A universal motor is a special type of motor which is designed to run
on either DC or single phase AC supply.
These motors are generally series wound (armature and field winding
are in series), and hence produce high starting torque.
That is why, universal motors generally comes built into the device
they are meant to drive.
Most of the universal motors are designed to operate at higher speeds,
exceeding 3500 RPM. They run at lower speed on AC supply than they
Share 56 run on DC supply of same voltage, due to the reactance
voltage drop which is present in AC and not in DC. There are two
basic types of universal motor :
(i)compensated type and (ii) uncompensated type
Speed control of Universal Motor
Figure . – Equivalent circuit of the universal motor
In figure, the parameters are
Ra - rotor winding resistance
Rf - field winding resistance
La - rotor winding inductance
L f - field winding inductance
u(t) - terminal voltage
e(t) - back emf
i(t) - current in the machine
J - moment of inertia of machine and
D - viscous damping constant
T(t) - electromagnetic torque
T L - load torque
ω (t) - angular velocity of machine
By applying Kirchhoff’s voltage law on the circuit in figure the
following equation can be derived.
This differential equation can be solved if an expression for the
induced back emf can be determined. This expression will be
derived in next section.
As the rotor rotates in a magnetic field, an electromotive
force is induced in the turns of the rotor winding. The emf induced
in one turn of a coil is given by equation. This is the general form
of Faraday’s law for a moving conductor in a time varying
Where v is the velocity vector of the conductor and B is the magnetic
flux density vector in which the conductor is moving. is the path along
the conductor in the magnetic field and is the surface bounded by .
The flux cutting back emf produced in one turn of a coil according to
figure together with the simplifications made, is given by equation.
The magnitude of the velocity of the conductors in figure 3 is given
v = ωm r
where ωm is the angular velocity of the rotor and is the radius of the
rotor. Further on, the magnitude of the electric field along the
conductor is r
Ea = ωm rBn
Bn is the normal-component of the magnetic flux density, i.e. the
component perpendicular to the velocity of the conductor. The
electric field Ea is in the opposite direction in conductor 2 compared
to conductor 1.
By integrating the electric field along the length of the conductor
moving in the magnetic field, the total emf voltage produced in the
conductor is given. Assuming a constant electric field along the
conductor, the emf induced in one conductor is:
e conductor= E ⋅l conductor = ωm r Bn⋅l conductor
l conductor is the length of the conductor moving in the magnetic field,
i.e. the active length of the machine. Since the electric fields in the
two conductors are of opposite directions, the total induced emf
voltage in one turn is:
The flux density seen by the conductors beneath each pole in the
machine is calculated according to:
Where p is the number of poles in the machine and c is the pole
coverage factor. rpole is the radius of the pole which
approximately is the same as the radius of the rotor i.e.:
rpole ≈ r
Further on, l pole is the same length as , i.e. l conductor
lpole = lconductor
Again, index n stands for the normal component, and n ψ is the
normal component of the flux beneath a pole. Substituting equation
8- 10 in eq.7 gives the expression of the induced emf in one turn of a
eturn = ωm
This is the induced emf in one turn of a coil due to a conductor
moving in a magnetic field. The total emf induced in the rotor
winding consisting of N turns is the single turn voltage multiplied by
the total number of turns. If the N turns are connected in parallel paths
a, the total emf induced is divided by the number of parallel paths .
Therefore, the total induced emf in the rotor winding is:
is called the armature constant (or the rotor constant).
This constant contains information on the configuration of the rotor
winding. Assuming non-saturated operating conditions, the flux n ψ
is directly proportional to the current in the field winding.
Since the field winding is connected in series with the
rotor winding, the flux is proportional to the rotor current
e =KaKψωm Ia
Where Kψ is the flux constant.