VFD and Soft Starter Application and Fault Reading
Choose between a soft starter and a drive, size a drive by current and duty, check that a soft start can move the load, and use fault logs and bus readings to find the real cause.
Table of contents
Most variable frequency drive (VFD) and soft starter problems are decided before anyone presses Start. The wrong device gets picked, a drive gets sized by horsepower, or a soft starter gets set up on a load it cannot break loose. Then the fault codes begin. A senior technician picks the right device, and when it trips, reads the data the device saved and does not simply reset it. This covers those decisions. It assumes you know how a drive's three sections work and what a soft starter does.
What You Will Be Able to Do
- Choose between across-the-line, soft start and drive for a given load.
- Check whether a soft starter's current limit leaves enough torque.
- Size a drive by current and duty, not horsepower alone.
- Apply the NEC input, output and bypass rules.
- Use a fault log, bus voltage and passive tests to find a cause.
Warning: The DC bus of a drive holds a lethal charge after power is removed. Follow the wait time on the label, then a qualified person verifies the bus with a suitably rated meter before anyone touches the inside. Soft starters and drives can both restart on their own, depending on settings, so lock out and confirm before you work on the motor.
Parameter names, fault codes and test values differ by brand. Use the manual for the unit in front of you. NEC sections are from recent editions, so confirm the one adopted for your job.
Picking the Device
| Need | Across-the-line | Soft starter | VFD |
|---|---|---|---|
| Cost and simplicity | Best | Middle | Highest cost |
| Cut starting current | No | Yes | Yes, the most |
| Starting torque | Full | Reduced | High, with low current |
| Speed control | No | No | Yes |
| Energy savings on fans and pumps | No | No | Yes, if speed is reduced |
| Soft stop to prevent water hammer | No | Yes | Yes |
| Harmonics added | None | Only during ramps | Yes, while running |
Do not buy a drive when the load only needs a gentle start. Do not use a soft starter when a heavy load needs high torque from zero. Soft starters reduce voltage, and torque falls faster than voltage.
Can a Soft Starter Move This Load
A soft starter limits current by lowering voltage. Current falls in step with voltage. Torque falls with the square of voltage. That is the whole issue.
Worked Example: A Loaded Conveyor
A motor draws 600 percent of its full-load amps (FLA) at locked rotor and produces 150 percent of full-load torque at full voltage. A loaded conveyor needs 50 percent of full-load torque to break loose. You set a 300 percent current limit.
- Voltage at that limit: 300 / 600 = 50 percent.
- Torque at that voltage: 0.50 x 0.50 = 25 percent of locked-rotor torque.
- Available torque: 150 x 0.25 = 37.5 percent of full-load torque.
The conveyor needs 50 percent, so it will not move. The motor sits and heats until the soft starter trips. Try a 400 percent limit:
- Voltage: 400 / 600 = 67 percent.
- Torque: 0.67 x 0.67 = 45 percent of locked-rotor torque.
- Available torque: 150 x 0.45 = about 67 percent of full-load torque.
Now it starts, with some margin. A kick-start setting can help break a sticky load free, but it brings back a short burst of high current. If the load needs high torque at low speed, use a drive.
Sizing a Drive
Drives are sized by output current, because current heats the transistors. Horsepower on the cover is a shortcut, and it assumes a standard motor.
Check these in order:
- Motor nameplate FLA. The drive's output rating must cover it. Older and low-speed motors can draw more than a standard motor of the same horsepower.
- Duty. Most drives have two ratings. Normal duty suits fans and centrifugal pumps, and it commonly allows about 110 percent overload for 60 seconds. Heavy duty suits conveyors, mixers and positive-displacement pumps, and it commonly allows about 150 percent for 60 seconds. The same drive has a lower current rating at heavy duty.
- Conditions. Heat, altitude and higher carrier frequencies reduce the rating. The manual gives the derating.
- Number of motors. One drive on several motors must carry their total current, in V/Hz mode, with an overload for each motor.
Worked Example: A 40 HP Conveyor
A conveyor uses a 40 HP, 460-volt motor with a nameplate FLA of 50 amps. Table full-load current (FLC) is 52 amps. The load is constant torque, so you need heavy duty.
- A drive with a label of "50 HP normal duty, 40 HP heavy duty" has a heavy-duty output of 52 amps in its manual. That covers 50 amps.
- The drive's input current in the manual is 55 amps.
- Input conductors, under NEC 430.122: 55 x 1.25 = 68.75 amps. 4 AWG copper at 75 degrees C.
- Output conductors: 52 x 1.25 = 65 amps. 6 AWG copper at 75 degrees C.
- Input fuses: the type and size the maker lists. The drive's own short-circuit rating depends on them.
Sizing the drive at its 50 HP normal-duty rating and running a conveyor on it invites overload trips the first time the conveyor starts loaded.
A Drive With a Bypass
A bypass lets the motor run across the line if the drive fails. That changes the rules. NEC 430.124 requires separate overload protection in the bypass, because the drive's built-in overload is out of the circuit. The conductors feeding the system must handle the larger of the drive input and motor currents. Check the listing.
What the Motor Needs
- Insulation. A drive's fast pulses create voltage peaks at the motor. NEMA MG-1 Part 31 sets inverter-duty insulation to withstand peaks of about 3.1 times rated voltage, or about 1,430 volts on a 460-volt motor. An older general purpose motor may not survive that on a long lead.
- Cooling. A shaft-mounted fan moves less air at low speed, so a constant-torque load at low speed overheats a standard motor. Check the motor's turndown rating.
- Service factor. Treat it as 1.0 on a drive, unless the motor maker says otherwise. The drive's waveform already adds heat.
Reading a Drive Fault
Most modern drives log the last faults with data. That data is more useful than the fault name.
Before you reset, record:
- The fault name and code.
- Output frequency at the trip.
- Output current at the trip.
- DC bus voltage at the trip.
- Time since last start, and the date and time.
- Any fault before this one in the log.
Worked Example: Same Code, Different Cause
An overvoltage fault appears on two drives.
| Log detail | Drive A | Drive B |
|---|---|---|
| Output frequency | Falling, 38 Hz | Steady, 60 Hz |
| Current | Low | Normal |
| When | During stops | Around 7 a.m. |
Drive A trips while it is slowing a heavy load. The motor is regenerating, and pushing energy back onto the bus. Fix it with a longer decel ramp, or a braking resistor if the stop time is fixed.
Drive B trips at steady speed. Nothing on the machine is regenerating, so the energy is coming from the supply. A utility capacitor bank switching on is a common cause. A line reactor can help, and a power quality recording will prove it.
The code was the same. The answer was not.
Reading the DC Bus
A healthy bus sits at about 1.35 times the supply voltage. On a 480-volt system, that is about 648 volts, and it can sit a little higher with no load.
| Bus reading on a 480 V drive | Likely meaning |
|---|---|
| About 650 V at idle | Healthy |
| Much lower, such as 580 V | Supply is low, or the input section has a fault |
| Rising during decel | Regeneration |
| Swinging with heavy ripple | Lost input phase or failing capacitors |
Worked Example: A Low Bus
A drive shows 580 volts on its bus at idle.
- 580 / 1.35 = about 430 volts of supply.
- Measure the three input phases. If they are 480, 480 and 475 volts, then the supply is fine, so suspect the drive's input section or its bus reading.
A lost input phase looks different. At light load the bus can stay near normal, because the capacitors still charge from the two good phases. Under load, ripple climbs and the average drops. Many drives detect this and trip, but a lightly loaded drive may run on two phases until the load comes up.
Common Faults, Deeper Causes
| Fault | Look at | Often missed cause |
|---|---|---|
| Overcurrent at start | Accel time, load, motor | Motor spinning backward from a flowing pump |
| Overcurrent at speed | Mechanical load | Failing bearing or coupling |
| Ground fault | Motor and output cable | Water in a junction box |
| Overtemperature | Filters, fans, room heat | A blocked heat sink, or a cooling fan past its life |
| Motor overload | FLA parameter, load | Wrong motor data entered |
| Undervoltage | Supply and fuses | A plant sag rather than a drive fault |
Passive Tests
With the drive locked out, the bus verified at zero and the motor leads and supply disconnected, a qualified person can test the power sections using a meter's diode test. Each junction should read a forward drop, commonly about 0.3 to 0.7 volt, one way, and open (OL) the other way. Near zero both ways means a shorted device. Open both ways means an open device. Compare the three phases against each other.
Do not use an insulation tester (megger) on a drive. Insulation testing is done on the motor and cable, with the leads disconnected from the drive.
Soft Starter Faults
A soft starter controls voltage with silicon-controlled rectifiers (SCRs), which are solid-state switches.
| Fault | Likely cause | First check |
|---|---|---|
| Phase loss | Blown fuse, loose lug, motor lead open | Line and load connections |
| Shorted SCR | Failed thyristor, past surge or overload | Lock out, test each pole to the maker's value |
| Overload | Long start, heavy load, low setting | Start time, setting, motor current |
| Overtemperature | Too many starts, poor airflow | Starts per hour against the manual |
| Stall or start timeout | Current limit too low for the load | Torque math above |
| Bypass fault | Failed bypass contactor or wiring | Bypass contacts, control voltage |
Two points catch people out:
- Starts per hour. Soft starters heat up during each ramp. The manual gives a limit that depends on start current, ramp time and ambient temperature. A process that cycles more than this will trip, or fail the SCRs.
- Capacitors. Power factor capacitors do not go between a soft starter and the motor, because they can destroy the SCRs. The manual shows where they belong.
A shorted SCR is a serious fault. In some two-phase-controlled designs, a shorted device can leave the motor connected to voltage while it looks stopped. Use the isolation contactor or disconnect the maker calls for.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Sizing a drive by horsepower on a heavy load | Overload trips on loaded starts |
| Setting a soft starter's current limit by habit | A motor stalled at reduced torque |
| Resetting before reading the log | The clue is gone |
| Using a service factor on a drive | A hotter motor than you think |
| Forgetting overloads on the bypass | An unprotected motor on bypass |
| Putting capacitors after a soft starter | Destroyed SCRs |
Check Your Understanding
1. Someone trimmed a pump's setting to cut peak demand on the electric bill, and now it sits and hums instead of spinning up. Why did a small change do that?
Torque falls with the square of voltage, so a small cut removes much more torque than it seems. Work out the setting the load really needs, and restore it.
2. A larger unit replaces a failed one, and the crew plans to reuse the existing wire and fuses. What must be rechecked?
The new unit's rated input current at 125 percent against that wire, and the fuse type and size its maker lists. The old numbers no longer apply.
3. A drive trips on overvoltage at steady speed each morning, with normal current and no regeneration. What do you suspect?
A supply event, such as utility capacitor switching. Record the supply to confirm it. A line reactor may help.
4. A conveyor ran fine while nearly empty, then tripped as it filled up, and you find one input fuse open. How did it run at all?
At light load, the capacitors still charged from the two good phases. As the load came up, ripple climbed and the drive tripped.
5. A drive with a bypass is being fed by new conductors. What must they be able to carry?
The larger of the drive's input current and the motor's current, because the motor may run from either path. Check the listing.
6. A replacement unit fails within a week of being fitted. What did the first repair probably miss?
The cause of the first failure. Test the motor and cable with the leads off, because a fault out there can take out the next unit too.
Key Takeaways
- Soft starters cut torque by the square of voltage, so check the torque before you commit.
- Size drives by current and duty, not horsepower alone.
- Input conductors are 125 percent of drive input current, and bypass circuits need separate overloads.
- Read the log before you reset. The same code can have different causes.
- A healthy bus is about 1.35 times supply voltage.
- Never megger through a drive, and keep capacitors off the output of a soft starter or drive.