DC Motor Drive - Imbalanced input (line/neutral) and output (motor + / - ) current

[PaulKraemer]

Hi,

I am trying to troubleshoot a blower drive system that includes a DC drive board [KB Electronics KBCC-255 (9940A)]. The DC drive board controls a 0-180 VDC output to a DC motor that drives a blower. The magnitude of the 0-180 VDC output is determined by a 0-10 VDC signal that the DC drive board receives from a closed loop speed controller. There are three identical blower drive systems on the same machine. Each blower drive system utilizes the same components and is wired the same way. The attached image shows how these components are wired. The drives I am focusing on are the #1, #2, and #3 Supply Blowers.

All three Supply Blower drives were working fine for a long time, but the 15 amp fuse that protects the #2 Supply Blower drive recently started blowing repeatedly. The #1 and #3 Supply Blowers are still working fine.

We were able to get the #2 Supply Blower running by replacing the 15 amp fuse with a 30 amp fuse, and we took some current measurements with a clip-on ammeter while all three blowers were running.

On the #1 and #3 Supply Blowers (which are working fine), we measured very similar current on the line (going into terminal L1) and the neutral (coming out of terminal L2) in the input power circuit for the drive. We also measured very similar current on the output power wires to the motor, one coming from terminal A1 (+) and the other coming from terminal A2 (-).

On the #2 Supply Blower (which is misbehaving), we measured noticeably different current on the line and the neutral of the input power circuit. We also measured noticeably different current on the A1 (+) and the A2(-) output power conductors to the motor. Our exact measurements are shown below....

	#1 Supply	#2 Supply	#3 Supply
Input Line Current	1.32 amps	16.5 amps	5.4 amps
Input Neutral Current	1.8 amps	4.5 amps	4.5 amps
Motor A1 (+) Current	2.4 amps	4.6 amps	4.6 amps
Motor A2 (-) Current	2.2 amps	12 amps	4.5 amps

When we took these measurements, #3 Supply Blower was running at a faster speed than #1 Supply, so the higher current does not surprise me. The seeming imbalance between line and neutral and A1 (+) and A2 (-) on #2 Supply does concern me.

If anyone here can give me an idea what might cause this, I would greatly appreciate it. I'd like to determine if the problem is more likely in the DC drive or external to the DC drive (perhaps the motor and/or the filter through which the input power circuit passes).

Any advice or suggestions will be greatly appreciated.

thanks in advance,
Paul

 

[Gr8blu]

1) All three DC systems are NOT wired the same, according to the image appended. Units 1 and 2 take power from "Phase 2" to neutral; Unit 3 takes it from "Phase 3" to neutral. I suspect the Unit 1 connection was intended to be from "Phase 1" to neutral - it would be a good idea to physically verify the drawing to the reality.
2) The fuse(s) in question are on the incoming power line (L1 termination) which means they are monitoring the incoming current, not the "return" current. If the fuse is operating (i.e. "blowing") then something is making that particular system (i.e. Unit 2) draw more power - to the point where it overloads the protection.
3) The DC machines driving the supply blowers are presumably 2 HP at 180 Vdc (and some operating speed - likely in the 1800 rpm vicinity). This would give a rated current at full load in the 5.2 A range. As a result, Unit 3 motor is (apparently) operating at nominally 100% electrical load, while unit 1 is operating at roughly 35% electrical load. Unit 2 is ... well ... unit 2.

Question(s)
a) Where does the output air from each blower go? Do they act as a series path to the exhaust - or are they parallel?
b) has anything changed on the process side - such as a position change on dampers/louvers, etc. - that might allow recirculation between different fans?
c) what does a thermal image tell you about the temperature(s) of all three motors and all three "drives"? Are they nominally the same or different?
d) is there a different acoustic noise to the impeller connected to Unit 2 (e.g. rattling, whistling, screeching, etc.) that might denote a mechanical issue with the blower itself?

 

[waross]

I suspect either a ground fault somewhere,
OR
Someone has grounded the neutral in one or more locations.
Look for the missing current on the protective earth conductor.
If you find the missing Amps on the PE, then try to follow the current back to either the drive or the motor, or possibly both.
You don't report the motor running hotter than normal so I expect a fault external to the motor.
Given that the motor currents are unbalanced, I would first look at the motor connections and look for a possible motor ground fault very close to the A2 end of the motor winding.
Depending on the brush arrangement, a brush lead may be shorting to ground.