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An induction motor cannot run at synchronous speed because it is the "slippage" of the rotor relative to the rotaing magnetic field that generates the current in the windings contained within the rotor. This induced (hence the term induction) current sets up the opposing magnetic fields that cause the rotor to rotate.

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14y ago
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9y ago

The torque produced by an induction motor depends on the current induced in the rotor. To have current induced in the rotor there must be a difference between the speed of the rotor and the speed of the rotating magnetic field. At synchronous speed the rotor current is zero and the torque must also be zero, so it never happens.

An induction motor runs at less than synchronous speed by an amount called the 'slip', defined by a formula:

s = (Ns - N) / Ns

Ns is the synchronous speed and N is the actual speed, so with a slip of 0.1 or 10% the actual speed is 10% less than the synchronous speed.

In a very simple model of an induction motor the current drawn and the output torque are both proportional to s.

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9y ago

An induction motor has to run at less than synchronous speed to allow currents to be induced in the rotor, without which there is no torque. The slip is defined as 1 - N/Ns where Ns is the synchronous speed and N is the actual speed. The torque is proportional to the slip, so no slip, no torque.

A typical induction motor runs at 2700 rpm on a 50 Hz system with a synchronous speed of 3000 rpm, so the slip is 1 - 2700/3000 or 10%.

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11y ago

Induction motors run on the principal of induction i.e the synchronous rotating magnetic field cuts the rotor windings inducing an emf in them. This emf causes current to flow in the rotor conductor which in turns produce a field which interact with that of the stator producing motion.

Now if the rotor was to run at synchronous speed,that would mean that the rotor would then be parallel to the rotating stator magnetic field. No rotor conductor would be cut,no emf induced in them,no current,no rotor magnetic field and hence no motion.

Synchronous motor run at synchronous speed because its rotor receive a DC current from an external source called an excitor.

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12y ago

The motor generates torque dependent upon the slip frequency (the lag between running speed and synchronous speed). Since the motor is not lossless, it takes some amount of slip to create the minimum amount of torque required to get the motor to turn.

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12y ago

The naturally-rotating magnetic field set up by the machine's field windings cuts the rotor bars of the rotor, inducing voltages into them which set up circulating currents. These currents link with the rotating field to produce the torque that drives the rotor. The rotating field rotates at synchronous speed. If the rotor turned at the same speed, then no voltages would be induced into the rotor bars, so no circulating current would flow, and no torque would be produced. So the rotor MUST rotate below synchronous speed.

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9y ago

The rotor must run at less than synchronous speed to allow low-frequency currents to flow in the rotor. At synchronous speed no currents are induced and no torque is produced.

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11y ago

if induction motor rotates at syn speed ...........there is no posibble to create relative motion between rotor and stator.....so net emf value is zero ....so motor cant run.......

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11y ago

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Q: Why induction motor does not runs at synchronous speed?
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If a three phase induction motor runs at a speed greater than synchronous speed?

When an induction motor is pushed over synchronous speed it will become a generator and will deliver power back to the utility.


Which motor rotate above synchronous speed?

An induction motor rotating at higher than synchronous speed would be generating power, thus would be a generator. No motor operating as a motor runs above synchronous speed.


Does a induction motor need excitation?

The induction motor is the special kind of motor which runs below and above the synchronous speed. which the synchronous motor runs nearly equal the synchronous speed. The operation of synchronous motor runs with dc field excited hence separate dc field current is given to the field circuit. where as the induction motor the field and main field is drawn from the same supply hence no excitation is required. But due to this separate starting mechanism has to be required in case of the single phase induction motor.


Why induction motor speed is always less then synchronous speed?

induction motor never runs at synchronous speed, if it does so there would not be any relative speed between stator flux and rotor and no emf will induce in rotor and we can not apply motor principle as we need current carrying conductor in magnetic field.


When the induction motor working as induction generator?

An induction motor connected to an existing ac supply normally runs at less than the synchronous speed by an amount described as the 'slip' which is proportional to the torque demanded at the shaft. If the same motor is instead coupled to an engine and driven at a speed higher than the synchronous speed, with negative slip, power then travels back into the ac supply and the motor has become a generator. The amount of power produced depends on the speed. However a normal induction motor will not behave as a stand-alone ac generator. For ac power generation synchronous generators are invariably used because it is necessary to set and maintain the frequency of the supply accurately.

Related questions

If a three phase induction motor runs at a speed greater than synchronous speed?

When an induction motor is pushed over synchronous speed it will become a generator and will deliver power back to the utility.


Which motor rotate above synchronous speed?

An induction motor rotating at higher than synchronous speed would be generating power, thus would be a generator. No motor operating as a motor runs above synchronous speed.


Does a induction motor need excitation?

The induction motor is the special kind of motor which runs below and above the synchronous speed. which the synchronous motor runs nearly equal the synchronous speed. The operation of synchronous motor runs with dc field excited hence separate dc field current is given to the field circuit. where as the induction motor the field and main field is drawn from the same supply hence no excitation is required. But due to this separate starting mechanism has to be required in case of the single phase induction motor.


Why induction motor speed is always less then synchronous speed?

induction motor never runs at synchronous speed, if it does so there would not be any relative speed between stator flux and rotor and no emf will induce in rotor and we can not apply motor principle as we need current carrying conductor in magnetic field.


What is slip of synchronous motor?

A synchronous motor runs at synchronous speed, so there is no slip, or zero slip.


Why single phase induction motor always runs below the synchronous speed?

For exactly the same reason as three-phase motors always run below synchronous speed. If they were to run at synchronous speed then no voltage and, therefore, no rotor current will be induced into the rotor to drive it.


What is the slip of synchronous motor?

A synchronous motor runs at synchronous speed, so there is no slip, or zero slip.


When the induction motor working as induction generator?

An induction motor connected to an existing ac supply normally runs at less than the synchronous speed by an amount described as the 'slip' which is proportional to the torque demanded at the shaft. If the same motor is instead coupled to an engine and driven at a speed higher than the synchronous speed, with negative slip, power then travels back into the ac supply and the motor has become a generator. The amount of power produced depends on the speed. However a normal induction motor will not behave as a stand-alone ac generator. For ac power generation synchronous generators are invariably used because it is necessary to set and maintain the frequency of the supply accurately.


Why induction motor runs slightly slower than supply frequency?

if the motor is at synchronous frequency there is no current induced in the shorted windings in the rotor so at no load its slightly less. if you drive the motor faster than synchronous speed it becomes a generator


What is the slip of a synchronous motor?

Synchronous motors have no slip. This is because the rotor runs at the same speed as the speed of the rotating magnetic flux of the stator.


Difference between synchronous and asynchronous motors?

1)Synchronous motors are those that run only at Synchronous speed i.e. constant speed. But Motor can run in various speeds. 2)Synchronous motors runs at same speed of magnetic filed. but if one consider case of induction motor the rotor rotates at different speed than revolving magnetic field. there is slip between field and rotor.


When induction motor is switched on the rotor frequency is?

The induction motor rotor has different frequency compared to it's stator. The rotor has slip ( s ) frequency. slip = ( Synchronous speed - rotor speed ) / Synchronous speed Synchronous speed = ( 120 * f ) / P where f = supply frequency to the stator. p = no of poles rotor speed is the actual speed the motor is running. Frequency in the rotor = slip * frequency in the stator At starting rotor speed is zero, so slip is one. Let us take the supply frequency is 50 Hz, then rotor frequency is also 50 Hz at starting. The motor attains speed and runs with its full speed at a point of time. Then let us take the slip is 0.04 then the rotor frequency will be 2 Hz.