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
David Castor
www.cvoes.com
RE: Motor startup and Voltage drop
"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.
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RE: Motor startup and Voltage drop
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.
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RE: Motor startup and Voltage drop
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
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
RE: Motor startup and Voltage drop
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor startup and Voltage drop
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(2B)+(2B)' ?
RE: Motor startup and Voltage drop
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If we learn from our mistakes I'm getting a great education!
RE: Motor startup and Voltage drop
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
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"Why not the best?"
Jimmy Carter