A 100 hp fan motor draws full running current through the soft starter’s SCR path whenever no bypass contactor is present. The SCR on-state voltage drop multiplied by current becomes heat inside the enclosure. In this installation, that loss amounted to several hundred watts. The number that matters for a normal bypass contactor is therefore the motor’s measured or nameplate full-load current, not horsepower alone.
The stopped-state 480 V indication is a different phenomenon. A high-impedance digital multimeter can display voltage passed by SCR leakage and RC snubber networks even though the starter is not gating the SCRs. The available current may be only milliamperes or less, but the reading does not provide isolation. Open the disconnect, apply the required lockout procedure, and verify absence of voltage before touching motor or starter conductors.
Current, Heat, and Switching Duty
A bypass contactor closes after acceleration and carries the motor’s continuous running current around the SCRs. Its contacts still conduct the complete motor current; bypass duty does not justify selecting a continuous-current rating below motor FLA. What changes is the required switching duty. Under a correctly sequenced transfer, the SCRs conduct before the bypass opens, so the bypass contacts are not expected to interrupt normal motor current.
The line contactor has a different job. It connects the soft starter and motor to the supply before starting and may have to make motor-starting current or break the running circuit following a fault or control action. If installed, rate it for full-voltage non-reversing motor duty—FVNR duty—not merely for continuous resistive current.
| Quantity or limit | Selection rule | Where to read it |
|---|---|---|
| Motor running current | Bypass thermal-current rating must equal or exceed it after applicable derating | Motor nameplate and a clamp-current measurement on each phase |
| Motor switching duty | Required for a line contactor and for any bypass expected to start or interrupt the motor directly | Contactor motor-duty table at the actual system voltage |
| Starter heat loss | Falls sharply after current transfers from the SCR path to the bypass path | Starter loss data and enclosure temperature measurements |
| Stopped output voltage | A high-impedance reading can result from SCR or snubber leakage | Starter schematic or manual plus an appropriately rated low-impedance test method |
| Overload coverage | Protection must remain active in both SCR and bypass operation | Starter listing label, manual, and overload-relay documentation |
Supported Contactor Arrangements
| Arrangement | Bypass requirement | Line-side behavior | Best fit |
|---|---|---|---|
| Bypass only | Continuous thermal or resistive-current rating at least equal to motor FLA; motor switching rating if it may interrupt load |
SCRs remain connected to the supply while stopped | Existing disconnect provides maintenance isolation and the bypass is used only for thermal relief |
| Line contactor plus bypass | Continuous thermal-current rating at least equal to motor FLA when sequencing prevents load switching |
Line contactor disconnects the starter when commanded off and takes the normal breaking duty | Isolation by control sequence or reduced exposure to supply transients is required |
| Motor-duty bypass for emergency operation | Full FVNR motor rating plus suitable overload protection |
Can run the motor without soft-start acceleration if the complete circuit is designed for that mode | A deliberate emergency full-voltage restart function is required |
For the stated objective—removing several hundred watts of SCR heat after acceleration—the preferred arrangement is a normally open bypass contactor selected from its published continuous thermal or resistive-current rating. Add a line contactor only when the operating philosophy calls for controlled disconnection, the starter manufacturer requires one, or supply transients justify removing the ungated SCRs from the line while idle.
An existing nearby disconnect remains the maintenance isolation device. A control contactor is not a substitute for a lockable disconnect or verified absence of voltage.
Bypass Contactor Rating
Select the bypass from motor nameplate current at the connected voltage and the contactor manufacturer’s continuous-current data. A 100 hp label alone is insufficient because motor current varies with voltage, efficiency, power factor, and design. Record the actual nameplate FLA, then check ambient-temperature, enclosure, terminal, and conductor limitations that may reduce the published contact rating.
An AC-1 or resistive/thermal rating can be used when the bypass closes only after the motor is near running speed and opens only after the SCRs have taken the current. The utilization label is not permission to undersize the contactor: its continuous rating must still carry all motor current. AC-1 and AC-3 ratings chiefly differ in the current and switching conditions assigned to the contacts; they are not interchangeable shorthand for fault-interrupting capability.
If the control sequence can open the bypass while its contacts carry the motor, select it for motor duty. Apply the same rule when the bypass is intended to start the motor across the line following a starter failure. That arrangement also needs overload protection and upstream fault protection coordinated for full-voltage operation.
Line Contactor Decision
The stopped output indication alone does not justify a line contactor. An SCR remains a semiconductor connection even when it is not gated, and its snubber can couple the output toward line potential. A digital meter presents so little burden that this leakage can display close to supply voltage. A lower-impedance instrument may collapse the reading, demonstrating that the circuit cannot sustain load current, but that test never turns the output into a safe work point.
| Symptom | Likely mechanism | Decision |
|---|---|---|
| Digital meter shows voltage at the stopped output | SCR off-state leakage and RC snubber current | Keep using the disconnect for isolation; add a line contactor only for the operating or protection policy |
| High enclosure temperature during steady running | Full motor current remains in the SCR path | Add and correctly sequence a bypass contactor |
| Repeated SCR damage while idle | Ungated SCRs remain exposed to supply transients | Check grounding and surge conditions, then consider a starter-controlled line contactor |
| Bypass contacts discolor or overheat | Continuous rating, derating, termination, or phase current is unsuitable | Measure all phase currents and inspect the complete current path before returning to service |
When used, the line contactor must tolerate the motor circuit’s starting and breaking demands. Control it from the soft starter’s designated line-contactor output or an approved interface relay. It should close before the starter gates the SCRs, remain closed throughout acceleration and running, and open only at the point defined by the starter’s stop sequence. Read the required sequence from the exact starter manual rather than assigning an arbitrary delay.
Overload and Fault Coordination
Bypassing the SCR power path must not bypass motor overload protection. Confirm whether the exact Baldor model’s built-in overload function is listed for branch motor-overload protection and whether it measures all required phases. The reported digital starter used a function that monitored only two of the three phases and was not listed to serve as the required overload protection.
Where the listing or manual does not authorize the built-in function for this purpose, install a separate overload relay in a common motor-current path so it measures current during both soft-start and bypass operation. Match its setup and trip class to the motor, driven load, acceleration time, and manufacturer instructions. Test its trip path through the complete control circuit.
Fault-current capability is a separate check. Verify the assembly short-circuit rating, upstream protective device, contactor fault-current data, conductor protection, and starter requirements as one system. A large interrupting-capacity value does not correct contacts that overheat because their continuous thermal rating is too low.
Selection and Installation Procedure
- Record the exact soft-starter model, motor nameplate voltage and
FLA, enclosure conditions, upstream protective device, disconnect rating, and available fault-current data. - Read the starter manual for an internal bypass, external-bypass wiring diagram, bypass output logic, required contactor category, and overload listing. Confirm that the replacement starter does not already contain a bypass path.
- Select a bypass contactor whose published continuous thermal or resistive-current rating equals or exceeds motor
FLAafter all applicable derating. Select full motor duty instead if the contacts may start or interrupt the motor. - Decide whether a line contactor adds a required function. If selected, use a contactor fully rated for
FVNRoperation at the installed motor voltage and current. - Place overload sensing where bypass operation cannot route motor current around it. Wire its trip contact into the starter and contactor control chain according to the approved diagram.
- Wire the bypass coil to the starter’s run or bypass command. The SCRs must first accelerate the motor, then the bypass closes; before a normal bypass opening, the starter must re-establish SCR conduction so the contacts transfer current without breaking it.
- Wire the optional line contactor through the starter’s specified line-control sequence. A loss of the permissive must remove gating and open the power circuit in the order prescribed by the manual.
- Check conductor sizing, terminal torque, phase order, coil voltage, auxiliary-contact feedback, upstream protection, and assembly fault rating before energizing.
Commissioning and Verification
- With power isolated, verify continuity through the intended SCR and bypass paths and confirm that no wiring routes around the overload sensor.
- Run the fan while measuring all three phase currents. Compare steady current with motor nameplate
FLA, bypass thermal rating, and the overload setting. - Observe the start sequence. The bypass must close only after acceleration reaches the starter’s transfer condition; current should move to the bypass without a speed or torque disturbance.
- Confirm that steady-state SCR heating falls after transfer. Measure enclosure temperature under a representative production run and inspect the contactor terminals for abnormal temperature rise.
- Command a normal stop. Verify that the starter follows its documented transfer and stop sequence and that neither the bypass nor line contactor is forced to interrupt current outside its rating.
- With the starter stopped but the disconnect closed, characterize the output indication using rated instruments and the approved test procedure. Then open and lock the disconnect and verify absence of voltage at every relevant conductor.
- Test overload and fault-control inputs using the manufacturer’s commissioning method. Confirm that the motor stops, the contactors reach their safe states, and a restart requires the intended reset action.
Frequently Asked Questions
What happens if the bypass contactor is rated below motor FLA?
The contacts and terminals can overheat even though they never switch the running motor. Select a thermal or resistive-current rating at least equal to measured or nameplate FLA after derating.
What happens if the bypass opens while the motor is running?
If the SCRs have already taken the current, the bypass opens with little switching stress. If they have not, the contactor may interrupt motor current and must have the corresponding motor-duty rating.
What happens if a meter reads 480 V at the stopped output?
SCR and snubber leakage can produce a near-line-voltage reading on a high-impedance digital meter. Treat the circuit as energized until the disconnect is opened, locked, and absence of voltage is verified.
When should I stop commissioning and contact official support?
Stop if the manual does not identify a valid bypass sequence, the overload listing is unclear, phase current exceeds the selected ratings, contacts overheat, or the starter cannot transfer current without a disturbance. Record the exact Baldor model, wiring diagram, nameplate data, current measurements, protection details, and observed sequence. Escalate those records to official manufacturer support before further operation.