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
Thanks in advance!
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
Thanks in advance!
RE: Erratic measurement of field current in DC motor
HOW are you measuring the current?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Erratic measurement of field current in DC motor
RE: Erratic measurement of field current in DC motor
Keith Cress
kcress - http://www.flaminsystems.com
RE: Erratic measurement of field current in DC motor
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
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
RE: Erratic measurement of field current in DC motor