Transformer sizing for motor starting 11

[gentech]

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.

 

[stevenal]

"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.

 

[artym]

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"

 

[jbartos]

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).

 

[Aquarius]

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

 

[RAMConsult]

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.

 

[artym]

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.&quot

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!)

 

[jbartos]

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.

 

[gentech]

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.

 

[gentech]

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.

 

[Marke]

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

 

[jbartos]

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).\\

 

[artym]

Gentech,

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