Motor starting voltage drop on 208V system 2

[rockman7892]

I work with a client that has a sorting facility that has multiple conveyors/sorters that operate on a 208V system. There are several of these conveyors located throughout the facility with groups of 2-3 conveyors served from a local 480-208/120V step down transformer and secondary 208V panelboards having branch circuits going out to each conveyor.

The conveyors have 208V motors which are started across the line and are started frequently throughout the day. I'm not sure of the exact HP but am told that the full load of each of these conveyors are 102A at 208V.

The client has stated that they experience voltage drop issues when starting these motors frequently thoughout the day resulting in issues with getting motors started and heating of cables.

As a solution to this voltage drop issue they are suggesting increasing the sizes of the step-down transformers mentioned above sizing the transformers to 20% more than required for steady state load to help with voltage issues (The customer is changing all these transformers anyway to re-configured layout of conveyors).

As an example a group of (2) conveyors is currenly fed from a 75kVA transformer. They are suggesting increasing this transformer to 150kVA based on the following...…37kVA per conveyors *2 Conveyors *120% = 89kVA. Next size up is 112kV however 150kVA is being selected.

Is increasing transformer capacity in this manner and effective solution to minimizing voltage drop issues with motor starting? Perhaps a more effective solution may be to provide some means of soft starting motors but client does not seem interested in exploring this option. Would it be more effective to have large step down transformer to group all 208V loads from a common switchboard?

Thanks

 

[waross]

If they are experiencing heating of cables it may be well to review the conductor sizing.
Typically 125% of the current given in the appropriate table for motor currents, at the 75 rating.
You must use the tables for motor current rather than the nameplate current to allow for future replacement with a less efficient motor.
If the customer wants 150 KVA, go with it.
There will be a lot of expense to re-cableing at 208 Volts.
It depends, but in a lot of plants, 480 Volt distribution to smaller step down transformers is common and economical.
You may want to explore changing the motors over to 480 Volts.
Many motor starters are rated for 480 Volts.
It depends.
Long ago and far away, I toured a plant that had all the 480 Volt motors fed from 230 Volt circuits through local transformers and all the 230 Volt motors from 480 Volts circuits through local transformers.
This was a country where the electrical wholesalers gave a nice cash kick-back to anyone ordering a transformer.
I got out of there as quick as possible.
In that time and place a guy could wake up dead, messing with a gravy train like that.
I am not going to guess at a 480 Volt layout without a lot more information, but it is doable and may be the best and cheapest.
It depends.



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. rockman7892
Q1. ...The facility ... served from a local 480-208/120V step down transformer and secondary 208V panelboards having branch circuits going out to each conveyor.
A1. Your existing power supply is 208/120V It would (involved considerable cost and down-time) to replace all the existing [208V motors to 480V ]. The starter coil voltage and the thermal over-load setting would have to be looked into.
Q2."... conveyors have 208V motors which are started across the line and are started frequently throughout the day. ..."
A2. (start across the line starters) may be replace by [soft-starter] or add it in series to the existing across the line starters.
Q2.1 "... are started frequently throughout the day".
A2.1 suggest to reduce the number of starts throughout the day. A (unloaded) motor [power-loss subsumption] would not be very high. On the other hand, a reduction in the number of starts would prolong the life of the contactor and the conveyor mechanism.
Q3. "... they experience voltage drop issues when starting these motors frequently throughout the day resulting in issues with getting motors started and heating of cables".
A3. If across-the-line starters are replaced by soft-starters, the starting current and the voltage-drop would be reduced. It is also advisable to maintain motor running free-loaded instead of frequent starting.
Q4. "...As a solution to this voltage drop issue they are suggesting increasing the sizes of the step-down transformers mentioned above sizing the transformers to 20% more than required for steady state load..."
A4. Carryout a simple voltage drop study. It would also help to ensure that a 150kVA transformer would resolve the voltage-drop problem.
Q5. "... (2) conveyors is currently fed from a 75kVA transformer. They are suggesting increasing this transformer to 150kVA based on the following..."
A5. Instead of (replacing the existing) 75kVA transformer to a new 150kVA, another option is to stall another new 75kVA [in parallel] to the existing. Some cost saving.
Che Kuan Yau (Singapore)

 

[edison123]

If they already have a 480 V system, it makes more sense to replace/rewind the 208 V motors to 480 V and eliminate the step down transformer altogether (or use to feed only 120 V single phase loads, if any). This will solve their cables heating problem as well as voltage dip problem since the motor current is more than halved at 480 V for the same KW.

Muthu

http://www.edison.co.in

 

[LittleInch]

Sounds to me like first you need to find the cause of the low voltage.

Is this in the cable from TX to motor?
the 480V system?
Excessive volt drop across the TX?

Maybe all three, but then making one a bit better might not solve the issue.

what is the starting current you're getting now?
What is your starting method - DOL?, star delta?
I can't believe the client isn't interested in reducing starting current.
is this 208V three phase?
which cables are getting hot?
Size of the different cables?
Lengths of the cables?

Have you done a network analysis?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[waross]

"A1. Your existing power supply is 208/120V It would (involved considerable cost and down-time) to replace all the existing [208V motors to 480V ]. The starter coil voltage and the thermal over-load setting would have to be looked into. "
Lack of local knowledge.
Little cost and time to change connections on a standard 208-230/460 volt motor.
Under normal circumstances, starting does not cause cable heating.
The cost of a soft start is generally only justified for high inertia loads that are unable to start DOL.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

The problem with soft starting on conveyors is usually one of load; you usually can't soft start a fully loaded conveyor because with the lower torque, it stalls. A VFD might work, but that's a lot of $$ for a glorified soft starter at the size you are looking at (assuming 102A is one motor).

Increasing the transformer size is possibly going to help, but not if the conductors are under sized. I would start there; look at the conductor sizing and distances to see if they created the problem there first.

It was probably a mistake to power motors that large from the 208V system in the first place if they had 480V available, for this very reason.I wonder why they did that? It's definitely worth looking into.

A lot of motors are 208-230/460V when looking at 50HP and under and at 102A, these are likely 40HP motors (again, assuming the 102A was for one motor). If so, the conductors would be over sized for 460V (a bonus situation here), the motor starters would be over sized (again, bonus) and all they would need to change would be the overloads, OCPD and distribution equipment. That alone is not a small undertaking, but neither is replacing the transformers, primary OCPDs etc. for those. I would just leave the transformers in place as is for the 120V loads and any other 208V stuff, then place a small 480V sub-panel next to it, using the transformer primary circuit as the feed, then tap off of the new 480V panel to feed the transformer and the motors. It would take running new conduits to J-boxes to tie into the existing feeder conductors that you remove from the 208V panelboard for these conveyors, but again, those conductors would now be BIGGER than they need to be because of the lower current.

If the motors are not tri-rated, then this is moot; the cost of replaving the motors would be too high. If that's the case, increase the conductor size FIRST and if that doesn't do it, bump up the transformer size.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

In this digital world VFD is the best solution.

Wow. I wish that I had your budget.
Not sure how VFDs help with conductor heating.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

jraef, thanks for writing out the details.
I was thinking very similar, but I didn't take the time to write it out.
lps

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Also it will reduce your power consumption drastically and give you the $$ return.

Not very likely on a conveyor.

There is LOTS of hype saying that simply installing a VFD will save energy and have a quick payback, but most of it is complete hogwash.

 

[waross]

I agree with Lionel here.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sn00ze]

Yeah exactly jraef, I am surprised that their first solution was changing the transformer before the conductor. They are going to have to do that anyway if they change the TX.

 

[jraef]

Not very likely on a conveyor.

There is LOTS of hype saying that simply installing a VFD will save energy and have a quick payback, but most of it is complete hogwash.

Agreed. I am in the VFD business, yet it still makes me cringe when I hear/see a sales pitch that touts energy savings on loads that will NOT realize it. There are valid arguments to be made for saving energy on some types of applications (centrifugal machines), but not all of them blanketly, which is how many people cast it.

For those who haven't thought it through;
On something like a conveyor, the energy (kWH) consumed is a measure of work performed across time, and kWH is what we pay for. Slow down the conveyor and the kW goes down, but you increase the time it takes to do the same work, so no energy is actually saved. You may improve the process, reliability, interactions with other machinery, consistency etc., but not the energy consumed in moving whatever is being moved. The mistake that a lot of people make is in conflating POWER (kW) with ENERGY (kWH), they are not the same thing. So yes, reducing the speed of a motor reduces the POWER, but preforming the WORK is kWH, and if the kW goes down, the H goes up so the net result remains the same.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

Nice explanation, Jeff.
lps

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Well, I didn't mean it that way. With a conveyor, I might want to move 50 tons an hour. This is the required amount of material that must be moved in an hour and it can't be changed.

So, I can run the belt fast so it is rather lightly loaded and there is only 1 ton of material on the belt at any instant in time.

Or, I can run the belt slower so it is fully loaded and there is 2 tons of material on the belt at any instant in time.

But, the weight on the belt doesn't make much difference to the energy used because the energy used is more or less proportional to the work being done and the work being done is moving 50 tons of material per hour.