Topic
Drives & Motion
Variable frequency drives, soft starters, servos and encoders: the parameters that matter, commissioning mistakes, tuning, and electronic gearing and camming.
6 articles
The twelve VFD parameters that actually matter
Motor nameplate data first: voltage, current, frequency, speed and power factor, because everything the drive calculates depends on them. Then control mode, command and reference source, acceleration and deceleration ramps, minimum and maximum frequency, current limit, and the stop method. Run an autotune after the nameplate data and before tuning anything else. Everything beyond those twelve is refinement.
8 min readSix ways a drive commissioning goes wrong
The recurring six are: motor cable too long or unshielded, so the drive causes network faults or bearing damage; ramps left at defaults, causing overcurrent or overvoltage trips; no braking resistor on a high inertia load; motor rotation not checked before coupling; the autotune skipped; and the drive set for network control while somebody is trying to start it from the terminals.
7 min readVFD or soft starter: what each one is actually for
Use a soft starter when the only problem is inrush current or mechanical shock at start, and the motor runs at full speed once it is going. Use a VFD when the process genuinely benefits from varying speed, which on a centrifugal pump or fan usually pays for the drive in energy alone. A soft starter is cheaper, smaller, produces no harmonics and cannot vary speed at all.
6 min readEncoders: incremental, absolute, and the wiring that fails
An incremental encoder outputs pulses as it turns, so position is counted from wherever it started and is lost on power down. An absolute encoder outputs a unique code for each position, so it knows where it is the moment power returns. Multi turn absolute encoders also remember how many revolutions, using gearing or a battery, which removes the homing routine entirely.
7 min readTuning a servo without a control theory degree
Set the inertia ratio first, because every gain the drive calculates is scaled by it. Then tune from the inside out: current loop, which is usually fixed by the manufacturer, then velocity loop, then position loop. Raise the velocity proportional gain until the axis buzzes, back off by a third, then add integral until steady state error disappears. Only then close the position loop.
8 min readElectronic gearing and camming, and when you need either
Electronic gearing makes a slave axis follow a master at a fixed ratio, replacing a mechanical gear train. Electronic camming makes it follow an arbitrary profile defined as a table of master position against slave position, replacing a mechanical cam. Both are position based rather than speed based, which is what keeps them synchronised over time instead of drifting.
7 min read