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22 Cards in this Set
- Front
- Back
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What is the principal of operation of a DC Motor |
DC motors operate on the principle of attraction and repulsion of magnetism |
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What is a motor? |
The motor is a machine that converts electrical energy into mechanical energy |
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What is the function of the commutator in a DC motor |
To convert AC into DC (rectified DC , fluctuating DC, pulsating DC) by maintaining polarity to the brushes and therefore the outside circuit |
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What two factors determine the amount of torque developed by a motor? |
1. Strength of the magnetic field of the armature. 2. Strength of the magnetic field of the pole pieces Torque is proportional to Armature current if field excitation current remains constant. |
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What type of motor is known as a constant-speed motor |
Shunt Motors are sometimes known as constant-speed motors. |
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What is CEMF? |
When the windings of the Armature Spin through the magnetic field produced by the pole pieces, a voltage is induced into the armature. This Induced Voltage is opposite in polarity to the Applied Voltage and is on his counter electromotive force. |
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What three factors Determine the amount of CEMF Produced in the Armature |
1. The strength of the magnetic field of the pole pieces 2. The number of turns of wire in the armature 3. The speed of the armature |
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What limits the amount of current flow through the Armature when power is first applied to the motor |
When a DC motor is first started the inrush current can be high because no CEMF is being produced by the Armature. The only current-limiting factor is the amount of wire resistance in the windings of the armature |
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What factor determines the speed regulation of a DC motor |
The amount by which the speed decreases as load is added is called speed regulation. The speed regulation of a DC motor is proportional to the resistance of the armature. The lower the armature resistance the better the speed regulation. The reason for this is that armature current determines the torque produced by motor if field excitation remains constant. The requirements of the load also determine the voltage regulation |
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What type of motor should never be operated at no load? |
Series motors must NEVER be operated under a no-load condition. momentary bumping ok to determine DOR. |
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In the general, what type of compound motor connection should be avoided? |
Differential compound motor connection. |
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What is the most common way of changing the direction of rotation of a compound motor? |
It is common practice to reverse the connection of the Armature leads to prevent changing a compound motor from a cumulative- to a differential- compound connection. |
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What is the most common way of changing the direction of rotation of a compound motor? |
It is common practice to reverse the connection of the Armature leads to prevent changing a compound motor from a cumulative- to a differential- compound connection. |
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How can a DC motor be made to operate at its base speed |
When FULL VOLTAGE is applied to BOTH the armature and shunt field, the motor operates at base speed. |
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How can a DC motor be made to operate above its base speed? |
When full voltage is applied to the armature and reduced voltage is applied to the shunt field the motor operates above base speed. |
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What device is used to disconnect power to the armature if the shunt field current drops below a certain level? |
The Field-Loss Relay FLR |
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Describe the function of a commutator in a DC machine used as a motor |
The commutator preforms the function of a rotary switch and maintains the correct direction of current flow through the armature windings. |
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Why do many industries leave power connected to the shunt field at all times even when the motor is not running? |
The resistance of the winding produces heat, which is used to prevent any formation of moisture inside the motor |
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Who was the first person to establish a measurement for hp |
James Watt |
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One horsepower is equal to how many watts |
746 (745.7) |
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1ft-lb per second = |
1.36 Watts |
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Motor effect |
Whenever a conductor carries current in the presence of a magnetic field a force will act on the conductor at a right angle to the field. |
