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Flashcards in General Deck (57):

What’s special about universal motor

Designed to run on both a DC or single phase AC supply


Do universal motors run faster on DC compared to AC for a given torque

Faster, due to the reactance drop that is present in AC and not DC


Are inductive loads lagging or leading



Equation for the thermal time constant, tau

c / k

Thermal capacity / dissipation coefficient


Equation for temperature at infinity

P / k

Power losses / dissipation coefficient


Equation for temperature difference at time t after being switched on from cold

Tinf (1 - e^-t/tau)

T inf = P/k


Equation for temp difference when switched off from hot

To e^-t/tau


Equation for temp rise when on overload from cold

Ps/Pc Tinf (1 - e^-t/tau)


Equation for temp difference when entering a new heating cycle when already hot

T initial + (Tmax - Tinitial) (1 - e^-t/tau)


If you’re running at 75% of rated torque what does that mean for your current

You are at 75% of rated current


Equation for the emf constant in DC motors

K = pZ / pi

p is pole pairs
Z is number of conductors on the armature (Therefore Z/2 coils)


What term gives the power out in dc motors

Excitation x armature current


In a dc Motor when does max speed occur

At 0 torque


In a dc Motor when does max torque occur

At 0 speed


What do you do to dc Motor equivalent circuit when given open circuit voltage?

Open circuit therefore Ia = 0 so Ea = Va


What’s the dc Motor equation for armature equivalent circuit

Va = Ea + IRa


Torque equation for pure inertial load

T = J x rate of change of rotor speed with time


Torque equation for constant power load

T is proportional to 1/ rotor speed


Torque equation for fan load

T is proportional to rotor speed ^2


Torque equation for viscous damping

T = constant x rotor speed


Torque equation for frictional load

T = torque due to friction


If you have a permanent magnet froviding the flux, how do the dc Motor equations change

Phi, the flux per phase is constant therefore equations become

T= K I and E = k w


What term in dc Motor equation represents back emf and what are conditions for motoring and generating

Excitation (Ea)

For motoring voltage supplied must be greater than the back emf generated and vice versa for generating


How does the equivalent circuit equation change when the motor is decelerating

The current Ia becomes negative as it flows out of the dc machine


How to find energy taken from armature voltage supply

Integrate the power (P=IV) being supplied over the time duration.


2 benefits of PMBLDCM

Greater efficiency
High torque and power density


2 disadvantages of brushed dc motors

Commutator and brush wear
Inefficiency due to field winding losses and frictional losses


What supplies the magnetic field on the rotor for a BLDCM

Permanent magnets that are shaped to produce a sinusoidally distributed magnetic field, and glued to surface of the rotor.


Two types of BLDCM

Standard three phase winding on stator (sinusoidal back emf induced in the stator winding)

Concentrated stator windings such that the back emf induced by the rotor field is trapezoidal in time


In a trapezoidal BLDCM why do number of stator and rotor poles have to differ

To a valid a cogging torque, to avoid the tendency of the rotor permanent magnets to align themselves with the stator poles to minimise stored magnetic energy.


In a trapezoidal BLDCM what shape are the phase currents



Are the phase currents in phase with the back emf in trapezoidal BLDCM, and what does this mean for the torque and
angle and what’s the benefit of this


The torque angle is the angle between the stator-driven field and the rotor field, and will be at 90 degrees

90 degrees gives the max torque per amp of stator current.


Equation to find torque from output power which was found from using the standard v = e + ir equation for each of the phases at a given point

Pout = Tw


Example of a sensor used to provide rotor speed and position feedback in a BLDCM and its purpose

Hall effect sensor (a transducer that varies its output voltage in response to a magnetic field)

Used to know the correct time to switch on/of the high/low voltage to the different phases.


How todo sensorless BLDCM work

Utilising the fact that at any point one of the three phases is floating, they can detect the zero crossing point of the floating phase and then add a further delay of 30 electrical degrees.

Tends to be better at high speeds


Torque equation for BLDCM

T = 2kI

Factor of 2 comes from the fact 2 phases are excited at a given time


When does rated speed occur in BLDCM and how to calculate it

Max speed it can attain whilst still delivering rated torque


How does the standard v = e + ir equation change for a trapezoidal BLDCM

V = the line to line voltage supply

The other terms on the right hand side both get doubled due to the fact that 2 phases are excited at any given time


What is the duty ratio for a DC chopper circuit

The ratio of the time the signal is at Vdc and the whole cycle time

Means: Va = duty ratio x Vdc


If the load is purely inertial and the Motor is running at steady state, what can you say about the operation of the motor

No torque is needed hence Ia =0


Equation for electrical time constant

Tau e = La / Ra

Inductance devided by resistance


Equation for electro-mechanical time constant

Tau em = Ra J / (k phi)^2


Significance of time constants

They give the amount of time for Ia to rise to 63% of its final value (its rated value)

A measure of how quickly Ia responds to a change in Va.


Which time constant tends to dominate

Tau em (electromechanical)


How to adapt standard armature voltage equation to apply for non steady state conditions

Include a L dI / dt term to take inductance into account


What does critically damped mean in terms of damping

Damping = 1


For trapezoidal BLDCM what’s the relation between E line and E phase

E phase = E line / 2

E phase is what you use to work out emf constants/ rotor speeds


For trapezoidal BLDCM what’s the relation between R line and R phase

R ph = R line / 2


How do we analyse sinusoidal BLDC drive

With the per phase equivalent circuit for the synchronous machine lie in IB, creating the phaser diagram with V, E and IXs.


Equation for output power of a lossless sinusoidal BLDCM

P = 3VI cos phi (power factor)


At what torque angle is output torque and hence power per amp maximised

90 degrees


Torque equation for sinusoidal BLDCM

T = 3kI


For sinusoidal BLDCM what’s the Excitation eqn

E = k ws (synchronous speed)


For sinusoidal BLDCM what’s the Power out eqn

P out = T ws (synchronous speed


What sides of the phasor diagram of a sinusoidal BLDCM change when running at higher speeds (field weakening)

E still equals K ws but this increases due to greater speed and then current increases to rated value to complete the triangle.


For a sinusoidal BLDCM when does max speed occur

When torque equals 0, so the phasor diagram is a straight line with the current/reactance line acting in the opposite direction to voltage


What’s the torque angle

The angle between the rotor driven field and the stator driven field (on phasor diagram this is the angle between E and I lines)