354 amps on a 70 amp 3 phs breaker

P

power 4 people

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Location
california
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electrician
Got a call, 20 hp Vacuum motor wont stay on. Newly installed after being moved from previous location that had commercial 3 phs power. Now that panel is fed from a 40hp phase converter. client turned on the breaker and flipped the switch. motor ran for about 3 seconds and breaker tripped. I turned off breaker and began to check wiring. Found a wire on the motor contact load side that missed the lug when tightened and was possibly in contact or near but not tightened under lug. I installed that wire correctly and started up the motor again. Ramped up to 354 on my amp clamp before breaker tripped.

Could it be damaged control form improper wiring?

Motor needs a Soft Start installed?

Damaged motor from improper wiring?


Voltage is 240 phase to phase. 208v A phase to ground, B, and C, both 120v to ground.

Motor diagram shows it is wired for 208-230v

Fan is spinning in correct rotation.
 
1) Motor started and _ran_ for 3 seconds, or motor tried to start but didn't spin?

2) Was the current rising or falling during the starting attempt?

3) Did you measure voltage during starting?

4) Is there any sort of valve or mechanical unloader on this system that needs to be set for proper startup?

5) What is the nameplate rated current, and what is the current breaker?

6) Does the motor have a proper overload/starter?
 
jim dungar said:
Effectively a rotary phase convertor creates a center grounded delta system.
Click to expand...
But the measured volts of the derived third phase isn't as consistent as high leg delta supply normally is and can change as loading changes.

The other two lines is direct connect of the incoming single phase supply.
 
Is this application starting the idler motor and allowing it to reach running speed before attempting to start the driven load motor?

That idler needs to be at full speed to be able to give the most starting torque for the driven load.
 
power 4 people said:
Got a call, 20 hp Vacuum motor wont stay on. Newly installed after being moved from previous location that had commercial 3 phs power. Now that panel is fed from a 40hp phase converter. client turned on the breaker and flipped the switch. motor ran for about 3 seconds and breaker tripped. I turned off breaker and began to check wiring. Found a wire on the motor contact load side that missed the lug when tightened and was possibly in contact or near but not tightened under lug. I installed that wire correctly and started up the motor again. Ramped up to 354 on my amp clamp before breaker tripped.

Could it be damaged control form improper wiring?

Motor needs a Soft Start installed?

Damaged motor from improper wiring?


Voltage is 240 phase to phase. 208v A phase to ground, B, and C, both 120v to ground.

Motor diagram shows it is wired for 208-230v

Fan is spinning in correct rotation.
Click to expand...
along with a bigger breaker maybe if you have some spare larger wire you can temp it up on top of the ground and connect shop vac and see if voltage drop is an issue upon start up and run. Also hook back up to old power and see if vacuum still currently works and get amp readings there.
 
70 amp breaker seems too small if this is a 20 HP motor @ 240 volts.

NEC FLA = 54 amps. Typical thermal magnetic breaker for this would be at least 100 amps, but more likely 125 amps.

If the 70 held at previous location you probably had high enough impedance in supply circuit to limit starting current enough to not exceed the trip level of the 70 amp breaker. I know I've had installs that sometimes would hold sometime would not hold when trying to start a 20 HP motor on a 100 amp breaker and had to increase or actually in my cases I went to 100 amp time delay fuses instead. They have different trip curve than thermal mag breakers and was the lesser cost option at the time.

I don't know where the 70 amp breaker is located in the circuit, since OP said three pole breaker I'm presuming on the three phase output of the phase converter. Remember that the single phase supply current will be 1.732 times the three phase output current before factoring in any inefficiencies so that supply circuit needs to have an ampacity of 93.5 and probably at a 125% factor making it 116 amps. To handle starting current reliably I'd bet you need at least a 175 or 200 amp breaker on the single phase supply.

Your rotary phase converter instructions will tell you what max overcurrent protection should be.
 
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