Direct current motor BDC - KTH · Direct current motor BDC ... Postscaler can be used to generate...

William Sandqvist [email protected]

Direct current motor BDC

lIBF

The current I turns the loop, when it has turned half a lap the

current direction is changed so it continnues to turn all the

way around and so on. – The motor principle!

Current direction

is changed!

Commutator


Direct current motor BDCTogether with the ”motorprinciple” also the

”generatorprinciple” is in effect. In a loop that

rotates in a magnetic field a AC voltage is induced

that is proportional to the rotational speed. The

switch, the commutator, changes this AC voltage to

a DC voltage.

DC voltage

Generator-

emf


The DC motor in idle

The motor reaches the speed 0 when the direct voltage emf

is exactly balanced by the generator emf. Then, ideally, the

current to the motor IA = 0.

The DC Engine idle speed is therefore

directly proportional to the supply

voltage UA.

Motorconstant:

]V/rad/s[

AUK Voltage constant

( commutator )


Current is connected to the coil with two "brushes" at right angle to the

motor permanent magnets. The coil current I and motor permanent

magnetic field generates a force to rotate the winding, but because the

brushes continually makes contact with the "new" windings the ring with

the spool will rotate, but the coil magnetic field will stay.

Today's DC motors have windings with other, more

efficient designs, but with the same commutation

principle.

I

Gramme ring

The right angle between the magnetfield and the current I gives the motor maximal and

constant torque.

The original winding from the 1800s was called

Grammes ring. It consisted of a coil wound around an

iron ring.

AU

90

( DC-motor Achilles Heel )


The commutator wear of sparking and has to be renovated after

some time. This is not an argument against the use of a DC motor

to a function prototype, but it can be a problem for a finished

product.

A commutator tacked in a lathe for renovation – bumps after sparking are

lathed away – then the engine runs smoothly again.

The motor under load


,n

Basically, you control the speed with voltage. To have the benefit of an engine

to the shaft must mechanically deliver a torque M. Then there will be a

prortional current IA through the winding and there will be a voltage drop

IARA in the resitance of the winding.

The voltage that now are balancing the generator emf E will be lover. E=UA-

IARA. Therby the rotainal speed will decrease. If we want the same speed we

now has to increase the voltage UA.]Nm/A[

AI

MK Torqe constant

Motor constant


ERIU AA

KIM A KE

In catalogs there are often used two different motor constants.

A voltage/speed constant and a torque/current constant. This

happens when one is not using SI units, otherwise it had

become one and the same constant.

rpm5700

Ex. An unknown motor?


An experiment. 12V idle, rotational speed n0 is measured to 5700 rev/min.

Motor is braked with a block of wood against the shaft and then the

current IAN is measured to 10 A and the speed nN to 4500 rev/min.

Calculate the motor constant K. What was the braking torque M?

Which resistance RA has the motor winding?

Tachometer

Unknown motor (but it was for free …)

V12

A10rpm4500

Power supply

02,0

60

25700

12

60

2)a

n

UUK AA

Unknown motor!


]Nm[2,01002.0)b A

A

IKMI

MK

26,010

60

2450002,012

60

2

60

2)c

A

NA

A

AAA

I

nKU

R

nKEEIRU

The motor is no longer unknown!

Current will

provide an

exact measure

on torque!

Resistance can

also be measured

directly with an

OHM-meter if

the motor shaft is

locked.

PWM-voltage


Current, Torque

SpeedVoltage

The DC motor speed is controlled with the

voltage. The motors's own inertia equalizes

the voltage pulses - so it goes equally well

with the mean value of a PWM voltage as

with a constant DC voltage

DutyCycle = UA = UD.

DU

DA UU

DA UU

PWM-voltage

Pulse operation free-wheeling


It's the free wheel that allows the cyclist to rest on the pedals in the downhill slope.

Hence the name "free-wheeling diode."

In pulse operation, we also need to include the motor winding inductance

LA. Current through an inductance must be continuous (as motor torque),

Therefore, there is a "free-wheeling diode" which current can continue

through during that part of the PWM time when the voltage is 0.

bicycle freewheel

PWM PWM

Pulse operation free-wheeling


DU

AU

AI

AAAD IUPIUP 21

Di Di Di Di

W50A1V50

W5066,0A1V75

V507566,0V75A166,0

2

1

AA

AD

DADA

IUP

IUP

UUUI

DUDU

DU DU66,0

V50AU

LA keeps IA constant

Motor inertia keeps UA constant

V75DU

A1AI

AU

Di

PWM

Gear


DC motors often have high speed n

and low torque M - the ordinary is

that we need just the opposite, low

speeds and powerful torque.

Gears can be used to shift down the speed and

to the corresponding proportion shift up

the torque

31

75

253

25

75

A

B

A

B

n

n

M

M

Lego motor


It is very possible that we now have

exaggerated the theory part of DC motors

when you consider what engine it is we are

going to experiment with!

PIC-processorn PWM


Controling the speed of a motor – one

rotary direction

PIC enhenced-PWM


PWM H-bridge


PWM H-bridge CW


CCP1CON.7 = 0; Change one bit in CCP1CON.

PWM H-bridge CCW


CCP1CON.7 = 1; Change one bit in CCP1CON.

PWM H-bridge at lab


BDC

V15DU

V5

ICL7667

CCW

CW

CCW CW


TIMER2 servo update

TMR2IF postscaler

overflow kan an-

vändas som samp-

lingsklocka.

If Timer2 is used with the ECCP-unit to generate a PWM-signal then the

Postscaler can be used to generate interrupt at eg. each 16:th PWM-pulse

(1:1 … 1:16). In the interruptoutine one can read the AD-converter and

the update the PWM DutyCycle.


Brushless DC motor

With electronic commutation one can avoid sparks in the motor.

By turning the engine "in and out", with the permanent magnets

on the rotor and the windings in the stator one avoids

transferring power to the rotor.

Commutator wear is a major

problem for the brushed DC

motor.

Rotor with windings

Permanent

magnets in stator

To the left the ordinary BDC-motor (with brushes),

to the right the brushless BLDC-motor.

Permanent

magnets in rotor

Stator with windings


Brushless DC-motor

The engagement of the windings is done with semiconductor

switches (transistors) and the rotor angle is detected by

magnetic Hall-sensors. The semiconductor switches can

simultaneously pulse width modulate the voltage so that the

speed is controlled.

Permanent

magnets in rotor

Stator with windingsElectronic

commutationStator winding

Hall

sensors Rotor

Logic


Brushless DC-motor

A

B

C

With the help of six switching transistor, a DC voltage is "chopped"

into pulses. Switching sequence consists of six steps. The switches are

controlled so that in each moment there are two switches in different

branches connected, one to supply and one to ground. The third branch

is "disconnected".

BLDC-motor

is common for

small motors,

but nowdays

also for bigger.


Torque-motor

With the magnets on the rotor one can increase the number

of magnetic poles – it has the same effect as a downshift of

the speed (and upshift of torque), but without the losses

that had been with a mechanical gear.

Is this a DC motor?


ERIU AA

KIM A KE

You can use exactly the same calculation

methods as for DC motors. It is therefore

justified to call the BLDC motor for a

"brushless dc motor" even though it is

obvious that it is a sort of AC motor.

Direct current motor BDC - KTH · Direct current motor BDC ... Postscaler can be used to generate...

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