The Core Question: What Happens to 3 Phase Motor Amps After a VFD?
The direct answer: The 3 phase motor amp after VFD output is dictated entirely by the mechanical load torque and speed, not the drive itself. If a 10HP, 460V motor requires 14A to turn a conveyor at 60Hz across-the-line, it will still demand approximately 14A from the VFD output terminals at 60Hz full load. The VFD does not "create" or "limit" the current the motor needs to do the work; it simply supplies it up to its rated capacity.
However, the electrical dynamics change significantly on the input side of the drive. Because the VFD’s internal DC bus capacitors correct the displacement power factor, the VFD will draw less current from the grid than it outputs to the motor. A motor drawing 14A from the VFD output might only pull 11A from the 480V mains input.
Motor Type Selection for VFD Loads
Not all 3-phase motors survive the harsh electrical environment created by a VFD. The rapid voltage transitions (dV/dt) of the PWM waveform can exceed 1600V/µs, causing micro-corona discharges that eat through standard motor winding insulation. Selecting the right motor type for your load profile is the first critical decision.
| Motor Type | Torque Curve & Profile | Control / Drive Needs | Cost & Application |
|---|---|---|---|
| Standard AC Induction (NEMA Design B) | Good starting torque, but torque drops linearly with speed below 30Hz due to cooling fan starvation. | Simple V/Hz (Volts per Hertz) control. No encoder required. | Lowest cost. Variable torque loads only (centrifugal pumps, HVAC fans). |
| Inverter-Duty Induction (NEMA MG 1 Part 31) | Constant torque down to 10Hz (with independent blower) or 30Hz (shaft fan). Magnet wire rated for spike voltages. | Sensorless Vector or Closed-Loop Vector control for high starting torque. | Moderate cost (+20% over standard). Conveyors, crushers, extruders, hoists. |
| PMSM (Permanent Magnet Synchronous) | 200%+ peak torque at zero speed. High efficiency across the entire RPM range. | Requires a drive with PM motor algorithms and usually absolute encoders. | Highest cost. High-precision indexing, dynamic hoists, robotics. |
For a comprehensive breakdown of insulation requirements for these motors, refer to the NEMA MG 1 Motors and Generators standard, specifically Part 31 which defines inverter-duty dielectric stress limits.
Sizing Rule of Thumb and Worked Load Example
The most common mistake in drive selection is sizing the VFD by horsepower. HP is merely a thermal rating; it tells you nothing about the actual current the motor will draw under load. Always size your VFD by the motor’s Full Load Amps (FLA) and the load torque profile.
Worked Example: 15HP Rock Crusher
You are retrofitting a 15HP, 460V, 3-phase rock crusher. The motor nameplate reads 21A FLA. A crusher is a heavy constant-torque load with frequent shock loading (jams).
- Identify Load Type: Constant torque with shock loads. The NEC and general drive engineering practice dictates sizing the VFD at 125% of the motor FLA for continuous constant-torque applications to handle thermal dissipation in the drive's IGBTs.
- Calculate Required VFD Amps: 21A (Motor FLA) × 1.25 = 26.25A.
- Select the Drive: You need a VFD rated for at least 27A continuous output at 460V. Looking at standard manufacturer charts, a 15HP drive is typically rated for 22A (Heavy Duty). You must step up to a 20HP drive, which is typically rated for 27A to 32A (Heavy Duty/Constant Torque).
- Verify Peak Current: Crushers jam. Ensure the selected 20HP drive has an overload capacity of 150% for 60 seconds (31.5A to 40A peak) to clear the jam without tripping.
Wiring, Terminals, and the Post-VFD Circuit
Correct terminal identification and cable selection prevent catastrophic ground faults and electromagnetic interference (EMI). The circuit is strictly divided into line-side and load-side.
| Circuit Section | Terminal IDs | Wire Type & Rules |
|---|---|---|
| VFD Input (Line Side) | R/L1, S/L2, T/L3 (or simply L1, L2, L3) | Standard THHN in conduit. Size per NEC 310.16 based on VFD input current rating, not motor FLA. |
| VFD Output (Load Side) | U, V, W (or T1, T2, T3) | Must use symmetrical shielded VFD cable (e.g., Belden 29512 or VFD-2X). Standard NM-B or unshielded THHN will radiate EMI and cause bearing fluting. |
| Grounding (PE) | PE (Protective Earth) on Drive, Motor Frame Ground | 360-degree shield termination at the VFD gland. Ground the shield at the VFD end only to prevent ground loops. |
Failure Signatures: Hum, Overheat, and Stall
When a motor fails on a VFD, the symptoms tell you exactly which parameter or physical limitation was violated. Do not blindly swap parts; read the failure signature.
1. The High-Pitch Hum or Whine
Cause: The VFD’s PWM carrier frequency (switching frequency) is set too low, typically at the default 2kHz. The motor stator laminations physically vibrate at this acoustic frequency.
Fix: Access the VFD parameters (e.g., Yaskawa parameter C6-02 or WEG P0298) and increase the carrier frequency to 4kHz or 8kHz. Warning: Higher switching frequencies increase heat in the VFD's IGBTs. If you push past 8kHz, you must derate the VFD's maximum continuous current output by 10-15%.
2. Motor Overheat at Low Speeds
Cause: You are running a standard TEFC (Totally Enclosed Fan Cooled) motor at 15Hz (25% speed). The shaft-mounted cooling fan is also spinning at 25%, moving only a fraction of the required cooling air, while the motor is still drawing full-load current to maintain torque.
Fix: You must either replace the motor with an Inverter-Duty model equipped with an independent, separately-powered blower (force-cooled), or reconfigure the VFD's V/Hz curve to reduce voltage/torque at low speeds if the load permits. For constant torque at low speeds, an independent blower is mandatory.
3. Motor Stall and Drive Trip (OC or OL1 Fault)
Cause: The mechanical load requires more torque than the VFD can supply, or the acceleration time is too aggressive. The VFD hits its internal current limit (usually 150% of rated amps) and folds back the frequency to protect itself, eventually tripping on Overcurrent (OC) or Motor Overload (OL1).
Fix: Check the mechanical load for binding. If the load is healthy, increase the VFD acceleration time parameter (e.g., from 5 seconds to 15 seconds) to reduce the inertial torque demand. If the application genuinely requires higher breakaway torque, you must upgrade to a larger VFD frame size.
The Final Decision Path: Concrete Picks for 2026
Stop guessing. Use this decision matrix to select your exact hardware combination based on your load profile.
| If Your Load Is... | Then Choose This Motor... | And Pair With This Drive... |
|---|---|---|
| Variable Torque (Centrifugal pumps, HVAC fans, cooling towers) | Standard NEMA Premium Efficiency Induction Motor (TEFC) | Basic V/Hz Drive (e.g., Yaskawa J1000 or ABB ACS150) |
| Constant Torque (Conveyors, crushers, extruders, positive displacement pumps) | Inverter-Duty Motor (NEMA MG 1 Part 31 rated, Class H insulation) | Sensorless Vector Drive (e.g., Yaskawa GA800 or WEG CFW110) |
| High Dynamic / Hoisting (Cranes, elevators, rapid indexing) | PMSM (Permanent Magnet) or Vector-Duty Induction with Encoder | Closed-Loop Flux Vector Drive with Braking Chopper (e.g., Yaskawa A1000) |
For further reading on system efficiency and harmonic mitigation when deploying multiple drives on a single facility bus, consult the U.S. Department of Energy's Advanced Manufacturing Office guidelines on VFD deployment. Properly calculating your 3 phase motor amp after VFD installation ensures your system runs cool, efficient, and trip-free for decades.
