Skill 7 (P. 38) 1. Device the motor is to drive: Wood lathe Device drive requirements: 120 VDC \( 1 / 3 \mathrm{Hp} \) Good speed control Low starting torque Full-load current- 3.5 A Correct Motor type Reason for Selection

To select the correct motor type for driving a wood lathe with the specified requirements, we need to analyze the conditions provided: ### Known Conditions: 1. **Device**: Wood lathe 2. **Drive Requirements**: - Voltage: 120 VDC - Power: \( \frac{1}{3} \) Hp - Speed Control: Good speed control required - Starting Torque: Low starting torque acceptable - Full-load Current: 3.5 A ### Motor Type Selection: Given the requirements, the most suitable motor type for this application would be a **DC motor**, specifically a **Permanent Magnet DC (PMDC) motor**. ### Reason for Selection: 1. **Voltage Compatibility**: PMDC motors can easily operate at 120 VDC, making them suitable for this application. 2. **Power Rating**: A PMDC motor can be found in the \( \frac{1}{3} \) Hp range, which matches the power requirement of the wood lathe. 3. **Speed Control**: DC motors, especially PMDC motors, offer excellent speed control through the use of variable resistors or pulse-width modulation (PWM) techniques. This is crucial for a wood lathe, where different speeds may be required for different operations. 4. **Starting Torque**: PMDC motors can provide sufficient torque at startup, and since low starting torque is acceptable, this type of motor can be a good fit. 5. **Current Rating**: The full-load current of 3.5 A is manageable for a PMDC motor, which can be designed to operate efficiently within this current range. ### Conclusion: A **Permanent Magnet DC motor** is the correct choice for driving the wood lathe due to its compatibility with the voltage and power requirements, excellent speed control capabilities, and suitability for the specified current and torque conditions.