Running current is only part of the starter duty
A motor starter can carry a motor continuously and still be unsuitable for the way a machine actually operates. Frequent starts, jogging, inching, plug stopping and reversing increase switching and thermal stress well beyond steady running. For those applications, selection has to consider utilisation category, operational current, starts per hour, acceleration time, switching frequency and expected electrical life.
The load profile matters just as much as the nameplate current. A conveyor that starts twice per shift is a different application from a positioning machine that jogs every few seconds. A hoist that reverses under load imposes another set of demands. Treating all three as the same “motor starter” duty can lead to premature contact wear, overheating or nuisance overload trips.
Where reversing is part of the cycle, BCH’s reversing contactor interlocking guide is a useful companion because the duty decision and the interlocking method need to work together.
Capture the real operating profile
|
Duty input |
What to record |
Why it matters |
|
Starts per hour |
Normal and worst-case frequency |
Sets cumulative thermal and switching stress |
|
Start duration |
Time from energisation to stable speed |
Affects heating during acceleration |
|
Jog/inch command |
Pulse length and repetitions |
Can create severe contact duty |
|
Reversing |
Delay, speed at reversal, interlocking |
Defines switching severity and control needs |
|
Load |
Inertia, breakaway torque, jams |
Changes acceleration and current |
|
Environment |
Ambient, altitude, enclosure ventilation |
Changes thermal margin |
Do not rely on an average operating cycle if the machine regularly has short bursts of severe duty. The worst credible production sequence often determines the correct selection. If the process has multiple recipes or operating modes, record the mode that creates the most starts, the longest acceleration or the highest reversal frequency.
AC-3 and AC-4 are not interchangeable
AC-3 duty commonly describes starting a squirrel-cage motor and switching it off after it is running. More severe operations, including inching and plugging, can fall under AC-4. The difference is important because contactors experience much greater electrical stress when they make or break current under these conditions. An AC-3 rating should not be applied to AC-4 service without the required derating or manufacturer approval.
Use the exact product data for the BCH contactor being considered. If the application is unusual, the BCH special-purpose starter range may also be relevant, but only after the duty is defined clearly.
Frequent starting is a thermal problem as well as a contact-life problem
Every start adds heat to the motor, contactor, overload relay and enclosure. If the next start occurs before the system has cooled sufficiently, temperature accumulates. This is why a starter may complete several starts successfully and then trip even though the load appears unchanged. The pattern often points to thermal duty rather than a single abnormal event.
Low supply voltage can make the problem worse by extending acceleration time. High load inertia, increased mechanical friction or a process jam can do the same. Poor panel ventilation raises the starting point for every cycle. The overload relay should therefore be reviewed together with the actual start duration and rest period, not as an isolated current setting.
Jogging and inching need special attention
Jogging uses short energisation pulses to move a machine in small increments. The motor may never reach normal running speed, so the contactor repeatedly switches during a high-current part of the operating cycle. That can consume electrical life much faster than normal start-run-stop operation. Inching can also raise motor temperature because repeated acceleration events occur with little cooling time between them.
When a machine relies heavily on jogging, document the pulse duration and number of operations per hour. Do not describe the duty only as “intermittent.” That word does not tell a selector whether the starter sees ten operations per hour or several hundred short commands during setup and production.
Reversing changes both the power circuit and control logic
A reversing starter needs two contactors arranged so that opposite directions cannot be energised together. Electrical and mechanical interlocking are used to reduce that risk. The control sequence should also consider whether the motor is allowed to slow or stop before reversal, because reversing while significant speed remains can create a much more severe electrical and mechanical event.
For cranes, hoists, positioning systems and other machines that reverse frequently, the delay between directions and the load condition at reversal should be part of the selection data. A control scheme that is safe for occasional direction changes may be unsuitable for repetitive production duty.
Use life curves, not intuition, for expected endurance
Contactor life is not a single universal number. Electrical endurance depends on operational current, utilisation category and switching conditions. Use the manufacturer’s published life information for the actual duty. Mechanical endurance is useful, but it does not replace electrical-life data in a motor application where contacts are switching load current.
This is especially important when production targets are high. If an application operates thousands of times per week, a small difference in expected electrical life can translate into a meaningful maintenance interval. Record the expected operation count so the maintenance team can plan inspection or replacement before contact wear becomes a process reliability issue.
A selection workflow for duty-cycle applications
- Record motor voltage, full-load current, connection and starting time.
- Define the machine cycle, including normal and worst-case starts per hour.
- Record jogging, inching, plugging or reversing operations separately.
- Identify the correct utilisation category for the actual switching duty.
- Check contactor operational current and electrical-life data at that duty.
- Confirm overload relay behaviour for repeated starts and real acceleration time.
- Review enclosure temperature, ventilation and heat from neighbouring devices.
- For reversing, verify electrical and mechanical interlocking and the control delay.
- Commission under a representative production cycle and record measured values.
For standard starter families and available configurations, review the BCH motor starter range. For product literature, the motor starter and submersible pump starter downloads can support the final reference check.
Commission with the machine doing real work
A no-load functional test confirms wiring and direction, but it does not prove that the starter can tolerate the production duty. Commissioning should include representative starts, running load and, where applicable, jogging or reversing sequences. Measure supply voltage and phase current under those conditions. If a problem appears only after repeated operations, record the count, timing and temperature rather than resetting immediately and losing the pattern.
Inspect the control logic as well as the power circuit. Verify that interlocks prevent simultaneous contactor operation and that the system responds safely to loss of control power. If auxiliary contacts are used for sequencing, prove the intended logic before production starts.
When a drive-based solution deserves evaluation
If the machine frequently changes speed or direction, requires controlled acceleration, or needs process speed regulation, a drive-based solution may deserve evaluation. That is not the same as saying a drive is always preferable. The decision should come from the machine requirement, supply conditions, duty and control objective. A conventional starter remains appropriate for many fixed-speed applications when its switching duty is correctly selected.
Information to leave for maintenance
- Normal and maximum starts per hour
- Acceleration time under representative load
- Jogging/inching frequency and pulse duration
- Reversal method and interlocking arrangement
- Utilisation category and contactor reference
- Electrical-life basis or curve reference
- Overload setting and trip class
- Measured commissioning current and voltage
This record helps a maintenance engineer distinguish genuine deterioration from a change in process duty. If production later increases the starts per hour or introduces more jogging, the starter can be rechecked against the new profile rather than simply replacing contacts more often.
Design for maintainable switching life
Frequent-duty machines should be designed with maintenance in mind. If the expected operation count is high, make the contactor accessible for inspection and replacement, keep the control wiring easy to trace and leave enough panel space for safe servicing. A starter that is electrically adequate but difficult to maintain can still create unnecessary downtime over the machine life.
Use the commissioning operation count as a baseline. If contact replacement intervals shorten later, compare the current machine cycle with the original duty. Production may have increased, a new recipe may use more jogging, or a mechanical change may have lengthened acceleration. These changes can consume electrical life without any obvious change to motor full-load current.
Watch for duty changes hidden in control software
On automated machines, the electrical hardware may remain unchanged while PLC logic changes the real starter duty. A software update can reduce delays, add retries after jams or increase positioning movements. Those changes can raise starts per hour and reversal frequency even though the panel drawing still looks identical.
For that reason, duty-cycle reviews should include control-sequence changes as well as hardware changes. When production logic is modified, confirm that the revised cycle still sits within the starter, overload relay and motor thermal limits.
FAQs
Is starts per hour the same as duty cycle?
No. Starts per hour is one input. Acceleration time, running time, rest time, stopping method, load inertia, jogging and ambient conditions all contribute to the real duty cycle.
Why can a starter trip after several successful starts?
Repeated starts accumulate heat. Low voltage, long acceleration, high load, short cooling periods or poor enclosure ventilation can push the motor and starter beyond their thermal capacity.
Why is jogging harder on a contactor than normal running?
Jogging repeatedly switches the motor during the high-current acceleration period. The contacts may see many operations without the long steady-running intervals typical of normal AC-3 service.
Does reversing always require mechanical interlocking?
The final design should follow the approved control architecture and manufacturer guidance. In reversing starters, electrical and mechanical interlocking are commonly used together to reduce the risk of opposite contactors closing at the same time.
Can I use an AC-3 contactor for AC-4 duty?
Do not assume the AC-3 rating applies. AC-4 is more severe and may require different ratings, derating or specific manufacturer approval.
When should the duty-cycle calculation be reviewed?
Review it when production rate increases, the load changes, the machine begins jogging or reversing more often, acceleration time increases, enclosure conditions change or contact life becomes shorter than expected.
