⚡ VFD Output Frequency
Hz

Read from your VFD display or program

🔧 Motor Nameplate Data
RPM

The RPM printed on the motor nameplate

2.8%

Typical induction motor slip: 2–4%

Hz
📈 Variable Torque Load (Fan / Pump)
— RPM
Live motor speed estimate
Enter frequency above

Calculating motor speed…

📊 Your VFD Speed Results
—
Actual Motor Speed
— RPM
Synchronous + actual with slip
🔄
—
RPM
Sync Speed
📉
—
RPM
Slip Loss
📊
—
%
% of Full Speed
⚡
—
%
Power Draw
📈 Speed vs. Power Curve (Variable Torque — Fan/Pump)
Motor Speed → Power % 0% 25% 50% 75% 100% —%

📋 Speed at Different Frequencies

Frequency
Sync RPM
Actual RPM
Speed %
Power %
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💡 VFD Application Notes

VFD Speed Control — Explained Simply

A VFD (Variable Frequency Drive) controls motor speed by changing the electrical frequency it sends to the motor. Understanding how frequency and RPM are connected saves time and money.

⚡

What Is a VFD?

A Variable Frequency Drive (also called an inverter, AC drive, or variable speed drive) converts fixed-frequency AC power into variable-frequency AC. By changing the frequency, it changes the motor’s speed. It also changes the voltage proportionally to maintain proper motor flux.

🔄

Frequency Controls Speed

The speed of an induction motor is directly tied to the electrical frequency it receives. A 4-pole motor runs at 1800 RPM on 60 Hz. Run it at 30 Hz and it runs at 900 RPM. Run it at 45 Hz and you get 1350 RPM. The relationship is linear below base speed.

💨

The Cube Law for Fans and Pumps

This is the big reason VFDs save so much energy on fans and pumps. Power varies with the cube of speed. Cut the speed in half and you use only 1/8 of the power. Drop to 80% speed and you use just 51% of the energy. This is called the affinity law or cube law.

📉

What Is Motor Slip?

An induction motor never quite reaches its synchronous speed (the theoretical maximum). The difference is called slip. A 4-pole 60 Hz motor has a sync speed of 1800 RPM but typically runs at 1740–1770 RPM. Slip is normal and necessary — it’s what creates torque in an induction motor.

🌡️

Low Speed Concerns

Motor cooling is a concern at very low speeds. Most TEFC (Totally Enclosed Fan Cooled) motors use a shaft-mounted fan that slows down with the motor. Below about 20–25 Hz, this fan can’t provide enough cooling for continuous operation. Consider externally-cooled motors for low-speed duty.

💰

Energy Savings in Practice

A 100 HP fan motor running at 75% speed instead of 100% uses only 42% of the energy. That’s a 58% energy reduction. Over a year, this kind of saving can easily justify the cost of a VFD installation. Most VFDs pay back their cost in 1–3 years on fan and pump applications.

📐 The Formulas Behind This Calculator

Sync RPM = (120 × Frequency) ÷ Number of Poles Actual RPM = Sync RPM × (1 − Slip fraction) Fan/Pump Power % = (Speed % ÷ 100)³ × 100 Torque (lb·ft) = (HP × 5252) ÷ RPM

Slip fraction is typically 0.02–0.04 (2–4%) for standard induction motors. The 120 factor comes from 60 seconds/minute × 2 poles per cycle. Fan and pump power follow the affinity laws (cube law). Constant torque loads (conveyors, compressors) use a linear relationship: P ∝ Speed.

VFD Speed Reference Chart — 4-Pole Motor at 60 Hz Base

VFD FrequencySync SpeedActual Speed% of Full SpeedFan/Pump Power
60 Hz (Full)1800 RPM1750 RPM100%100%
54 Hz (90%)1620 RPM1575 RPM90%72.9%
48 Hz (80%)1440 RPM1400 RPM80%51.2%
42 Hz (70%)1260 RPM1225 RPM70%34.3%
36 Hz (60%)1080 RPM1050 RPM60%21.6%
30 Hz (50%)900 RPM875 RPM50%12.5%
24 Hz (40%)720 RPM700 RPM40%6.4%

Frequently Asked Questions

Use the formula: Sync RPM = (120 × Frequency) ÷ Poles. For a 4-pole motor at 45 Hz: (120 × 45) ÷ 4 = 1350 RPM synchronous speed. Subtract slip (typically 2–4%) for actual speed: 1350 × (1 − 0.028) = about 1312 RPM. This calculator does all of that automatically — just enter the frequency and pole count.
Yes — this is one of the main advantages of a VFD. The VFD converts your incoming power to the exact frequency the motor needs. A 60 Hz motor can run at 50 Hz output — it will just run slightly slower. Many facilities use this deliberately when importing equipment rated for a different line frequency. The VFD handles the conversion cleanly.
Above the base frequency (60 Hz for USA motors), the motor enters the “field weakening” zone. Speed continues to increase but torque decreases. Power stays roughly constant. The motor can run above base speed, but mechanical limits of the load (bearings, fan blades, pump impellers) may limit how high you can go. Always check the motor and driven equipment specs before running above base frequency.
The savings follow the cube law. Reducing fan speed to 80% of full speed cuts power consumption to (0.8)³ = 51.2%. Reducing to 70% speed cuts power to (0.7)³ = 34.3%. On a 100 HP fan running 8,000 hours per year at $0.10/kWh, dropping from 100% to 75% speed saves roughly $50,000 per year in electricity. This is why VFDs on fans and pumps typically pay back in under 2 years.
Most VFDs can output from 0 Hz up to their maximum (often 400 Hz). But practically, running a standard TEFC motor below 15–20 Hz continuously can cause overheating because the shaft-mounted cooling fan slows down with the motor. For applications requiring sustained low-speed operation, use a motor with an external forced-cooling fan, or a vector-duty motor designed for variable speed. Some VFDs have a “minimum frequency” parameter you can set to prevent damage.
V/Hz (volts per hertz) is the simpler control mode. It maintains a constant voltage-to-frequency ratio to keep motor flux stable. It works well for fans, pumps, and centrifugal loads. Vector control (also called field-oriented control) is more sophisticated — it controls flux and torque independently, giving better low-speed performance and faster response. Use vector control for applications needing precise speed regulation, high starting torque, or dynamic load changes.
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