Basics of Regenerative Braking

Basics of Regenerative Braking:

* In Regenerative braking, the motor operates as a generator, while it is still connected to the supply, here N>Ns.

* Mechanical energy is converted into energy, part of which is returned to the supply & rest of the energy is last as heat in wdg & bearings.

* Most of electrical m/c pass smoothing from motoring region to generating region, when over driven by the load.

* Here electric motor is drawing a trotley bus in the uphill & downhill direction. The gravity force can be resolved into 2 components in the uphill direction. One is perpendicular to load surface F & another one is parallel to the road surface Fl. The parallel force pulls the motor towards the bottom of the hill.

* If we neglect the rotational losses, the motor must produce a force F_m opposite to F_l, to move the bus in the uphill direction. This operation is in I quadrant. Here motor torque is motor speed in same direction. But T_L is opp to T_m. The power flow from motor to mech. Load.

* Now consider same bus to travelling downhill. The gravitational force does not change its torque direction, but load torque pushed the motor toward the bottom of the hill. The motor produces a torque in the reverse direction because of the direction of the motor torque always opposite to the direction of the load torque.

* Here rotation of the motor is still in the same direction on both sides of the hill. This operation in the second quadrant. This is known as “regenerative braking”.

In this regenerative braking mode, motor torque & speed are in the opposite direction. T_L is opposite to T_m. The energy is exchange under regenerative braking operation is power flows from mechanical load to source.

Hence the load is driving the m/c and the m/c is generating electric power that is returned to the supply.

Regenerative braking of Induction Motor:-

An induction motor is subjected to regenerative braking, if the rotor rotates in same directions as that of stator magnetic field, by N>NS.

* Such a state occurs during any one of the following processes.

i) Downward motion of a loaded hoisting mechanism.

ii) If variable freq is available or if the motor is of pole change type.

* Under regenerative braking mode , the m / c acts as an induction generator and this power fed back to the supply. The m/c taking only the reactive power for excitation.

* If active load is present, motor speed > Ns and regenerative braking may be obtained. In this case, slip and torque developed become -ve.

* Here Regenerative max torque > max motor torque.

Regenerative braking of DC Motor :

In regenerative braking , generated energy is supplied to the source, the condition is

E>V and –Ia ---(1)

Fild flux increases not beyond rated because saturation.

In series motor, as N increases Armature current , flux decreases so (1) not achieved. Thus braking possible.

Multiquadrant Operation – Operation of Hoist in Four Quadrant

Multiquadrant Operation

* A motor operate in 2 modes – Motoring and braking

* Motoring - electrical energy to mechanical energy, support its motion.

* (generator) braking – mech energy to electrical energy, oppose the motion.

* Motor can provide motoring & braking for both forward & reverse direction.

* Power developed by a motor is given by the product of speed & torque.

* Quadrant I – Power +ve, m/c working as a motor, supplying mech energy. So called “forward motoring”

* Quadrant II – Power –ve, m/c works under braking opposing the motion. So called “forward braking”

* Quadrant III & IV – reverse motoring and braking.

Operation of hoist in four quadrants

* Direction of motor & load torques and direction of speed are marked by arrows.

* A hoist consists of a rope wound on a drum coupled to a motor shaft. One end of a rope is tied to a cage which is used for transporting material. Other end of the rope has a counter weight.

* Weight of the counter weight chosen higher than the weight of an empty case but lower than a fully loaded cage.

* Load torque TL2 in quadrants I & IV represent speed torque charal of the loaded hoist. This torque is the diff. of torques due to loaded hoist & counter weight.

* Load torque TL2 in quadrants II & III is the speed-torque charal of an empty hoist. This torque is due to the diff in torque of counter weight & empty hoist. This is –ve because the counter weight is always higher than the empty cage.

* The quadrant I operation – hoist requires the movement of the cage upward, which corresponds to the +ve motor speed which is in CCW (counter clockwise) direction. It will be obtained if motor produce +ve torque in CCW direction equal to TL. Since developed power is +ve, this is forward motoring operation.

* Quadrant IV operation is obtained when a loaded cage is lowered. Since the weight of the loaded cage is > the counter weight. In order to limit the speed of the cage within a safe value, motor must produce a +ve torque T = TL2 in anti clockwise direction. Both power & speed are –ve, drive is in reverse braking.

* Quadrant II is obtained when an empty cage is moved up since a counter weight is heavier than a empty cage, it is able to pull it up. In order to limit the speed to safety value, motor must produce braking torque = TL2 in clockwise direction. Since speed is +ve, developed power is, -ve. It is forward breaking operation.

Quadrant III – empty cage is lowered since empty cage weight is < counter weight motor produce a torque in clockwise direction. Since speed is –ve & developed power is +ve, this is reverse motoring operation.

Types and Characteristics of Load Torque

Classification of load torques :

1.Active Load torques

2.Passice Load torques

Active Load Torques:

Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques.

Load torques usually retain sign when the drive rotation is changed.

Active Load Torques

Passive Torque:

Load torques which always oppose the motion and change their sign on the reversal of motion are called passive load torques.

Torque due to friction cutting – Passive torque.

Components of load torques:

1.Friction Torque (TF)

The friction torque (TF) is the equivalent value of various friction torques referred to the motor shaft.

2.Windage Torque (Tw)

When a motor runs, the wind generates a torque opposing the motion . This is known as the winding torque.

3.Torque required to do useful mechanical work ( Tm)

Nature of the torque depands of type of load.

It may be constant and indeoendent of speed, Some function of speed, may be time invariant or time variant.

The nature of the torque may change with the change in the loads mode of operation.

Characteristics of different types of load:

In electric drives the driving equipment is an electric motor.

Selection of particular type of motor driving a m/c is the matching of speed-torque charal of the driven unit and that of the motor.

· Different types of loads exhibit different speed torque charal.

· Most of the industrial loads can be classified into the following 4 general categories:

1. Constant torque type load.

2. Torque proportional to speed (generator type load)

3. Torque proportional to square of the speed (fan type load)

4. Torque inversely proportional to speed (const power type load)

1.Constant Torque Characteristic :

The speed – torque characteristic of this type of load is given by T=K.

Working motor have each mechanical nature of work like shaping , cutting, grinding or sharing, require constant torque irrespective of speed. Similarly cranes during the hoisting.

Similarly cranes during the hoisting and conveyors handling constant weight of material / unit, time also exhibit this type of characteristics.

Torque proportional to speed:

Separately excuted dc generators connected to a constant resistance load, eddy current brakes and calendaring m/cs have a speed torque characteristics m/cs have a speed – torque characteristics given by T= Kw.

Torque propositional to square of the speed :

Load Torque Square of speed

Example : Fans , Rotary pumps , compressors , ship propellers. The speed – torque characteristics of this type of load is given by

Torque inversely propositional to speed:

· In such types of loads , torque is inversely proportional to speed or load power remains constant.

· Eq: Lathes, boring m/cs, milling m/cs , steel mill colier and electric traction load.

· This type of characteristics is given by

· Most of the load require extra effort at the time of starting to overcome static friction. In power application it is known as brake away torque and load control engineers call it “stiction” . Because of slition , the speed torque characteristics of the load is modified near to zero speed.

Classification of Electric Drive Based on Power Supply

Classification of Electric Drive:

Main classification of electric drives are :

1. DC drives and

2. AC drives

 

DC drives :

A dc motor speed can be controlled by using a power converter. Here motor is coupled with load.

 

AC drives:

An AC motor speed can be controlled by power converter. Here motor is coupled to the load.

The main purpose is to vary the motor speed.

 

Comparison between DC and AC drives:

                    DC drives                                                                  AC drives

1. Power circuit and control circuit is simple 2. Frequent Maintenance. 3. Commulator makes bulky , costly and heavy. 4. Speed and design rating are limited due to commutation. 5. This is used in certain location 6. Fast response and wide speed range smooth achieved by conventional and solid state control 7. Poor PF, harmonic distortion of the current. 8. Power / weight ratio is small.

1. Power and Control circuit is complex. 2. Less Maintenance. 3. Problems are not there, particularly squirrel cage motor. 4. Ratings have no upper limits. 5. Used in all location 6. In solid state control speed range is wide and conventional method it is stepped and limited. 7. For Regenerative drives the line pf is poor, for non-regenerative drives the line PF is better. 8. Power / weight ratio is large.

Solid State Drives – Parts of Electric Drives

Parts of Electric Drives:

1. Electrical motors and load

2. Power modulator

3. Source

4. Control unit

5. Sensing unit.

Electric Motors:

Most commonly used electric drives are

DC motos – Shunt , Series. Compound and permanent magnet.

Induction Motors – Squirel cage , wound rotor and linear

Synchronous motor - Wound field and permanent magnet

Brushless Dc motors , stepper motors, and switched reluctance motors can be used.

Power Modulator

It can classified as

1. Controlled rectifier (AC to DC converters)

2. Inverters (DC to AC converters)

3. AC voltage controllers (AC to AC converters)

4. DC choppers (DC to DC converters)

5. Cyclo converters (Frequency conversion)

Electrical Source:

Single Phase and Three Phase , 50Hz readily available in most locations.

Very low power drives fed from single phase source. Rest of the drives from three phase source.

Low and medium power motor are fet from 400v supply.

Higher rating , motors are fed from 3.3kv , 6.6 kv , 11kv.

Some drives are power from a battery , battery voltage may be 24v, 48v and 110v.

Sensing unit :

1. Speed sensing is required for implementation of closed loop speed control schemes.

2. Speed sensing by tachometers , high speed accuracies required digital tachometers are used.

3. Current sensing methods are

· Use of current sensor employing hall effect.

· Involves the use of a non-inductive resistance shunt in conjunction with an isolation amplifier which has an arrangement for an amplification and isolation bet power and control units.

Control Units :

· Control for power modulator is provided in the control unit.

· When semi conductor converts are used, the control unit will consist of firing circuits, which employ linear and digit integrated circuits and transistor and a up when sophisticated control is required.

Power Modulator and Simple Electric Drive

Block Diagram of a simple electric drive:

· System employed for motion control are called Drives.
· Drives employed electric motors are known as “Electric Drives”
· Load is usually a machinery designed to accomplish a given task eg: fans, pumps, robots, trains and drills.


Power Modulator:
· Modulates the flow of power from the source to the motor in such a manner that the motor will modulate the speed – torque characteristics required by the load.
· During transient operation , such as starting , breaking and speed reversal , it restricts source and motor currents within permissible values.
· Converts electrical energy of the source in the form of suitable to the motor.
· Select the mode of operation of the motor (i.e) Motoring or Braking.

Classification of Electric Drive with Advantage and Disadvantage

Classification of Electric Drive

Generally classified into 3 categories:

I. Group drive

II. Individual Drive

III. Multimotor Drive

 

Group Drive :

If several group of mechanisms or machines are organized on one shaft and driven or actuated by one motor, the system is called a group drive or shaft drive.

 

Advantage :

Most Economical

 

Disadvantage:

1. Any Fault that occurs in the driving motor renders all the driving equipment idle.

2. Efficiency low because of losses occurring in the energy transmitting mechanisms (Power loss)

3. Not safe to operate.

4. Noise level at the working spot is high.

5. Flexibility.

Single motor drives a no of machines through belt form common shaft.

 

Individual Drive:

1. If a single motor is used to drive or actuate a given mechanism and it does all the jobs connected with this load , the drive is called individual drive.

2. All the operations connected with operating a lathe may be performed by a single motor.

3. Each motor is driven by its own separated motor with the help of gears , pulleys etc.

 

Disadvantage:

Power loss occurs.

 

Multi Motor Drive:

· Each operation of the mechanism is taken care of by a separate drive motor.

· The System contains several individual drives each of which is used to operate its own mechanism.

· Separate motors are provided for actuating different parts of the driven mechanism.

 

Advantage :

1. Each Machine is driven by a separated motor it can be run and stopped as desired.

2. Machines not required can be shut down and also replaced with a minimum of dislocation.

3. There is a flexibility in the installation of different machine’s.

4. In the case of motor fault, only its connected machine will stop where as others will continue working undisturbed.

5. Absence of belts and line shafts greatly reduces the risk of a accidents to the operating personnel.

 

Disadvantage:

Initial high cost.

SOLID STATE DRIVES (SSD)–April / May 2011 Question Paper

Anna University

B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2011

Sixth Semester

Electrical and Electronics Engineering

EE 2352 — SOLID STATE DRIVES

(Regulation 2008)

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Time : Three hours

Maximum : 100 marks

Answer ALL questions

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PART A — (10 × 2 = 20 marks)

1. What are the types of load torques?

2. Write down the fundamental torque equation of motor load system?

3. List out the drawbacks of ac-dc converter fed dc drive.

4. What is TRC scheme?

5. Write the transfer function of converter.

6. Write the real and reactive power equations of a balanced 3 phase ac system.

7. What are the various applications of stator voltage control scheme?

8. Give the advantages of vector control method.

9. What is the necessity of delay unit in a open loop v/f control of synchronous motor?

10. Define self control of synchronous motor.

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PART B — (5 × 16 = 80 marks)

11. (a) Explain in detail the multi quadrant operation of low speed hoist in speed torque plane. (16)

Or

(b) A motor drives two loads. One has rotational motion. It is coupled to the motor through a reduction gear with a = 0.1 and efficiency of 90%. The load has a moment of inertia of 10 kg-m2 and a torque of 10 N-m. Other load has translational motion and consists of 1000kg weight to be lifted up at a uniform speed of 1.5 m/s. coupling between this load and the motor has an efficiency of 85%. Motor has inertia of 0.2 kg-m2 and runs at a constant speed of 1420 rpm. Determine equivalent inertia referred to the motor shaft and power developed by the motor. (16)

12. (a) Explain the operation of single phase fully-controlled converter fed dc separately excited motor in continuous and discontinuous modes of operation with necessary waveforms and steady state analysis. (16)

Or

(b) (i) Explain the different control techniques of chopper in detail. (8) (ii) Discuss the four quadrant operation of DC-DC converter. (8)

13. (a) Explain the closed loop operation of armature voltage control method with field weakening mode control in detail. (16)

Or

(b) Explain the design procedure of current controller in detail. (16)

14. (a) Explain the theory of v/f control in detail. (16) Or

(b) Explain the principle of vector control in detail with block diagram. (16)

15. (a) (i) Explain the open loop v/f control of synchronous motor in detail. (8)

(ii) Explain the concept of self controlled synchronous motor drive. (8)

Or

(b) Explain the construction and working of permanent magnet synchronous motor. (16)

Anna University - Solid State Drives - Two Marks with Answers

Anna University Solid State Drives - Important Two Marks with answers.

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