Practical Procedure · Safety Systems Control Valve Partial Stroke Testing (PST) for Safety Valves Applies to: Emergency shutdown (ESD) valves and safety instrumented function (SIF) final elements | Equipment needed: PST-capable positioner/logic solver, position feedback monitoring, coordination with process/operations Full Stroke vs. Partial Stroke — Why PST Exists Tap a tab to see the difference between a full functional test and a partial stroke test, and why PST lets you test without shutting down. Full Stroke Test Partial Stroke Test (PST) ESD Valve — Process Running 0% → 100% → 0% process fully interrupted during test ESD Valve — Process Running ...
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Motor Selection & Starting Methods in the Field
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M1.L1 — Motor Selection & Starting Methods in the Field
Module 1 · Lesson 1
Motor Selection & Starting Methods in the Field
Advanced Motor Control & Drives Mastery — Book 3 of the Electrical Series
Why this lesson matters: Before you ever touch a VFD or soft starter, you need to answer one question correctly:
what does this motor actually need to start and run this load safely? Get this wrong and every downstream
decision — wiring, protection, drive sizing — inherits the mistake.
1. Reading a Motor Nameplate — Click Each Field
Every field on a nameplate is a decision input. Click on a field in the diagram below to see what it tells you
and where technicians commonly misread it.
Click a nameplate field to see the field-relevant meaning and common misreads.
Field mistake seen often: A technician replaces a motor using only the HP/kW rating as the match criterion,
ignoring the Service Factor and Insulation Class. The replacement runs hotter under the same load and trips on
thermal overload within weeks — because the original motor had SF 1.15 headroom the replacement didn't.
2. Matching Motor to Load — the Real Checklist
In the field, "does this motor fit this job" comes down to five checks, in this order:
#
Check
What you're actually verifying
1
Torque profile match
Does the motor's torque-speed curve clear the load's torque curve at every speed, especially breakaway?
2
Duty cycle match
Continuous (S1) motor on a start-stop-start load will overheat — check S-rating against actual duty
3
Ambient & altitude derating
Nameplate rating is at 40°C, sea level. A hot cement plant floor demands derating or a higher SF/Class
4
Starting current tolerance
Can the supply (transformer, generator, or busbar) handle the inrush without unacceptable voltage dip?
5
Enclosure & environment
TEFC vs explosion-proof vs washdown — dust and moisture exposure decide this, not preference
3. Starting Method Decision Guide
Click each method to expand the practical decision criteria.
Direct-On-Line (DOL)
Use when load allows it
Use it when: Motor is small relative to supply capacity (rule of thumb: motor kW under roughly 5% of
transformer kVA), load has low starting torque demand, and mechanical shock at start is acceptable.
Field check: Watch for voltage dip on other loads during start — a dimming light or a PLC brownout
during motor start is your signal DOL is stressing the supply.
Star-Delta
Use with care
Use it when: Motor is too large for DOL on the available supply, but the load can tolerate the torque
gap during the star-to-delta transition (torque drops to roughly 1/3 in star).
Field check: If the load stalls or dips speed hard at the transition, the load's breakaway torque is
higher than star-connection torque can supply — this method is the wrong fit, not a wiring fault.
Soft Starter
Use for smooth ramp, fixed speed
Use it when: You need smooth acceleration/deceleration (pumps, conveyors, fans) but the process runs
at one fixed speed once started — no need for speed control during run.
Field check: A soft starter that's undersized for the load's inertia will show extended ramp times and
high case temperatures — check the starter's kW rating against actual load inertia, not just motor kW.
VFD
Use when speed control is needed
Use it when: The process needs variable speed (rate control, energy savings on fans/pumps), precise
torque control, or frequent start/stop cycling that would wear mechanical starters.
Field check: Motor must be VFD-rated (inverter-duty insulation) for long cable runs or high switching
frequency — a standard motor on a VFD with long cable runs is a bearing-current failure waiting to happen.
4. Common Starting Mistakes — Field-Verified
Mistake 1 — Sizing the starter to the motor nameplate kW alone
The starter (soft starter or VFD) must be sized to the actual starting/running current under
the real load and duty cycle, not just the motor's nameplate FLA. A high-inertia load (large fan, ID fan with damper
closed at start) can demand a starter one frame size larger than "motor kW" would suggest.
Mistake 2 — Ignoring supply voltage dip during DOL starts
DOL starting current is typically 6–8× FLA. On a weak supply or long feeder run, this causes
a voltage dip that can nuisance-trip nearby VFDs, PLCs, or contactors. Always check available fault current /
transformer capacity before defaulting to DOL "because it's simple."
Mistake 3 — Using star-delta on high-breakaway-torque loads
Star-delta reduces starting torque to about a third of DOL torque. On loads like loaded conveyors,
positive-displacement pumps, or crushers, the motor can stall in the star connection and never reach transition
speed — repeatedly tripping on overload while looking like "the starter is faulty."
Mistake 4 — Standard motor + VFD + long cable run
Fast PWM switching edges on long cable runs (typically over 30–50m without mitigation) cause
voltage reflection spikes at the motor terminals, stressing standard insulation and driving bearing currents.
Fix: inverter-duty motor, or output filter/reactor sized for the cable length.
5. Field Case Study
Scenario: A 37 kW ID fan on a cement kiln line was starting DOL, tripping the upstream breaker roughly one
start in five. The panel showed nothing wrong — breaker rated correctly for FLA.
Root cause found: The fan damper was left open at start (interlock bypassed by operators to "save time"),
so the motor was starting against near-full load torque instead of the near-zero torque a closed damper gives.
Starting current stayed at inrush level for several seconds longer than the breaker's thermal-magnetic curve allowed.
Fix: Restored the damper-closed-at-start interlock (matches Check #1 — torque profile — from the checklist
above) and no starting-related trips have recurred.
Knowledge Check
1. A motor's nameplate shows Service Factor 1.15. What does this tell you?
The motor can run at 15% under-voltage indefinitely
The motor can handle 15% above nameplate load briefly without immediate damage, but it isn't a continuous rating
The motor efficiency improves by 15% at rated load
2. Which starting method risks a stalled motor if the load's breakaway torque is too high?
Direct-On-Line (DOL)
Star-Delta
VFD with full vector control
3. A standard (non-inverter-duty) motor is run on a VFD with a 60m unfiltered cable run. What's the main risk?
The motor will run backward intermittently
Voltage reflection spikes and bearing currents stressing the insulation and bearings over time
The VFD will automatically underpower the motor to compensate
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