Direct Current Motors-2a Control of small brushed DC

Direct Current Motors-2a Control of small brushed DC.

Control of DC Motors. Switch ON-OFF Speed: - Analog - Pulse With Modulation (PWM) - Continuous Control Direction.

Switch. Diod Switch.

BJT.

BJT Switching Characteristics. When the transistor is saturated, it acts as a closed switch. When a transistor is in the cutoff region, it acts as an open switch. When it is in the active region, it acts as a current ( i B ) controlled current ( i C) amplifier. Realistically, transistor switching is not instantaneous. The turn-on time t ON of the transistor is the sum of the delay time t D and the rise time t R . Similarly, the turn-off time t OFF is the sum of the storage time t S and the fall time t F . The turn-on and turn-off time of a transistor limits the maximum switching frequency. Typical switching frequency for a power BJT is between 2 and 20 kHz. BJTs can switch at a higher frequency than thyristors but can handle less power . Power BJTs can handle currents up to several hundred amperes and V CE up to about 1 kV..

MOSFET. When operating in the enhancement mode, a MOSFET behaves very similar to a BJT. Instead of base current, the MOSFET behavior is determined by the gate voltage. When carefully controlling the gate voltage of a MOSFET, the transistor can be made to operate as a voltage controlled switch that operates between the cutoff (point A) and the Ohmic (point B) region. One advantage of a MOSFET device is that the MOSFET has significantly larger input impedance as compared to BJT. This simplifies the circuit that is needed to drive the MOSFET since the magnitude of the gate current is not a factor. This also implies that a MOSFET is much more efficient than BJTs as well as it can be switching at a much higher frequency. Typical MOSFET switching frequency is between 20 and 200 kHz , which is an order of magnitude higher than BJTs. Power MOSFETs can carry drain currents up to several hundreds of amperes and V DS up to around 500 V..

Insulated-Gate Bipolar Transistor (IGBT). IGBT is a voltage-controlled transistor that has the terminals identified in the same way as BJTs. IGBT is a four-layer device that has the similar construction of a MOSFET with an additional p layer. IGBT has the combined characteristics of the BJT and MOSFET. Similar to MOSFET, it has high input impedance and high switching frequency. It also has high power handling capacity like the BJT..

The IGBT has the high input impedance and high-speed characteristics of a MOSFET with the conductivity characteristic ( low saturation voltage ) of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current that will cause a large enough voltage drop to activate the thyristor . Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch-up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded. Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum junction temperature of the device under all conditions for guaranteed reliable operation. The on-state voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied..

Thyristors. GTOs. IGBTs. 5 5 5 kV. 4 kV. MCTs. 3 kV.

Control of Direction. Reversing the PM Motor To reverse the rotation direction of the PM motor , the polarity of the applied voltage must be reversed. One way to accomplish this is to have a motor-driver amp capable of outputting a positive and negative voltage. When the drive voltage is positive with respect to ground, the motor turns clockwise (CW). When the drive voltage is negative with respect to ground, the voltage polarity at the motor terminals reverses, and the motor rotates counterclockwise (CCW). The LM12 power op-amp is capable of providing positive and negative output voltages..

Relays In many applications, the drive amplifier cannot output both positive and negative voltages, in which case a switching circuit must be added to reverse the motor. One approach is to use a double-pole relay . When the relay contacts are up the positive voltage is connected to terminal A of the motor, and terminal B is connected to the negative voltage. When the relay contacts are down, the positive voltage is connected to terminal B, and terminal A goes to the negative voltage, thus effectively reversing the polarity..

H-Bridge Forward-reverse switching can also be done with solid-state devices using four FETs. When Q1 and Q4 are on, and Q2 and Q3 are off the current I 1,4 causes the motor to turn clockwise. When Q2 and Q3 are on, and Q1 and Q4 are off the current I 3,2 flows in the opposite direction and causes the motor to turn counterclockwise. The entire switching operation can be performed by a single IC, such as the Allegro A3952 . This IC contains four separate driver transistors that are controlled by internal logic to operate in pairs . The A3952 controls a motor-supply voltage of up to 50 V with up to 2 A of output current..

L293 H-bridge chip.

DC Motor Analog Drive Speed Control. Using a single power transistor: The circuit could be either: 1- the common emitter (CE) configuration, which gives current and voltage gain, or 2- the common collector (CC) configuration, which gives only current gain. When the base voltage ( V B ) is increased (beyond the forward-bias voltage), the transistor begins to turn on and let the collector current ( I C ) flow. The collector current is 30–100 times greater than the base current, depending on the gain of the transistor. Once the transistor starts to conduct, I C increases with V B more or less linearly. Note that all of I C goes through the motor, providing the drive current.

Heating Problem with BJT. Power transistors are physically bigger than signal transistors and are designed to carry large currents. In control systems, they are used to provide the drive current for motors and other electromechanical devices. When a transistor has a large current and voltage at the same time, the resulting power ( V C I C ) must be dissipated in the form of heat. A typical power transistor is designed to operate up to 200°C (360°F) above ambient temperature. However, its power capacity is derated proportionally for temperatures above 25°C . The power transistor case has a flat metal surface to provide a thermal escape path for the heat. Therefore, to operate at anywhere near the rated power, the transistor must be mounted firmly to the chassis or a metal heat sink—a piece of metal with cooling fins to dissipate the heat into the air Many times the case itself is the collector terminal. If the collector must be kept electrically insulated from the mounting chassis, then a special mica insulator is used, together with a thermally conducting white grease..

Power IC Drive (LM12). The power IC driver is a single-package DC amplifier with a relatively high current output. An example is the LM12 (National Semiconductor) The high-power operational amplifier can supply up to 13 A with a maximum voltage of ±30 V. As in any op-amp circuit, feedback resistors are added to adjust the gain to any desired value..

Darlington Power Transistor Drive. The Darlington configuration consists of two CC amplifiers connected in such a way that the first transistor directly drives the second. Although the voltage gain is only 1 (maximum), the current gain can be very high. The transistor shown in the Figure is a TIP 120, which has a current gain of 1000 and a maximum output current of 5 A. The motor must be placed in the emitter path of the output transistor. A separate small-signal amplifier, probably an op-amp, would be needed to provide any voltage gain required..

Power MOSFET. Notice the output current ( I D ) is 0 A when the input voltage (V GS ) is in the 0-5-V range but then climbs to 12 A when V GS rises to 13 V. Using a power MOSFET, the motor is in series with the drain, which means the FET will provide both voltage and current gain. The gate voltage is supplied from an op-amp circuit that is designed to interface the controller with the FET ..

DC Motor Control Using Pulse-Width Modulation. Pulse-width modulation is an entirely different approach to controlling the torque and speed of a DC motor. Power is supplied to the motor in a square wavelike signal of constant magnitude but varying pulse width or duty cycle. Duty cycle refers to the percentage of time the pulse is high (per cycle)..

PWM Control Circuits.