Why to limit LRC in a Motor 6

[NickParker]

What is the reason to reduce the LRC of the motor? Isn't the LRA inherent characteristic of a general purpose induction motor.
Why do some specifications insist to reduce the LRA, for example, to limit within 3-3.5 times of the FLA (Full load Amperes).

Is it to reduce the voltage drop at the bus during starting or to reduce the mechanical stresses of the driver/driven equipment?

Will it affect short circuit rating of the switchgear?

If LRA to be reduced, will it affect other parameters like Power factor, Efficiency?

 

[jraef]

Both reasons are valid alone or together. I would say that most of the time people use them because they need to reduce the starting current, either because the serving utility REQUIRED some sort of reduced voltage starting method and Reduced Voltage Solid State (RVSS) starting is a good choice, or because they are using a generator that cannot handle the LRC. But reducing starting torque is also going to be beneficial in most cases an RVSS starter does so without a transition spike. One RVSS manufacturer used to use the slogan “Electrical solutions to mechanical problems” in their advertising.

No it has no effect on the short circuit rating of switchgear or anything else.

Power factor and efficiency are very poor, but so temporary that both are ignored since this only happens at starting.






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

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)7 Jul 22 04:55
" #1. What is the reason to reduce the LRC of the motor? Isn't the LRA inherent characteristic of a general purpose induction motor. #2. Why do some specifications insist to reduce the LRA, for example, to limit within 3-3.5 times of the FLA (Full load Amperes). #3. Is it to reduce the voltage drop at the bus during starting or to reduce the mechanical stresses of the driver/driven equipment? #4. Will it affect short circuit rating of the switchgear? #5. If LRA to be reduced, will it affect other parameters like Power factor, Efficiency? "
#1. Yes, the LRA inherent characteristic of a general purpose induction motor. You may had mistaken. The (reduce LR current) is during [Starting Not during running].
#2. This is applied to Starting only.
#3. Yes. During starting only.
#4. No. It affects the breaker and starter size.
#5. Since the reduction of LRA is for a short starting period only say not exceeding 30s, the affect other parameters like Power factor, Efficiency is of not consequence/immaterial/no issue.
Che Kuan Yau (Singapore)

 

[Gr8blu]

NickParker The main reason to specify reduced current during starting (e.g., locked rotor current, also known as "inrush" current) is because the upstream feed - be it cabling, transformer, or generation source - is insufficient to handle the "normal" motor inrush draw. Sometimes it is a requirement of the utility rather than the process facility, particularly for very large power ratings at the end of a long transmission line.

There are multiple ways to achieve a low inrush result, ranging from basic machine design (often entailing a physically larger machine) to some form of reduced voltage starter (either a solid state or autotransformer approach). Reducing the applied voltage reduces the inrush current linearly (i.e., 80% volts = 80% LRC) ... BUT affects torque in square law fashion (i.e., 80% volts = 64% start torque). For the latter reason, close examination of machine developed torque vs actual load torque is required to ensure there is enough difference to achieve acceleration at ALL voltage and speed points up to rated conditions.

Although the machine has a terrible power factor and efficiency during the start sequence, the duration of the start attempt is so much less than the normal operating profile that it is insignificant and can be ignored ... UNLESS the facility happens to be tied to a utility where "peak usage" determines the cost of electricity. In that case, having a "low inrush" design can offer fairly large savings over the long haul, as the utility bills on the "peak demand" within some time period - which is often quite a bit longer than a simple start sequence.

The short circuit capability of the protective gear is not affected by the inrush requirement.

Converting energy to motion for more than half a century

 

[NickParker]

Regarding the LRC not affecting the short circuit levels of the switchgear, If there are multiple motors at the plant and have high LRC (typical 6-7 times the FLA), Would the initial symmetrical short circuit current go higher?

 

[Gr8blu]

NickParker Nope. For one thing, the chances of starting them ALL SIMULTANEOUSLY is essentially nil - especially if they are large enough motors to produce a voltage dip on the plant distribution bus. Think about how many motors you actually have in your theoretical facility: if it is a process plant (of any type), you likely have several HUNDRED machines with that 6-7 pu LRC. Individually, the motors may not be rated all that high (sub-200 hp, for example) but the combined load is significant. And the switchgear isn't "upsized" to handle that, is it?

The possible short circuit current (generated by the motor in the event of a system fault where it can backfeed into the system) is a different current than the lock rotor value. You'd have to look at individual motor capabilities in that case.

Converting energy to motion for more than half a century

 

[bacon4life]

Yes, the LRC affects the short circuit contribution from a motor, which in turn will impact the short circuit levels of the switchgear. IEEE C37.010-1999 figure 16 shows example calculations for motors with various LRC.

From a practical perspective, I suspect that fault currents hardly ever influence a choice to purchase a low LRC motor.

 

[che12345]

Dear Mr. NickParker (Electrical)(OP)8 Jul 22 11:46
" ... Regarding the LRC not affecting the short circuit levels of the switchgear, If there are multiple motors at the plant and have high LRC (typical 6-7 times the FLA), Would the initial symmetrical short circuit current go higher?.."
1. Per IEC 60909 documents:
a) when calculating the maximum short-circuit current, ....motors are to be taken into consideration.... See 2. below.
b) when calculating the minimum short-circuit current, ....motors are to be disregarded ...
2. The momentary (few cycles) Motor contribution current level of a running motor, during short-circuit; is related to its LRC of the motor by design.
Che Kuan Yau (Singapore)

 

[edison123]

Deep bar, inverted T bar, L bar and double cage rotors reduce the LRA, which improves the PF but reduces the efficiency a bit (due to higher rotor resistance and hence higher rotor copper loss).

Muthu

http://www.edison.co.in

 

[LionelHutz]

I've only seen specs like that being used to force a soft-starting means for the motor. It was not meant to apply to the actual motor design. But, you can still vary the starting torque vs starting current to get a more desirable motor starting characteristic which is more suitable to soft-starter use. Just generally not nearly as low as 300% or 350%. Generally speaking, the current falls within a certain range to get the desired motor efficiency, that just seems to be the way it is. A lower locked rotor current does affect the short circuit contribution.

A soft-starter could eliminate the short circuit contribution if it was using SCRs with no bypass. Otherwise, it doesn't affect the short circuit contribution. A VFD eliminates the short circuit contribution. Other methods like wye-delta or autotransformer have no effect on short circuit contribution.

Now, in theory you could design the motor itself to limit the current to 300%, but it wouldn't be very efficient and might not meet that part of the specification. I've seen data from the synchronous motors at a pump station that were designed to start the matching pump by making their torque curves almost perfectly matched to the pump torque curve plus some accelerating torque but that was a specially designed large motor, not a general purpose induction motor.

 

[RRaghunath]

If voltage drop at the switchgear bus during motor starting is an issue, it is preferred to use dedicated transformer at motor terminals rather than specify LRC design for the motor.

 

[che12345]

@ RRaghunath (Electrical)12 Jul 22 06:10
"...If voltage drop at the switchgear bus during motor starting is an issue, it is preferred to use dedicated transformer at motor terminals rather than specify LRC design for the motor"
1. I don't understand what you mean by use dedicated transformer at motor terminals rather.... You did not stress that dedicated transformer shall by bypassed after starting. Do you mean auto-transformer starting ? See 2 below.
2. There are numerous ways to lower the starting current by e.g. auto-transformer starting (in US), Star-delta (in Europe), Soft-starter and VFD etc.
3. It is not practical to insist on all general squirrel-cage induction motors shall have LRC by design to 3-3.5 of FLA. It would be a special design, not readily available on the market.
4. If it is understood that the [LRC to be limited to 3-3.5 of FLA] is for starting only, then the case is closed. It is widely practised. Attention. it is not calling for special motor with low LRC.
Che Kuan Yau (Singapore)