Coil Inductance
The property by which a winding resists change in current, measured in henries. It determines how fast the current can rise, how much current ripple there will be when driven with PWM, and how large a back EMF arises when the coil is switched off.
Inductance is like an inertia that slows the current. The moment voltage is applied the current starts at zero and rises exponentially; the rate of rise depends on the time constant determined by the ratio of inductance to resistance. This behavior has three practical consequences:
- A coil with high inductance responds more slowly; to overcome this, drives apply a high voltage at the first instant and establish the current quickly.
- As the PWM frequency rises the current ripple decreases, because inductance allows the current to change only a little in each period.
- When the current is cut abruptly the inductance tries to sustain it and produces a reverse voltage that can reach hundreds of volts.
This last item is the reason a freewheeling diode is placed at the drive output. If you drive a valve coil through a relay or switch without a protection element, contacts wearing out in a short time and nearby electronics failing is an expected result.
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