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February 15, 2021

400 Hz tutorial: generation and conversion of 400Hz power

Introduction

Various frequencies have been used for AC transmission, 60Hz and 50Hz are the most common. Low frequencies are used because radiation losses are lower at lower frequencies. Radiation losses for 400Hz are 40 times as high as 60Hz, and 60 times as high as 50Hz. However, transformers for 60Hz are bigger and heavier. So for the short distances involved in aircraft, and the advantage of smaller and lighter transformers, motors and filter capacitors, 400Hz was adopted, particularly for military aircraft: MIL-STD-704F states: “Equipment designed for use on military aircraft should not use 60 Hz power, since it requires more weight and prime mover capacity per unit of power consumed by the utilization equipment.”

Specifications for normal operation of 400Hz equipment.

Voltage range

108 to 118VAC RMS

115VAC nominal

Voltage range 3 phase

Wye configuration

108 to 118VAC RMS

115V line to neutral

200V line to line

Frequency range 

393 to 407 Hz

400HZ nominal

Frequency range for helicopters

380 to 420 Hz

400Hz nominal

DC offset

-0.10 to +0.10 volts

Zero volts nominal


Rectification

To prevent problems in the aircraft power system half wave rectification is forbidden. Thus for low voltage power supplies full wave rectifiers or switchmode power supplies are preferred.

Three phase requirements

phasor diagram for 400Hz aircraft electronics

When higher powers are required three phase is mandated. In military aircraft powers greater than 500VA require 3-phase power.

Skin depth at 400Hz

Alternating current likes to distribute the current in a wire in such a way that the maximum current is at the outside of the wire. For high frequencies and large wires the current can be mostly on the “skin” of the wire. The skin depth is the distance from the surface where the current density is 1/e of the maximum at the surface. 60Hz has a skin depth in copper of 8.4 mm, so even with a 0000 AWG wire (11mm diameter) the skin effect can be neglected. At 400Hz the skin depth is 3.3 mm, so wires thicker than 8 gauge will have more resistance than expected. Litz wire, which is individual strands of thin insulated wire woven into a cable can be used to avoid the skin effect.

Power conversion equipment

Power conversion equipment can take the 400Hz input and create 60Hz AC (frequency conversion), DC (transformer-rectifier).

1. Start with 60Hz and transform to 400Hz. Mechanical motor-generator rotary converters used to be widely used. Now, with modern power semiconductors, these frequency converters consist of a rectifier to create DC and an inverter to make the 400Hz all in one unit.

100 Watt 60Hz to 400Hz convertersingle phase frequency converter, 100W 400 Hz
250 Watt 60Hz to 400Hz converter250VA 115V to 115VAC 400Hz AC/AC frequency converter
500 VA 60Hz to 400Hz converter500 watt AC/AC frequency converter 60Hz to 400Hz
1000VA 60Hz to 400Hz converter60Hz to 400Hz converter, 1000VA 115VAC output

1a. Start with 60Hz single phase and transform to 400Hz 3-phase.

600VA 60Hz single phase to 400Hz three-phasesingle phase 60Hz to three phase 400 Hz frequency converter

2. Start with 400Hz and transform to 60Hz. To use 60Hz equipment in aircraft a similar converter rectifies the 400Hz and then converts the DC to 60Hz using an inverter.

500 Watt 400Hz to 60Hz convertersingle phase frequency converter, 500W 400 Hz to 60 Hz
1250 Watt 400Hz to 60Hz converter400 hertz to 60 hertz frequency converter

3. Start with 400Hz and transform to DC voltage. A number of switchmode power are available to make any DC voltage, or even charge batteries when plugged into 400Hz.

DC power supplies from 3V to 48V with 400Hz AC input400 Hz input DC power supplies

4. Start with DC and create 115VAC 400Hz. These are just inverters with 400 Hz output.

DC to 400Hz AC inverters 40VA to 1000VA400Hz inverters

5. DC to 400Hz 3-phase

24VDC, 48VDC, 125VDC or 250VDC to 3-phase 3kVA or 6kVA 400Hz6000VA 3-phase DC/AC inverter rack mount, heavy duty, 3 phase power inverter

6. 60Hz single phase to 400Hz 3-Phase

115VAC or 230VAC 60Hz single phase input, to 3kVA three-phase 400Hz3000VA single phase to 3-phase converter and frequency converter

7. 3-phase 60Hz to 3-phase 400Hz.

Three phase 50Hz or 60Hz to three phase 400Hz 1.5kVA to 6kVA3-phase to 3-phase frequency converters, 1500 watts, 50Hz, 60Hz, 400Hz,

8. Transformers.

Can I use a transformer designed for 60 Hz at 400 Hz? Yes, a transformer that is designed for 60Hz operation will work at 400Hz, but may have a loss in efficiency due to eddy currents and skin-effect increase in resistance.

Can I use a transformer designed for 400Hz at 60Hz? Again, yes, but the voltage must be derated by the ratio (60/400). This is to prevent the core from saturating. I have beautiful little 3-phase Variac designed for 400Hz. It is so cute and unique that I can't bear to surplus it, so I am waiting for an application where we have to test three phase 60 Hz at 17 volts.


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