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The effect of transformers
5

The effect of transformers

The effect of transformers

(OP)
hi,

I wanted to know please in comparison, when comparing 2 electrical fixed speed motors, of same power & rpm , but different voltages.

in other words, is it more efficient to have a motor rated at 3300 volts via transformer from the 11000 volts feeder, or to have a 11kv motor directly ?

i do understand that lower voltage means higher current, but are there loses in the case of lower voltage via transformer ? worse efficiency? lower power factor ? how to count my loses if any to know if its worth it to change the motor to 11kv in next replacement opportunity ?

Thanks & Regards

RE: The effect of transformers

2
Efficiency varies. You must refer to the motor data sheet.
If you are using a transformer, you must also factor the transformer losses.
Tariffs; If you are worried about efficiency, I hope that you are also worried about cost of energy.
Your total energy costs may not be the same at 3300 Volts as at 11,000 Volts.
A very rough rule of thumb is that when the motor HP exceeds the motor rated voltage a larger motor may be more cost effective.
Is your motor larger than 3300 HP?

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: The effect of transformers

Bill,
"A very rough rule of thumb is that when the motor HP exceeds the motor rated voltage a larger motor may be more cost effective." I can relate to this though heard it for the first time. A simple rule of thumb. Thanks for sharing.
Somacast,
I don't think adding a transformer because a 3.3kV motor is slightly more efficient than a 11kV one is going to be beneficial. You can check specifics with manufacturer data to confirm.
In my experience, transformer is added when voltage drop at the bus is unacceptably high during motor starting. Here, transformer acts like a reactor to limit the upstream voltage drop while motor starting.
The other reason, I have seen transformer being added is when there is no HV motor voltage bus (say only 33kV is available) and only one HV motor is required in the plant. Under such circumstances, it is economical to add a transformer at motor primary (to step down the grid voltage to motor level)rather than creating an all new HV bus (motor voltage).

RE: The effect of transformers

(OP)
thank you very much for your replies.

the 3.3kv motor is just less than 1000kw , and 11kv is about 1000 kw.

the feeder is 11kv, anf the transformer to step down to 3.3kv (motor voltage).

so is there a numerical way to calculate if the cost of energy is higher for which motor ? 11kv without transformer , or 3.3kv with transformer..

RE: The effect of transformers

If you do not have 3.3kV bus available in the plant, you can go for 11kV motors and connect to the available 11kV bus.
As per IEC 60034-1, minimum kW rating for 11kV system is 800kW. The preferred ratings listed are based on design and manufacturing considerations, IEC says.
The lowest rated motor I have come across in 11kV system is 330kW which is non-standard going by the preferred rating table of IEC.

RE: The effect of transformers

(OP)
Thank you very much for the information

now in my case 3.3kv bus is there, however it comes through a transformer from the 11kv

so i was thinking if in future replacement of motor if buying an 11kv to avoid the the transformer, will I have any benefits in terms of power savings, better power factor, etc ?

thnx

RE: The effect of transformers

Remember that you will also have to buy new switch-gear, including PTs and CTs. The protection relay may be usable, if you want to install an old relay into new switch-gear.
Installing the new witch-gear may be expensive.
Are your electricians qualified to work safely on 11 kV?
Do your people now service the 11 kV system or is that the responsibility of the supply utility?
Servicing an 11 kV motor may be more hazardous than simply living with an 11 kV feeder.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: The effect of transformers

While the 11 kV motor may be more efficient as it will avoid the transformer losses in addition to the motor losses, I doubt that the energy saving will pay back the cost of conversion in a reasonable time.
But, I may be wrong.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

RE: The effect of transformers

Voltage does not play a role in a motor's efficiency. A 3.3 KV motor can be as efficient as a 11 KV one.

Since the motor (and generator) windings are not designed to withstand line surges (lightning strikes, traveling waves etc.), it is recommended to have a transformer as a buffer. Typical transformer efficiencies for that size will be around 98 to 99%. Your 11 KV/3.3 KV trafo with a 3.3 KV motor is just fine. It would have been better if it were 6.6 KV for 1 MW motor.

Muthu
www.edison.co.in

RE: The effect of transformers

(OP)
thank you very much for all the informative replies

@waros they are as we do have other 11kv motors

@edison: so if 6.6 kv is more like better, then 11kv would be also an upgrade from 3.3kv in terms of saving ? can the difference be estimated? in terms of new pf or efficiency or so ?

thnx

RE: The effect of transformers

Somacast,
" in my case 3.3kv bus is there" - is the bus capable of taking the additional motor loads??
If yes, my suggestion is to connect the ~1000kW motors to 3.3kV bus. These will tend to be more robust.

RE: The effect of transformers

As I said, the motor voltage does not dictate efficiency or the pf. In the IEC world, recommended voltages are 3.3 KV up to 200 KW, 6.6 KV for 200 to 1000 KW and 11 KV over 1000 KW. This is due to difficulty in handling the higher currents at lower voltages.

Again, connecting the motor directly to the grid is not a good idea for reasons already given. Minuscule increase in efficiency vs reliability, your call.

Muthu
www.edison.co.in

RE: The effect of transformers

(OP)
noted, thank you very much,

Last question, you refered to IEC in your replies, any specific volume/number covers the scope of such matter please?

Thanks again

RE: The effect of transformers

(OP)
thanks again , I will go through it .

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