Flashcards in General Deck (57):

1

## What’s special about universal motor

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

2

## 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

3

## Are inductive loads lagging or leading

### Lagging

4

## Equation for the thermal time constant, tau

###
c / k

Thermal capacity / dissipation coefficient

5

## Equation for temperature at infinity

###
P / k

Power losses / dissipation coefficient

6

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

###
Tinf (1 - e^-t/tau)

T inf = P/k

7

## Equation for temp difference when switched off from hot

### To e^-t/tau

8

## Equation for temp rise when on overload from cold

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

9

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

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

10

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

### You are at 75% of rated current

11

## 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)

12

## What term gives the power out in dc motors

### Excitation x armature current

13

## In a dc Motor when does max speed occur

### At 0 torque

14

## In a dc Motor when does max torque occur

###
At 0 speed

15

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

### Open circuit therefore Ia = 0 so Ea = Va

16

## What’s the dc Motor equation for armature equivalent circuit

### Va = Ea + IRa

17

## Torque equation for pure inertial load

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

18

## Torque equation for constant power load

### T is proportional to 1/ rotor speed

19

## Torque equation for fan load

### T is proportional to rotor speed ^2

20

## Torque equation for viscous damping

### T = constant x rotor speed

21

## Torque equation for frictional load

### T = torque due to friction

22

## 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

23

## 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

24

## 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

25

## How to find energy taken from armature voltage supply

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

26

## 2 benefits of PMBLDCM

###
Greater efficiency

High torque and power density

27

## 2 disadvantages of brushed dc motors

###
Commutator and brush wear

Inefficiency due to field winding losses and frictional losses

28

## 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.

29

## 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

30

## 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.

31

## In a trapezoidal BLDCM what shape are the phase currents

### Rectangular

32

##
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

###
Yes

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.

33

## 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

34

## 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.

35

## 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

36

## Torque equation for BLDCM

###
T = 2kI

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

37

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

###
Max speed it can attain whilst still delivering rated torque

38

## 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

39

## 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

40

## 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

41

## Equation for electrical time constant

###
Tau e = La / Ra

Inductance devided by resistance

42

## Equation for electro-mechanical time constant

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

43

## 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.

44

## Which time constant tends to dominate

### Tau em (electromechanical)

45

## 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

46

## What does critically damped mean in terms of damping

### Damping = 1

47

## 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

48

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

### R ph = R line / 2

49

## 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.

50

## Equation for output power of a lossless sinusoidal BLDCM

### P = 3VI cos phi (power factor)

51

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

### 90 degrees

52

## Torque equation for sinusoidal BLDCM

### T = 3kI

53

## For sinusoidal BLDCM what’s the Excitation eqn

### E = k ws (synchronous speed)

54

## For sinusoidal BLDCM what’s the Power out eqn

### P out = T ws (synchronous speed

55

## 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.

56

## 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

57