Squirrel cage induction motor - Negative phase sequence current withstand 2

[RRaghunath]

Hi!
IEC 60034-1 Table-2 gives negative phase sequence capability (I2squared-t) ratings for Synchronous machines. However, I couldn't find reference for squirrel cage induction motors, neither in IEC or IEEE.
I am looking for typical I2squared-t values for HV motors.
I know the capability should be much better than that for Synchronous motors, don't know how much better!
Appreciate any guidance. Thanks.

 

[waross]

I am under the impression that the negative phase sequence capability of synchronous speed machines is only an issue when an overhauling load drives the motor over speed.
Then I believe that the capability is the same as the positive phase sequence capability.
eg. Rated Full Load Amps, or starts per hour.
Zero sequence capability due to unbalanced voltages and or phase angles is more of a concern with induction motors.
Zero sequence capability should be less than √(A1[sup]2[/sup]+A2[sup]2[/sup]+A3[sup]2[/sup])
There may be additional above normal rotor heating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

The only thing I know of is NEMA MG1 section 14.36 - Effects of Unbalances on the Performace of Polyphase Induction Motors which has a voltage unbalance vs derating curve.

 

[7anoter4]

On IEC 60034-1 chapter 7.2 Form and symmetry of voltages and currents 7.2.1 AC motors there is this paragraph:
Three-phase a.c. motors shall be suitable for operation on a three-phase voltage system having a negative-sequence component not exceeding 1 % of the positive-sequence component over a long period, or 1,5 % for a short period not exceeding a few minutes, and a zero-sequence component not exceeding 1 % of the positive-sequence component.

 

[RRaghunath]

As such, there seems to be no standard that gives I2squaredxt values and I2 continuous withstand capability values for Squirrel cage induction motors it seems!!
What are the typical values that can be expected?
Could some motor experts help please!
Thanks

 

[waross]

Memory fails.
have I said this before?
Negative sequence power of a motor.
That is when an overhauling load is driving the induction motor overspeed and the motor is acting as an induction generator.
For an induction motor rated at 1450 RPM, (50 RPM slip) full load negative sequence power will occur at 1550 RPM (50 RPM slip).
Generators are normally brought up to speed before they are connected to the grid so starts per hour may not be an issue with negative sequence power.
However if the machine is started as a motor before it is driven overspeed, then starts per hour are applicable.
The capability is basically the same as when acting as a motor.
The motor protection will also provide protection when generating.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Overhauling would still be positive sequence. Negative sequence would be the same imbalance as it would be anywhere else in the system and would cause additional heating of the rotor. I don’t know the answer to the original question though.

 

[Skogsgurra]

Yes, Bill. I think your memory fails. You need to review Steinmetz' texts about symmetrical components to get the definition of negative sequence right. It is NOT about phase sequence.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

There are Negative Sequence Current Ratings of generators because it is EXPECTED that a generator will not be connected to a perfectly balanced load, hence the 51Q protection scheme found in Generator Relay systems. That rating of the generator is all about the extra cooling techniques used in the generator construction.

There are no "Negative Sequence Current Ratings" of induction motors per se because they are MOTORS, not generators, so they are expected to deliver mechanical power, not produce electrical power.

But Negative Sequence Current caused by voltage imbalance does affect induction machines in that it heats up the rotor at a rate disproportionate to shaft load, meaning that given the same shaft load, a motor running from an unbalanced line voltage will consume more energy and heat the rotor faster than it would if the line voltage was balanced. How that affects the motor will therefore vary by how much mechanical load is connected to it and how much voltage imbalance exists; if the motor is fully loaded to its maximum mechanical kW, then even a small amount of NSC due to a small voltage imbalance will cause severe rotor heating, but if the shaft kW load is only a small fraction of the motor rating, the effect of a small level of NSC may be inconsequential, but the effect of NSC from a high voltage imbalance will be. So the way induction motors are "rated" for this is in the basic design considerations. So for example in NEMA MG-1, the "service conditions" for which a standard motor is designed allow for up to 1% voltage imbalance. More than that (referred to as "Unusual Service Conditions" in MG-1) and the user must consult the motor mfr for de-rating instructions.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[wroggent]

From IEEE 242:

10.3.3 Phase unbalance protection (Device 46, current) (Device 47, voltage)
10.3.3.1 Purpose

The purpose of phase unbalance protection is to prevent motor overheating damage. Motor
overheating occurs when the phase voltages are unbalanced. A small voltage unbalance
produces a large negative-sequence current flow in both synchronous and induction motors.
The per-unit negative-sequence impedance of either motor is approximately equal to the
reciprocal of the rated voltage per-unit locked-rotor current. When, for example, a motor has
a locked-rotor current equal to six times rated current, the motor has a negative-sequence
impedance of approximately 0.167 per unit (16.7%) on the motor rated input kilovoltampere
base. When voltages having a 0.05 per-unit negative-sequence component are applied to the
motor, negative-sequence currents of 0.30 per unit flow in the windings. Thus, a 5% voltage
unbalance produces a stator negative-sequence current equal to 30% of full-load current. This
situation can lead to a 40% to 50% increase in temperature rise.