Induction Motor

[mary20]

Hey guys stuck on another exam question..

Q. In the case of an induction motor,the magnitude and frequency of the induced rotor e.m.f are at a maximum at the instant the motor is turned on. Explain why this is the case and why both these values reduce as the motor runs up to speed??

Thanks

 

[malcolmsanford]

It's all to do with Mr Lenz's law and magnectic fluxes

 

[telectrix]

in my case it's inertia. can't be bothered to move.

 

[mary20]

mr lenz,s was a smart man but i cant understand him

 

[telectrix]

only lens i worry about is the one on my camera for when i photograph my supberb work and other people's bodge-ups. ( sorry about that but i am in need of a confidence boost today. sitting here waiting for the phone to ring )

 

[malcolmsanford]

I was hoping that some of my more learned friends would get me off the hook here, such as that arsenal supporter lurking around somewhere, never find a gooner when you want one ........so without the aid of a safety net or books here goes my tuppence worth .......and bear in mind when things like this came up in my learnings I was not impressed enough to listen lol

The sart up on a motor or a motor changing direction, I believe induces what is called a back emf and that back emf increases the resistance of the motor windings becasue it's producing an opposite voltage. So becasue of this when a motor starts up it uses the maximum load and as the speed increases it reduces that load, and that leads us on to why we start large loads in Star and then run in delta .................

So now disappear into the back ground awaiting the flax

 

[telectrix]

well, malc. that beats myh knowledge of green button = Go, red button = Stop, and if it goes backwards, change 2 of the pretty coloured wires over.

 

[RogertheBodger]

Someone mention Bodge-ups?

Malcolm that was good to say it's from memory, I've cheated and looked on the Net, even so it's turned my Brain to mush, Going for a lie down..............



"The Emf induced in the rotor depends upon the rotor frequency which further depends on relative speed between rotor and synchronous speed of rotating magnetic field. The relative speed at the time of starting or standstill is maximum and hence a large Emf in induced in rotor conductors or winding due to which very high current flows in rotor which is generally 5 to 7 times of the full load or running current of an indication motor.


The starting current, sometimes called inrush current will be 8-10 times higher than the motor's rated current because the motor is at rest.
When at rest, the motor appears to be a transformer, with it's secondary winding short circuited, i.e. the rotor winding or squirrel cage is a closed circuit. This results in a low impedance to the system voltage and the motor has a "locked rotor" current of typically 6 times full load current,
This starting value is independent of any load attached; however, the inertia of the motor and load has to be overcome. This is especially true when the motor is connected to a load, like a conveyor belt with product on it. The larger the inertia, the longer the motor takes to reach full speed.
As the motor accelerates, part of the starting current power overcomes this inertia and is converted to kinetic energy. The remaining power of the starting current heats the rotor, up to possibly 250 deg C for a "long" start (20 seconds!!).

V (voltage) stays pretty much constant, therefore I (current) must rise (but is limited to the locked rotor value for the specific motor) as P (power) is required to start the motor and to keep it running.
Some voltage drop will occur for most power systems during a motor start, thus with a constant locked rotor impedance, the starting current will reduce proportionally.
I start = V / 1.732 / Z (locked rotor) for a three phase motor. (Z is fixed)
When nearly full running speed is reached, the current drops rapidly to full load current or less, depending on the actual load attached".

 

[malcolmsanford]

Someone mention Bodge-ups?

Malcolm that was good to say it's from memory, I've cheated and looked on the Net, even so it's turned my Brain to mush, Going for a lie down..............



"The Emf induced in the rotor depends upon the rotor frequency which further depends on relative speed between rotor and synchronous speed of rotating magnetic field. The relative speed at the time of starting or standstill is maximum and hence a large Emf in induced in rotor conductors or winding due to which very high current flows in rotor which is generally 5 to 7 times of the full load or running current of an indication motor.


The starting current, sometimes called inrush current will be 8-10 times higher than the motor's rated current because the motor is at rest.
When at rest, the motor appears to be a transformer, with it's secondary winding short circuited, i.e. the rotor winding or squirrel cage is a closed circuit. This results in a low impedance to the system voltage and the motor has a "locked rotor" current of typically 6 times full load current,
This starting value is independent of any load attached; however, the inertia of the motor and load has to be overcome. This is especially true when the motor is connected to a load, like a conveyor belt with product on it. The larger the inertia, the longer the motor takes to reach full speed.
As the motor accelerates, part of the starting current power overcomes this inertia and is converted to kinetic energy. The remaining power of the starting current heats the rotor, up to possibly 250 deg C for a "long" start (20 seconds!!).

V (voltage) stays pretty much constant, therefore I (current) must rise (but is limited to the locked rotor value for the specific motor) as P (power) is required to start the motor and to keep it running.
Some voltage drop will occur for most power systems during a motor start, thus with a constant locked rotor impedance, the starting current will reduce proportionally.
I start = V / 1.732 / Z (locked rotor) for a three phase motor. (Z is fixed)
When nearly full running speed is reached, the current drops rapidly to full load current or less, depending on the actual load attached".

Where were you 30 minutes ago mate when I needed you ...................... Nice one much more professional than my ramblings

 

[MarkieSparkie]

Hi Mary, I hope my explanation is what your looking for:

The rate at which the rotor conductors are cut by the flux (generated by the rotating field) and hence their induced emf is directly proportional to the slip, with no induced emf at synchronous speed and maximum induced emf when the rotor is stationary.
The frequency of the rotor emf is also directly proportional to slip, since the rotor effectively slides with respect to the flux-wave, and the higher the relative speed, the more times in a second each rotor conductor is cut by a North and a South pole. At synchronous speed the rotor frequency is zero, while at standstill, the rotor frequency is equal to supply frequency.

 

[Knobhead]

The easiest way to look at a motor at stand still is as a transformer. The stator being the primary, the rotor squirrel cage being the secondary. As speed increases the “slip” increases lowering the induced voltage in the cage due to back EMF. Now bare in mind that there is no voltage as such because the “cage” is a dead short, it’s induced current you need to look at.
If you want to confuse your tutor’s look up double cage or deep slot rotors, they are used for high torque starting. 2 to 3 times FLT.