Direct current motor BDC Commutator Current direction = is - - PowerPoint PPT Presentation

William Sandqvist william@kth.se

Direct current motor BDC

l I B F ⋅ ⋅ =

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

William Sandqvist william@kth.se

Direct current motor BDC

Together 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

William Sandqvist william@kth.se

The DC motor in idle

The motor reaches the speed ω0 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 [ ω

A

U K =

Voltage constant

( commutator )

William Sandqvist william@kth.se

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 Φ 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.

Φ I

Gramme ring

• The right angle between the magnetfield

Φ 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.

A

U

° 90

( DC-motor Achilles Heel )

William Sandqvist william@kth.se

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

William Sandqvist william@kth.se

ω , 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 IA⋅RA in the resitance of the winding. The voltage that now are balancing the generator emf E will be lover. E=UA- IA⋅RA. Therby the rotainal speed will decrease. If we want the same speed we now has to increase the voltage UA.

] Nm/A [

A

I M K =

Torqe constant

Motor constant

William Sandqvist william@kth.se

E R I U

A A

+ ⋅ = K I M

A ⋅

= ω ⋅ = K E 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.

rpm 5700

• Ex. An unknown motor?

William Sandqvist william@kth.se

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 …)

V 12 A 10 rpm 4500

Power supply

02 , 60 2 5700 12 60 2 ) a = = = = π π ω n U U K

A A

Unknown motor!

William Sandqvist william@kth.se

] Nm [ 2 , 10 02 . ) b = ⋅ = ⋅ = ⇒ =

A A

I K M I M K Ω = ⋅ − = ⋅ − = ⇒ = ⋅ = + ⋅ = 26 , 10 60 2 4500 02 , 12 60 2 60 2 ) c π π π ω ω

A N A A A A A

I n K U R n K E E I R U

• The motor is no longer unknown!

Current will provide an exact measure

• n torque!

Resistance can also be measured directly with an OHM-meter if the motor shaft is locked.

William Sandqvist william@kth.se

PWM-voltage

William Sandqvist william@kth.se

Current, Torque Speed Voltage

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 = α UA = α⋅UD.

D

U

D A

U U ⋅ = α

D A

U U ⋅ = α PWM-voltage

Pulse operation free-wheeling

William Sandqvist william@kth.se

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

William Sandqvist william@kth.se

D

U

A

U

A

I

A A A D

I U P I U P ⋅ = ⇔ ⋅ ⋅ =

2 1

α

D

i

D

i

D

i

D

i

W 50 A 1 V 50 W 50 66 , A 1 V 75 V 50 75 66 , V 75 A 1 66 ,

2 1

= ⋅ = ⋅ = = ⋅ ⋅ = ⋅ = = ⋅ = ⋅ = = = =

A A A D D A D A

I U P I U P U U U I α α α

D

U

D

U

D

U

D

U 66 , = α V 50 =

A

U

• LA keeps IA constant
• Motor inertia keeps UA constant

V 75 =

D

U A 1 =

A

I

A

U

D

i PWM

Gear

William Sandqvist william@kth.se

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 25 , 75 25 4 25 75 = = = =

A B A B

n n M M

Lego motor

William Sandqvist william@kth.se

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!

William Sandqvist william@kth.se

PIC-processorn PWM

William Sandqvist william@kth.se

Controling the speed of a motor – one rotary direction

PIC enhenced-PWM

William Sandqvist william@kth.se

PWM H-bridge

William Sandqvist william@kth.se

PWM H-bridge CW

William Sandqvist william@kth.se

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

PWM H-bridge CCW

William Sandqvist william@kth.se

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

PWM H-bridge at lab

William Sandqvist william@kth.se

BDC

+ V 15 <

D

U + V 5

ICL7667

CCW CW

CCW CW

William Sandqvist william@kth.se

William Sandqvist william@kth.se

TIMER2 servo update

TMR2IF postscaler

• verflow 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.

William Sandqvist william@kth.se

William Sandqvist william@kth.se

Brushless DC motor

With electronic commutation one can avoid sparks in the motor. By turning the engine "in and out", with the permanent magnets

• n 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

William Sandqvist william@kth.se

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 windings

Electronic commutation Stator winding Hall sensors Rotor Logic

William Sandqvist william@kth.se

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.

William Sandqvist william@kth.se

Torque-motor

With the magnets on the rotor one can increase the number

• f 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?

William Sandqvist william@kth.se

E R I U

A A

+ ⋅ = K I M

A ⋅

= ω ⋅ = K E 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

• bvious that it is a sort of AC motor.

William Sandqvist william@kth.se