Starting I in induction motor at synchronous speed
Starting I in induction motor at synchronous speed
(OP)
If I have an induction motor (500 kW) being driven at exactly synchronous speed, what sort of starting currents would I be likely to get if I start it DOL?
What happens if the speed slightly varies from synchronous?
I have been told by an engineer that at synchronous speed the starting transient won't be higher than the rated current of the machine and would be less than 1 sec.
I don't understand why the starting current is lowered just because the machine is synchronous. At time 0+ the stator windings would still basically appear as a short and as there is little remanence in the core the steel still needs to be magnetised.
What happens if the speed slightly varies from synchronous?
I have been told by an engineer that at synchronous speed the starting transient won't be higher than the rated current of the machine and would be less than 1 sec.
I don't understand why the starting current is lowered just because the machine is synchronous. At time 0+ the stator windings would still basically appear as a short and as there is little remanence in the core the steel still needs to be magnetised.





RE: Starting I in induction motor at synchronous speed
I don't have quantitative answers for you, but induction generators are typically started in this fashion and the resulting inrush is quite brief - similar to energizing a transformer, I suppose. Energizing an induction motor from a standstill is a little like energizing a transformer with the secondary shorted.
Hope that helps.
RE: Starting I in induction motor at synchronous speed
Current in a given phase will look like
I(t) = sqrt(2)* V/|Znormalstart|* [ A*exp(R/(L1+L2)*t) + cos(2Pi*f*t) ]
It rmeains like this until speed begins to increase close to sync, at which time sinusoidal current drops further.
It has a sinusoidal component and an exponential component.
|A| <=1, depending on phase of voltage at time of closing.
V/|Znormalstart| = LRC
At sync speed the R2/s term will be an open circuit. Now the magnetizing branch becomes more important. The impedance seen is Zsyncstart = j * (X1+Xm)
Where Xm>>X2 or R2
Now the current will look like:
I(t) = sqrt(2)* V/|Zsyncstart|* [ A*exp(R/(L1+L2)*t) + cos(2Pi*f*t) ]
The current has the same general initial shape with exponentially decaying dc plus sinusoidal. The magnitudes of both of these components are lower by ratio of (Zsyncstart/Znormalstart) >>1.
Since V/|Zsyncstart| ~ no-load current, the sinusoidal starting current has magnitude approximately same as no-load current (20-60% of FLA) which can increase by factor of up to 2 during the initial period before the dc decays.
RE: Starting I in induction motor at synchronous speed
For motors 6-pole speed or faster,it is safe to can assume no-load current <50% of FLA. Assume you have one of these faster motors.
Then the ac component of your current will be <50%FLA after start. When you add in the exponentially decaying component the peak can increase by factor of 2 to a peak current equivalent to that seen under full load.
RE: Starting I in induction motor at synchronous speed
Also, a clarification is needed regarding R2 and X2 whether they are on the rotor side or the stator side.
RE: Starting I in induction motor at synchronous speed
If I have an induction motor (500 kW) being driven at exactly synchronous speed, what sort of starting currents would I be likely to get if I start it DOL?
///Clarification is needed. Normally, the induction motors run at subsynchronous speeds since they experience a small slip (for all practical purposes, the slip is different from zero).\\\
What happens if the speed slightly varies from synchronous?
///It depends whether you mean synchronous machine or induction machine.\\\
I have been told by an engineer that at synchronous speed the starting transient won't be higher than the rated current of the machine and would be less than 1 sec.
///Some synchronous machines have a rotor cage that enables them to be started as a squirrel-cage induction motor.\\\
I don't understand why the starting current is lowered just because the machine is synchronous.
///It depends on the design of the synchronous motor starting arrangement, i.e. rotor cage.\\\
At time 0+ the stator windings would still basically appear as a short and as there is little remanence in the core the steel still needs to be magnetised.
///Not quite, it depends on the rotor cage resistance.\\\
RE: Starting I in induction motor at synchronous speed
RE: Starting I in induction motor at synchronous speed
RE: Starting I in induction motor at synchronous speed
I mentioned in the original post that the induction motor is being DRIVEN at synchronous speed. This is being achieved with a VSD and a smaller motor coupled to the large motors through shaft.
This is probably enough clarification to answer your other points.
What may help is a description of what is being done. This design is for a gearbox test rig. The induction motor drives a step down GB which then drives the step up GB being tested. The tested GB is connected to a synchronous generator.
The induction motor has a through shaft, one end coupled to the GB the other end to a smaller induction motor. The small motor has a VSD which enables us to rotate the system to synchronous speed and put the generator online. Once online we then intend to power the large drive motor to put 500 kW through the system. Hence the question on starting currents at synchronous speed.
Torque will be applied to the generator via the first step down gearbox which has a variable and controllable mechanical slip charecteristic.
RE: Starting I in induction motor at synchronous speed
""I don't understand why the starting current is lowered just because the machine is synchronous.""
There are big differences between the synchronous machine (e.g. motor) and induction machine (e.g. motor).
RE: Starting I in induction motor at synchronous speed
jb, other posters realize the context implies that the sentence reads as "...the induction machine is at synchronous speed."
RE: Starting I in induction motor at synchronous speed
Compare with the original posting and my remarks ///\\\:
"I don't understand why the starting current is lowered just because the machine is synchronous. At time 0+ the stator windings would still basically appear as a short
///There is not exactly short since many starting machines have Zm~0.17p.u. to about 0.28p.u.\\\
and as there is little remanence in the core the steel still needs to be magnetised."
RE: Starting I in induction motor at synchronous speed
Perhaps I should have written my sentence to you as "the context of the opening sentence implies that the sentence in question reads as...", but I'm more interested in engineering rather than semantical precision.
RE: Starting I in induction motor at synchronous speed
Clarifications.
I mentioned in the original post that the induction motor is being DRIVEN at synchronous speed. This is being achieved with a VSD and a smaller motor coupled to the large motors through shaft.
This is probably enough clarification to answer your other points.g
///Thank you for clarification.\\\
What may help is a description of what is being done. This design is for a earbox test rig. The induction motor drives a step down GB which then drives the step up GB being tested. The tested GB is connected to a synchronous generator.
///This was not so apparent in the original posting.\\\
The induction motor has a through shaft, one end coupled to the GB the other end to a smaller induction motor. The small motor has a VSD which enables us to rotate the system to synchronous speed and put the generator online.
///Please, clarify the relationship of the generator to the induction motor with the through shaft.\\\
Once online
///What is online? The generator started by the induction motor and rotating the generator at synchronous speed?\\\
we then intend to power the large drive motor
///What kind of motor is it synchronous or induction one?\\\
to put 500 kW through the system. Hence the question on starting currents at synchronous speed.
///Once, the synchronous machine is driven to be rotating at synchronous speed, one may connect power supply to its terminals. If the machine is synchronous, the field winding also has to produce sufficient field flux to have the rotating machine (motor) rotating on its own without the propulsion through the shaft.\\\
Torque will be applied to the generator via the first step down gearbox which has a variable and controllable mechanical slip characteristic.
///Essentially, there is a Motor-Generator Set, sometimes called M-G set that will be powering the 500kW to the system. Or am I missing anything?\\\
RE: Starting I in induction motor at synchronous speed
///Please, clarify the relationship of the generator to the induction motor with the through shaft.\\\
Assume that the ratio is 1:1, i.e., the induction motor is coupled to the synch genny.
///What is online? The generator started by the induction motor and rotating the generator at synchronous speed?\\\
In this case the generator would be put online (using auto sync). The 500kW induction motor would still be offline and driven by the small motor with VSD. So if I take the VSD offline, the sync genny will drive the induction genny at sync speed.
///What kind of motor is it synchronous or induction one?\\\
Both motors are induction and the generator is synchronous.
///Essentially, there is a Motor-Generator Set, sometimes called M-G set that will be powering the 500kW to the system. Or am I missing anything?\\\
No this is probably very similar to what you are talking about except that we will apply load through the gearbox. We are sure the test rig will work but I needed to supply starting current values to the network owner to determine if we were going to effect the voltage for other LV customers.
RE: Starting I in induction motor at synchronous speed
If I have an induction motor (500 kW) being driven at exactly synchronous speed, what sort of starting currents would I be likely to get if I start it DOL?
///The only current supplied to the motor will be the motor magnetizing branch mostly inductive current that is relatively small fraction of the motor rated current, e.g. 10% or so depending on the motor design. This magnetizing branch current is often neglected in the full load motor analysis.\\\
What happens if the speed slightly varies from synchronous?
///The supplied current to the motor terminals will be slightly higher than the current supplied to the motor driven at synchronous speed.\\\
I have been told by an engineer that at synchronous speed the starting transient won't be higher than the rated current of the machine and would be less than 1 sec.
///Yes, the starting current transient exponentially increases on the mostly inductive load, which the motor magnetizing branch is, and the motor magnetizing branch current will level off at the steady state magnetizing branch current level.\\\
I don't understand why the starting current is lowered just because the machine is synchronous.
///There is no load on the synchronously rotated motor to be powered over the motor terminals, only the motor magnetizing branch current.\\\
At time 0+ the stator windings would still basically appear as a short and as there is little remanence in the core the steel still needs to be magnetised.
///To the contrary. The inductor is starting with the essentially zero current. The capacitor is starting with the short circuit current. See the basic equations:
http://www.eng.hull.ac.uk/teaching/57022/13207_Handouts...
etc.\\\