Earlier, a variable DC voltage was obtained from a fixed DC voltage by two methods:
·Resistance Control Method: In this method, a variable resistance is used in between the fixed-voltage DC source and the load. This method is still used in very few applications, but it is inefficient because of high-energy losses in the resistance.
·Motor-Generator Set Method: A variable DC output voltage is obtained by controlling the field current of the DC Generator. This system is still used in some industrial drives. Three machines of the same power rating are involved, and therefore the system is bulky, costly, slow in response, and less efficient.
Nowadays, high power solid-state devices are available to make the solid state DC power converters practical and cost-effective for DC drives powered from DC source. These converters offer greater efficiency, faster response, smooth operation, minimal maintenance, smaller size, and lower weight and cost. There are two types of solid state DC-to-DC converters:
·Inverter Rectifier System: In the Inverter-rectifier system, the DC is first converted to AC, which is then stepped up or down by a transformer and then rectified back to DC. The conversion is in two stages, DC to AC and AC to DC and therefore is costly, bulky and less efficient.
·DC Chopper System: The DC chopper converts directly from DC to DC and is a relatively latest technology with good efficiency. It may be visualized as a DC equivalent to an AC transformer, because, in performing DC-to-DC conversion, its behavior is similar to that of a continuously variable turn-ratio transformer.
The chopper can replace the resistor commonly used in series with the armature of DC motors for speed control. Therefore, it can be used in battery operated vehicles and applications where energy saving is a prime consideration. Choppers used in subway cars also reduce tunnel heating.
Choppers can also provide regenerative braking of the motor and return the energy back to the supply. This results in energy saving for transportation systems with frequent stops. Choppers therefore find wide application in transaction system all over the world. Choppers are successfully used in BART (Bay Area Rapid Transit) system in San Francisco and in subway cars in Toronto and Montreal. Choppers are also used in other applications such as interurban and trolley cars, marine hoists, forklifts trucks and mine-haulers.
Choppers are most likely to be used in future electric vehicles for speed control as well as braking. Nowadays, these also find lots of applications in continuous process plants like glass, fertilizer and tire manufacturing industries. A few good features of the chopper drives are: smooth control, high efficiency, fast response and regeneration.
Principles of Chopper Operation
A chopper is a solid-state On/Off switch, which connects load to and disconnects it from the supply and produces a chopped load voltage from a constant-input supply voltage. During the period t-On. When the switch is on, the supply terminals are connected to the load terminals. During the interval t-Off, when the switch is Off, load current flows through free-wheeling diode Dfw and the load terminals are shorted. A chopped DC voltage is thus produced at the load terminals. The average load voltage Eload is given by,
Eo= E*Ton/Ton+ Toff
T= Ton+Toff =Chopping Period
a= Ton/T=Duty cycle.
So we see, the load voltage is controlled by the duty cycle of the chopper.
There are two methods for it:
f=l/T The chopping frequency (and hence the chopping period T) is kept constant and the On-time Ton is varied. This may be called Pulse-Width Modulation.
2. Variable-Frequency System:
The chopping period T is varied, and either (a) On-time Ton is kept constant or (b) Off-time Toff is kept constant. This may be called Frequency Modulation. The frequency has to be varied over a wide range to provide the full output voltage range. So, filter design for frequency modulation is difficult. Secondly, the possibility of interference with signaling and telephone lines is greater. Thirdly, the large Off-time at low output voltage will make the current of a DC motor load discontinuous.
The constant-frequency system with pulse-width modulation is thus the preferred scheme for chopper drives. The chopper control is also dropping in speed nature as in phase control system for DC motor. However, the drop in speed is less for chopper control than for phase control because of the nature of the supply voltage, which does not change with time. Secondly, the region of discontinuous motor current operation can be reduced with the chopper control by increasing the chopping frequency or time constant of the motor circuit (i.e., adding an inductor in the motor circuit).
The chopper is an electronic On-Off switch. When it is on, supply current and motor current are the same. When the chopper is Off, motor current flows through the free-wheeling diode, and the supply current is zero. The chopper therefore produces chopped current for the supply. The supply current therefore has very high harmonics, which will produce undesirable effects, such as source voltage fluctuation, signal interference, supply distortion, ucidilional healing and so on. A simple capacitor filter will reduce ripple from the supply current. The capacitor will provide the ripple current of the chopper, and only the average current is drawn from the supply. The high peak power demand from the supply is thus eliminated.
In practice, however, instead of using only a large capacitor filter, an L-C filter is used to reduce the size of the capacitor. The filter inductor has the additional function of providing transient isolation between supply and load during a short-circuit condition.
If two or more choppers are operated in parallel called multiphase chopper, with phase shifted from each other, the ripple amplitude decreases and the ripple frequency increases. As a result, the supply harmonic current is hugely and effectively reduced. This is used for very high load current requirement.
To stop the drive motor, a braking mechanism is necessary. Classically, applying friction (mechanical) brakes stops a motor. The amount of braking is normally controlled pneumatically by air pressure. Friction brakes have been very reliable and have been used extensively over the years. However, the brake shoes are subject to considerable wear and tear. If braking is required frequently, as in transit vehicles, frequent maintenance and ultimate replacement of the brake shoes may make this type of braking uneconomic and undesirable.
Fortunately, a motor can also be operated as a generator in chopper control. As a result two other types of braking is also possible:
Dynamic braking and regenerative braking. In both cases, the kinetic energy of the drive system is converted to electrical energy. In this process, the motor operated as a generator, experiences a rotary force opposing its motion, and therefore slows down. At lower speeds, friction brakes are normally used. However, use of friction brakes is hugely reduced and consequently energy loss as well as wear and tear also gets reduced.
In dynamic braking, electrical energy is scattered in a stack of braking resistors. The scheme is simple, but if it is used in subway cars, it may produce considerable tunnel heating. In regenerative braking, electrical energy is fed back to the supply. The scheme is complex; however, it saves energy. When regeneration fades out at lower speeds, friction brakes can be blended with the fading electrical braking.
Chopper Drives Braking
The chopper controls the motor power in the driving mode. In a same way, it can be used to control regenerative power in the regenerative braking mode. The position of switch and diode will be interchanged. The function of the chopper in motoring and braking is the same. The On-Off ratio of the chopper is regulated in a closed-loop control system to maintain the desired braking current. In regenerative braking, during chopper is on, the motor terminals are shorted. The armature current builds up, and energy stored in the reactor connected in series with the armature. When the chopper is off, armature current is forced in to the supply. So, energy stored in the reactor from the armature is thus released to the supply.
If a series motor is used, an additional reactor in series with the armature may not be necessary. The series field winding stores energy during the chopper on interval and releases energy to the source during the chopper Off interval.