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Transformer sizing for motor starting
11

Transformer sizing for motor starting

Transformer sizing for motor starting

(OP)
What size transformer is necessary to full voltage start a 75 hp, 460v induction motor for a propeller fan? Must I use the motor nameplate locked rotor amps to size the transformer? The only load on this transformer is this motor.

RE: Transformer sizing for motor starting

2
Gentech,

Suggest you identify the minimum voltage you can accept at the motor terminals for a successful start, then calculate volt drop in cables at locked rotor current, then calculate the maximum acceptable transformer impedance which will allow you to start. Calculate based on low supply voltage to the transformer.

I think the transformer size will be determined by impedance rather than rating.

RE: Transformer sizing for motor starting

Socttuk is correct.

If you do your calc correctly, you may find that a transformer rated 120% to 150% of kVA (HP) of the motor would provide acceptable voltage dip during the starting.

15 to 20% voltage dip during starting should normally be acceptable. Of course lesser is better.

RE: Transformer sizing for motor starting

I somewhat disagree with scottyuk and rbulsars.  I’m not sure about all the theoretical impedance stuff mentioned, however, if you were to size all of your system components based on locked-rotor current, you would GROSSLY overestimate.  The following works from a practical standpoint:

For virtually all industrial applications that I am aware of, Locked-Rotor current is never used to size the equipment feeding the motor.  The full-voltage starting current of a motor is approximately 6 times rated current (or approximately the locked rotor current).  The only time that this may be a consideration is if the motor is “plugged” very frequently (start-stop-start-stop-start-stop-you-get-the-picture).  Your application appears to be “normal duty” (assuming this is a fan, the motor will be running continuously for long periods of time), thus locked rotor amps would NOT be necessary to size the transformer (or starters, fuses, overloads, cabling, conduit, etc.) to feed the motor.  The rated current on the nameplate is sufficient.  In reality, the RMS (root-mean-squared) heating produced by the (very short duration) starting current will not harm the transformer (or the motor for that matter).  By the way, concerning the fuses, you must use “dual-element, time-delay” fuses that are designed to “pass through” the starting current without melting.

For your specific application, a 3-phase, 460V, 75 hp motor requires approximately 95 amps (rated amps).  Thus the kVA required to meet the motor demand is (460)(95)(sqrt(3))/1000 = 75.7 kVA.  My recommendation would be to install a transformer rated a minimum of 100 kVA.

You indicate that this is the only motor to be fed.  One other consideration is that it is extremely more expensive to have to engineer, design, purchase, install and checkout a transformer twice.  If there is a good chance that this transformer will be used to supply future loads (like a second fan), I would take that into account.

RE: Transformer sizing for motor starting

SJK14,

I'm not suggesting to size the transformer or switchgear to be able to continuously supply locked rotor current. Normal design practise would be to use motor starting gear to suit the motor rating, unless there was something unusual about the load starting characteristics.

The permissible volt-drop through the transformer impedance depends on what starting torque the load requires: if the load requires a certain torque at standstill or during acceleration to full speed, then there will be a minimum voltage at which this torque can be developed. If the voltage available at the motor is too low to achieve the required torque because the volt-drop due to [motor current x xfmr impedance] is large, the motor will stall. The motor current remains high until it is almost at full speed, and consequenctly the volt-drop is large for much of the acceleration period. Thus the transformer, cable, and source impedances are important characteristics which should be considered when sizing the transformer.

In the specific case of a fan, the torque required is low and the motor will probably start with the transformer you suggest. But it is a quick and easy calculation to check that the motor will start, so isn't it better to be sure?




RE: Transformer sizing for motor starting

I usually check if the full load current of the motor is below transformer size serving it then I check the IEEE red book fig "Approximate Voltage Drop in a transformer Due to full load voltage starting of a motor" I will select a transformer from the curves that will allow only 85 percent voltage drop on the motor. In this case- it is a 100 kva transformer.


Discussion: The 85 percent voltage drop is used because the NEMA rating on coils of starters and relays is specified as a minimum of 85 percent of rated voltage before dropping out. coils will usually drop out at a lower voltage but that is a safe minimum.

Some motors will start as with as low as a 35 percent voltage dip (see REA Bulletin 160-3 Oct 1969 "Service to Induction Motors") and still work, but the holding coils will fail long before the motors ever see this this drop.


The only place I could find this 85 percent requirement for motors is ANSI C50.41-1982. It indicates that motors are required to be able to start as long as the voltage is not less than 85 percent of rated. But this ANSI standard is for motors in power generating plants  

RE: Transformer sizing for motor starting

2
(OP)
Thanks for the information eyeryone. I haven't sat down and went through the calculations yet, but plan to. The Red Book should prove helpful also. Thanks again!

RE: Transformer sizing for motor starting

Suggestion: When it comes to larger motor sizes, computer software may be used for modeling. The transformer upstream will be included in the motor starting model showing the motor starting transients. Then, the actual design follows the results of the motor starting from the computer simulation.
Normally, industry standards are the design basis for the computer software.

RE: Transformer sizing for motor starting

(OP)
What may further complicate things is that this motor powers the engine cooling fan in a 2000KW diesel generator. Under black start conditions the generator voltage will rise to normal, energize the transformer and then start the fan motor. My assumption is that the time it takes the motor starter to transfer, 25-50 mS, is enough time to ride out the transformer inrush and not melt the transformer primary fuses. I am considering placing a 0-5 second timer in the circuit. Any thoughts?

RE: Transformer sizing for motor starting

Suggestion to the previous posting: The posting is more related to the protective relay selective coordination study rather than to the transformer sizing.

RE: Transformer sizing for motor starting

jb,

The transformer is protected by fuses. The transformer load is a trivial part of the output of a 2000kW gen-set, so the generator protection relay settings should be irrelevant to this problem. I agree that it would be relevant if discussing the unit auxiliary transformer of a large multi-MW set, which would be covered by a complex and overlapping set of protection functions, but that isn't the case here.

Gentech,

There seems little reason why you can't include such a delay timer, as the gen-set will have little or no cooling demand for the first few seconds of operation. I personally don't think the timer will be necessary but if you make allowance for the timer, you can always remove it without loss of face, whereas retrofitting one to cure a problem that your customer will think you should have foreseen will do you no favours. Spend the extra $20 and fit one.



RE: Transformer sizing for motor starting

Suggestion to the previous posting marked ///\\\
ScottyUK (Electrical) Sep 14, 2003
jb,
The transformer is protected by fuses.
///It is good to know.\\\
 The transformer load is a trivial part of the output of a 2000kW gen-set,
///I did not address or comment on this issue.\\\
 so the generator protection relay settings should be irrelevant to this problem.
///I do not see anywhere that I mentioned it relevant to this problem.\\\
 I agree that it would be relevant if discussing the unit auxiliary transformer of a large multi-MW set, which would be covered by a complex and overlapping set of protection functions, but that isn't the case here.
///Agree.\\\
///I see that the main concern is in the protection of transformer and its sizing. Typically, the protection of the transformer (e.g. melted fuses) are covered under protective devices selection and coordination study. It appears the discussed time delay 0 - 5 second is being considered for the motor starter to have the motor starting delayed from 25 - 50 msec to (25 - 50 msec) + whatever will be set on the 0 - 5 second timer.\\\

RE: Transformer sizing for motor starting

gentech:

I think that transformer inrush is not your problem. As the generator comes to speed and developes the voltage, tranformer will be energized. If the voltage is building slowly ,  the inrush, if anything, will be less (much like reduced voltage motor starting).

It will be a good idea to have a few seconds (3-10 sec) of delay before the motor is started, so the full (or maximum)voltage is applied to the motor for starting.  I would think this radiator fan motor is very small compared to the gen size so DOL starting should be acceptable,even preferred.

RE: Transformer sizing for motor starting

See IEEE 399.

RE: Transformer sizing for motor starting

Suggestion: Try to simulate it by the computer software that covers transient and motor starting or have it done for you by someone.

RE: Transformer sizing for motor starting

Wow, what a lengthy thread!

I think that some of the respondents are not recognizing that a fan load is the easiest of all loads to start (NEARLY zero starting torque).

Contrary to some posts, I'd have to say that the transformer could be sized  based on the continous rating and that the transformer impedance is desirable to reduce the starting current of the motor.

Does anybody want to venture a guess on a worst-case fan, bearing (and belt?) starting torque; maybe 10 percent?

I wouldn't think that this would take a lot of calculation to be sure of a safe start.

RE: Transformer sizing for motor starting


Let remind ourselves the basics, since we went a bit off track and some suggestions are not correct:

(By the way fully agree with scottyUK and rbulsara)

The commonly held misconception about motor starting is that:
The start current of a motor is dependant of a motor driven load. (artym)

The fact is:

The start current of a motor is dependant on motor design, rotor speed and stator voltage from zero speed until full speed is reached.
The load will influence the time taken for the motor to reach full speed.
The current/speed curve of the motor is  independent of all external influences other then stator voltage

When a motor is atationary, the motor current is limited by the effective series impedance of the rotor and stator.

At very low speeds, the dominant impedance is the rotor.
At high speeds, the stator impedance can become influential.

Therefore:

The rotor determines the starting characteristics of the motor while the stator has an influence on the full speed characteristics.

Motors, including large motors, are design for DOL full voltage starting(except most Slip Rings as they have external rotor resistance).
It is the allowable voltage drop on the plant/power system that may require other starting methods. Hence, limitation of power source must always be considered when applying motors, especially large motors, all up stream imedances must be known and starting volts drop calculated.

The motor in the question is small in kW and the original question was not re. volt drop but the transformer size; the question was correctly answered in the first replay by scottyUK.

 

RE: Transformer sizing for motor starting

Aquarius is certainly correct about the starting current drawn by a motor being independent of the load.

ScottyUK did answer it in his first line of the very first post:
 
"Suggest you identify the minimum voltage you can accept at the motor terminals for a successful start, then calculate volt drop in cables at locked rotor current, then calculate the maximum acceptable transformer impedance which will allow you to start. Calculate based on low supply voltage to the transformer."

The next question is: "the minumum voltage you can accept at the motor terminals for a successful start".

What I hope to add is: that the minimum acceptable voltage at the motor terminals for reliable starting IS load-dependent and very low for a fan load.


Aquarius also correctly pointed out that the main consideration of the questioner was transformer sizing.

I'd have to differ with ScottyUK regarding the sizing being  likely driven by impedance rather than rating  (in this special case of a fan load).

RE: Transformer sizing for motor starting

"The start current of a motor is dependant of a motor driven load. (artym)"

I fail to see where he suggested this. He did state that source impedance affects starting current. And he suggested that the nature of the load determines whether staring will be successful.

I think artym makes a some good points.

RE: Transformer sizing for motor starting

I'm sorry, I neglected to credit ScottyUK for recognizing the  specific case  of the fan  re start torque requirements:

"In the specific case of a fan, the torque required is low and the motor will probably start with the transformer you suggest"

RE: Transformer sizing for motor starting

Suggestion: Reference:
IEEE Std 399-1997 IEEE Recommended Practice for Industrial and Commercial Power Systems Analysis, (Brown Book),
Chapter 9 Motor-Starting Studies beginning by page 231

A motor-starting study should be prepared if the motor horsepower exceeds approximately 30% of the supply transformer(s) base kVA rating, if no generators are present.

If generator is present, and no other sources are involved, a study should be considered whenever the motor horsepower exceeds 10-15% of the generator kVA rating, depending on the actual generator characteristics. The study should also recognize contingent condition(s), i.e. a loss of a source (if applicable).

RE: Transformer sizing for motor starting

Stevenal wrote : - I fail to see where he (artym) suggested this "The start current of a motor is dependant of a motor driven load."

I agree, my apologies artym, I've jumped to conclusions

RE: Transformer sizing for motor starting

Although this thread appears to have converged on a solution I have a few questions based upon a similar situation that I encountered many years ago:

1. Is the diesel generator isolated continuously or will it paralleled at a later time?
2. It is stated that the fan motor is the only load on the transformer; are there any other loads or sources on the generator bus?
3. If the answer to 2. is yes, are they sequenced or randomly applied?
4. What is the size and length of the total cable run from the generator, through the MCC, to the motor terminals?

The questions are relevant since:

1. isolated systems require different analyses than those connected to an infinite bus,
2. the speed-torque curves of the prime mover, motor and  fan may need to be considered along with the settings for the governor and voltage regulator,
3. it is possible for a fan to fail to accelerate past a subsynchronous speed (usually half of rated) if the cable is very long and/or the voltage is severely depressed.

This combination of conditions actually existed in a power plant forcing a redesign of the cabling to the motor.

RE: Transformer sizing for motor starting

Hey, I think Ramconsult has raised some really interesting issues:

I think that the paralleled/infinite bus/isolated question is interesting due to the effect that a heavy lagging 500KVA+ load from a DOL start 75 hp motor might have on an isolated 2MVA genset.


I especially agree with ramconsult's concern with the voltage regulator response to the  DOL starting of the fan motor(I have to differ w/ Rbulsara that the "radiator fan motor is very small compared to the gen size so DOL starting should be acceptable,even preferred.")

I also agree with RAMconsult that the  fan/motor accelleration to full speed deserves a close look. (although maybe the  voltage regulator will catch up by this time and send a nice voltage overshoot to help the accelleration!)

RE: Transformer sizing for motor starting

Suggestion: Referring to my previous posting and Reference, there are different types of genset regulator/exciter systems, namely,
1. High-speed regulator/exciter system (TE+TR=.25+.5 sec), which converges to the generator output voltage level fast, e.g. with one oscillatory wave only, and voltage dip smaller than in 2. below.
2. Low-speed regulator/exciter system (TE+TR=2.0+2.5 sec), which converges to the generator output voltage level slowly, e.g. with two or more oscillatory waves, and voltage dip lower than in 1. above.

The motor applications are better aligned with 1. above.

RE: Transformer sizing for motor starting

(OP)
Ramconsult,
1.    Is the diesel generator isolated continuously or will it paralleled at a later time
    
It will run as a single unit with an isolated load. It will parallel with other sets to an isolated load. It will parallel as a single unit and with other like sets to the grid. I must size the transformer assuming the genset alone (2MW) is the primary source.

2.    It is stated that the fan motor is the only load on the transformer; are there any other loads or sources on the generator bus?

The generated voltage is 4160V. The transformer is to step down, to 480V, only control power for the engine cooling fan. There will be 2000KW of other loads on the 4160V gen bus, but not at the time the motor starts. My intention is to supervise the motor starter through the generator controls.

3.    If the answer to 2. is yes, are they sequenced or randomly applied?

The loads will be randomly applied. The genset can be soft loaded in utility parallel or when paralleled with other gensets serving an isolated load.

4. What is the size and length of the total cable run from the generator, through the MCC, to the motor terminals?

I haven’t performed the motor feeder and branch circuit calculations yet. I plan on using the largest size conductor that is practical, which will exceed the NEC requirements and keep voltage drop as low as possible. Cable lengths: generator terminals to transformer primary=10ft, transformer secondary to motor terminals=20ft.

The questions are relevant since:

1. isolated systems require different analyses than those connected to an infinite bus,

I agree. I don’t fully understand the process of those analyses; however I do understand why they are necessary.

2. the speed-torque curves of the prime mover, motor and  fan may need to be considered along with the settings for the governor and voltage regulator,

I agree. Engine BHP@1800RPM=2990.0 Motor curve is matched to the fan curve.
The voltage regulator and governor are integrated within a single microprocessor based and networked control system made by the engine manufacturer.
 
3. it is possible for a fan to fail to accelerate past a subsynchronous speed (usually half of rated) if the cable is very long and/or the voltage is severely depressed.

My initial reason for asking this question was that I knew the genset, when isolated, was a relatively finite source of power. That fact made me question what size transformer would be needed to ensure adequate voltage at the motor terminals.

RE: Transformer sizing for motor starting

(OP)
artym,
   The KVA required by the fan motor is well below the manufacturers published motor starting KVA rating.

jbartos,
   My genset was production tested, and subsequently rated, to accept rated motor starting KVA with out dropping below .85p.u. voltage and returning to .97p.u. voltage within 1.5 seconds. The alternator is separately excited from a permanent magnet generator (PMG) pilot exciter and uses a current boost system. The current boost system uses CT/VT’s and supporting circuitry to sense high demand and assists the normal exciter “force” the field through transients.

RE: Transformer sizing for motor starting

An interesting collection of ideas here, but there are some definite points that need to be clarified.
1. When a motor is Direct On Line started, it will initially draw Locked rotor current. The start current will remain high until the motor is at almost full speed. Typically the start current will drop by around 10 - 20% up to about 80% speed. The Full Voltage start current is independant of load. The load determines the Full Voltage start time. Locked Rotor currents are generally in the order of 550% - 900%
2. Fans are quite a heavy start due to the inertia of the fan. On a speed drive, they are considered easy loads, but on starters, the inertia reduces th rate of acceleration and extends the start time.
3. Most supply transformers have an impedance in the order of 4 - 6 percent. That means at their rated load, the voltage drop will be in the order of 4 - 6%. If you apply a motor to a transformer that has the same rating, then the voltage drop will be perhaps 4 - 6 times that during start due to the high start current.
4. A lot of electrical control equipment is rated to operate down to 85% of rated voltage only. Dropping the voltage more than this can lead to welded contacts and burnt coils. This suggests that you need to size the transformer such that the voltage drop will be no more than 15%
5. In many cases, it is posible to use a reduced voltage starter to reduce the start current. In this situation, you could probably use a soft starter to reduce the start current from 6 - 8 x rated current, down to 3.5 - 4.5 x rated current. As the transformer is going to be selected on the voltage drop during start, the reduction in start current will reduce the voltage drop during start and allow a smaller transformer to be employed.

If we ignore the voltage drop of the generator, (The AVR should be able to correct this) and assume a transformer impedence of 5%, then I would suggest a maximum voltage drop of 15% during start. At a start current (DOL) of say 600%, that would mean that the transformer would need to be twice the KVA rating of the motor, i.e. about 150KVA
If we reduct the start current to say 400%, then the transformer size can be reduced to about 100KVA. If a start current of 300% was possible, then the transformer could be rated to the KVA rating of the motor.
Best regards,

Mark Empson
http://www.lmphotonics.com

RE: Transformer sizing for motor starting

Suggestion to gentech (Military) Sep 21, 2003 marked ///\\\
jbartos,
   My genset was production tested, and subsequently rated, to accept rated motor starting KVA
///i.e. 75HP\\\
 with out dropping below .85p.u. voltage and returning to .97p.u. voltage within 1.5 seconds.
///These parameters are probably for much higher kVA rated motor(s).\\\
 The alternator is separately excited from a permanent magnet generator (PMG) pilot exciter and uses a current boost system. The current boost system uses CT/VT’s and supporting circuitry to sense high demand and assists the normal exciter “force” the field through transients.
///The generator is designed to accept much higher motor load than the 75 HP motor.
Actually, my posting pertaining to generator was intended to reveal how many kVAs of motor load are the industry standard. Approximately, 2000kVA x (.1 to .15) = 200kVA to 300kVA for the DOL start. Evidently, this number will increase with soft or reduced voltage starter(s) or with VFD(s) aligned with the motor(s).\\\

 

RE: Transformer sizing for motor starting

Gentech,

Looks like you've got that start more than covered (that exciter is pretty slick)

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