods of starting Wound rotor motors

Dol starting

Rotor resistance

Stator resistance

Auto transformer

Methods of improving starting torque

1. Wound rotor motor

the star connected rotor winding end connections are brought out to slip rings and brushes by which external 3phase star rheostat is connected. By increasing the resistance, the starting torque will be increased.

2. Double cage rotor

 The inner bars or deep bars are of large cross section, low resistance cu bars. also inductive reactance is high due to the high rate of cutting of magnetic field with the stationary rotor.

The outer bars have high resistance and low inductance with small cross section.

Due to the high inductive reactances of the inner bar during starting, the current flows in the outer cage. Thus more torque due to more resistance offered by it.

As speed increases and slip reduced, the rate of cutting drops due to which frequency of the rotor current and induced emf get gradually decreased consequently​ the rotor reactances decreases. Hence inductive reactances in the deep bar decreases. Thus the current faces less reactances and less resistance as a whole.

The advantages of squirrel-cage motors compared with the wound rotor type are that they:
(i) are cheaper and more robust
(ii) have slightly higher efficiency and power factor
(iii) are explosion-proof, since the risk of sparking is eliminated by the absence of slip rings and brushes.
Less starting torque as external resistance cannot be added.
The advantages of the wound rotor motor compared with the cage type are that they:
(i) have a much higher starting torque
(ii) have a much lower starting current
(iii) have a means of varying speed by use of external rotor resistance.

Comparison between single cage and double cage motors:

1.    A double cage rotor has low starting current and high starting torque. Therefore, it is more suitable for direct on line starting.

2.    Since effective rotor resistance of double cage motor is higher, there is larger rotor heating at the time of starting as compared to that of single cage rotor.

3.    The high resistance of the outer cage increases the resistance of double cage motor. So full load copper losses are increased & efficiency is decreased.

4.    The pull out torque of double cage motor is smaller than single cage motor.

5.    The cost of double cage motor is about 20-30 % more than that of single cage motor og same rating.

Speed variation

1. frequency,
2. cascade system

In this two number 3phase IM – main motor and auxiliary motor are connected to a common shaft and thus called cascadecontrol. 3phase supply is given to stator of main motor  while the aux motor is derived at a slip frequeny from the slip rings of main motor. 4 speeds are obtained – main alone (120F/P1), aux alone(120F/P2), cumulative cascading (120F/P1+P2)and differential cascading(120F/P1-P2)

3. adding resistance in rotor winding

4. Pole change arrangement

i)             MULTIPLE STATOR winding where stator is provided with two seperate windings which are wound for two different poles equal to the poles of stator winding. One winding is energized at a time

ii)            Consequent Pole CHANGING

In the slow speed version, the stator poles are in series but delta connected. For high speed, poles r in parallel and star connected.

In practice, stator winding is divided into only two coils to get two different speeds.

iii)                 POLE AMPLITUDE MODULATION if speed other than 2:1 is reqd.

MOTOR PROTECTION

1. under voltage protection 
2. Overload Relay
3. Dual element fuse or HRC fuse for short Circuit Protection
4. Resistance heaters inside the motor- space heaters
5. For large motors, Thermistors or bimetallic strips are provided to sense overload and give alarm and trip with a time delay.

UNKNOWN MOTOR

SINGLE PHASE IM

Stator is provided with a single phase winding which creates only alternating flux and not synchronously revolving. This cannot produce rotation. That is why it is not self starting

To make it self starting, it is temporarily connected into a two phase by providing a starting winding inaddition to main winding. The two windings are spaced 90degrees electrically apart and connected in parallel across the single phase supply. Phase difference btw the two windings is made large abt 90… these two currents produce revolving flux which makes the motor self starting.

Starting torque is prop to sin@, so larger the angle higher the starring torque.

HOT SPOT IN I.M.

When centre of the coil or winding is higher in temp than at the surface

What is Single Phasing

Loss of current thru one phase. Causes – broken fuse, loose contact in contactor, broken wire.

Will draw huge current and if not tripped… it keeps running till it burn the motor. Stopped one doesn’t start.

AIR GAP

If air gap more, high reluctance for magnetic path, flux strength reduces which inturn reduces induced emf and torque produced by the rotor. Cos of the reduced torque, starting surge current is higher. Power factor of the rotor current is low, which also hampers the running torque.

EFFECT

Uneven air gap can cause unbalanced magnetic pull which may result in vibration. Pulsation loss also reduces effective motor torque. 

Protection motor

INGRESS PROTECTION by two or three digit code

First digit says the protection against solid bodies and the second digit designates for protection against liquids

DRIP PROOF-IP22 ;   TOTALLY ENCLOSED IP54; DECK WATER TIGHT IP56

FIRST DIGIT

FIRST DIGIT

SECOND DIGIT

1 large sized foreign bodies

1 drops of condensed water

2 protection against solid bodies larger than 12mm

2 protection against fallings drops and not exceeding 15degrees angle 

3   small sized solid impurities, wires and tools of thickness greater than 2.5mm

3 ingress of rain

4   small sized solid impurities, wires and tools of thickness greater than 1mm

4 protection against water falling from all directions.

5 against dust of all sizes

5    jets of water

6   

6 protection against jet of water equivalent to heavy seas

Purpose of motor Varnish

Secondary layer of insulation to protect the coils and windings. To immobilize the insulated windings. It also increases dielectric strength of the windings which is more important in transformer than motor.

Steering gear protection

Each electro hydraulic s/g with two independent power supply shall be fed from two circuits from MSB and ESB

Short circuit protection and overload alarm 

Protection against excess current shall not be less than twice the FLC.

Main & aux s/g power units shall be arranged to restart once power restores

In the event of power failure, audio-visual alarm sounds 

Negative phase sequence for single phasing

Motor provided with 200% insulation

Overload and also short circuit trip

Power failure alarm

No voltage trip

Hydraulic oil low alarm

Windlass safeties

Overload relay

Short circuit protection

Electro magnetic brake

Delayed restart timer 

     

SARKAR            MOTOR

E/R crane motor construction

Principle of IM?

Slip? Why?

Material of winding in stator? 

Types of rotors. Construction of squirrel cage rotor? 

Terminal connections of IM with diagrams? – star delta connection

How to change the direction of rotation in 3 phase motor?  - change any two phase terminals

Starting of AC motor? – DOL, Star-Delta, Auto Transformer,  Soft starting.

Types of starting, DOL starter draw

Pole change motor?

Various method of AC motor speed change? 

Freq, cascade system, multiple stator, pole changing arrangement, rotor resistance.

Why SM is not self starting? 

Diff btw IM and SM?

SM construction is complicated and IM is simple

Economical below 300rpm for SM; economical above 600rpm for IM

SM is Not self starting torque.

S.M requires dc excitation

S.M - speed remains constant while for IM decreases with increase in load

Power factor can b unity, leading or lagging for S.M, and lagging for I.M 

Why IM cannot run as SM?

If IM runs at synchronous speed, then there will be no relative speed btw the rotating magnetic flux and the rotor, n thus no magnetic field will be induced and no current and no interation…. So rotor will stall

In an induction motor, the field winding in the rotor are shorted by end rings, so it cannot act as SM.

Dual core rotor advantages? Comparison between single cage and double cage motors:  high torque and low starting current

Which type of starter you prefer onboard and then cross questions?

New motor for sewage pump is installed what kind of connection to be done?          Delta

Cogging of motor? 

Crawling of a squirrel cage motor means motor running slower, 1/7th of the Synchronous speed. due to the fact that harmonics fluxes produced in the gap of the stator winding of odd harmonics like 3rd, 5th, 7th etc. These harmonics create additional torque fields in addition to the synchronous torque. The torque produced by these harmonics rotates in the forward or backward direction at Ns/3, Ns/5, Ns/7 speed respectively. Here we consider only 5th and 7th harmonics and rest are neglected. The torque produced by the 5th harmonic rotates in the backward direction. This torque produced by fifth harmonic which works as a braking action is small in quantity, so it can be neglected. Now the seventh harmonic produces a forward rotating torque at synchronous speed Ns/7.

Hence, the net forward torque is equal to the sum of the torque produced by 7th harmonic and fundamental torque. The torque produced by 7th harmonic reaches its maximum positive value just below 1/7 of Ns and at this point slip is high. At this stage motor does not reach up to its normal speed and continue to rotate at a speed which is much lower than its normal speed. This causes crawling of the motor at just below 1/7 synchronous speed and creates the racket. The other speed at which motor crawls is 1/13 of synchronous speed.

COGGING of the motor refusing to start at all because of low supply voltage, the slots of the stator get locked up with the rotor slots. When the slots of the rotor are equal in number with slots in the stator, they align themselves in such way that both face to each other and at this stage the reluctance of the magnetic path is minimum and motor refuse to start. Apart from this, there is one more reason for cogging. If the harmonic frequencies coincide with the slot frequency due to the harmonics present in the supply voltage then it causes torque modulation. As a result, of it cogging occurs. This characteristic is also known as magnetic teeth locking of the induction motor.

This problem can be easily solved by adopting several measures. These solutions are as follows: The number of slots in rotor should not be equal to the number of slots in stator & Skewing of the rotor slots,

Deck motor immersed in SW, action?   Pg71

Machine is disconnected from power sources and dismantled

Terminal  box removed and earthed toavoid shock due to accumulated voltage.

All salt deposits washed out with freshwater

If deposited with oily bilge water, wash out with Electro Cleaner.

Check polarization index test, R10minutes Cranking/R1min Cranking >1.5, then it indicates insulation low is due to moisture and can be improved by external lamp heating or low voltage “heat run” drying of the winding. Moisture should be escaped, by lifting the cover continuously or periodically

Hottest part of the machine shall not exceed 90°C, while heating

IR readings and temperatures taken regularly, until constant value reach about 1 mega ohms. 

Then spray the machine with insulation vanish

Assemble and out in service with low load, if possible

Main SW pumps maintenance, tell electrical safeties?

Safeties starter with overload current safety trip

Dual element fuse or HRC fuse for short circuit protection

Resistance heaters to avoid moisture

Short circuit winding how to find?

Continuity by multimeter – 0 and beeps gud; if 1/OL, then broke

Short circuit by checking the resistance in btw phases and also btw phase and earth. Best method is to use a megger. And to test resistance wen motor is hot as it has more leaking current wen it is hot. Insulation resistance is inversely proportional to the rise in temp

Current has to be measured in series with the circuit

Voltage measured across the load circuit

Insulation test and improvement?

Check resistance by megger. 

If more, then visual check for any damage in windings, use vacuum for dirt and heating for moisture (current injection method or lamp method)

MITRA MOTOR

SM used onboard, working principle? 

SM torque zero, why? 154

Methods of starting SM? PONY MOTOR and DAMPER WINDING

Cogging of IM, what effect of lengthy conductor due to skewing?

Rotor slots are lightly skewed to reduce the magnetic hum and help in reducing the locking tendency btw the rotor and stator teeth, therferoe giving a smooth start.

Principle diff btw soft starter and star delta starter

What are criteria for using star delta motors? Which pumps not to be star delta connected?     

motors larger than 7.5kw (10hp) rated at 220V are required to use reduced voltage starters like the Wye-delta starter. Motors less than 7.5kw are usually started direct-on-line, or the motor already connected in Delta permanently.

Synchronous motor donot use star delta. Double cage sq cage motor is best suitable for star delta. 

Which motor has the highest IR in ER and why? (SG motor)

Electric motor used in electric propulsion with reason – SYNCHRONOUS MOTOR

IP protection of deck motor and testing? 

IP 56. All screws have sealing washers and a labyrinth sealing gland at the shaft exit from end casing

Star delta starter change over time delay how much from star to delta?

In star Delta usually we consider that the load and motor torque are equal when the motor reach 75-85% of the speed. At this time you stop the star connection to go with delta connection, the time is around 30 to 50 milliseconds, to avoid short-circuit. But when the motor is connected in delta, at start you've got a temporary current peak due to back electromotive force. So to avoid this you have different option, by these, you can delay of 1 or 2 seconds the time between star and delta, but it depend on inertia of the load, another solution is to put a resistor in series with the delta connection for 3 seconds to limit the current peak, another is to put the resistor directly in series with the motor at star disconnection.

By seeing a motor , how can u tell which type of connection without opening the terminal box

When the three-phase motor is connected to the mains, the data on the rating plate must correspond with the main voltage and mains frequency. The connection is implemented via six screw terminals (standard version) in the terminal box of the motor and  distinguishes between two types of circuit, the star connection and the delta connection. 

Example : Let the mains supply voltage of 3 AC 400 V, 50 Hz.

And we have 2 Specifications for 2 different motor –

For Star connection:

•With its voltage specification of 230/400 V, this motor must

be connected to the three-phase supply (LN = 400 V) in a

star configuration.

•The voltage in a motor winding is designed for a maximum

of 230 V here.

•The three winding phases (W2-U2-V2) are configured in the

terminal box to the so-called star point. The voltage for the

individual phases to the star point is 230 V.

For Delta connection:

•With its voltage specification of 400/690 V, this motor must

be connected to the three-phase supply (LN = 400 V) in a

delta configuration.

•Every motor winding is configured for a maximum phase

voltage of 400 V and can be directly connected.

•For direct-on-line starting, the ends of the phase windings

are connected in the terminal box (U1-W2, V1-U2, W1-V2)

to the individual phases.

Below shows terminal box connection how it should be made.

HV motor and normal motor both having same voltage rating give constructional difference

Which machinery in ER takes maximum current?

High starting torque motor?

What types of motor used on decks?

SG telemotor, electric type. How getting reversed

What is the type and rating of motor used in LNG cargo tank

If one SG motor running in 3 phase, taking over current? Reasons – short circuit, single phasing, 

High winding temp alarm comes what ll be ur action and what are the reasons?

Stator purpose?

Name plate details of IM?       

Freq,power in kw, cos@, rpm, protection, voltage for star/delta.

BHOUMIK MOTOR

Starting system for SM and IM?

Can SM can be made IM, how?

IM details? 

SM details?

Where DOL and star delta used onboard and why? 

What are the common features btw transformers and IM?

BHATTA MOTOR

IM why starting current is high? 178

Cogging of IM? 134

Hysteresis losses everything?

SW entered DC motor, how to bring back to service condition?

What is no volt coil and why you need it?