# Assistance required for induction motors

#### Landon

Joined Oct 3, 2007
9
Hi guys,

I would like to know if there's someone out there who can enlighten me on some issue regarding induction motor. The induction motor is acting like a generator as a dc motor drives it. This is obtain by varying the Nr > Ns. However, Nr can only be increase to a certain limit for eg, once the current hit the rated current. To increase the range of Nr, we can add some resistance to the rotor terminals. by a 3 phase variable resistor. so in theory wise, where do i add these resistance to? Wrt the exact equivalent circuit of the induction motor? If this generator (induction motor) is connected to a grid, there is bound to be losses. Can someone tell me what are the losses apart from stator loss, core loss & copper loss?

#### SgtWookie

Joined Jul 17, 2007
22,230
Why don't you start out by stating your issue that you need help with?

Like, "I have this induction motor that won't start; made by the blah blah corp, rated at nn volts, yy hp, zz amps, driving a _something_

Otherwise, you're just delaying possible responses.

#### recca02

Joined Apr 2, 2007
1,212
if the induction motor is having slip ring rotor then it might be possible to add external resistance to rotor circuit ,if it is squirrel cage then tough luck if u are not going to tamper with its design.
how come u are connecting such a generator to grid. grids are at very high voltages and the frequency is the determining factor. i havent read in details about induction generators but i think the frequency might become a problem
and it wont work in synchronism with other generators.
anyways why not use a synch generator they are much better and BTW why are u using dc motor as a prime mover?

#### mikey

Joined Oct 11, 2007
2
Hi recca02, i was wondering if i was to use a rectifier to convert the ac output to dc then use a inverter to convert back to ac, why will the frequency be a problem?

#### recca02

Joined Apr 2, 2007
1,212
one problem i can see with that is that u can not have a control on the grid frequency,
it is the load on the grid that will decide its frequency and hence the grid frequency varies a little from the specifications even after having efficient governing systems.
and since you are using perhaps IGBT's for the frequency control in inverter your frequency will be independent o the grid frequency.
i think however there may be a way to synchronise this frequency using some feedback
what i dont get is why use induction generator?