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23 Cards in this Set
- Front
- Back
- 3rd side (hint)
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How does a commutator work? |
Reverses direction of current when magnetic field changes. This keeps motor rotating in one direction |
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How does a DC motor work? |
Like poles repel and unlike attract. Stator poles generate fixed magnetic field with permanent magnetics. The rotor is commutated to cause changes in direction of its magnetic field. The constantly changing direction of field produce torque. |
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Torque relationship of motor? When does commutator change direction of current? |
When torque is zero |
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Parts of a motor? |
Rotor or armature Commutator Brushes Axle Field magnet Dc power supply |
6 parts |
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Draw speed torque characteristics graphs. What is starting torque and no load speed? |
Starting torque = max torque motor can produce. No load speed wmax = max speed of motor with no load. |
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What are the four types of DC motors? |
Shunt wound Series wound Compound wound Permanent magnet |
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Characteristics of series wound motor? |
Armature and field windings connected in series ans thus carry equal currents. High starting torques Highly variable speed depending on load Very high speed when load is small Can have speed runaway problem Use thicker wires for low R |
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Describe shunt motors? Draw speed torque graph |
Armature and field windings connected in parallel. Constant speed for large loading range Fine wire higher R Lowest starting torque produced (bad) Low no load speed |
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Describe compound motors? Draw torque speed and current torque graphs. |
Has both shunt and series. Gets best features from both High start torque - series Good speed regulation - shunt wound |
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If polarity changes which motors result in change of direction of rotation? |
Permanent magnets will change. Shunt series and compound will not change. |
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Describe permanent magnets? Draw 2 graphs |
Stator field provided by permanent magnets Can switch direction Has linear torque speed therefore ideal for computer control app. |
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Draw DC motor armature equivalent. |
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Why is back emf V produced in windings? |
Because of changing magnetic flux due to armature rotation. |
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Draw the DC motor power characteristics. Where is the max power? Show on graph |
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Draw the DC motor block diagram and name its parts |
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What are the two types if electronic speed controllers? |
Linear amplifiers Pulse width modulation |
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How does pulse width modulation work? |
Using a bipolar transistor or FET to switch DC power supply rapidly at fixed frequency. Changes in duty cycle cause variable current output which controls torque and speed. |
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What causes motor to stutter? How do you also avoid noise? |
If PWM frequency is too low, below 1khz. Above 20khz to avoid noise. |
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How do H bridges work with a motor? |
Allows bidirectional control. |
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How do we measure motor performance? |
Current sensing Back emf Tachometer Encoders |
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What is current sensing? |
Tells how much torque the motor is producing. T=Ktia |
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What does a tachometer do in a motor? |
Measures back emf which can be used to measure speed (requires current to be at 0). Adding small extra motor to output shaft, acts like generator. Voltage is proportional to rotational velocity. |
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Using encoders, how do you distinguish between forward and backward rotation? |
Use two tracks that are 90 deg out of phase. |
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