Module 7745 Chapter D

With reference to Torque in a dc machine:

The magnitude of the force is proportional to

• The strength of the magnetic field, Flux Density,

  The current in the conductor,

  The length of the conductor influenced by the magnetic field.

• The value of back emf,

  The current in the conductor,

  The length of the conductor influenced by the magnetic field.

• The value of back emf,

  The current in the field windings,

  The length of the conductor influenced by the magnetic field.

• The value of back emf,

  The current in the conductor,

  The number of poles on the stator.

Armature torque is proportional to which of these statements

Select 1 or more

• The force experienced by each conductor

• The distance of each conductor from the centre of the axis

• The number of conductors

• The Cross Sectional Area (CSA) of all the conductors

• The distance of each conductor from the North Pole of the main magnetic field

Torque balance equation can be expressed as;

• Shaft torque = Armature torque - Lost torque

• Shaft torque = Armature torque + Lost torque

• $Shaft\ torque=\frac{Armature\ torque}{Lost\ torque}$

• $Lost\ torque=\frac{Armature\ torque}{Shaft\ torque}$

Define the term "back emf"

• In a motor an emf is induced in the conductors of an armature as it cuts lines of magnetic flux. This emf opposes the supply voltage.

• In a motor an emf is induced in the conductors of an armature as it cuts lines of magnetic flux. This emf enhances the supply voltage.

• In a motor an emf is induced in the poles of an armature as current increase in the windings. This emf distorts the main fields magnetic flux.

• As supply voltage increases, a capacitance is present between the supply line and the return line of the motor. The discharge of this capacitance is called back emf.

How can Back emf be calculated

• $E_b=V_s-I_aR_a$

• $E_b=V_s+I_aR_a$

• $E_b=\frac{V_s}{I_aR_a}$

• $V_s=\frac{E_b}{I_aR_a}$

What is back emf proportional to?

• The back emf is proportional to the speed of rotation and the strength of the magnetic field

• The back emf is proportional to the speed of rotation and the current in the conductor

• The back emf is proportional to the number of poles in the main field and the current in the conductor

• The back emf is proportional to the supply voltage and the current in the conductor

Calculate the step angle of a stepper motor with 24 stator poles and 12 rotor poles.

• 24^o

• 15^o

• 30^o

• 12^o

With respect to a Stepper Motor, which of these statements is true.

• The rotor must have an even number of poles

• The rotor must have an odd number of poles

• There must be more rotor poles that stator poles