chpt 23 Permanent Magnet Brushed DC Motors Applications Flashcards
what causes inductive kickback
the inevitable effect of trying to abruptly turn off the current flowing in an inductor.
what is inductive kickback
a large voltage spike whenever the current flowing through an inductor is switched off
what is the equation for the voltage potential across an inductor
V = L . dI/dt
what is the time constant characterising the current rise time in an inductor
t (tau) = L/R
what is the energy stored in the inductors magnetic field given by
E = 1/2 I^2 L
what is the voltage developed through inductive kickback
kickback voltage or flyback voltage
inductive kickback can be dangerous as it can spike voltages to over 1KV, how can this be dealt with
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
what is the most straight forward means of reducing inductive kickback
place a diode across the terminals of the motor in a reverse-biased orientation
what does placing a diode across the terminals of the motor in a reverse-biased orientation
it will limit the voltage spike caused by inductive kickback to the forward voltage drop of the diode plus the positive voltage supply
what is the name of the diode in the reverse biased orientation across the motor terminals
kickback diode or flyback diode or diode snubber
when does the diode snubber conducts
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
what is the voltage spike limited to with the kickback diode in place
Vc = Vs + Vt
positive voltage supply level plus the forward voltage drop of the diode
when selecting a kickback diode, what two characteristics should be considered
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
what is the stall current
the maximum current through the motor
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
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
what is the expression of the dissipation of energy
E stored = Integral ( Power(t) ) dt
or restated as
E stored = Average power x delta T
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
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
why should a zener diode be selected
a zener diode should be selected so that the voltage at transistors collector terminal wont exceed the devices maximum collector to emitter voltage
standard and zener are not ohmic devices
what does this mean
they have no resistance only an inherent voltage drop when conducting
what is the expression for the power dissipated resulting from kickback that takes into account how frequently a motor is switched on and off
P = (1/2.I^2.L) x frequnecy
where is the power dissipated during kickback
dissipation will be distributed among the motor, the standard diode and the zener diode
what is the least and most desirable configuration for snubbing
the least = diode only
the most = zener diode added to the diode
what is a dual power supply
one with both a positive and negative voltage supply
how can you make a motor rotate in both directions using a dual power supply
by having terminal on current can flow in either directions
what are the transistors in a circuit for bidirectional current through a DC motor using a dual power supply
PNP transistor
NPN transistor
what is an H bridge and what is it used for
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
what is shoot through current
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
what is dynamic breaking
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
what is good practise for diode specification for peak intermittent current
should be above the maximum current that the circuit will encounter
what is each side of the H bridge called
the half bridge
what is an H bridge driver
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
what does PWM stand for
Pulse width Modulation
what is PWM
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
what is duty cycle
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
what is duty cycle equation
Duty Cycle(%) = Ton/T x 100%
what is the PWM frequency
PWM frequency = 1/Period
how can you tell if the PWM is done properly
the switching occurs too rapidly for the mechanical system to follow
when is torque ripple become less noticable
at higher frequencies
what is torque ripple
torque ripple is when the motor will accelerate and decelerate noticeably
what can the PWM speed control be combined with to make a servomotor controller
position or velocity feedback
