Figure 11: A diode is used to prevent voltage spikes on direct current coils. The metal diode limits the voltage spike to approximately 140 volts. The magnetic field around the coil collapses and induces a voltage into the coil. Because the induced voltage is opposite in polarity to the applied voltage (Lenz's Law), the induced voltage causes the diode to become forward biased. A silicon diode exhibits a forward voltage drop of approximately 0.7 volts. This limits the induced voltage to a value of about 0.7 volts. The energy of the voltage spike is dissipated as heat by the diode.
Review Questions:
1. What is the simplest of all starting methods for a motor?
2. Explain the difference between jogging and inching.
3. When installing a motor control system, list four major factors to consider concerning the power system.
4. What is the most common method of controlling the speed of an alternating current motor?
5. Where is the best place to look to find specific information about a motor, such as horsepower, voltage, full load current, service factor, and full load speed?
6. Is the National Electrical Code a law?
7. What agency requires employers to provide a workplace free of recognized hazards for its employees?
8. What is meant by the term ramping?
9. What is the most important function of any control system?
10. Explain the difference between manual control, semiautomatic control, and automatic control.