A variable speed DC motor

 

 

With a PWM controller, this plate can be measured very precisely the energy supplied (and thus the speed of rotation of the axis) to an electric motor, both in vacuum load. The adoption of a special MOSFET with integrated sensor can suspend the supply current when the engine uses too.

SPECIFICATIONS
- Power supply: 12 to 24 VDC
- Peak current: 10 A
- Designed for low voltage motors
- Frequency adjustable from 300 Hz to 2 kHz
- Protection current adjustable from 0 to 10 A
- Speed ​​adjustable between 0 and 100%
- MOSFET with integrated current sensor
- Motor voltage: 12 to 24 VDC
- Dimensions: 15 x 8.5 x 2.5 cm.


Component technology and design techniques are constantly evolving and fixtures that we offer and are the refl. The variable speed motor with brushes DC which is the subject of this article is a good example: the component that constitutes the heart is a truly remarkable novel. The controller couples to the traditional technology PWM (pulse width modulation) current limiter operated by the fact that the MOSFET, which is confi ed power impulse engine has an auxiliary terminal can provide a current whose intensity is proportional to that across the drain-source junction. A component belonging to a new class of "POWER-MOSFET" (produced by International Rectified st) and is controlled just like a traditional or IRF540 BUZ10, etc..
The assembly proposed here will suit anyone who wants to control the speed of rotation of the shaft of an electric motor: the regulation is fairly accurate because it is based on the amplitude variation of the voltage applied to motor brushes connected to points "OUT" or on the power output. This regulation is performed by sending the motor current pulses whose duration is directly proportional to the desired speed. Compared to the voltage regulation, the production of pulses of amplitude equal to those which the engine needs, but of variable width, provides a fairly uniform rotation, even supported, because the torque does not change : in fact the shaft does not rotate in steps (or facets) but uniformly with an angular velocity dependent parity applied load, the pulse / pause ratio of the pulse train supplied by the regulator.

The circuit diagram

 

Figure 1: Diagram of the variable speed DC motor.

The electrical diagram of the fi gure 1 shows a circuit quite complex in appearance, but actually quite simple. The generator consists of a rectangular pulse width modulation, a voltage translator, a fi nal of power and a floor current sensor capable of responding to the pulse generator to block. A voltage regulator U2 7809 to stabilize the power supply 9 V to the circuit required: indeed, this voltage powers the PWM controller and current protection, which require reference potentials, demand stability. Thanks to the venerable fi ltering capacity obtained with C7, C8 and C10, U2 is a wall that peaks and drops in voltage due to MOSFET switching on the motor armature can hardly pass. The fuse operates, cutting the main supply line when the circuit or the engine tends to consume more current than the limit allows.
The heart of the PWM generator is achieved by coupling a particular astable multivibrator and a comparator operational amplifier with: in fact, to produce the pulse-width modulated, we compare the potential for a continuous waveform produced by a quasi triangular U1 pin, mounted in a NE555 astable confi guration. This timer produces a square wave loading and discharging a capacitor inserted in the network delay, that is to say, by letting it charge through R1, D1, R2 and R23, and then discharging it through R2, R3 , R23 and D2, when the logic level of the output is inverted. Pin 3, not used, is not in the diagram.
The exponential component is taken across C1: comparison, devoted to the operational U3a (confi ured as a comparator), the quasi triangular waveform (pin 2) and the voltage fed to pin 3 by T4 and the trimmer R24, determines the output (pin 1) a rectangular waveform whose duty cycle depends strictly on the amplitude of the voltage due precisely to T4 and R24.
Let's see how the comparator: the output is at logic high (about the potential of the positive supply) when the value of this component across C1 is lower than that applied to pin 3 and, conversely, it starts at about 0 V if the voltage taken from the quasi triangular astable is more amplitude than the reference potential. It follows that the duty cycle is directly proportional to the amplitude of the voltage delivered by T4 because if it increases, the periods during which the pin 2 is positive with respect to the 3 are reduced, and the length of pauses between two consecutive pulses of the rectangular wave. Of course we are talking about relative pulse / period of the component out of the pin 1 because, at the end, that is to say at the motor terminals, the situation is reversed: the higher the voltage reference the lower the width of current pulses produced.
The diagram shows that the reference of the operation is achieved through two components: a fi xed potential, customizable when setting a variable and R24 from the circuit current limiting output.
To understand the operation of the latter, we need to know where is fi ne the wave produced by the PWM comparator.
Pin 1 drives the power MOSFET T3 through a translator and amplifier stage consists of complementary symmetrical T1 and T2 (PNP and NPN, respectively). This circuit provides a behavior equal to the negative half-wave and the positive half-wave: it produces control pulses perfectly square (with rising edges and falling net) to the gate of the MOSFET. The collectors of T1 and T2 drivers then send impulses to the leg 1 (trigger) of T3. Note that the zener D1 is mounted essentially as extreme protection of the MOSFET for the event, by destruction of U2, the voltage on the supply line regulator exceed 12 V. Whenever it receives a positive pulse, the fi nal power leads between drain and source and thus leaves through the current with virtually any resistance: indeed, its Rdson (measured electrical resistance between drain and source in full conduction ) is typically 0.077 ohm, which n'infl uence not the voltage applied to the motor.
At the same time the leg 2 through R18 is a current directly proportional to browsing the motor windings and therefore the drain-source circuit, which allows the floor to limit its role. Next to each positive pulse at the terminals of R18, another pulse is created (in phase and duration proportional to the current product): the result is a rectangular waveform which, properly fi lter by a cell low-pass ( R17/C9) becomes a continuous component, then sent to the second operational U3 (a CA3240).
U3B is the element in the system, decides when the current protection must be made: by adjusting appropriately the trimmer R25 is in setting the switching threshold and hence the level that the current supplied to the engine can reach without the Regulator suspends the production of pulses.
It is obvious that the greater the potential in the trim slider, the higher the intensity allowed in the MOSFET and vice versa. For a precise and effi cient triggering protection, we made stable switching threshold by feeding R25 through a network of resistive voltage is stabilized by D3 (polarized directly it gives exactly 0.7 V). In addition, the comparator with hysteresis is of type. Functionality rétroactionnant obtained in the positive operational U3B: thus, once the governor intervened, the current released by the MOSFET must fall below the value that caused the outbreak, if the output stage allows the engine off.
Let one last detail: how the protection operates. The output of the command U3B NPN T5, mounted inverter logic and interface to the regulation stage. Its collector feeds the base of the PNP T4. We can see that when the current in the motor exceeds the limit set by R25 and pin 7 of the CA3240 from the high logic level, the saturation force T5, the current in the collector of the latter determines the R20 ends of a potential difference suffi cient to saturate as T4. Its collector current, fed to R7, raises the potential U3a reference to a level above the maximum reached by the wave almost triangular, which prevents the output of the comparator to the logic low level and to drive the MOSFET Output T3.
Ultimately, when the intensity set as the limit is exceeded by R25, U3a passed the state of its pin 1 of logic low to logic high and holds it there. T1 and T2 is prohibited conduct, maintaining almost to ground the gate of the MOSFET. Note that the drain current is missing, the current sensor and also stops at the ends of R18 there is no more voltage drop: the comparator U3B may again raise its output logic high to logic low in prohibit leaving T5 and T4.
Thus blocking the PWM modulator is released and the MOSFET can restart and carry the load until a new ... any excess power consumption, in which case the protection will intervene again and stop again T3. The current limiting circuit has a dynamic behavior: it is able to "feel" at every moment what is happening to output, that is to say in the engine.
Conclude the analysis with the zener U4, playing the dual role of suppressors of potential peaks and suppressing reverse voltages, two phenomena caused by switching "ON / OFF" on charges for inductive ment (as are the engines) . In fact, this zener prevents from spreading in the supply line voltages higher than his own and especially of opposite polarity (in this case the zener is practically a short circuit).

Figure 2a: Layout diagram of the components of variable speed DC motor.

Figure 2b: Design, scale 1, the PCB of the drive motor speed.

Component List
R1 .... 4.7 kOhms
R2 .... 4.7 kOhms
R3 .... 10 k
R4 .... 47 kOhms
R5 .... 4.7 kOhms
R6 .... 4.7 kOhms
R7 .... 10 k
R8 .... 4.7 kOhms
R9 .... 470 kOhms
R10 .. 6.8 kOhms
R11 .. 3.9 kOhms
R12 .. 3.9 kOhms
R13 .. 6.8 kOhms
R14 .. 10 k
R15 .. 220 Ω
R16 .. 220 Ω
R17 .. 1 k
R18 .. 220 Ω
R19 .. 10 k
R20 .. 4.7 kOhms
R21 .. 10 k
R22 .. 10 k
R23 .. trimmer 25 kOhms
R24 .. trimmer 10 k
R25 .. 1 k trimmer
C1 .... 10 nF multilayer
C2 .... 10 nF polyester
C3 .... 100 uF 50 V electrolytic
C4 .... 100 nF multilayer
C5 .... 4.7 uF 63 V electrolytic
C6 .... 1 uF 100 V electrolytic
C7 .... 100 nF multilayer
C8 .... 100 uF 50 V electrolytic
C9 .... 2.2 uF 100 V electrolytic
C10 .. 1000 uF 25 V electrolytic
C11 .. 100 nF multilayer
C12 .. 10 uF 63 V electrolytic
C13 .. 1000 uF 25 V electrolytic
D1 .... 1N4148
D1 .... 1N4148
D1 .... 1N4148
DZ1 .. 12 V zener
DZ2 .. 3.9 V zener
DZ3 .. 3.9 V zener
U1 .... NE555
U2 .... L7809
U3 .... CA3240
U4 .... MBR745
T1 ..... BC557
T2 ..... BC547
T3 ..... IRC540
T4 ..... BC557
T5 ..... BC547

Misc:
2. 2-pole terminal blocks
2. supports 2 x 4
3. 3 10 mm bolts MA
1. sink
1. fuse horizontal
1. 10 A fuse
Cation unless specifically noted, all resistors are 1 / 4 W 5%.


The practical realization
We can now proceed to the construction of the device. The plate fits on a single sided PCB with a fi gure 2b gives a scale drawing. When you have before you, get first all the components not going to sink in helping you to fi gures 2 and 3 of the list. Then fi xez U4 and T3 on the sink (not to mention the insulation kit: bolt nylon + + mica or silicone grease bolt nylon + Tefl is one) and U2 (without insulation kit), if the heatsink on xez the circuit board and fi nally solder all the legs with a small iron and waiting a few seconds between the welds. Sink (finned aluminum rail with two flat parts) must have a thermal resistance of 4 ° C / W.

Settings
Installation is complete and verified to use cations made, connect the device to the motor and power supply: battery or AC 230 V capable of supplying a voltage from 12 to 24 VDC and a current equal to at least what the engine consumes to order. But in no case exceed a voltage of 25 V and a current of 10 A.
Afi n to avoid any problems, you should first connect the power and then, after putting the cursor R25 towards its end connected to R5 (counterclockwise) to connect the motor in accordance with the polarity of many terminal (if it will run in the opposite direction).
Put the cursor halfway to R25 in order to maintain the current protection disabled, at least micharge and do the same with the frequency trimmer (R23). Then turn clockwise until you see the R24 motor shaft turn, then go in the end achieved: you can make a comparison by disconnecting the terminal ls fi and connecting them in parallel with food). If rotation begins too late, try to influence the frequency trimmer (R23) by turning the cursor in one direction or another, until you find the position that allows the engine to start shooting shortly after the position minimum of R24. The frequency is important because the DC motors when they are controlled by PWM, react differently depending on the pulse frequency.
So find the optimal frequency and adjust the trimmer of the current limiter empirically or by making more scientific as: turn off the power then in series with the motor, connect a multimeter (DC, 10 A full scale) reconnect the power and overload the motor to increase the current.
At some point, an indication of the meter must be fi ger to a value determined corresponding to the threshold set: for the modifi er, play R25 (counter-clockwise: the protection occurs for currents always lower, meaning time: it occurs for currents ever closer to the boundary 10 A). Load the engine by preventing its rotation: attention, do not hurt! For example with a drill press DC powered, try to break a hard material with a large drill bit (do not try to curb the chuck by hand!).

Figure 3: Photograph of a prototype of the plate of variable speed motor.

The controller U2, U4 the diode and the MOSFET T3 should be fi xed to a sink Rth 4 ° C / W. Between the heatsink and the components do not forget to insert a sheet of mica insulation (coated with white grease silicone) or TEFL is gray. For fi xing components to the heatsink, use small nylon bolts.

Figure 4: Connections.

There are two terminals on the stage with two poles to connect to the power and motor control.
The drawing also shows the adjustment controls present in the circuit: frequency, duty cycle, current limitation. Their function is explained in detail in the text.