87 Hz Motor Operation Explained: More Power From the Same Motor
Run a standard 230/400 V motor in delta from a 400 V VFD and it keeps full torque up to 87 Hz instead of 50 Hz, giving about 1.73 times the power from the same motor frame.
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What is 87 Hz operation
Most small induction motors are wound for 230/400 V: 400 V when connected in star, 230 V when connected in delta. On a 400 V supply they are normally connected in star and run at 50 Hz.
In 87 Hz operation the same motor is connected in delta and fed from a 400 V VFD. The VFD raises the voltage with frequency and reaches its full 400 V at 87 Hz, so the motor delivers constant (100%) torque from 0 to 87 Hz instead of only up to 50 Hz.
230/400 V motor + delta connection + 400 V VFD → constant torque to 87 Hz
Where the extra power comes from
- An induction motor needs a constant voltage to frequency ratio (V/f) to keep full magnetic flux, and so full torque.
- In delta the winding is rated 230 V at 50 Hz = 4.6 V/Hz.
- A 400 V VFD can hold that same 4.6 V/Hz all the way to 400 / 4.6 = 87 Hz.
- Torque stays at 100% up to 87 Hz. Since power = torque × speed, power rises to 87 / 50 ≈ 1.73 times (√3).
- In standard star operation the VFD runs out of voltage at 50 Hz. Above that the motor is in field weakening: torque falls and power stays flat.
P87 = Prated × √3 · n87 ≈ nrated × √3
Torque and power against frequency
Why the VFD must be sized up
- In delta the motor draws the delta current from the nameplate, about 1.73 times the star current.
- The VFD must be rated for that current, not just the motor kW.
- So the VFD is usually one or two sizes bigger than the motor's kW rating.
Worked example
A typical 1.1 kW 4 pole motor, 230/400 V, nameplate 4.4 A delta / 2.5 A star, 1440 rpm (typical values; use your own nameplate).
| 50 Hz star | 87 Hz delta | |
|---|---|---|
| Power | 1.1 kW | 1.1 × 1.73 ≈ 1.9 kW |
| Speed | 1440 rpm | 1440 × 1.73 ≈ 2500 rpm |
| Torque | 7.3 Nm | 7.3 Nm (unchanged) |
| Motor current | 2.5 A | 4.4 A |
| VFD needed | 1.1 kW, ≥ 2.5 A | ≥ 4.4 A, often a 2.2 kW VFD |
Advantages
- Smaller motor for the same power.
- Wider constant torque speed range (0 to 87 Hz).
- Often a cheaper or more compact motor and gearbox.
- Better speed adjustment, with full torque available above 50 Hz.
Disadvantages and checks
- Bigger, more expensive VFD.
- Higher motor speed: check the motor and gearbox input speed limits.
- More fan noise and bearing wear.
- Higher iron losses and heating.
- The motor must be wound 230/400 V and suitable for inverter use.
- 2 pole motors are usually not suitable (speed far too high).
- Always check that the motor manufacturer allows 87 Hz operation.
When to use it
Good for conveyors and machines that need a wide speed range or a compact drive, where a smaller motor and gearbox save space and cost.
Avoid it when the motor or gearbox is already close to its speed limit, or when the extra VFD cost outweighs the motor saving.
60 Hz countries: 104 Hz operation
On a 480 V, 60 Hz supply the equivalent is a 277/480 V motor connected in delta. Its winding is rated 277 V at 60 Hz = 4.62 V/Hz, so a 480 V VFD can hold that ratio up to 480 / 4.62 ≈ 104 Hz.
The power gain is the same, about 1.73 times, and the same VFD current and speed checks apply.
Quick 87 Hz calculator
- Power at 87 Hz
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- Speed at 87 Hz
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- Minimum VFD current
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- Suggested VFD size
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- Torque (constant)
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- Delta / star current
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Suggested VFD size uses typical 400 V (480 V at 60 Hz) heavy-duty current ratings; ratings vary by brand, so check the VFD datasheet.
Frequently asked questions
- Does 87 Hz operation damage the motor?
- Not if the motor is suitable and the manufacturer allows it. The winding sees its normal 4.6 V/Hz, so flux and current are as designed. The motor runs about 1.73 times faster, so fan noise, bearing wear and iron losses rise; check the maximum speed, the bearing rating and that the motor is rated for inverter duty.
- Why do I need a bigger VFD for 87 Hz operation?
- In delta the motor draws its delta current, about 1.73 times the star current. The VFD must be rated for that current, which usually means a VFD one or two sizes above the motor kW.
- Can any motor run at 87 Hz?
- No. It needs a 230/400 V (delta/star) winding, so it can be connected in delta for 230 V, and it must be suitable for inverter use and for the higher speed. 2 pole motors are usually not suitable because the speed would be too high. Always check with the motor manufacturer.
- How much more power does 87 Hz operation give?
- About 1.73 times (the square root of 3), because torque stays at 100 percent up to 87 Hz and power is torque times speed. A 1.1 kW motor gives about 1.9 kW.
- Does the gearbox ratio change with 87 Hz operation?
- Yes. The motor runs about 1.73 times faster at full speed, so the gearbox ratio for the same output speed is about 1.73 times higher, and the gearbox input speed limit must be checked.
- What is the 60 Hz equivalent of 87 Hz operation?
- In 60 Hz countries a 277/480 V motor connected in delta on a 480 V VFD keeps 4.6 V/Hz up to 480 / 4.62 = 104 Hz, which gives the same 1.73 times power gain.
Disclaimer: general guidance only. Always check that the motor and VFD manufacturers allow 87 Hz (104 Hz) operation and confirm current, speed and thermal limits from their data.
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