Unexpected Volt/Amp readings on 7.5 hp motor

Status
Not open for further replies.
B

bburn

New member
Location
Crawfordsville IN
Had some higher than expected electric bills in my shop so decided to check a few devices.

I have 490 line volts feeding a 30kva delta (480/240) transformer to run my 240V 3ph equipment. When checking voltage/amperage on my 240V 7.5hp table saw (no load) I had the following readings.

T1, 9.12A - 0V :?
T2 5.81A - 235V
T3 6.00A - 235V

I've never bothered checking before but doesn't zero volts seem odd. Or maybe it's normal for transformed power. It's not just that machine either. I also checked another 240V 3.5KV motor running off the same transformer and found a zero voltage leg on it as well.
 
Welcome to the forum. I'm not sure where or how you checked the voltage, or between what two points for all three readings, but 0V between a phase and ground would either be a blown fuse on that leg or a corner grounded delta where one phase is 0V; the former would mean your 3ph machines would be running on one phase and probably not happy at all.
 
bburn said:
Had some higher than expected electric bills in my shop so decided to check a few devices.

I have 490 line volts feeding a 30kva delta (480/240) transformer to run my 240V 3ph equipment. When checking voltage/amperage on my 240V 7.5hp table saw (no load) I had the following readings.

T1, 9.12A - 0V :?
T2 5.81A - 235V
T3 6.00A - 235V

I've never bothered checking before but doesn't zero volts seem odd. Or maybe it's normal for transformed power. It's not just that machine either. I also checked another 240V 3.5KV motor running off the same transformer and found a zero voltage leg on it as well.
Click to expand...
Welcome from me too.
I'm with JFletcher on this. It may be a supply problem. Measure between phases. T1-T2, T2-T3, T3-T1.
 
Besoeker said:
Welcome from me too.
I'm with JFletcher on this. It may be a supply problem. Measure between phases. T1-T2, T2-T3, T3-T1.
Click to expand...
Then disconnect (de-energize) this motor and take the same measurements on the line side of the disconnect (still energized).

Wear appropriate PPE for the entire process.
 
The mismatched currents seem odd. But the voltage just suggests that the isolated 240V delta is corner grounded.
With a corner ground some mid-winding shorts in the motor might not draw enough current to trip the OCPD.

Sent from my XT1585 using Tapatalk
 
GoldDigger said:
The mismatched currents seem odd. But the voltage just suggests that the isolated 240V delta is corner grounded.
Click to expand...
That's why checking the line to line voltages has been suggested. That's probably the quickest way of showing whether it is a supply problem or a motor problem.
GoldDigger said:
With a corner ground some mid-winding shorts in the motor might not draw enough current to trip the OCPD.
Click to expand...
And he's running it off load.
 
Status
Not open for further replies.
Top