For plant engineers and system integrators, selecting a variable frequency drive is rarely about picking the closest kW rating from a catalog. It is a sizing exercise. Oversize the drive, and you pay for unused capacity and larger footprint. Undersize it, and you face nuisance trips, shortened motor life, or catastrophic drive failure during peak load.
This guide treats the FST-650L not as a product pitch, but as a reference platform to walk through the calculations, derating rules, and peripheral decisions that turn a motor nameplate into a correctly specified drive order.
1. Start with the Load Profile, Not the Catalog
Before you touch a specification sheet, classify the mechanical load. The torque characteristic determines the control mode, the overload margin, and whether you need a heavy-duty (G-type) or light-duty (P-type) rating.
| Load Profile | Torque Behavior | Typical Machinery | FST-650L Control Mode | Sizing Rule |
|---|---|---|---|---|
| Variable Torque | T ∝ n² | Centrifugal pumps, fans, blowers | V/F or SVC | Match drive kW to motor kW; current check only |
| Constant Torque | T = constant | Conveyors, mixers, compressors, cranes | SVC (sensorless vector) | 150% overload for 60 s required; verify low-speed torque |
| Constant Power | P = constant | Machine tool spindles, wire drawing | FVC (closed-loop vector) | Match by base-speed torque; check max frequency |
Key Insight: The FST-650L delivers 180% starting torque at 0.5 Hz in SVC mode and 150% rated current for 60 seconds. For constant-torque loads, this means you can often size the drive 1:1 with the motor kW without upsizing—provided the motor current does not exceed the drive's continuous rating.
2. The Current-First Sizing Method
Rule #1: Amps beat horsepower. A 30 kW motor running at 400 V may draw 54 A, while another 30 kW motor at 690 V draws only 32 A. The drive must be sized to the motor full-load current (FLA), not the nameplate kW.
The Sizing Formula
Where:
- = Drive continuous output current (A)
- = Motor full-load amps from the nameplate
When to Add Margin
| Condition | Recommended Margin | FST-650L Action |
|---|---|---|
| Standard pump or fan | 1.0–1.1× FLA | Standard G/P model |
| Heavy starting (crushers, ball mills) | 1.1–1.2× FLA | Select next size up or G-type heavy-duty |
| Multi-motor parallel | 1.1–1.15× sum of all FLA | Verify total current against drive rating |
| Long motor cable (>50 m) | 1.1× FLA + output reactor | Add reactor or filter; upsize if voltage drop is critical |
FST-650L Example:
A 22 kW, 400 V motor with FLA = 42.5 A.
Minimum drive current = 1.1 × 42.5 = 46.8 A.
The FST-650L-022G/030P-T4 provides 45 A / 60 A (G/P), so the G-type (heavy-duty) 22 kW model is acceptable, but if the application involves frequent shock loads, stepping up to the 030G/037P unit (60 A / 75 A) provides thermal headroom.
3. Control Mode Selection: Matching Performance to Process
The FST-650L offers three control modes. Choosing the wrong one is a common source of "the drive works, but not well" complaints.
V/F Control (Open Loop)
- Best for: Centrifugal pumps, HVAC fans, simple conveyors
- Speed accuracy: ±0.5%
- Advantage: Simple setup, no motor autotune required
- Limitation: Torque drops at low speed; poor dynamic response
Sensorless Vector Control (SVC / SFVC)
- Best for: General machinery, mixers, extruders, material handling
- Speed accuracy: ±0.2%
- Advantage: Automatic motor parameter identification; high starting torque without encoder
- FST-650L benefit: 180% torque at 0.5 Hz; supports 4 independent motor parameter sets switchable by terminal command
Closed-Loop Vector Control (FVC / CLVC)
- Best for: CNC spindles, elevators, winding/unwinding, high-dynamic positioning
- Speed accuracy: ±0.02%
- Requirement: Incremental encoder (PG card) on motor shaft
- FST-650L benefit: Compatible with multiple encoder types via plug-in PG cards; torque control mode available for tensioning applications
Engineer's Note: If your process requires torque control or zero-speed holding (e.g., hoists), SVC is insufficient. Budget for a PG card and encoder wiring from day one.
4. Voltage, Frequency & Environmental Derating
Voltage Matching
The FST-650L spans 0.75 kW to 630 kW across four voltage platforms:
| Input Voltage | Available Power Range | Typical Market |
|---|---|---|
| 1-phase 220 V | 0.75 kW – 2.2 kW | Light industrial, retrofit |
| 3-phase 220 V | 0.75 kW – 110 kW | Special machinery, legacy systems |
| 3-phase 380 V | 0.75 kW – 630 kW | Global industrial standard |
| 3-phase 660 V | 11 kW – 630 kW | Mining, heavy industry |
Critical: Never match a 380 V drive to a 660 V motor, even if the kW rating aligns. The IGBT module voltage class is fixed.
Environmental Derating
| Factor | Standard Condition | Derating Rule |
|---|---|---|
| Ambient temperature | -10 °C to +40 °C | Above 40 °C: derate 2.5% per °C; above 50 °C requires external cooling |
| Altitude | ≤ 1000 m | Above 1000 m: derate 1% per 100 m due to reduced air density |
| Switching frequency | Factory default (typically 2–4 kHz) | Increasing carrier frequency above 4 kHz reduces continuous current by 5–10% |
| Dust / moisture | Clean panel, IP20 | For IP54 or harsh environments, ensure cabinet ventilation or select protected chassis |
5. Peripheral Sizing: The Forgotten 30% of the Bill of Materials
A drive is not a standalone device. For a reliable system, size the peripherals at the same time.
Braking Systems
| Drive Power | Braking Requirement | FST-650L Configuration |
|---|---|---|
| ≤ 18.5 kW | Occasional deceleration | Built-in braking unit; add external braking resistor sized by duty cycle |
| 22–75 kW | Frequent stop/start | Built-in braking unit on most models; verify resistor ohms and wattage |
| ≥ 90 kW | High inertia, fast stop | External braking unit required; consult factory for resistor bank sizing |
Braking resistor formula (approximate):
For a 400 V system, 540 V. Always stay within the drive's stated minimum and maximum resistance values.
Input / Output Reactors & Filters
- Input reactor: Recommended when supply transformer kVA > 10× drive kVA, or when multiple drives share a bus. Limits inrush and harmonics.
- Output reactor: Required for motor cable runs > 50 m to limit voltage reflection (dV/dt stress on motor windings).
- EMI filter: Mandatory for CE compliance in residential or light-industrial environments; recommended near sensitive PLCs or sensors.
AnyHz FST-series filters (5 A – 1600 A) use a three-stage design and are cross-compatible with all inverter brands if you are retrofitting an existing panel.
6. Real-World Sizing Examples
Example A: Municipal Water Booster Station (Variable Torque)
- Motor: 30 kW, 400 V, 54 A, centrifugal pump
- Duty: 24/7 operation, PID pressure control
- Selection: FST-650L-030G/037P-T4 (G-type, 60 A continuous)
- Rationale: Pump is variable torque; G-type provides 150% overload headroom for occasional debris blockage. PID sleep/wake function built-in.
- Peripherals: Input reactor (supply is 1000 kVA transformer), no braking resistor needed (coast-to-stop acceptable).
Example B: CNC Machine Tool Spindle (Constant Power)
- Motor: 11 kW, 400 V, 22 A, max speed 8000 RPM
- Duty: Frequent acceleration/deceleration, rigid tapping
- Selection: FST-650L-011G/015P-T4 + incremental PG card
- Control mode: FVC (closed-loop vector)
- Rationale: ±0.02% speed accuracy required for thread quality; FVC maintains constant power above base speed.
- Peripherals: Braking resistor (3 kW, 75 Ω, 10% duty cycle); output reactor (motor cable 35 m).
Example C: Mining Conveyor (Constant Torque / Heavy Start)
- Motor: 75 kW, 400 V, 140 A, loaded start on an incline
- Duty: Continuous, dusty environment, ambient 45 °C
- Selection: FST-650L-090G/110P-T4 (up-sized to 90 kW heavy-duty, 176 A)
- Rationale: 150% starting torque required to break static friction; 45 °C ambient demands ~12.5% derating, so 75 kW drive would run at thermal limit.
- Peripherals: IP54 cabinet with forced ventilation; input and output reactors; DC choke for harmonic mitigation.
7. FST-650L Quick Sizing Reference (380 V Class)
| Motor kW | Motor FLA (approx.) | Recommended Model (G-Type) | Current (A) | Built-in Brake Unit |
|---|---|---|---|---|
| 0.75 | 2.1 | FST-650L-0R7G/1R5PT4 | 2.5/3.8 | Yes |
| 1.5 | 3.6 | FST-650L-1R5G/2R2PT4 | 3.8/5.5 | Yes |
| 2.2 | 5.0 | FST-650L-2R2G/4R0PT4 | 5.5/9.0 | Yes |
| 3.7 | 7.5 | FST-650L-4R0G/5R5PT4 | 9.0/13.0 | Yes |
| 5.5 | 11.0 | FST-650L-5R5G/7R5PT4 | 13.0/18.0 | Yes |
| 7.5 | 15.2 | FST-650L-7R5G/011PT4 | 18.0 / 25.0 | Yes |
| 11 | 22.0 | FST-650L-011G/015PT4 | 25.0 / 32.0 | Yes |
| 15 | 29.0 | FST-650L-015G/018PT4 | 32.0 / 38.0 | Yes |
| 18.5 | 35.0 | FST-650L-018G/022PT4 | 38.0 / 45.0 | Yes |
| 22 | 42.5 | FST-650L-022G/030PT4 | 45.0 / 60.0 | Yes |
| 30 | 56.0 | FST-650L-030G/037PT4 | 60.0 / 75.0 | Yes |
| 37 | 70.0 | FST-650L-037G/045PT4 | 75.0 / 90.0 | No* |
| 45 | 85.0 | FST-650L-045G/055PT4 | 90.0 / 110.0 | No* |
| 55 | 103.0 | FST-650L-055G/075PT4 | 110.0 / 150.0 | No* |
| 75 | 140.0 | FST-650L-075G/090PT4 | 150.0 / 176.0 | No* |
| 90 | 165.0 | FST-650L-090G/110PT4 | 176.0 / 210.0 | No* |
| 110 | 200.0 | FST-650L-110G/132PT4 | 210.0 / 250.0 | No* |
*External braking unit required for 22 kW and above on certain chassis sizes; verify with factory datasheet.
Conclusion: Sizing Is Engineering, Not Shopping
The difference between a drive that lasts 15 years and one that fails in 18 months usually comes down to what happened before the purchase order: the load analysis, the current verification, the derating calculation, and the peripheral specification.
The FST-650L is designed to absorb real-world industrial margin—150% overload tolerance, four motor parameter sets, and a control algorithm that maintains torque where simpler drives stall. But no drive can compensate for a fundamentally undersized selection.
If you have a motor nameplate in hand and need a second opinion on the exact FST-650L model, voltage class, and peripheral bill of materials, the AnyHz application engineering team can turn your specs into a finalized BOM within one business day.
About AnyHz
AnyHz (Anyhertz Drive Shenzhen Co., Ltd.) has designed and manufactured variable frequency drives since 2005. The FST-650L series is a general-purpose flux-vector inverter covering 0.75 kW to 630 kW, deployed in pump stations, machine tools, conveyors, and HVAC systems across more than 100 countries. For technical documentation, 3D models, and application support, visit www.anyhzvfd.com.
