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# Erratic measurement of field current in DC motor

## Erratic measurement of field current in DC motor

(OP)
Hi, everyone...

I have an issue regarding a DC motor with the following dataplate:
Armature: V = 460 V, I = 130 A
Field; V = 310 V, I = 1.55 A
Power: 54 kW
RPM: 50-2,400

The problem is when I measure the field current I get 1.9 A on one terminal and I get 1.26 A on the other terminal...

1) Shouldn't the current be the same on both terminals since it's a serial circuit?
2) How concerned should I be of the 1.9A measurement on one terminal if the motor is supposed to take 1.55A at most

### RE: Erratic measurement of field current in DC motor

Tell us more.
HOW are you measuring the current?

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

### RE: Erratic measurement of field current in DC motor

(OP)
I'm measuring using a hook ammeter at the F+ and F- terminals of the drive

### RE: Erratic measurement of field current in DC motor

DC ammeters see the DC earth's magnetic field. That's why they often have zero-adjusts. Slightly moving them can change the ambient field the probe is intercepting. That's can be why you're seeing different values on different terminals. It can also be strong leakage flux from the motor itself being seen differently on the various terminals.

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

### RE: Erratic measurement of field current in DC motor

erickgl: As itsmoked mentioned, the measurement device you're using can easily be affected by strong magnetic fields, such as that of the Earth itself and (almost certainly) by the electromagnetic fields in the DC machine stator. When your "true" reading is down below about 10 A, this is definitely a known issue. The measurement device uses Hall effect sensing to determine the current flowing through the conductor - in essence it is measuring the magnetic field produced by the conductor - which is MUCH weaker than either the Earth or "main field" influence.

To answer your other question: if the actual field current is that much higher than rated condition, then yes - you should definitely be worried. You're putting in (1.9 / 1.55 = 1.22) per unit current, which translates roughly to (1.22)^2 = 1.5 per unit thermal stress on the winding. That means that if the main field winding is intended to operate at 70 C above ambient, it will actually be operating at about (1.5 * 70 = 105 C) above ambient. Depending on the ambient, that may very well be enough to severely shorten the life expectancy of the insulation system. This is because the "life" is halved for every 10 C above design point. So - if we start with 10 year life (which is about normal for something with this kind of rating), at 105 we would have roughly (((10/2)/2)/2) = 1.2 years on the insulation. Minus however long it has already been in operation.

Converting energy to motion for more than half a century

### RE: Erratic measurement of field current in DC motor

Clamp on hall effect ammeters are not that accurate in measuring DC current, especially at such low range. As others have pointed out, moving the clamp meter diametrically from one end to other end will affect the reading. Move the meter and see if the same current difference is seen between + and - leads. If yes, then it is likely your shunt field(s) has interturn shorts with part of + current shunted locally at the shorted area.

If you are seeing increased sparking at your brushes, then most likely it's interturn shorts in the shunt field(s). Do an 110 / 220 V AC voltage drop test across the shunt field. The shorted coil will show significant low voltage when compared to healthy coils.

Muthu
www.edison.co.in

### RE: Erratic measurement of field current in DC motor

Would clamping both wires to see if there is a current difference work? I'm thinking maybe if both wires are held together and centered in the meter window.

### RE: Erratic measurement of field current in DC motor

I think about the other option is leakage current through the fraim.

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