Motor Load on Transormer

[tom baker]

I have an existing 40 hp submersible 460 V motor on a 45 KVA utility tranformer 12,500 v primary. We are changing to a 50 hp submersible. The other load at this site is a 5 KW single phase transformer. The POCO says the existing transformer is large enough as they can load it to "300%". I have always used 1 KW/HP as a rule of thumb. Any thoughts on the transformer size? If the motor goes bad due to undervoltage its a pain to pull as its set 300 feet deep....

 

[taylorp]

Re: Motor Load on Transormer

Tom:

Utility transformers are not my specialty but:
I would guess that the transformer feeding your
40hp motor is barely adequate. I would lean towards a 75kVA for your new motor. It would depend on how a person could figure the voltage drop from the locked-rotor current of the motor and the impedance of the transformer. Need more technical data to do the math. I would guess that your new 50hp motor will have hard starts and perhaps might even stall depending on the load with a 45kVA transformer. Maybe you could start your 50hp once and just leave it running? As far as "300%" on the POCO transformer, I think they are being highly optimistic. :roll:

[ July 16, 2004, 07:58 PM: Message edited by: taylorp ]

 

[charlie]

Re: Motor Load on Transormer

No, he is being truthful. A transformer can, for very short periods of time, carry 300%. We do not change out our transformers that feed residential loads until they are loaded to 170%. Having 200% is not unusual but we don't like to see it.

It depends on the load profile for the installation. In the case I mentioned about residential loads, the load peaks around dinner time and people have dinner at different times, dish washers are going along with water heaters, etc. Other than that, there is not much load on a transformer. By the time it gets warm, the load goes away.

This same line of reasoning applies to your pump. the heavy load is at the startup. After the motor starts, you are now dealing with only the running amperes (nameplate, the from the tables). It doesn't hurt the transformer to see that kind of load.

You mentioned the 5 kVA transformer, that is not the load that the electric utility will see. the electric utility's transformer will see the load that is running and being fed from that transformer if it is on at the time.

The interesting thing is that I have never seen a 45 kVA pad mounted or pole mounted transformer. To the best knowledge I have, a 45 kVA is only built as a dry type and most electric utilities do not use dry types.

Tom, is this a dry type, pole mount, or pad mount transformer and are you sure of the size? The reason for the question is that dry types are much more sensitive to overloads that the mineral oil filled types (it takes oil a while to heat up).

 

[tom baker]

Re: Motor Load on Transormer

Its a pad mount and I have verified the size. If I remember correctly the motor is a design code H. The FLA is 67, which is above the table value of 62. There is no seria number or mfg on the transformer, but if its oil cooled it may have the fins and oil reservoir on the back.


Charlie here is the interesting part about the dry transfomer.
The orginal transformer was a 50 KVA. It lasted about 5 years, and then one rainy sunday it burnt up, as it had never been filled with oil. I didn't know the rotation on site, I had to go back to my shop and get the rotation (these are vertical turbine pumps, not good to run backwards, sometimes the current is almost as much in reverse as it is forward). From that day forward I always mark the rotation and request clockwise on a new installation.

I didn't add that I had discussed the existing size with the POCO's contract engineer (they contract out their O&M)he was kinda sure on the size, but he didn't realize he needed to check the inrush of the transformer to see how the starting would affect the adjacent customers - it can cause a dip in the voltage and the lights can flicker (is that right Charlie?)

This well is in the middle of a residential area.

[ July 16, 2004, 11:37 PM: Message edited by: tom baker ]

 

[templdl]

Re: Motor Load on Transormer

My information shows that a 45kva transformer for a 40hp motor, which should be close to 52fla, requires a minimum 43.2kva transformer.
kva=(volts x current x 1.732) / 1000
50HP at about 65FLA would require a minimum of 52kva.
HOWEVER, if the motor sf is greater than one, increase the fla proportionally.
IN ADDITION, for motors that are started more than once an hour the transformer kva should also be increased usually a minimum of 20%.
As you can see even a 45kva for a 40HP application may be some what marginal not even to consider 50HP.

 

[charlie]

Re: Motor Load on Transormer

Tom, I am confused. That size transformer would not have an oil reservoir on it at all. Now it is a 50 kVA single phase instead of a 5 kVA transformer. The bottom line though is how much load is on the single phase transformer? Was this a 50 kVA oil filled utility transformer or a customer owned transformer fed from the 45 kVA utility transformer? Lastly, if this was feeding other customers, what kind of customers were they?

Templdl, it is common for electric utilities to size their transformers on a HP to kVA basis. If no other major loads are present and the electric utility is using oil filled transformers (they almost always do), it is no problem. Even with a higher service factor, most motors are not loaded to their rated HP while running. The only real problem is if the motor is trying to start under a big head of pressure and is a distance away from the transformer and the impedance of the transformer is too high, there may be too much voltage drop through the transformer. This could result in a burned up motor.

Each installation has to be looked at by an experienced engineer and even they make mistakes although I never have . . . HMMM

 

[templdl]

Re: Motor Load on Transormer

Thanks Charlie,
I agree with you that a 45kva would work with a 40hp motor if the if you had a known load and that you could start the motor without a problem. Also the number of starts per hour apparently were not an issue as it worked.
If you think about it if the 40hp motor wasn't loaded to 40hp it's strange why they would be going to 50hp.
However, going to 50hp may be a bit more tenuous when paired with the 45kva.
The life expectancy of a transformer can be significantly reduced when operated at its rated temperature rise.
Because the person asking for input into his/her application issue often fails to provide enough information it's not all that uncommon that when attempting to answer an application question one often has to read between the lines and venture to provide a calculated guess based upon some assumptions.

 

[charlie]

Re: Motor Load on Transormer

Many times an electric utility will just roll the dice when selecting a transformer. If you have a load that is a large motor and it is starting often, the starting current will tend to splay the windings in the coils in the same manner as a fault. We have to look at a lot of things when making that decision, it is not for the untrained engineer. A lot of the complaints you have is the direct result of untrained or partially trained engineers.

Where we are running equipment to the edge of its capabilities, it is necessary to use the available experience. I have written several training manuals to help new engineers with those decisions. Keep in mind that we can do that because we guarantee our work forever. If it is undersized, we will replace it even 20 years from now.

 

[templdl]

Re: Motor Load on Transormer

Interesting Charlie. 20 years is not too bad. In this day and age it is not all that uncommon that a person will not be in a particular job long enough to take the responsibility of his/her recommendations of which quite often are based upon the lowest cost. Client is happy at the time, the contractor gets the business, the person who has made the recommendation has long moved on to another job elsewhere before the failure.
I'm not all that familiar with the life expectancy of liquid cooled transformers but what I do know is that there is a 30 life to be expected out of a dry type distribution transformer which is based upon 35% avg. load which my referrence shows as an accepted industry average. Expect 3-5 years max at 100% loading based upon an ambient temp of 30degC w/40degC max.
Raising the amb. temp. another 10degC reduces the transformer life up to 50%.
These are from the notes that I was fortunate to have gleaned from a long time transformer design engineer before he retired. And, what a brain trust he was.

 

[charlie]

Re: Motor Load on Transormer

You are correct about reducing transformer life. We have some that have been hanging on poles so long that we need to replace them just because they are old and inefficient (40 years plus). On the other hand, we have had bad transformers from the manufacturer and we have had poor engineering.

Generally speaking, if an engineer follows my training manuals, the transformer will last and not have wasted capacity.