Electric Braking: Merits and applications

Induction motors are used at various places. Speed control of induction motors is quite difficult and that’s why their use was restricted and DC motors had to be used as their speed regulation was possible. But when induction motor drives were invented and implemented, they were given preference because of many advantages over DC motors. Whenever controlling of motors is done, braking is the most important term, so as with induction motors. Induction motor braking can be done by different methods, which are i. Regenerative braking of induction motor ii. Plugging Braking of induction motor iii. Dynamic braking of induction motor is further categorized as a) AC dynamic breaking b) Self excited braking using capacitors c) DC dynamic braking d) Zero Sequence braking We know the power (input) of an induction motor is given as. Pin = 3VIscosφs Here, φs the phase angle between stator phase voltage V and the stator phase current Is. Now, for motoring operation φs < 90° and for braking operation φs > 90°. When the speed of the motor is more than the synchronous speed, relative speed between the motor conductors and air gap rotating field reverses, as a result the phase angle because greater than 90° and the power flow reverse and thus regenerative braking takes place. The nature of the speed torque curves are shown in the figure beside. It the source frequency is fixed then the regenerative braking of induction motor can only take place if the speed of the motor is greater than synchronous speed, but with a variable frequency source regenerative braking of induction motor can occur for speeds lower than synchronous speed. The main advantage of this kind of braking can be said that the generated power is use fully employed and the main disadvantage of this type of braking is that for fixed frequency sources, braking cannot happen below synchronous speeds. When the rotor of an induction motor turns slower than the speed set by the applied frequency, the motor is transforming electrical energy into mechanical energy at the motor shaft. This process is referred to as ‘motoring’. When the rotor turns faster than the synchronous speed set by a drive output, the motor is transforming mechanical energy from the motor shaft into electrical energy. It may be a ramp to stop, a reduction in commanded speed or an overhauling load that causes the shaft speed to be greater than the synchronous speed. In any case this, condition is referred to as ‘regeneration”. Essentially, mechanical energy is converted to electrical energy. The case is much the same for a DC drive and motor. The increase in DC voltage for the DC drive occurs at the armature connection. Some DC drives have not only a forward but also a reverse bridge. The reverse bridge allows the DC energy from the armature to be transferred to the utility line. If the DC drive has only the forward bridge, a shunt regulator can be used in parallel with the armature to dissipate this energy into heat. For an AC drive and motor in a regenerative condition, the AC power from the motor flows backward through the inverter bridge diodes shown in figure 1 below. On most AC drives, utility power is first converted into DC by a diode or SCR rectifier bridge These bridges are very cost effective, but only handle power in the “motoring” direction. Dynamic brake or Chopper – What’s the difference? From an electrical stand point they both do the same thing. The major difference is in the construction. Dynamic brakes have the controller, switching device and resistor housed in one self contained unit. It is rated in horsepower and has only a 20% duty cycle rating. A chopper contains only the regulator circuit and switching device and is rated in amps. The resistors are treated as a separate component. This gives the user several advantages. First, the resistors can be accurately sized for a given application. Also, the chopper module can be mounted in an enclosure while the resistor, with the large amount of heat energy to dissipate, can be remotely mounted up to 100 feet away. A close look at the application is needed before a decision to use a brake or chopper is made. Some rotational and linear loads with a low regenerative duty cycle can be handled with a brake while overhauling loads, and loads with a duty cycle greater than 20% are more suited to a chopper. In general, the chopper is a more “heavy duty” solution. Exceeding the 20% duty cycle rating, a condition that may be tough to prove after the fact causes many dynamic brake failures. Another avoidable cause of failure for dynamic brakes and choppers that warrants mention is misconnection. These devices need to be connected at the capacitor bank nodes of the DC bus. Many drives also provide a DC bus connection point at the input bridge rectifier nodes. Applications Electrical drive systems are being used more and more on ships, oil rigs, crane barges and offshore vessels of all types, for every type of powered application: main propellors and bow thrusters, driving winches and windlasses, cranes, lifts, conveyors and jacks, cable laying and tensioning. Advantages of electric braking An important benefit from using an electric drive is that reliable systems of regenerative and dynamic braking resistors are available to complement or replace traditional mechanical braking systems. The advantages of electric braking include control, reliability, mechanical simplicity, weight saving and in some cases the opportunity to make use of the regenerated braking energy. Range Cressall Resistors’ wide range of resistor technologies and long experience in this field means that we have suitable brake resistor designs for all of the above applications, with braking powers from a few kW up to many MW (needed, for example, for some crane and main propulsion brakes) and cooling methods which include liquid, forced air and natural convection. We have standard products of all types. Design considerations The majority of our designs make use of suitably rated Incoloy-sheathed mineral insulated elements. These are less vulnerable to physical damage, prevent accidental contact with live, possibly high voltages, and are thus considered much safer to use in these environments.