chpt 23 Permanent Magnet Brushed DC Motors Applications

Question 1

what causes inductive kickback

Answer 1

the inevitable effect of trying to abruptly turn off the current flowing in an inductor.

Question 2

what is inductive kickback

Answer 2

a large voltage spike whenever the current flowing through an inductor is switched off

Question 3

what is the equation for the voltage potential across an inductor

Answer 3

V = L . dI/dt

Question 4

what is the time constant characterising the current rise time in an inductor

Answer 4

t (tau) = L/R

Question 5

what is the energy stored in the inductors magnetic field given by

Answer 5

E = 1/2 I^2 L

Question 6

what is the voltage developed through inductive kickback

Answer 6

kickback voltage or flyback voltage

Question 7

inductive kickback can be dangerous as it can spike voltages to over 1KV, how can this be dealt with

Answer 7

a strategy can be to add components to the control circuit that provides a low voltage current path, thereby reducing or ‘snubbing’ the magnitude of kickback voltage to safe levels

Question 8

what is the most straight forward means of reducing inductive kickback

Answer 8

place a diode across the terminals of the motor in a reverse-biased orientation

Question 9

what does placing a diode across the terminals of the motor in a reverse-biased orientation

Answer 9

it will limit the voltage spike caused by inductive kickback to the forward voltage drop of the diode plus the positive voltage supply

Question 10

what is the name of the diode in the reverse biased orientation across the motor terminals

Answer 10

kickback diode or flyback diode or diode snubber

Question 11

when does the diode snubber conducts

Answer 11

the diode only conducts when the collector voltage of the transistor exceeds the diode’s forward voltage drop of ~ 1V which only occurs during inductive feedback

Question 12

what is the voltage spike limited to with the kickback diode in place

Answer 12

Vc = Vs + Vt

positive voltage supply level plus the forward voltage drop of the diode

Question 13

when selecting a kickback diode, what two characteristics should be considered

Answer 13

First the diode should be able to switch very rapidly from the forward biased condition when it is conducting current to the case where it is reversed biased and blocking the flow of current.
this is the reverse recovery time
the second being to consider the maximum allowable current flow
the maximum current that the diode will experience under normal conditions is the maximum current through the motor: the stall current

Question 14

what is the stall current

Answer 14

the maximum current through the motor

Question 15

how is the energy from the collapsing magnetic field causes current to flow in the in a loop that includes the motor and the kickback diode dissipated

Answer 15

both in the motor since torque is generated by the current flow and I^2R losses occur in the coil windings and VxI losses across the diode

Question 16

what is the expression of the dissipation of energy

Answer 16

E stored = Integral ( Power(t) ) dt

or restated as
E stored = Average power x delta T

Question 17

by limiting the kickback voltage and power dissipation to the highest possible safe values, the duration required to dissipate the energy will be minimised and this aspect of system performance will be optimal.
what is a way of implementing this

Answer 17

add a resistor to the diode in the kickback loop. the additional resistance increases the energy dissipation rate by increasing the resistance R in the I^2R loss term. the value of the resistor should be selected so that the voltage at the resistor should be selected so that the voltage at the transistors collector terminal cant exceed the devices maximum collector-to-emitter voltage specification

Question 18

why should a zener diode be selected

Answer 18

a zener diode should be selected so that the voltage at transistors collector terminal wont exceed the devices maximum collector to emitter voltage

Question 19

standard and zener are not ohmic devices

what does this mean

Answer 19

they have no resistance only an inherent voltage drop when conducting

Question 20

what is the expression for the power dissipated resulting from kickback that takes into account how frequently a motor is switched on and off

Answer 20

P = (1/2.I^2.L) x frequnecy

Question 21

where is the power dissipated during kickback

Answer 21

dissipation will be distributed among the motor, the standard diode and the zener diode

Question 22

what is the least and most desirable configuration for snubbing

Answer 22

the least = diode only

the most = zener diode added to the diode

Question 23

what is a dual power supply

Answer 23

one with both a positive and negative voltage supply

Question 24

how can you make a motor rotate in both directions using a dual power supply

Answer 24

by having terminal on current can flow in either directions

Question 25

what are the transistors in a circuit for bidirectional current through a DC motor using a dual power supply

Answer 25

PNP transistor

NPN transistor

Question 26

what is an H bridge and what is it used for

Answer 26

by using a network of transistors in the shape of an H that will allow the current to flow through the motor in either direction using only one power supply

Question 27

what is shoot through current

Answer 27

shoot through current is when either the left or the right side of the H is on, current will flow from directly power to ground without any resistance

since there is no load or other components to resist the flow of current, it is essentially a short circuit
if sustained for a prolonged period of time one of the components are likely to be destroyed by excessive current levels

this unrestrained current is referred to as shoot through current

Question 28

what is dynamic breaking

Answer 28

a configuration of the H bridge that resists the rotation of a motor

when the leads of a spinning motor are shorted together, the current that is generated by the back-EMF causes current to flow in a loop

the direction of the current flow during dynamic breaking depends on the direction of the motor

Question 29

what is good practise for diode specification for peak intermittent current

Answer 29

should be above the maximum current that the circuit will encounter

Question 30

what is each side of the H bridge called

Answer 30

the half bridge

Question 31

what is an H bridge driver

Answer 31

a type of device that combines control logic and gate driver outputs designed to be connected to the gates of external N channel MOSFET in both the high side and low side positions to create a full H bridge

Question 32

what does PWM stand for

Answer 32

Pulse width Modulation

Question 33

what is PWM

Answer 33

a way of rapidly turning on and turning off the power supply to the motor, at high enough rates so that the effects of switching are negligible

Question 34

what is duty cycle

Answer 34

the duty cycle is the drive signal when switched on and off with a given period and is in the ‘on’ state at voltage Von for a friction of the period

Question 35

what is duty cycle equation

Answer 35

Duty Cycle(%) = Ton/T x 100%

Question 36

what is the PWM frequency

Answer 36

PWM frequency = 1/Period

Question 37

how can you tell if the PWM is done properly

Answer 37

the switching occurs too rapidly for the mechanical system to follow

Question 38

when is torque ripple become less noticable

Answer 38

at higher frequencies

Question 39

what is torque ripple

Answer 39

torque ripple is when the motor will accelerate and decelerate noticeably

Question 40

what can the PWM speed control be combined with to make a servomotor controller

Answer 40

position or velocity feedback