What exactly is d c motor running on no-load?

  • Thread starter dummie
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  • #1
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hey, can someone please explain what exactly is d c motor running on no-load?
 

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  • #2
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Nothing is attached to the shaft. It's spinning freely.
 
  • #3
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does no load means that the armature draws no current from the supply.as current drawn by armature is dependent on load torque
 
  • #4
davenn
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its still drawing some current ... as the motor is operating... its spinning.
its just that under full load its drawing max current

Dave
 
  • #5
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what happens if commutator is not placed on magnetic neutral plane?
 
  • #6
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what happens if commutator is not placed on magnetic neutral plane?
there would be excessive sparking where the brushes make contact with the commutator
 
  • #7
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does no load means that the armature draws no current from the supply.as current drawn by armature is dependent on load torque
Ideally yes!.... but practically there is always a small current drawn by the armature due to frictional losses at shaft bearing and due to windage and some amount of iron losses.
 
  • #8
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does the air gap flux in induction motor remain constant while it s speed changes depending on load? because over all flux is resultant of stator mmf which remain constant as long as voltage is not changed and the rotor mmf which depends on rotor current . now a s the speed increases, the rotor current decreases and hence the rotor mmf decreses. so according to this shouldn't the over all air gap flux change?
 
  • #9
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No the air gap flux does not decrease because it depends on stator terminal voltage. The stator just draws the appropriate current to counteract the rotor's effect.
 
  • #10
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okay. so the stator current increses when ever the rotor current increases on the demand of load.
thanks for making it clear .
 
  • #11
jim hardy
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DC motor? Stator?

usually the stator is the field
and the field can be any of several configurations:
permanent magnet (no windings)
Shunt wound, ie field current is set externally
Series wound, ie field is wound with large wire and placed in series with armature, which gives a motor with high starting torque
Compound wound - there are two field windings, a shunt and a series and they can aid or oppose one another to give desired speed-torque characteristic.

your two basic formulas are:
Volts = K X phi X RPM
and
Torque = 7.04 X K X phi X Ia
where
K is an empirical constant for machine under study, usually determined from a no load test
phi is flux usually lumped in with K from above test
Ia is armature current
RPM, Torque and Volts are self explanatory.

airgap flux is vector sum of flux from field and from current in armature.
latter is called 'armature reaction'
for lightly loaded machine it can be neglected

but it is interesting to read up on the three brush machine to see how clever old timers (model T Ford days) used armature reaction for control.
DC machines are interesting i suggest you spend some time in old textbooks.

old jim
 
  • #12
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well, actually i was talking about the flux in induction motor . but the information on dc motor by you is quite helpful.
actually i have been spending time reading books on ac and dc machines but its always so confusing and this forum really helps.
thanks jim
 
  • #13
jim hardy
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oops -
i answered wrong question......

hey, can someone please explain what exactly is d c motor running on no-load?

sorry about that

i found it helpful to visualize an induction motor as a transformer with a rotating secondary

so secondary sees slip frequency
and flux must support primary voltage minus i x r drop.

one can refine ad infinitum, leakage flux increases as internal mmf's oppose etc etc

but i think you are on right track.
 
  • #14
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load

i have this very stupid doubt but i need to ask this.
what does we actualy call load. In some texts that i have referred , the current supplied by,say, alternator is called load while in others the active power supplied by the alternator is called load.
so what should i understand the load to be?
 
  • #15
davenn
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i have this very stupid doubt but i need to ask this.
what does we actualy call load. In some texts that i have referred , the current supplied by,say, alternator is called load while in others the active power supplied by the alternator is called load.
so what should i understand the load to be?
very simply...

the load is what the power is being supplied to eg

for a RF transmitter the load will be the antenna

for a power supply (AC or DC) the load is what equipment is placed on the output of the supply drawing current. This is why we talk about the Voltage reading of the power supply when its loaded or unloaded. eg the unloaded Voltage of a solar panel is quite a bit higher than what its loaded Voltage is

for a motor, as referred to in this thread, the load is what the motor is driving (turning)

hope that helps :)

Dave
 
  • #16
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so , what should i call a constant load on alternator , when it is supplying constant current or when the power output is constant.
i think it should be the power?
thanks
 

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