peteuk1
Electrical
- Nov 30, 2004
- 1
I am currently doing experiments with a separately excited, brushed DC machine, 24V, 18 Commutator Slots, 2 poles, 1 pair of brushes. From theory, two salient stator poles and a non-salient armature cause rotor position dependant variation in mutual inductance between each rotor winding and the stator windings. The mutual inductance will then periodically vary between a positive and negative maximum with single rotor speed. The self-inductance of each winding of the armature also varies with rotor position. However the self-inductance will vary around a mean amount with a frequency twice the rotor speed.
In practice we do not have access to each individual winding of the rotor. Merely the brushes placed 90 degrees relative to the stator poles provide access in order to feed current to armature windings. In order to maintain a torque producing, steady misalignment of the m.m.f axis of stator and rotor field the commutator switches the voltage accordingly. Hence the armature inductance is now seen from a fixed point, namely the q-axis where the brushes are placed on. The self-inductance and mutual inductance will only slightly vary locally within each rotor slot. However with respect to one revolution of the apparent armature inductance remains constant.
Carried out experiments have shown that for the given motor that the apparent inductance/impedance significantly varies periodically with frequency twice the rotor speed. This has been measured by means of chopping the supply voltage every 6ms and measuring the growth current at a fixed time within each probing period.
The current varies from 100mA – 175mA with frequency twice the rotor speed.
Furthermore, the armature resistance varies periodically from 30 -35 Ohm with each revolution. It can also be seen that the switching between slots causes a change in resistance. Therefore, 18 local extremes can be seen during each revolution.
Likely reasons for that may be that the armature exhibits some saliency and thus a change in the air-gap causes variation in self-inductance. Furthermore, rotor eccentricity results in a periodically changing air-gap and thus variable inductance.
QUESTIONS:
What causes usually, in practice this variation in apparent inductance/impedance?
May that also be due to a fault in the armature (e.g. one non-conducting/faulty coil)?
Many thx,
pete
In practice we do not have access to each individual winding of the rotor. Merely the brushes placed 90 degrees relative to the stator poles provide access in order to feed current to armature windings. In order to maintain a torque producing, steady misalignment of the m.m.f axis of stator and rotor field the commutator switches the voltage accordingly. Hence the armature inductance is now seen from a fixed point, namely the q-axis where the brushes are placed on. The self-inductance and mutual inductance will only slightly vary locally within each rotor slot. However with respect to one revolution of the apparent armature inductance remains constant.
Carried out experiments have shown that for the given motor that the apparent inductance/impedance significantly varies periodically with frequency twice the rotor speed. This has been measured by means of chopping the supply voltage every 6ms and measuring the growth current at a fixed time within each probing period.
The current varies from 100mA – 175mA with frequency twice the rotor speed.
Furthermore, the armature resistance varies periodically from 30 -35 Ohm with each revolution. It can also be seen that the switching between slots causes a change in resistance. Therefore, 18 local extremes can be seen during each revolution.
Likely reasons for that may be that the armature exhibits some saliency and thus a change in the air-gap causes variation in self-inductance. Furthermore, rotor eccentricity results in a periodically changing air-gap and thus variable inductance.
QUESTIONS:
What causes usually, in practice this variation in apparent inductance/impedance?
May that also be due to a fault in the armature (e.g. one non-conducting/faulty coil)?
Many thx,
pete