* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Explain the two methods off speed control for a DC motor
Power engineering wikipedia , lookup
Electrification wikipedia , lookup
Power inverter wikipedia , lookup
Electrical ballast wikipedia , lookup
Distributed control system wikipedia , lookup
History of electric power transmission wikipedia , lookup
Current source wikipedia , lookup
Commutator (electric) wikipedia , lookup
Opto-isolator wikipedia , lookup
Resistive opto-isolator wikipedia , lookup
Pulse-width modulation wikipedia , lookup
Control theory wikipedia , lookup
Electrical substation wikipedia , lookup
Electric machine wikipedia , lookup
Power MOSFET wikipedia , lookup
Surge protector wikipedia , lookup
Stray voltage wikipedia , lookup
Resilient control systems wikipedia , lookup
Three-phase electric power wikipedia , lookup
Electric motor wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Brushless DC electric motor wikipedia , lookup
Buck converter wikipedia , lookup
Control system wikipedia , lookup
Distribution management system wikipedia , lookup
Power electronics wikipedia , lookup
Alternating current wikipedia , lookup
Rectiverter wikipedia , lookup
Mains electricity wikipedia , lookup
Voltage optimisation wikipedia , lookup
Induction motor wikipedia , lookup
Stepper motor wikipedia , lookup
Explain the two methods off speed control for a DC motor and calculate the speeds for the following motor. A 480v shunt wound motor has a base speed of 1000rpm (16.67 rev/s) with an armature current of 150A and resistance of 0.2ohms. calculate the speed if the armature current is 75A and additional resistance of 0.6ohms is connected in series with the armature. (field current remains constant) calculate the speed when the armature is 75A and field current is reduced by 80% . There are mainly two methods of speed control of DC motors, that is, armature control and field control. Speed of a DC motor is given by V I a Ra k Where ω is speed(rad/sec), V is supply voltage across armature, is field flux, Ra is armature resistance and k is constant. We note that we can control speed by varying numerator or denominator. When we vary numerators, we basically vary voltage across the armature; hence, it is called armature control of speed of DC motors. When we control denominator, we basically control field flux. Armature control method is used for controlling speeds up to rated speed. Filed control method is used when speed is to be controlled beyond rated speed. Speeds up to rated speed can be achieved by controlling armature voltage. Speeds beyond rated speed can be achieved by increasing armature voltage but it will reduce the motor life unnecessarily. Thus, speeds beyond rated speed are never achieved by armature voltage control. For this purpose, field current is reduced. Reduction of field flux results in less produced torque. However, power remains same. Hence, this is also called constant power control. Armature voltage results in constant torque. Hence, armature voltage control is also called constant torque control method of controlling speed. 460V DC shunt motor, 1000 rpm rated speed, 0.2 ohms armature resistance. From the figure, we have back EMF Eb = 460 – 150*0.2 = 430 volts We know that Eb ZNp 60a Hence, kI f ZNp 60a 430 if field flux is proportional to field current I f . kI f Zp 1000 60a --- (1) Ia = 75 amps, armature resistance = 0.2 +0.6 = 0.8ohms. Hence, back EMF Eb = 460 -75*0.8 = 400 volts. If speed is N rpm, we have 400 kI f Zp N 60a --- (2) Dividing (1) by (2), we get 430 1000 400 N Or, N = (400/430)*1000 = 930.232 rpm Ans. If field current is reduced by 80%, we have 400 Dividing (1) by (3), we get k 0.2 I f Zp N 60a --- (3) 430 1000 400 0.2N Or, N = (400/430*0.2)*1000 = 4651.162 rpm Ans. Explain using a minimum of 200 words the concepts of speed control using. a)a chopper circuit from a DC source (b) a full bridge circuit from a DC source (c) compare a) and B) with reguards a controlled AC to DC rectifier circuit and how forward and reverse operation is possible. (d) the comparisons between speed control and torque control using closed loop feedback. (a)CHOPPER FED DC DRIVES Choppers basically reduce the voltage to suit the requirements. Voltage is applied to the motor terminals in pulses. The motor responds to the average voltage and not the maximum voltage. A chopper basically acts as a fast acting switch. When switch is on, the supply voltage is applied to the motor terminals. The motor accelerates during on period. When chopper is off, the motor decelerates. The current during coasting period is circulated through a freewheeling diode. As armature has generally quite high inductance, the operation, in most of the cases, results in continuous current mode. Four quadrant operation is possible through choppers. There are five types of choppers, that is, class A,B,C,D and E. Class A chopper operates in quadrant I only of torque-speed diagram. Type B chopper operates in 1st and 2nd quadrants. Four quadrant choppers are used for very high loads. Choppers are generally voltage impulse controlled or current pulse controlled. Fast acting, that is, inverter grade thyristors were initially used in choppers but, at present, MOSFETS or IGBTs are used in choppers. This became essential as switching off of thyristors is quite difficult. When duty ration is high, thyristors can be used. Design of commutating circuit for choppers is challenging job. A chopper fed motor has to dissipate more losses because of the presence of ripple in the output of the choppers. This requires special design of the motors. Choppers find extensive use in railway traction. (b) and (c)FULL BRIDGE CIRCUIT AC power, 1-phase or 3-phase, is generally available. We convert AC power into DC power for operating DC drives. When DC is available, it may directly be used for operating DC drives. When the available voltage level is higher than the drive can use, we reduce the voltage available voltage level by using choppers or any other means. DC series motors are frequently used in railway traction where available voltage is quite higher than what motors require. Separately excited motors are extensively used in industry as we can control precisely control speed as well as torque. Induction motors can not be used where precise control of speed and torque is required. This property of DC motors has made them to survive them in the era of AC transmission and distribution. Vector control drives are coming in the market but time will tell whether they will be able to replace DC drives or not. As the name indicates, a full bridge may be single phase or three phase bridge. When load is high, three phase bridge circuit is used. A three phase bridge circuit may be full controlled or half controlled. A three phase bridge, fully controlled or half controlled, is 6 pulse converter. Many times 12 pulse converters are also used to simplify the design of filter circuit and control ripple. Bridge circuits also suffer from the disadvantage of ripple. Harmonics are introduced in supply as well as load side. Power factor control is also a problem in these drives as these suffer from poor power factor. Supply voltage goes to zero in every half cycle; hence, line commutation is used. As a result, control circuit design becomes simple. Thyrsiors were the devices used earlier but MOSFETS or IGBTS are used as controlling devices. Four quadrant operation is possible with back to back connected bridge circuits. In case of 4 quadrant operation, torque as well as speed direction can be controlled. Four quadrant operation can be achieved by reversing the field current or dual converters (1phase as well as 3-phase) are used. Single-phase dual converters are used up to 15kW and 3-phase dual converters are used beyond 15 kW. Both circulating and non-circulating type dual converters are used as each has advantages and disadvantages. Circulating type dual converters require inter phase reactance. This increases the cost and incurs losses but provides faster response. Regenerative braking is the problem with these drives. (d) Open loop and closed loop control These are basically speed control methods. As mentioned above, constant torque or constant power control is possible through closed loop control. With open loop control, it is not possible. Separately excited motors are generally used for this purpose in industry. In traction, DC series motors are used. For position control, DC servo motors are used. These find extensive used in robots. Size of DC servo motor is generally less compared to AC servo motors. Also AC servo motors incur more losses. In DC servo motors, design of commutator is quite challenging task. Time constants of field as well as armature circuit play important role in closed and open loop control. Time constant of field circuit is generally quite high. Large time constant of field circuit does not allow speed to change fast when field current is varied. Hence, where fast response is desired, open loop control is preferred. Armature control is achieved by varying the supply voltage generally and not by inserting resistance in the armature circuit as it leads to in-efficient operation. Tachometer is required for open as well as closed loop control. It generates voltage proportional to the speed.