Direct current motor BDC Commutator Current direction = ⋅ ⋅ is changed! F B I l 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! 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 William Sandqvist william@kth.se
The DC motor in idle The motor reaches the speed ω 0 when the direct voltage emf is exactly balanced by the generator emf. Then, ideally, the current to the motor I A = 0. • The DC Engine idle speed is therefore Generator- emf directly proportional to the supply voltage U A . Motorconstant: U K = A [ V/rad/s ] Voltage constant ω William Sandqvist william@kth.se
( commutator ) Gramme ring Φ • The right angle between the magnetfield Φ and the current I gives the motor maximal and I constant torque. ° 90 The original winding from the 1800s was called Grammes ring. It consisted of a coil wound around an iron ring. 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. U A • Today's DC motors have windings with other, more efficient designs, but with the same commutation principle. William Sandqvist william@kth.se
( 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. William Sandqvist william@kth.se
The motor under load 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 I A through the winding and there will be a voltage drop I A ⋅ R A in the resitance of the winding. The voltage that now are balancing the generator emf E will be lover. E = U A - I A ⋅ R A . Therby the rotainal speed will decrease. If we want the same speed we now has to increase the voltage U A . M K = [ Nm/A ] Torqe constant I A William Sandqvist william@kth.se
Motor constant = ⋅ + U I R E A A = A ⋅ = K ⋅ ω M I 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. William Sandqvist william@kth.se
Ex. An unknown motor? An experiment . • 12V idle, rotational speed n 0 is measured to 5700 rev/min. • Motor is braked with a block of wood against the shaft and then the current I AN is measured to 10 A and the speed n N to 4500 rev/min. • Unknown motor (but it was for free …) Power supply 12 V 5700 rpm 4500 rpm 10 A Tachometer • Calculate the motor constant K . • What was the braking torque M ? • Which resistance R A has the motor winding? William Sandqvist william@kth.se
Unknown motor! U U 12 = = = = A A a ) K 0 , 02 π π ω 2 2 n 5700 60 60 Current will M = ⇒ = ⋅ = ⋅ = provide an b ) K M K I 0 . 02 10 0 , 2 [ Nm ] A I exact measure A on torque! π 2 = ⋅ + = ⋅ ω ω = ⇒ c ) U R I E E K n A A A 60 Resistance can π π 2 2 − ⋅ − ⋅ also be measured U K n 12 0 , 02 4500 A N 60 60 = = = Ω directly with an R 0 , 26 A I 10 OHM-meter if A the motor shaft is • The motor is no longer unknown! locked. William Sandqvist william@kth.se
William Sandqvist william@kth.se
PWM-voltage PWM-voltage Voltage Speed = α ⋅ U U A D Current, Torque The DC motor speed is controlled with the voltage. The motors's own inertia equalizes U the voltage pulses - so it goes equally well D with the mean value of a PWM voltage as = α ⋅ U U A D with a constant DC voltage DutyCycle = α U A = α⋅ U D . William Sandqvist william@kth.se
Pulse operation free-wheeling bicycle freewheel PWM PWM In pulse operation, we also need to include the motor winding inductance L A . 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. It's the free wheel that allows the cyclist to rest on the pedals in the downhill slope. Hence the name "free-wheeling diode." William Sandqvist william@kth.se
Pulse operation free-wheeling = i U 75 V D D U U U U U α = D D D D 0 , 66 D = U U 50 V • Motor inertia keeps U A constant A A = I 1 A A i i i i D D D D PWM I A U • L A keeps I A constant A = ⋅ ⋅ α ⇔ = ⋅ P U I P U I 1 D A 2 A A α = = = = α ⋅ = ⋅ = 0 , 66 I 1 A U 75 V U U 0 , 66 75 50 V A D A D = ⋅ α = ⋅ ⋅ = P U I 75 V 1 A 0 , 66 50 W 1 D A = ⋅ = ⋅ = P U I 50 V 1 A 50 W 2 A A William Sandqvist william@kth.se
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 M 75 n 25 = = = = B B 4 0 , 25 M 25 n 75 A A William Sandqvist william@kth.se
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! William Sandqvist william@kth.se
William Sandqvist william@kth.se
PIC-processorn PWM Controling the speed of a motor – one rotary direction William Sandqvist william@kth.se
PIC enhenced-PWM William Sandqvist william@kth.se
PWM H-bridge William Sandqvist william@kth.se
PWM H-bridge CW CCP1CON.7 = 0; • Change one bit in CCP1CON. William Sandqvist william@kth.se
PWM H-bridge CCW CCP1CON.7 = 1; • Change one bit in CCP1CON. William Sandqvist william@kth.se
PWM H-bridge at lab CCW CW BDC ICL7667 < CW + U 15 V + D 5 V CCW William Sandqvist william@kth.se
William Sandqvist william@kth.se
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. William Sandqvist william@kth.se
William Sandqvist william@kth.se
Brushless DC motor Rotor with windings Stator with windings Commutator wear is a major problem for the brushed DC motor. Permanent Permanent magnets in stator magnets in rotor To the left the ordinary BDC-motor (with brushes), to the right the brushless BLDC-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. William Sandqvist william@kth.se
Brushless DC-motor Stator with windings Electronic Stator winding commutation Logic Permanent Hall magnets in rotor sensors Rotor 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. William Sandqvist william@kth.se
Brushless DC-motor A B C BLDC-motor is common for small motors, but nowdays also for bigger. 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". William Sandqvist william@kth.se
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. William Sandqvist william@kth.se
Is this a DC motor? = ⋅ + U I R E A A = A ⋅ = K ⋅ ω M I 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 obvious that it is a sort of AC motor. William Sandqvist william@kth.se
William Sandqvist william@kth.se
Recommend
More recommend
