Need help with a transformer . Riser size

[nickelec]

Here is my issue i need to run a riser for an elevator. The elevator company is telling me they will be installing a step up transformer in the motor room. The problem is they dont know what transformer yet but they do know that the motor needs 460 v 45.7 hp and 76 amps. My question here is can i figure out what amperage is needed on the primary side with just that information it is a 208 3 phase 4 wire application .. any information would be great thanks

 

[rlundsrud]

The approximate primary current would be 168 amps at 208 Volts. I would think you would want a 75kva transformer for this, you may need to go larger that is a minimum on the transformer size.

 

[nickelec]

Thanks for the timely response. Im currently in school learning all this now so i was just a litte unsure. Can tou give me a breif break down on how you came up with that what formula did you use

 

[jumper]

Thanks for the timely response. Im currently in school learning all this now so i was just a litte unsure. Can tou give me a breif break down on how you came up with that what formula did you use

The kVA on the primary side of the tranny is equal to the kVA on the secondary.

Since this is an elevator I would assume 3 phase unless told otherwise.

You also need to use the tables in article 430 for motor FLA.

 

[augie47]

This is a situation where the services of an engineer could save you some heartache especially since this is an elevator motor.

Your basic HP vs amps indicates the motor is not the "normal" table 430.250 motor, possibly a lower speed motor. You are looking in the neighborhood of 65 kva load but with a motor involved, a 20% added factor is often recommended so a 75 kva might still be a bit undersized.

 

[Ingenieur]

Are you sure you are not feeding a DC motor controller
the fractional hp looks like a westinghouse (ge?) gte series dc elevator motor/machine

 

[nickelec]

Are you sure you are not feeding a DC motor controller
the fractional hp looks like a gte series dc elevator machne

To be completely honest with you im not sure elevator contractor basically said to give him a 150 amp 3 phase 208 disconnect and he will deal with the rest . Problem is the current feed for this elevator is a 3 phase 208 150 amp feed and i was trying to use the same bucket as opposed to finding another feed somewhere in the building. The new install also needs about 60 additional amps for lighting, receptacles and an Ac so trying to get an exact number for the the elevator motor!

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[kwired]

To be completely honest with you im not sure elevator contractor basically said to give him a 150 amp 3 phase 208 disconnect and he will deal with the rest . Problem is the current feed for this elevator is a 3 phase 208 150 amp feed and i was trying to use the same bucket as opposed to finding another feed somewhere in the building. The new install also needs about 60 additional amps for lighting, receptacles and an Ac so trying to get an exact number for the the elevator motor!

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Very possible the elevator contractor has already figured the lighting, AC, etc. into the load used to determine 150 amp 208 volts, especially if it is within the elevator shaft. Might want to ask him to make certain though.

 

[Ingenieur]

If we assume 80% x 150 = 120 A 208/3, 0.9 pf, 0.9 eff
47 hp
take 60 a for aux loads and < 25 hp
need more info on elevator

 

[nickelec]

So in theory 120 amps to feed primary should be sufficient to supply a motor of that size

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[infinity]

Since he asked for 150 amps that's what I would give him. Is the elevator controller a VFD if so you might have slightly higher input current for the drive.

 

[nickelec]

I was thinking the same thing instead of racking my brain just give him the 150 and be done with it. But i do appreciate the info

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[Ingenieur]

hp means little with an elevator
you need speed, rpm, gearing

eg
25 hp dc at 55 rpm direct drive (common)
36" drive sheave
lift = 5252/55 x 25 / 1.5 = 1600 lbs
depending on car wt and cwt a good load

speed 55 x 3 x Pi = 500 fpm

 

[nickelec]

That makes sense im not an elevator guy buy any means lol. I was just trying my best to answer my questions with the information i was given

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[jeremy.zinkofsky]

Here is my issue i need to run a riser for an elevator. The elevator company is telling me they will be installing a step up transformer in the motor room. The problem is they dont know what transformer yet but they do know that the motor needs 460 v 45.7 hp and 76 amps. My question here is can i figure out what amperage is needed on the primary side with just that information it is a 208 3 phase 4 wire application .. any information would be great thanks

S = VI*1.732

You will be supplying the motor via a 208Y/480V XFMR I imagine and you know what load is needed. Taking 125% of the motor full load will push you just over 75kVA for the XFMR. If there is additional load as you suspect then it would be covered as you have to go to the next size up XFMR anyway.

Showing the calculation:

Secondary VA:

S = VA = VI*1.732
= (480V)(76A*125%)*(1.732)
=78975VA
Next size up XFMR = 112.5kVA

Full Load Current of the Primary:

I = S/(V*1.732)
= (112.5kVA)/(208*1.732)
= 312A

 

[Ingenieur]

S = VI*1.732

You will be supplying the motor via a 208Y/480V XFMR I imagine and you know what load is needed. Taking 125% of the motor full load will push you just over 75kVA for the XFMR. If there is additional load as you suspect then it would be covered as you have to go to the next size up XFMR anyway.

Showing the calculation:

Secondary VA:

S = VA = VI*1.732
= (480V)(76A*125%)*(1.732)
=78975VA
Next size up XFMR = 112.5kVA

Full Load Current of the Primary:

I = S/(V*1.732)
= (112.5kVA)/(208*1.732)
= 312A

imo the 25% upsize is not required ~63 kva, could be wrong
that is typically for conductor sizing
a 75 kva should be fine
assuming only the motor is served
and the xfmr is suitable for repeated motor starting