time varying fields applicationsdc machines: dc motor braking 1. plugging the armature terminals or...

DC Machines

Introduction Theory of Operation Construction Induced EMF & Developed Torque Types of DC Machines Equivalent Circuit DC Generator Characteristics DC Motor Characteristics Effect of Armature Reaction DC Motor Loading & Stability DC Motor Starting DC Motor Speed Control DC Motor Braking Practical Aspects

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DC Machines: DC Motor Braking

There are three types of electrical braking done in a DC Motor:1. Plugging2. Dynamic Braking3. Regenerative Braking

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1. Plugging

The armature terminals or supply polarity of a separately excited DC motor or shunt DC motor when running are reversed. Therefore, the supply voltage V and the induced back emf (Ea) will act in the same direction. Thus, the armature current is reversed and a high braking torque is produced.

s aa

a

V EIR

s ab

a

V EIR

db a bT k I

b db LT T T

m LdT T Jdt

m LdT T Jdt

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1. Plugging

Separately excited Motor

LT

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1. Plugging

Series Motor

LT

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2. Dynamic Braking

It is also known as rheostatic braking. In this type of braking, the DC motor is disconnected from the supply and a braking resistor Rb is immediately connected across the armature. The motor will now work as a generator and produces the braking torque.

s aa

a

V EIR

ab

a b

EIR R

db a bT k I

b db LT T T

m LdT T Jdt

m LdT T Jdt

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2. Dynamic Braking

The mechanical energy is dissipated as heat in the braking resistance and armature circuit resistance.

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3. Regenerative Braking

It is a form of braking in which the kinetic energy of the motor is returned to the power supply system. To achieve that the motor back emf Eb should be greater than the supply voltage Vs , which reverses the direction of the motor armature current. The motor begins to operate as an electric generator.

s aa

a

V EIR

a sb

a b

E VIR R

db a bT k I

b db LT T T

m LdT T Jdt

m LdT T Jdt

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DC Machines: DC Motor Braking9

DC Machines: DC Reversing Rotation

Reversing Direction of Rotation

This could be done by either:1. Reversing the polarity of the armature winding.2. Reversing the polarity of the field winding.

s aa

a

V EIR

s aa

a

V EIR

m LT T

m LdT T Jdt

s aa

a

V EIR

m LdT T Jdt

m LT T

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Reversing Direction of Rotation

LT

LT

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Four Quadrants Operation

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Four Quadrants Operation

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DC Machines: Practical Aspects

Enclosure and Cooling

DPFG (Drip-Proof Fully Guarded)The drip-proof fully guarded (DPFG) type of enclosure is self-ventilated and has no external means of cooling. In many cases, these types of motors can be modified to accept additional outside air. Most DPFG designs can generate 100% rated torque down to 50% of base speed.

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DPBV (Drip-Proof Blower Ventilated)The drip-proof blower ventilated (DPBV) type of enclosure has an integral blower, which may or may not include a filter. The blower is typically mounted on the commutator end to provide constant cooling airflow. It is not uncommon for blower ventilated motors to deliver 100% rated torque down to 10 or 5% of base speed.

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DPSV (Drip-Proof Separately Ventilated)The drip-proof separately ventilated (DPSV) type of enclosure uses ducted air in the CFM amount required to cool the motor. This type of motor is capable of delivering 100% torque down to 10 or 5% of base speed.

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The totally enclosed non-ventilated (TENV) Has no external cooling, but uses an internal fan to circulate the air within the motor. This type of motor is capable of delivering 100% torque down to 10 or 5% of base speed. Due to the cooling effects, these types of enclosures are not practical for large horsepower ratings.

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TEFC (Totally Enclosed Fan Cooled)The totally enclosed fan cooled (TEFC) type of enclosure has an externally mounted fan on the commutator end shaft. Air flow is a direct result of the speed of the motor. Because of this fact, this type of enclosure is not suitable for low-speed applications. These types of motors are capable of delivering 100% torque down to 60% of base speed.

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TEUC (Totally Enclosed Unit Cooled)The totally enclosed unit cooled (TEUC) type of enclosure uses an air-to-air heat exchanger and receives its cooling through an external blower. These types of motors are able to deliver 100% rated torque down to 3 or 2% of base speed

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time varying fields applicationsdc machines: dc motor braking 1. plugging the armature terminals or...

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