Voltage drop issues

[a10jp]

I would like to better understand the effect of voltage drop has on equipment and insulation. Would running a piece of equipment at lower voltage (due to voltage drop on improperly sized cables) necessarily damage that piece of equipment permanently? Does anyone have any real-life experience?

 

[ScottyUK]

Running induction motors below nameplate voltage increases slip, which heats the rotor. Decrease the voltage too much and the rotor will be damaged. This is a well-documented condition.



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If we learn from our mistakes,
I'm getting a great education!

 

[RalphChristie]

On a few occasions voltage drop is fine, but most of the times it is not recommended.

Motors are designed to start with a specified voltage at their terminals. The developed torque is proportional to the square of the voltage. A high starting voltage drop (remember a motor can draw up to 6x full load current during start-up and some motors even more) can result in low terminal voltage and inadequate torque to accelerate the load to rated speed in the desired time. This can result in trips during start-up or even damage to the motor due to the excessive heat during acceleration. Other loads between the motor and the source my also be affected by the reduction in voltage during start-up. (malfunctioning of electronic circuits, contactors dropping out, etc.)
Voltage drops are associated with heat, and heat is responsible for damage in motors, cables, etc.


On the other hand, voltage drop on your system will affect lamp life and the amount of light lamps will deliver. With a slight voltage drop light loss is hardly noticeable to the naked eye while the lamp life is greatly extended. Too much voltage drop lowers the light output and changes the colour of the light from white to yellow or orange. Too little voltage drop burns the lamps too hot and shortens lamp life.

 

[dbaird]

Most electrical control devices here in the USA are designed to NEMA specifications which allow an operating votage range of +/- 10% rated nameplate. See: NEMA ICS 1-2000 6.1.3

http://www.nema.org/stds/results.cfm?srchString=ICS+1-2000&UserSelectedSubSites=12

Relays, contactors and like devices will drop out and/or not seal in if the voltage deviates from this.

David Baird
mrbaird@hotmail.com
Sr Controls Designer
EET degree.
Journeyman Electrician.

 

[a10jp]

Thanks for this great info. So there are different kind of damages for too much voltage drop and too little voltage drop? NEC specifies 5% as allowable VD (or 3% from source to distribution). Sounds like that is not acceptable depends on the applications.

In our case, we have a 10HP motor, used for vacuum pump, that was designed to operate on 230V. Without knowledge of this by anyone, it has been used for a long time under 208V "without problems"(according to customer). Then some time ago our contractor were replacing the feeders to this area that were clearly undersized, which added to the VD problem (5.7%, or 197V at the distribution). It was then the customer started to realise dropped performance, and not until 4~5 months later, the performance is getting worst that they are finally complaining. From all your input, it looks like the problem was slowing developing overtime, but exacerbated by VD.

As a recap, if the motor is designed to operate +/- 10%, then 208V is within tolerance of 230V. But for how long can this limit extreme be stretched?

 

[alehman]

Clarification - NEC doesn't specify maximum voltage drop. It only recommends a maximum for best efficiency. The NEC values seem conservative because they are intended to take into account voltage drop that will occur upstream (utility transformer) as well. Too little voltage drop is generally not a problem, unless the source voltage is too high for some reason - a situation that should be correct at the source anyway.

Motors are designed (when in good condition) to run anywhere within +-10% indefinately. 230V motors are commonly rated to operate at 230 or 208 (with +-10% applied from either voltage).

What kind of performance problems are you having?

 

[a10jp]

The vacuum pump that uses these 2 10-hp motor has been running under-performance since then cable feeds been replaced. That was about 6 months ago, according to the user. The suction has become weak over time. Before the motors were run alternately. Now even with 2 motors the suction has been weaker than ever with 2 motors on simultaneously. We measured the voltage on the ckt that feeds these motors, and resulted 197V. The cables that was replaced was definitely undersized (#4).

 

[ccjersey]

This sounds like a problem with the pumps, not the motors or other electrical components unless there is a variable speed drive or some such that may have been damaged or mis calibrated.

Check the belt tension if it has belt drive. What amperage do the motors draw. If it is low, the drive may be slipping or the pump worn so badly it can no longer do the work. If amperage is normal, there may be a leak in the system admitting air or a vacuum controller may be defective causing the same effect.

Alternatively there may be a blockage between the pump and the point of use for the vacuum. Manytimes a small amount of mist or dust will carry over past any traps designed to collect it and accumulate in elbows especially.

Check out the pump!

Jim

 

[alehman]

I agree with Jim. Unless the motor is not starting or it is stalling, poor performance is probably not an electrical problem. AC motors don't slow down significantly when voltage is low - they just draw more current and get warmer.

In terms of current capacity, if both 10hp motors are on one circuit, #4 should be fine. Assuming each motor draws about 32 amps, you should be able to go about 180 feet with #4's for a maximum of 3% (6V) drop.

If the motor has only a single 230V rating, 197 is a bit on the low side. If it is dual rated (230/208), then 197 is fine.

 

[jraef]

No, if the voltage has dropped to 197V, and the motor is rated for 230V, then it is operating at 86% of design voltage, and the output torque of that motor will drop to roughly 74% of normal. it may havebeen oversized slightly in the original design, which is why it ran OK for so long at 208V, but the additional drop has not consumed that margin of error and you have tangible results. In a vaccum pump, lower torque = lower vacuum, plain and simple.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[a10jp]

Actually, the #4 was for the entire load panel, rated at 100A. And the Vacuum pump motors are fed off from 30A breakers. Sorry my previous description were bad! Apology.
My understanding was the motors were old style motor, made by Basler??? Don't remember correctly. I do not think they are dual rated..the motor nameplate listed 240 (but written 230V elsewhere) and 415V. That's why I was curious to know if +/- 10% capacity can be correcly assumed for older style motor.

 

[alehman]

If the motor nameplate says 240, that's what you should use. 240 - 10% = 216V. So your supply is too low.

I think 10% has been the standard for a long time. You may wish to consider a boost transformer to increase the voltage to the motors.

As for the performance problem, I'm still not sure what's going on. As I said, induction motors will typically either run close to rated speed or stall, and not much between. As jraef said, torque capacity is dependent on voltage. Perhaps the pump load is cyclic and the motor comes close to stalling at peak torque and is able to recover between cycles. In this case I would say the overload relay should trip if it's set correctly.

 

[jraef]

that 10% is a NEMA spec (North America), but if your motor is 240/415V, that is NOT a NEMA motor, so all bets are off.

 

[ScottyUK]

415/240V sounds like a UK spec. Most of Europe used 380/220V as their standard, although Europe now notionally uses 400/230V because Brussels has dictated so.

UK spec for distribution voltages, prior to the EU meddling in our affairs, was 415V +/-6%. If the motor is of UK origin and is more than say 5 years old, then for a delta connected winding the operating range would be 226V - 254V.



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[a10jp]

Would you now think the problem has nothing to do with voltage drop as measurement shows?

 

[alehman]

Good point jraef.

If the source is truly 208V, then the voltage drop is about 5%, which is significant, but it may not be a severe problem if the source voltage were correct. Clearly the source voltage being 208 is the more significant problem.

 

[a10jp]

I have gone to the pit and this time take a good at the motor. I realised 240V is not a standard 3 ph rating. I went to check the name plate on the motor itself:
- Baldor Industrial Motor:
10HP,
240/415V
28 / 14A
RPM:2850
Frame: 213TCZ, 50Hz, 3PH
Serv F: 1.00 Code J,Des B, Class F
Usable at 208V: N/A

So the nameplate says it is 240V, 3Ph. Now NEC did not have ratings for 240V, 3PH AC motors, so I use ~ 230V, FLA = 28A.

Question is when we measured the amperes at the panel board terminals that feed these pumps motor, it measured 4.55A, 5.8A, and 4.2A acorss the 3 phases. The motor(pump) was not operating when we measured. So what are these readings we are reading? The 4.55A/5.8A/4.2A are residual amps when the pumps are not operating? If the motor is running, it should be like 28A or more, isn't that correct? For vacuum pump, shouldn't the motor be running all the time to create that suction when you need it? I am confused how this work?

 

[davidbeach]

You say NEC, and you have a motor with a 50Hz rating. The NEC tables are for 60Hz motors. Where are you, and where did the equipment come from? You may be dealing with equipment not intended for use on your power system.

 

[a10jp]

I am in Japan. The motor manufacturer is by Baldor motors, a US company. You could be right, the motor is made for 240/415 3-ph system, unusual for US applications. However, the motor has been running under 208V long before I work in Japan.

The thing is that the performance for the system is steady declining, i.e. suction is getting weaker. I am inclined to think what the others had mentioned above, that perhaps the pump is also experiencing problem, not just the motor. But I am not sure how a suction pump works.

 

[alehman]

Perhaps the pump has an oil heater or some other non-motor load that accounts for the non-running current. The motor should take somewhere around 28A at rated input voltage and rated output power. Since the input voltage is low, the current will be higher that at rated voltage. If the load is less than 10 HP, then the current would tend to be lower.

Motor running loads in the NEC tables tend to be higher than nameplate ratings. Nameplate is usually more accurate.