Electrical- Motors Flashcards
Formula for force on a current-carrying wire
F=BIl
Where l is length of wire
Only for current at right angles to field
What happens when a current-carrying wire moves at right angles to a magnetic field?
There will be a voltage in the wire that opposes the current. Physically makes sense as it takes work to move the wire.
Finding kinetic energy of current-carrying wire only subject to magnetic field
Need velocity. Equate F=BIl and F=ma to get a. Integrate assuming velocity is initially zero. Get velocity=BIlt/m.
Use normal KE formula to get
KE=(BIlt)^2/2m
Finding electrical work done when current-carrying wire only subject to magnetic field
Need equation for voltage. Know EMF=Blv (v is velocity). Know velocity=BIlt/m from finding kinetic energy equation. Subbing in means EMF=(Bl)^2It/m. Power=IxEMF to get the I^2. Work done is the integral of power. So
W=(BIlt)^2/2m (the same as kinetic energy)
Describe the layout of a DC motor using permanent magnets
Looking at the cross-section. Has a circular iron casing split into two parts by a gap at the top and bottom. Has a North Pole at the top and a South Pole at the bottom inside the casing so the field goes vertically down. There are windings of wire going into and out of the page with a pivot in the centre. A commutator is used to swap the direction of current flow in the wires. There are two brushes on opposing sides, one to deliver a positive potential and the other a negative. Every 180° the connections swap (due to the gaps) so current changes direction.
Electrical symbol for a motor
Circle connected to wires by two rectangles on opposing sides.
Describe the circuit diagram for a wound field motor
A transformer arrangement, but the right coil is replaced by a motor. The left side is the ‘field’ and has a separate power supply (not always drawn) and has a resistor and a current coming from the top. The right side is the ‘armature’ which has a resistor and a current coming down from the top.
Formula for back emf of a motor involving flux
Ea=kaφω
Where a is always subscript and refers to armature
ka is motor constant
Back emf is caused by spinning of the motor
Derivation starts from E=Blv
Formula for back emf of a motor involving current
Ea=kIfω
Where k is not ka
If is the current in the field winding
Derivation comes from φ=NIf/S meaning the φ in other equation is proportional to If
Formulae for torque, power, Vf and Va for all DC motors
P=ωT P=EaIa Therefore T=EaIa/ω Vf=IfRf Va=Ea+IaRa Where P is power, T is torque, f means field, a means armature
What can be said about the flux in a permanent magnet motor?
It is constant
Describe the circuit for a series DC motor
Same as for wound field but below the field winding and motor, the two are connected in series and there is no separate voltage source for the field side. Means the field and armature currents are the same. Still has current going in from top of armature.
Formula for angular frequency in relation to torque for a series DC motor
ω=V/rt(kT)
Where V is potential difference across normal armature branch.
Describe the circuit for a shunt-connected DC motor
The field and armature sections are connected by two wires so the field winding is in parallel with the motor
Why can a series DC motor (universal motor) run on DC or AC?
If the field current is reversed, the field changes direction which would normally reverse torque. However the armature current is also reversed so the torque actually stays in the same direction.
