208 Voltage too low for machine

[HEXx99]

I ran 208 v power to a rottler em69p cnc machine. it calls for 208/240 v 3 phase. 60amp rottler em69 installation manual
everything is wired using 3 #6 hot and #8 ground
to 6/4 SO cord drop to the machine . inside the machine again the wiring is 3 #6 and #8. I checked voltages and got 208-209v across two etc.
when the machine tech. came he said the voltage is too low that its calling for 210V and his readings are only 205v. I have to go back tomorrow to check it out but im not sure where to start. thanks in advance for any advice .

 

[HEXx99]

The information on this plate enables us to calculate the voltage drop at the output terminals of the transformer as the load current increases. The transformer is a 75kVA with 3 phase Y output of 208V/240V.

The rated secondary phase current is 75000/(3 x 208) = 25000/208 = 120A

The % Impedance figure of 5.54 tells us the percentage of the rated winding voltage which must be applied to it with the the other winding short circuited for the rated currents to flow in each winding. Thus if the primary is short circuited one would have to apply a voltage of 208 x 5.54/100 = 11.5V across the secondary for 120A to flow in the secondary and the transformed down current in the primary winding.

The effective impedance then of each of the secondary phase windings Zs is 11.5/120 = 0.096 Ohms

The volt drop for a particular phase current is Iphase x Zs. This is why I asked you to measure the phase currents.

At full load of 75kVA, or 120A per phase in the secondary windings the voltage drop is 11.5V, which means the terminal voltage is the secondary induced emf (208V) - 11.5V = 196.5V.

If we just consider just the 30kVA load of the Rottler machine fed by the transformer and assume it is balanced 10kVA per phase, the volt drop just due to the supply transformer will be (10/25) x 120 x Zs = 4.6V ...The secondary voltage will be 208 - 4.6 = 203.4V which is out of specification.

There are of course further voltage drops due to all the cabling and switchgear between the transformer secondary terminals and the terminals of the Rottler machine and also the currents for the rest of the site load.

In summary then, primary winding tap changing may be sufficient alone to achieve voltages within specification for the Rottler but be prepared that to supply the site load and the Rottler may require a new higher Kva site transformer or a dedicated transformer for the Rottler - 30kVA is specified. Interestingly the manual states a 60A per phase supply is required which is 208 x 60 x 3 = 37kVA. Whether 30 or 37kVA this is half of the rated capacity of the present 75kVA supply transformer. A review of the site load and power factor may be necessary to see if they can contribute to improve the site voltage using the present transformer.

This transformer is running power to at least two panels the machines these panels are powering are not ran constantly. they run maybe one of them about 8 times a day and maybe 10 min at the mist each time. the cnc wouldnt be running all day either . but there are also lights getting power from these panels . Im now thinking a 30 kva transformer 3 phase just for this machine is the best bet . unfortunately these are not cheap. unless i just find 240v andpower it from that, right? but then it looks like i made a mistake of supplying inefficient voltage to the machine . which is true, its not enough , but their own manual says 208 v ..im going to look for 240v

 

[marconi]

This transformer is running power to at least two panels the machines these panels are powering are not ran constantly. they run maybe one of them about 8 times a day and maybe 10 min at the mist each time. the cnc wouldnt be running all day either . but there are also lights getting power from these panels . Im now thinking a 30 kva transformer 3 phase just for this machine is the best bet . unfortunately these are not cheap. unless i just find 240v andpower it from that, right? but then it looks like i made a mistake of supplying inefficient voltage to the machine . which is true, its not enough , but their own manual says 208 v ..im going to look for 240v

Do not beat yourself up too much. The rottler electrical spec is far from clear and certainly not definitive in the way the acceptable voltage is written.

Please measure some phase currents and voltages at main breaker for different states of site load to establish what margins exist to supply the demanding rottler load. Would site accept load shedding to allow rottler to power up?

 

[marconi]

HEXx99. I have been reviewing everything you have said and the information in the Rottler manual on the electrical requirements. I have made a mistake because I thought the 205V you mentioned as a low voltage and the 208V mentioned by Rottler were phase voltage. They are actually line voltage. The Rottler requires a 3 phase 3 wire 60Hz supply with a line voltage in the range 208-240V. In terms of phase voltages, this is a range of 208/rt3 to 240/rt3 = 120 to 139V.

The supply transformer line voltage of 205V is a phase voltage of 118V. Tap changing out to be able to bump this up a few volts. At 125V phase voltage the line voltage would be 216V, or 8V above the minimum.

The cable calculation redone using 10kVA per phase at 120V is a current of 10000/120 = 83A.

the #6AWG cable run will drop between its ends 3.9V or a machine phase voltage of 116V or 201 line voltage. Clearly #6AWG is too thin. A double run of #6AWG would halve the volt drop to circa 2 V, thus a machine line voltage of (120-2) x rt3 = 204V - still not good enough.

#2AWG gives a volt drop of 1.71V; (120-1.7) x rt3 = 204V line voltage at the machine.

And this is before we consider the transformer volt drop and volt drop due to site loads. It will be very difficult to achieve the required machine voltage using the current transformer.

I think you are at the stage of placing a transformer 480V/240V - both line voltages - close to the machine to keep the machine's supply cable voltage drop as small possible without having to use a very thick cable and yet stay well within the 208 - 240V specification.

 

[marconi]

The 75kVA transformer with a secondary phase induced voltage of 120V, will have a full load phase current of 25000/120 = 208A.

5.54 percent impedance means 120 x 5.54/100 = 6.5V applied to secondary with primary short-circuited would circulate full load phase current. This Zs is 6.5/208 = 0.031 Ohms.

The Rottler machine when drawing 83A per phase would create a secondary terminal phase voltage of 120 - (83 x 0.031) = 117V.

117V is a line voltage of 202V - unsatisfactory for the Rottler even before any other loads connected to transformer.

The Rottler requires its own transformer. Hope this helps. Happy to discuss.

Regards

Marconi

 

[marconi]

Here is an 8 minute video on the Rottler 5 axis machine cutter - very impressive.

 

[HEXx99]

Here is an 8 minute video on the Rottler 5 axis machine cutter - very impressive.

First, I have to thank you for all the help on this. Its greatly appreciated.
Heres what it came down to : limited time , and trying to avoid paying for another transformer. The Rottler technician will be there Monday. So i basically had one shot to get that boost i needed. I decided to explain to the customer my plan of changing taps on the transformer to get some voltage increase I told them that 5% was the max deviation. that its mot safe foe other machines if the difference is greater than that.
They said to try that. So changing the taps wasnt as hard as i thought . I changed the taps from position 5 (480) to position 3 (456)z
the result was a 10V increase at the panel and at the machine. i showed the customer the voltage at the points in the manual and now we wait for the technician mondqy,

 

[pc1966]

Good luck with Monday visit!

 

[marconi]

Greetings from London. Thank you for the update- much appreciated by all in the EF. The photo is welcome because it shows some of our less experienced members what tap changers look like inside a power transformer.

We looking forward to the next update.

 

[Heriberto Batchellor]

Very useful advice. I think I'll try it，I have encountered a similar problem recently, and I am trying to solve it

 

[marconi]

Very useful advice. I think I'll try it，I have encountered a similar problem recently, and I am trying to solve it

Greetings from London. Please would you tell us more and how you fixed the problem? Some background with voltage measurements before and after would be appreciated. And we love pictures. I suggest you edit your profile to remove your address.

 

[marconi]

HEXx99 - I just wondered if the power supply issue for the Rottler had been resolved and how?

 

[HEXx99]

HEXx99 - I just wondered if the power supply issue for the Rottler had been resolved and how?

Hello I apologize for not updating. I get sidetracked. Let me see where I left off...oh right, so I changed the taps in the transformer and was able to increase voltage. (if anyone reading doesn't know when you decrease Voltage on the primary side, it will increase on the secondary, and vice versa) I didnt know and had to change the taps twice . So that Monday the technician did come and everything looked good for the inspection and passed. Meaning there's still a warranty if anything should happen to the machine which was the main concern as these are pretty expensive pieces of equipment. I was also there yesterday and they told me happily that they had used it since id been there last and everything worked great. So, again, I really appreciate yours and everyone elses help. It means a lot. I was almost in a panic for a bit there as time was a factor, and not much of it to spare. THANK YOU!