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Motor startup and Voltage drop
2

Motor startup and Voltage drop

Motor startup and Voltage drop

(OP)
I have a 5Hp single phase 240Volt motor that is being fed off a 25KVA Hydro transformer. The main service panel for this motor is 105 meters from the transformer with 3/0 triplex  and then 410 meters to the irrigation pump with 4/0 ACWU cable. With no load the motor starts up normally with a draw of 3 - 4 amps however when the pump is primed the motor stays on the start winding pulling the voltage at the motor to 200V with a current of 130Amps. The voltage at the panel is pulled from 245Volt down to 220V. Any ideas given these parameters on how to start motor under load.

 

RE: Motor startup and Voltage drop

I don't know what a hydro transformer is, but I'd say you need to start the motor unloaded or get a bigger motor.  The motor is not getting up to speed under load.  

David Castor
www.cvoes.com

RE: Motor startup and Voltage drop

Dave,
"Hydro" is a short cut term used to describe power utilities in places where they are mainly Hydro-generating systems. BC Hydro in Canada is a prime example.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
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RE: Motor startup and Voltage drop

gerlewis,
The problem is in the motor being single phase. There is no reliable and safe way to "soft start" a single phase capacitor start motor. You are right now experiencing the worst problem with that, the starting cap stays in too long and the motor never finishes accelerating, but draws too much current. So as dpc said, you need a larger motor or a way to ensure it never starts loaded.

If neither situation is possible, you can replace that motor with a 3 phase motor and use a 10HP 230V VFD to power it. The VFD needs to be double size in order to do the phase conversion from single phase in to 3 phase out, because the single phase current will be 1.732 x the 3 phase current, plus you need extra bus capacitance to smooth out the added ripple. It's done all the time in situations like yours.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln  
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies  

RE: Motor startup and Voltage drop

You can try starting it with the outlet blocked.  Don't leave it blocked for more than a few seconds!  If that works you can figure out some quasi-automated system to do it each start.

Keep in mind that you should let that motor cool at least 30 minutes after one of those horrid non-starts.

I have a 5hp 1ph on a V4 compressor.  If that motor doesn't reach speed within about 4 seconds smoke starts pouring out of it.  If at all possible abort your starts the instant it looks like a no-go.

You don't want this situation to continue because it's hell on your motor and your switchgear.  The next failed start could be the motor's last.


Otherwise I'm in with jraef on the 3ph solution.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor startup and Voltage drop

Apparently, you have proven that you can't start the motor under load unless you reduce the voltage drop.

You could try something like about 30kVAR of capacitor on the line while the motor is starting. This may cause a over-voltage when the motor reaches full speed.

 

RE: Motor startup and Voltage drop

Assuming you have checked your motor to be in good order, (if it's a split-phase capacitor motor -check the speed cut-off switch and the starting capacitor; if it is a plain capacitor motor - check the capacitor, etc.) you can throttle down the discharge valve opening to barely minimum (not totally shut but just a small opening)in order not to load-up your small motor. If you are not sure of the proper opening, try closing the discharge valve full, and then open the valve a little and start your motor. I guess you will be able to start it because you said "With no load the motor starts up normally with a draw of 3 - 4 amps".

RE: Motor startup and Voltage drop

2
Your voltage drop in the 410 meters of motor feed is 220-200 or 20 Volts. If you use a transformer to boost the voltage to 480V and then drop it back to 240V with another transformer at the motor location you will reduce the line drop to about 5 Volts. Going to 600 Volts will reduce the line loss to 3.2 Volts. You will get some voltage drop in the transformers but you should still get a net improvement in starting with adequately sized transformers.

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

RE: Motor startup and Voltage drop

Wouldn't it be nice if there was some kind of electronic switching unit that you could pair up in series with a voltage-boosting shunt capacitor that would provide vars during the pump start.  When the circuit is energized (motor start), the capacitor switch is closed.   The unit has some intelligence to know when the motor is almost up to speed  (timer... voltage sensing... current sensing... speed sending) so that it opens before overvoltage occurs.   

=====================================
(2B)+(2B)'  ?

RE: Motor startup and Voltage drop

Active power factor correction unit connected at the motor terminals? Seems kinda expensive but would work in principle.
  

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

RE: Motor startup and Voltage drop

Probably not fast enough, Scotty. Rather a dedicated circuit to connect the Caps to the motor before it is energized and then quickly disconnect as the motor comes up to speed.
The theory is the same.
As my aged memory slowly returns I solved a single phase starting torque issue by doubling the start capacitor value.
This was two 1/2 HP motors that should have been 3/4 HP motors. The 1/2 HP motors would handle the running loads quite well but did not have the breakaway torque needed to start the loads.
These motors have been running for years now with extra starting capacitors.
gerlewis You may try doubling the capacity of the starting capacitor. It is by far the cheapest and easiest possible solution.
The effect is two fold;
The capacity will supply some of the starting current and so reduce the line loss and voltage drop.
The extra capacity should increase the starting torque and help the motor get up to speed.
No extra control gear needed. Connect a capacitor in parallel with the existing starting capacitor. The starting switch will handle the switching.

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

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