20HP 208V motor with issues on 208V service 1

[prmmel]

I have a treatment system my company built in NC last year. The connected power is 120/208V 3phase. There is a 20HP 3phase pump that draws from the system. The control panel has branch circuit protection for a 20HP drive and a 25HP rated drive operates the pump. The issue is that site voltage drops consistently and the drive trips out on overload which is a nuisance. The power company is indicating that they are allowed 208V +/- 10% which would allow the voltage down to ~187 volts. We have thousands of applications across the US, 100s with similar setups but never this level of problems.

I am recommending a buck/boost transformer to up the voltage to 240AC.

Recommendations on the power issues???

 

[waross]

At 208 Volts you should be using a 200V rated motor in North America.
First, check that the motor is similar to motors used successfully on other projects.
Second, recheck the settings on the drive for possible configuration errors.
Then if you have to boost the voltage, I recommend the open delta auto transformer connection. I have used this connection many times over the years with good results.
I would start with two 240:12V dry type transformers.
That will boost 208V up to 218V. That should be enough to cover voltage dips without going too high when the voltage is high.
Sizing is easy, the secondary winding will take rated motor current. My tables show 62Amps for a 20HP, 200Volt motor.
Adding the normal 25% safety factor for motor feeds, gives us 77.5 Amps. 77.5A x 12V = 930VA, use two 1 KVA transformers.
If the supply voltages are balanced the motor voltages will be balanced. When the motor current causes a voltage drop in one transformer, it will cause an equal voltage drop in the other transformer. If you do a vector sketch you will see that the resultant voltage on the open side of the delta will drop an equal amount and the voltages will remain equal.
You may consider the wye auto transformer connection. The transformers will be smaller as you will be boosting the wye voltage rather than the line to line voltage.
A 120:6V transformer will give a boost to 126 Volts (or 218Volts line to line). That will be 465 VA. Use three 500 VA transformers.
You will have to mount three transformers instead of two and you will have to run a neutral conductor if there is not one in place.


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

 

[itsmoked]

I agree with Bill but would instead go for as close to 240 as I could get. I'd use 240:24V.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[edison123]

Bill

Does your method work in buck mode ?

Muthu

http://www.edison.co.in

 

[DickDV]

After re-reading the original post, I find it curious that the drive is tripping on OVERLOAD when the power supply dips low.

I checked with a couple of drive brands and the all rate their drives to 208 - 10%.

Of course, if the motor is working hard at or near full speed, the output voltage will also sag and overload could occur. But only at or near full speed.

I think that a careful review of the drive parameters could fix this without raising the input voltage. Following are a couple of suggestions:

1. Set the drive current limit low enough so the motor can't go into the overload zone. Instead, the motor and pump will slow down just enough to get back to full load amps.

2. Set the drive parameters to do "undervoltage ridethru". This may be described by another name but the idea is to use load inertia to keep the DC bus up under brownout conditions. It won't do that here because the pump probably doesn't have much inertia but it should stop the faulting.

3. As a last resort, let the drive fault but set up automatic reset and restart so a manual reset isn't required.

Hope one of these helps.

 

[waross]

Hi edison123;
First let's agree on the definition of "Buck Mode".
Consider a 120:12V transformer.
To me:-
Auto transformer, the two windings are connected in series with like polarity.
Auto Transformer Step Up; 120V is supplied to the 120V winding or tap and 132V is available from the combined windings or the 132V tap.
Auto Transformer Step Down; 132V is supplied to the combined windings or the 132V tap and 120V is available from the 120V tap.
Buck Connection: 120V is supplied to the 120V winding and the 12V winding is connected in reverse polarity to subtract 12V for 108V available.
On a 120V system the auto transformer step down connection will give 120 X (10/11) = 109.1V and the buck connection will give 108V.
I have seen textbook treatments of Buck/Boost voltage regulators for distribution lines. The buck/boost winding rotates. In one extreme position the series winding is in the same polarity as the main winding and adds to the voltage. In the center position the series winding is at right angles to the main winding and core and acts as a small series reactor but neither bucks nor boosts the voltage. In the other extreme position the series winding is in opposition and subtracts from the applied voltage.
I can't remember using the buck connection, but I am confident that it would work.
I have used these connections the most with 480:120V transformers switching between 480V and 600V.
We had a customer with a mix of 480V machines and 600V machines. The plant that we serviced in the west had a 480V service and their plant in the east had a 600V service. (Canada) They built a new plant in the west with a 600V panel and a 480V panel. 600V machines were fed from the 600V panel and 480V machines were fed from the 480V panel.
This worked fine until they started changing machines. They would get a special order and ship a machine out from the east that could handle the order more efficiently.
9 times out of 10 the new machine was the wrong voltage. We would use a pair of small dry type transformers at the machine to adjust the voltage.
The service to the plant was 600V and two dry type auto transformers in open delta were installed in the panel and used to supply the 480V panel.
This was a manufactured unit panel. The incoming main, metering, 600V panel, auto transformers and 480V panel were in one unit.

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

 

[jraef]

I think DickDV brings up some good points. Although it's true that the VFD will drop the output voltage at full speed if the line voltage drops, it will not necessarily do that if the commanded speed only needs as much voltage as is available, or less. In other words, if your line voltage is 187 and the VFD is programmed for a 208V maximum voltage (more on that below), then if the speed is commanded to be 53Hz or under, there would not be any problem at all. And as he said, many of these issues can be compensated for in the VFD programming anyway, without buying extra hardware.

But speaking from experience, I'm willing to bet (a small amount) that it is a REALLY common problem on VFDs used in 208V applications. VFD mfrs have factory default settings. In the case of a 240V VFD, the default voltage would be 240V, not 208V. Someone would have had to go into the programming at commissioning to purposefully program the VFD's maximum voltage to be 208V, or better yet, 200V. Even if it were set correctly, someone may have come alog later and got messed up, reset everything back to factory defaults to start over, and forgot to change the maximum voltage.

Not doing so causes the V/Hz ratio to be incorrect, so that means at 60Hz the VFD is under fluxing the motor, which causes it to lose torque and more easily overload. Happens all the time. So first things first, check the VFD programming for Maximum Motor Voltage.


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