Motor Inductance
The ability of the winding to store magnetic energy. It sets the current ripple on a PWM-driven motor: the larger the inductance, the smaller the ripple.
Motor inductance is the ability of the winding to store magnetic energy and is expressed in millihenries. On a motor driven by PWM it is the main factor that decides how smooth the current will be: the larger the inductance, the smaller the current ripple. This is the chief reason why the same driver on the same settings does not behave the same way on every motor.
What makes speed control with PWM possible
The winding inductance acts as a natural filter against the PWM pulses. Because the current cannot change instantly, the motor responds to the average of the chopped pulses; between pulses the current does not collapse completely, it only leaves a small ripple. This filtering effect is the basic reason why speed control with PWM is possible at all.
The size of the ripple depends on two things: the inductance and the switching frequency. If the inductance halves, the ripple roughly doubles; if the frequency doubles, the ripple roughly halves. That is why raising the frequency on a motor with low inductance is the way to win back the filtering that has been lost.
How much it differs by motor type
The values differ greatly by motor type. On large brushed DC motors with many turns the inductance is typically of the order of a few millihenries, while on small ironless rotor motors it can fall below a hundred microhenries. Motors with low inductance draw a far larger ripple at the same frequency and therefore call for a high switching frequency; otherwise the motor heats up needlessly.
- High inductance: The current is smooth and the motor stays cool; it causes no trouble even at a low frequency. The price is a slower response to a current command.
- Low inductance: The response is fast, the ripple is large. The extra heat builds up both in the winding and in the power stage of the driver.
| Motor type | Inductance | Symptom seen at the driver |
|---|---|---|
| Large brushed DC (winch, crane, door) | High | Current smooth; no trouble even at a low frequency |
| Medium sized geared DC | Medium | Marked hum if the frequency is in the audible band |
| Small ironless rotor | Low | Heating and ripple marked at the same frequency |
There are three ways of learning the value: the technical documentation of the motor, a measurement across the terminals with an LCR meter, or indirect observation. If there is no documentation the indirect route is enough in practice; when you compare two motors on the same driver, the one that heats up more and hums more loudly is the one with the low inductance.
The price of raising the frequency
Raising the switching frequency suppresses the ripple but it is not free: every switching pulse leaves a loss in the power stage, and a high PWM frequency heats the driver more. The right setting is the lowest frequency that keeps the noise and the temperature of the motor acceptable together; pushing the frequency blindly to the top strains the driver. If the frequency of the board is fixed, this is not a matter of setting but a matter of motor and driver match.
The check to make when you change the motor
A module running quietly and cool on one particular motor does not mean it will give the same result on a very different motor. When you change the motor, repeat these three measurements: the continuous current drawn under load, the driver body temperature at the end of half an hour of running, and the noise the motor makes off load. If one of the three has gone bad, review first the switching frequency, then the supply cable cross section, and last of all the choice of motor. The step that gets overlooked but costs the most is never making this measurement at all.
The context in which this term is used: DC Motor Driver Selection Guide