Electric Motors in Space Mechanisms
Contents
Typical Motor Selection Factors
- Application: The actual function that the motor has to perform.
- Environment: The conditions that prevail in the surroundings.
- Commutation method: The method of switching current to cause motion of the rotor.
- Duty cycle: The time occupied by the operation of a motor as a percentage of total time, when the motor does not operate all the time. D= W/T*100 ; W is the time elapsed while the motor is running, T is the total time.
- No-load speed: The maximum speed achieved by a motor with no load applied.
- Weight Stall torque: Stall torque gives a measure of the torque at which the motor will fail to rotate the load.
- Lifetime Load (operating) point: This is the point of intersection of the torque-speed curve of motor and machine. Machine can be any sort of an actuator, pulley or fan.
- Torque ripple: It is the periodic increase and decrease in the torque output by the motor on the rotor.
- Power source: The power budget of the system or the power allocation to the motor in the given scenario.
- Controllability Envelope (volume): It is the upper limit of a system size or complexity that we can control.
- Heat dissipation: The amount of heat dissipated by the motor and the effectiveness of the thermal control.
Commonly Used Motors in Mechanisms
The following is the list of some motors used commonly in a variety of mechanisms:[1]
- AC Induction motors: These motors work by the principle of changing the power to different pairs of magnets placed diametrically opposite to each other, as the power changes, according to the Lenz’s law there is opposition to the change in the loop which is suspended between the pairs of these diametrically opposite magnets. Thus they respond by rotating as well. So as we keep changing the pair that is powered, the loop continues to rotate to catch up with it. This is how this motor works.
- Brushed DC motors: These motors have a part called a commutator that reverses the current when the loop completes one half cycle. Thus when the current reverses it acts in a similar way as powering different pair of magnets in an AC Induction motor and thus the cycle continues.
- Brushless DC motor: A Brushless DC motor uses multiple sets of coils in place of a single coil as in the case of a DC motor. The motor powers the coils in a manner so as to keep the permanent magnets on the outside rotating and thus keep the rotor going. It incorporates a Hall effect sensor and an electronic circuit to determine which coil to power up, thus removing the need for brushes.
- Stepper Motor: It works by the principle of rotating current in the coils placed diametrically opposite on the outside and then using these to align the pair of magnets on the rotor of the motor.
Application of the motor types:[2]
- AC Induction motor: High RPM and high torque requirements.
- Brushed DC motor: Limited life applications, low RPM and high torque requirements.
- Brushless DC motor: High RPM requirements, light weight and low thermal emission applications.
- Stepper motor: Low torque applications, precise control applications.
- High RPM applications: Thrust Vector Control actuators (TVC), Fuel valve control actuators, Control moment gyroscopes.
- Low torque applications: Gimbal positioning, Timer switching, Open loop micropositioning, Solar array deployment.
Brushless DC and Stepper motors are the most predominant motors used for aerospace. Stepper motors are a special case of Brushless DC motors. AC Induction motors are too big to be used in smaller satellites.
Advantages and Disadvantages of Brushed DC motors
| Advantages | Disadvantages |
|---|---|
| 1. Low cost | 1. Brush dust |
| 2. Simplicity | 2. Brush to commutator arcing and wear |
| 3. Availability | 3. Electromagnetic interference |
| 4. Mechanical noise | |
| 5. Short motor life | |
| 6. Low efficiency | |
| 7. Limited speed | |
| 8. Poor thermal characteristics in vacuum |
Advantages and Disadvantages of Brushless DC Motors
| Advantages | Disadvantages |
|---|---|
| 1. High speed (up to 100,000 RPM) | 1. Higher electronic cost |
| 2. High torque at high speed | 2. Greater motor drive complexity |
| 3. Approximately double the output torque over Brush DC motor of the same size | |
| 4. Windings on the stator instead of the rotor improves heat dissipation | |
| 5. No brushes, so motor lastsas long as the bearings hold up | |
| 6. Higher efficiency | |
| 7. Vacuum compatible |
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References
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