Difference between synchronous and induction motor

Well a three phase cage rotor induction motor is formerly called squirrel cage motors and they work on this principle.

This motor has a fixed part called a "stator" which houses the three phase windings that produce the rotating magnetic field and moving part called a "cage-rotor" which revolves within the stator. OK r u with me??

The stator core is built up from steel laminations with slots to receive the windings, the laminations are punched from steel and are lightly insulated on one or both sides.

The rotor core is also built up from steel laminations, having longitudinal slots into which lightly insulated copper or aluminium conductors called?? u guessed it "rotor bars" which are fitted. The rotor bars are short circuited at each end by a heavy copper or alloy ring so forming a closed circuit.

The rotor bars are skewed at an angle to the end rings, (this type of design reduces i thing magnetic noise or "huming" and crawling tendencies during starting and run up.

I would draw you pictures if i knew how to on the computer and time is an issue aswell.

Advantages of the cage rotor induction motor and why the industrial use are:

• Due to the simple construction only the stator winding is connected to the supply and very cheap
• Due to the design and construction of the rotorthey are very strong and robust, thats why the use the in industrial drives
• normally self starting
• little maintance asa there is no rubbing contacts i.e brushes on the rotor
• Almost constant speed.

Hope this helps.

________________________________________________

Thank you and good night

Synchronous and induction machines both produce torque through the interaction of a rotor magnetic field and a stator magnetic field. The differences between the two types of machines arise because of the differences in the way the rotor magnetic field is generated.

Synchronous machines have a stationary (relative to the rotor) magnetic field on the rotor. This field can be generated either by permanent magnets, or by a field winding powered through slip rings. The interaction of this field with the rotating field on the stator creates torque and causes the motor to rotate. A synchronous motor always rotates at some multiple, determined by the number of poles, of the line frequency. If a synchronous motor loses lock with the line frequency, e.g., by torque overload, it will stall. A synchronous motor cannot start by itself on a fixed frequency AC source. It either needs to be fed a variable frequency source, or it needs to be brought up to speed by an auxiliary motor, sometimes called a pony motor, so that it can generate torque. Synchronous machines usually require some form of control to keep the rotor speed locked to the line frequency.

Induction machines have a rotating (relative to the rotor) magnetic field on the rotor. In a squirrel cage motor, this field is created because the motion of the stator field relative to the shorted rotor cage induces currents in the rotor. These currents generate the rotor field, which interacts with the stator field to create torque. A wound-rotor induction machine has rotor windings similar to a synchronous machine, in which currents are induced by the rotating stator field. Induction motors always rotate in some narrow speed range that is less than synchronous speed. This speed difference, which is necessary to generate the rotor field, is called the "slip." Low slip machines, which turn at very near synchronous speed, are more efficient than high slip machines, but have lower starting torque. Induction machines can produce some torque at zero speed, so they are capable of starting themselves if the load torque is low enough at zero speed. The torque-speed characteristic of induction machines at rated speed has a negative slope (as speed decreases, torque increases). As a result, induction machines do not require controls to operate - the feedback mechanism is built into the machine.

The winding resistance a wound-rotor induction machine can be varyied by connecting resistors to the rotor windings via the slip rings. This allows the torque-speed characteristics of the wound-rotor machine to be varied as needed (e.g., high resistance (= high slip) for high starting torque and then low resistance (= low slip) for high efficiency at rated speed).

The absence of a rotor winding makes squirrel cage induction machines significantly cheaper to manufacture than synchronous machines (or wound-rotor induction machines). Squirrel cage machines are extremely rugged because of the lack of a wound rotor (the cage is usually cast right into the rotor laminations), and the lack of slip rings makes them more suitable for explosive environments because there is no arcing mechanism. The circulating currents in an induction machine rotor lead to resistive losses that make induction machines less efficient that synchronous machines.

Summary of differences:

Synchronous machines:

• Wound-rotor or permanent magnet to generate the rotor magnetic field.
• Rotor magnetic field is stationary with respect to the rotor.
  
• Always turn at synchronous speed.
• Require some form of control to operate.
  
• More expensive to produce than squirrel cage induction machines.
• Not self-starting.
• More efficient than induction machines.

Induction Machines:

• Wound-rotor or squirrel cage to generate the rotor magnetic field.
• Rotor magnetic field rotates with respect to the rotor.
  
• Always turn at less than synchronous speed.
• Do not require control.
  
• Much cheaper to produce (true for squirrel cage machines).
• Self-starting.
• Less efficient than synchronous machines.
• More suitable for explosive environments.
• No maintenance (for squirrel cage machines).

Their robustness, low cost, and freedom from control makes induction machines preferable for most industrial applications.

Another note: Induction motors have tremendous starting torque (and, on the down side, tremendous starting current).

Hi Guest,

Induction motor starting torque depends on the rotor design. Low slip machines, designed for high operating-point efficiency, can have very low starting torque. I agree however, inrush current is high because at standstill there's no back-EMF.

thanx man!

Is any of your homework ever done on your own? This is your 4th thread asking questions that are clearly out of a homework assignment.

http://cr4.globalspec.com/thread/7891
http://cr4.globalspec.com/thread/7888
http://cr4.globalspec.com/thread/7848

Yes he/she should do their own homework. On the flip side though, I learned some new things about the motors in question so maybe it's good for some of us and not just the questioner.

I learned from this post as well.

Both machines run on AC at a speed depending on the number of poles and mains frequency.

Both motors have coils (poles) fixed to the frame - the 'stator' - which is connected to the mains - say 3-phase (because it is simpler) that causes a rotating magnetic field that spins at exactly mains frequency.

The induction motor has a rotating armature - the rotor - made up of copper bars laid into slots. If you take away the steel you are left with a framework that looks like a squirrel cage - hence the name.

The rotating field induces a voltage in the conductors of the squirrel that in turn causes a current to flow that creates a magnetic field in the rotor - that in turn is attracted to the stator field - and it tries to catch it up. But it can't - because if it did there would be no induction, no induced voltage, no current, and no magnetic field - so the rotor slows down - by an amount known as 'slip' - until the magnetic field is strong enough again to overcome the torque of the load attached to the shaft - and then it speeds up. Everything being equal, it settles down to a constant speed - but because of the essential slip it always runs slower than the main frequency. and because it is an induction motor, it has a lagging power factor.

It is easy to see that the slip increases as the load increases, but there comes a point when the magnetic field 'saturates' so that the output torque does not increase so the motor slows down. This has a knock-on effect that reduces the back-emf and causes a higher current to flow in the stator windings - which eventually can become high enough to burn the coils out.

Now take a modified version of the rotor and inject a (excitation) current into the windings (via slip rings). This creates it's own field and it locks on to the mains frequency and runs in synchronisation with it. The excitation current is adjusted to give the required running characteristics. Where, for other (complex) reasons, an over-excited field will cause a leading power factor - which could be useful for compensating for lagging power factors elsewhere in the electrical system.

Dear Mr. mohd12meeran,

You should not feel up-set for my answer and take it sportively.

ARE YOU STUDYING IN THE COLLEGE.? IF so, your Professor will explain.

Many members have given answers. I would like to add to this answers that

1.SYNCHRONOUS MOTOR WILL IMPROVE THE POWER FACTOR on its own.

2.Induction Motor will have a poor Power Factor at low loads, and improve at Higher higher Load and at 80 to 85% of Load P.F will be good. To improve P.F you have to add capacitor bank.

3.The loci of current of Induction Motor will follow the Circle and known as CIRCLE DIAGRAM.

THANKS.

DHAYANANDHAN.S