VFD and Gearbox: Matching a Drive to a Gear Reducer
A variable frequency drive changes the speed of the motor, not the torque capability of the gearbox behind it. The mechanical rating of a gear reducer is set by tooth bending, tooth contact stress and bearing life, and none of those improve because the motor now runs at 30 Hz. What the drive does change is the shape of the duty the gearbox sees: lower output speeds for long periods, higher torque per unit of motor current, faster accelerations, and a switching waveform on the motor terminals that finds its way into the bearings.
Matching a drive to a gearbox is therefore a set of specific checks rather than one calculation. This guide works through the ones that decide whether an installation survives: the speed-torque envelope, lubrication at low speed, thermal derating, the torque limit setting, torsional behaviour and bearing protection.
What a VFD Can and Cannot Change
A drive controls motor speed by controlling frequency and voltage together. Below base speed it holds the voltage-to-frequency ratio approximately constant, which keeps flux, and therefore available torque, roughly constant. Above base speed the voltage is capped and torque falls away as speed rises. That is the whole envelope: constant torque below base speed, constant power above it.
The gearbox sits downstream and multiplies whatever torque reaches it by the ratio, minus losses. Service factor, tooth geometry and bearing ratings are fixed at the drawing stage: if the application needs 40 percent more torque than the gearbox was selected for, the drive cannot supply it. It can only deliver more motor current up to its limit, and the gearbox sees that overload multiplied by the ratio.
The practical consequence is that the drive is selected against the gearbox duty, not the other way round. If the machine must run at half speed with full torque for hours, the gearbox has to be rated for that torque continuously, and it has to lubricate and cool itself at that speed.
The Speed-Torque Envelope the Gearbox Sees
Write the duty down before choosing anything: output torque at each speed band, hours per band, starts per hour, and acceleration torque. Three load shapes cover most machines.
| Load type | Torque behaviour | Typical machines | What it means for the gearbox |
|---|---|---|---|
| Constant torque | Torque flat, power falls with speed | Conveyors, positive displacement pumps, screw feeders | Full rated torque available down to low speed; thermal and lubrication limits dominate |
| Variable torque | Torque falls roughly with speed squared | Centrifugal pumps and fans | Little torque at low speed; the drive saves energy, the gearbox is rarely the constraint |
| Constant power | Torque rises as speed falls | Centre winders, some machine tool spindles | High torque at low speed; often exceeds gearbox rating below a threshold speed |
The third row is where most surprises occur. A constant power load at 25 percent speed demands four times the torque it needs at full speed, and a gearbox selected on full-speed torque will be overloaded for the whole slow phase of the cycle. Either upsize the gearbox for the low-speed point, or change the control scheme so the machine does not run there continuously. This is the most common variable frequency drive gearbox mismatch on retrofits.
Minimum Speed Is a Lubrication Question
Splash lubrication depends on a gear or an oil slinger throwing oil upward. Throw distance falls with speed, and below a certain point the top bearings and the oil seals stop receiving oil. There is no universal number, but for most horizontal splash-lubricated units continuous operation below about one third of rated input speed deserves a check with the manufacturer, and below roughly 20 percent it normally needs a different lubricating arrangement.
Worm units are more sensitive than helical units because the worm mesh relies on a hydrodynamic oil film at the sliding contact. At low sliding velocity the film thins, friction rises, and the loss turns into heat exactly when the housing has the least oil circulation to carry it away.
Options, in order of preference:
- Keep the minimum operating speed above the splash limit and use the drive only for trim and soft starts.
- Fit an independent oil pump or oil circulation for sustained low speed.
- Use a larger sump or an oil cooler where heat is the binding constraint rather than distribution.
Synthetic lubricants help with oxidation life and cold start-up but they do not create oil flow. An oil upgrade is not a substitute for a lubrication arrangement.
Thermal Power Rating Falls With Speed
A gearbox has a mechanical rating and a thermal rating. The mechanical rating is what the teeth and bearings will carry; the thermal rating is the power the housing can dissipate without exceeding the permissible oil temperature. At low input speed both churning losses and oil circulation fall, while the torque - and therefore the heat generated per revolution - stays high on constant torque duty. The thermal rating is usually what limits a slow-running drive.
| Input speed band | Likely constraint | Usual measure |
|---|---|---|
| 100–50 percent of rated | Mechanical rating | None; standard selection |
| 50–30 percent | Thermal margin shrinking | Check thermal rating at actual speed; verify oil level |
| 30–20 percent | Lubrication distribution and thermal | Consult manufacturer; consider oil pump or cooler |
| Below 20 percent, continuous | Both | Forced lubrication plus cooling, or a purpose-built slow-speed unit |
The motor runs into the same physics from another direction. A standard totally enclosed fan-cooled motor has its fan on the shaft, so at 30 percent speed it receives roughly 30 percent of its design cooling air. Inverter duty motors bring thicker insulation with a wider thermal margin, or an independent blower. If the motor runs hot at low speed, the answer is a blower or a bigger frame, not a higher drive current limit.
Overload, Acceleration and the Torque Limit Setting
Drives are usually set to deliver 150 percent of motor rated current briefly, and 150 percent current is not 150 percent shaft torque. The current includes magnetising current, and the torque produced depends on flux and slip. On a loaded machine the useful torque overload may be rather less than the current figure suggests.
Two settings decide whether the gearbox is protected:
- Torque limit. Set it below the gearbox mechanical limit expressed in motor torque terms, so a jam stalls the drive instead of stripping teeth. The gearbox limit equals its permissible output torque divided by the ratio and by the transmission efficiency.
- Acceleration ramp. A short ramp asks for high torque immediately after the start command, and on a high-inertia load that torque can exceed running torque by a large factor. Ramp times should come from the inertia and the available torque, not from a wish to reach speed quickly.
High starting torque is why gearboxes fail on inverter retrofits while years of direct-on-line service gave no trouble. Direct-on-line starting is gentle in torque terms, because breakdown torque arrives only briefly at the end of acceleration, whereas a drive can hold a high torque limit from zero speed for as long as the ramp lasts.
Torsional Vibration, Shaft Currents and Bearing Protection
A drive adds a controller to a mechanical system that already has natural frequencies, and it changes the electrical path through the machine. Two failure modes follow.
Torsional excitation. Torque ripple at the switching frequency and its sidebands can excite a drivetrain resonance that fixed-speed operation never reached. Symptoms are a growl or beat at one particular speed, coupling and seal wear, and in bad cases shaft fatigue. The remedy is usually procedural: find the offending speed with a speed sweep, then set a skip band so the controller accelerates through it instead of settling on it. Raising the carrier frequency reduces audible ripple but does not remove a mechanical resonance, only move the excitation.
Shaft currents. A switched output creates common-mode voltage that couples capacitively to the rotor and discharges through the bearings to earth. Above roughly 30 kW, and on any 400 V motor with a long cable run, this is a recognised cause of fluting, grease breakdown and early bearing failure. The gearbox is part of the same electrical path, and its bearings carry current too when the motor is flange-mounted to it. Cheapest measures first: short screened cable bonded at both ends; a shaft grounding ring at the non-drive end; an insulated bearing on larger frames; a dv/dt filter or output reactor on long runs. On a gearmotor supplied as one unit this belongs in the order, because an insulated bearing cannot be retrofitted into a flange machined without one.
Braking, Holding and Zero-Speed Duty
A drive can decelerate a load electrically but cannot hold it indefinitely without help. Dynamic braking turns the energy into heat in a resistor, regenerative braking returns it to the supply, and a mechanical brake holds at standstill. Holding duty matters to a gearbox for a specific reason: at zero speed there is no oil film in the mesh, so a load that creeps or oscillates while stopped puts the contact on the same few teeth with boundary lubrication only.
On inclined conveyors and hoists, size the brake or backstop for the holding torque including the ratio, and interlock brake release with the drive enable so the two cannot fight each other. For a load that must hold position for long periods, a mechanical brake is the correct component - holding torque at zero speed from a drive is a thermal risk to both motor and gearbox.
Commissioning Checklist
| Check | Target | Why it matters |
|---|---|---|
| Minimum continuous speed | Above the lubrication limit, or oil pump fitted | Bearing and seal starvation |
| Thermal rating at lowest speed | Above actual transmitted power | Oil overheating and seal failure |
| Torque limit | Below gearbox mechanical limit | Jam protection instead of tooth damage |
| Acceleration ramp | Set from inertia and available torque | Avoids peak torque at every start |
| Skip bands | Any identified resonance avoided | Torsional fatigue |
| Motor cooling at low speed | Independent blower where needed | Insulation life |
| Bearing current measures | Grounding ring, insulated bearing as required | Fluting and early bearing failure |
| Brake and backstop | Sized for holding torque at the output shaft | Load drift and mesh damage at standstill |
Work through this list with the duty in front of you and most inverter-related gearbox failures disappear before commissioning. A helical gear motor such as the H226 series is a convenient starting point because motor and reducer are matched at the factory, so the thermal and bearing questions are answered once rather than twice.
FAQ
Can any gearbox be run on a VFD?
Mechanically, yes, provided the duty stays inside the mechanical and thermal ratings at every speed in the range. The limits that catch people out are lubrication below about one third of rated speed, thermal rating at low speed on constant torque duty, and peak torque during acceleration. Trim and soft starting are safe on a standard unit; sustained low speed needs a lubrication and cooling review.
Does a VFD increase gearbox torque?
No. It increases the torque the motor can deliver at low speed compared with a fixed-speed motor, up to the motor's thermal limit. The gearbox still multiplies that torque by its ratio, and its teeth and bearings still have the same rating. If the machine needs more output torque, the gearbox must be upsized; raising the drive current limit only shortens the life of what is already there.
Why does the gearbox overheat only at low speed?
Because the heat input stays high while heat removal falls. On constant torque duty the mesh transmits full torque at any speed, so the power lost in the mesh is roughly constant, but the oil is thrown around less and internal circulation drops. The result is a rising sump temperature. An oil cooler or a larger sump solves the heat; an oil pump also solves distribution.
Should I use an inverter duty motor?
If the operating range goes below about half speed for long periods, or the cable run is long, yes. An inverter duty motor brings insulation rated for the switched waveform and either a wider thermal margin or a separately driven fan. A standard motor will run, but its insulation life and low-speed cooling are both compromised, and the saving is small against the cost of a rewind.
How do I protect the gearbox from a jam?
Set the drive torque limit below the gearbox mechanical limit, and add a mechanical torque limiter or shear pin coupling on duties where the jam can arrive faster than the drive can react. Verify the setting at commissioning by stalling the machine deliberately and confirming that the drive trips before anything in the drivetrain yields.
Write the duty down - speed bands, torque, hours, starts, environment - and send it through our online selection system. It sizes the mechanical side against real catalogue data, and our engineers will confirm the lubrication and thermal limits for the low-speed end of your range before the unit is built.
