# 400Hz L/C filter for 3 Phase 115v supply

#### AK 7466

Joined Aug 25, 2021
13
Hi.

Like so many before me I am try to build 3 phase 115v, 400Hz supply to drive an ex-aircraft gyroscope (or two).

My starting point is this ready made module
http://www.bldcmotor-driver.com/sal...r-board-high-voltage-motor-control-board.html

which comprises a JY02A Sensorless brushless DC motor control IC
https://docplayer.net/146908738-Jy02a-sensorless-brushless-dc-motor-control-ic.html

and a JY213 Motor Driver MOSFET / IGBT Driver High Speed 3 - Phase Half - Bridge
http://www.bldcmotor-driver.com/sal...bt-driver-high-speed-3-phase-half-bridge.html

To give these old gyros the easiest possible life I would like the drive waveforms to be as close to sinusoidal as is realisticaly possible so I am envisgaing incorporating 400 Hz L/C low pass filters between the output of the drivers and the gyro windings.

I have looked at various LC filter design tools such as

PI Filter http://www.calculatoredge.com/electronics/ch pi low pass.htm
Chebyshef https://rf-tools.com/lc-filter/

and understandably all of these design tools require a source and load impedances to be specified.

In my application the source impedance will hopefully be very low, and I have no idea what the gyro winding impedance will be when rotating.

Please can somebody suggest the best way forward, or point me in the direction of some helpful documentation?

Many thanks

Alan K.

#### Ian0

Joined Aug 7, 2020
6,976
How much power do they require?

#### AK 7466

Joined Aug 25, 2021
13
How much power do they require?
Thanks. Relatively little, between 5 and 10 watts, depending on the application

#### nsaspook

Joined Aug 27, 2009
10,895
A MIL-SPEC 3 phase 115v, 400Hz supply is usually a Wye configuration and most low-cost motor control systems supply Delta configuration 3-phase power. A BLDC motor ESC is designed to use back EMF from the motor windings or HALL sensors to is used to detect phase rotation. This may or may not work with varying generator non-motor electrical loads depending on the feedback loop and commutation logic. A PMSM motor controller might work better for 3-phase sine-wave power generation if you can program static torque and speed settings.
https://www.infineon.com/cms/en/app...nd-drives/permanent-magnet-synchronous-motor/

https://navyaviation.tpub.com/14329/Figure-7-33-Wye-Delta-Voltage-Relationships-345.html
https://navyaviation.tpub.com/14329/Ungrounded-Systems-350.html

https://youtu.be/ilN-DauXVFM

This program and hardware generates static a SVPWM Space Vector waveform to drive very low speed motors for star tracking.

Last edited:

#### Ian0

Joined Aug 7, 2020
6,976
Thanks. Relatively little, between 5 and 10 watts, depending on the application
Does it need the neutral connection?
If not, you can probably use a class-D audio amplifier IC, and a couple of step up transformers.
If if does need neutral, then it would be three transformers and one-and-a-half class-D amplifier ICs.

#### AK 7466

Joined Aug 25, 2021
13
Thank you for the suggestions. No neutral connection required or available.

I am happy with the driving circuitry, I just need some help in choosing the best type of L/C filter (PI or Low Pass etc.) and calculating the component values.

#### AK 7466

Joined Aug 25, 2021
13
A MIL-SPEC 3 phase 115v, 400Hz supply is usually a Wye configuration and most low-cost motor control systems supply Delta configuration 3-phase power. A BLDC motor ESC is designed to use back EMF from the motor windings or HALL sensors to is used to detect phase rotation. This may or may not work with varying generator non-motor electrical loads depending on the feedback loop and commutation logic. A PMSM motor controller might work better for 3-phase sine-wave power generation if you can program static torque and speed settings.
https://www.infineon.com/cms/en/app...nd-drives/permanent-magnet-synchronous-motor/
View attachment 246586
https://navyaviation.tpub.com/14329/Figure-7-33-Wye-Delta-Voltage-Relationships-345.html
https://navyaviation.tpub.com/14329/Ungrounded-Systems-350.html

https://youtu.be/ilN-DauXVFM

This program and hardware generates static a SVPWM Space Vector waveform to drive very low speed motors for star tracking.
Thank you for your help and the various links. I am not planning to use a BLDC motor driver.
I am happy with the driving circuitry, I just need some help in choosing the best type of L/C filter (PI or Low Pass etc.) and calculating the component values

#### Ian0

Joined Aug 7, 2020
6,976
I presume that the load is inductive.
What is the output of the drive circuit like? Is it inductive or capacitive?
Or is the output from a transformer?

#### AK 7466

Joined Aug 25, 2021
13
Thank you for your response.

As the load is a very small 3 phase motor, star connected, I think it is safe to presume that it will be primarily inductive, although I do not have any figures for this as yet.

The driver output is a 3 phase mosfet H bridge switching between 115v and 0v. No transformer

#### Ian0

Joined Aug 7, 2020
6,976
The driver output is a 3 phase mosfet H bridge switching between 115v and 0v. No transformer
PWM or just plain squarewaves?

#### Ian0

Joined Aug 7, 2020
6,976
I'd go for a 2nd order lowpass, and choose the inductor so that it has an impedance of about 5% of the load impedance at the operating frequency. That gives about 25mH
Then I'd choose the resonant frequency of the filter to be
$$f=\sqrt{f_{output}f_{pwm}}$$
That puts the resonance as far away as possible from both the PWM frequency and the operating frequency, because you don't want to excite the resonance.
That gives f=2250Hz
and from that
$$C=\frac{1}{4 \pi^2 f^2L}$$
which is about 220nF.
Attenuation at 13kHz is about 35dB, which will give about 2V of residual.

#### AK 7466

Joined Aug 25, 2021
13
Thank you for your contribution Ian, - it is very helpful.

I have ordered a small Hantek LCR meter to measure the winding inductance of the gyro motor.

As soon as I have this parameter I will use your suggestions and formulae to confirm the required LC values for a Low Pass filter.

Please could you tell me why you did not recommend the PI filter configuration which is seen in so many commercially available filters?

Thanks again for your input.

A.K.

#### Ian0

Joined Aug 7, 2020
6,976
Please could you tell me why you did not recommend the PI filter configuration which is seen in so many commercially available filters?
Because the PWM output would have to drive a capacitive load.
. . . And I didn’t recommend T configuration because the load is already inductive.

#### AK 7466

Joined Aug 25, 2021
13
Thanks. I will give it a try as soon as I get my L.C.R. meter

#### nsaspook

Joined Aug 27, 2009
10,895
How's the 400Hz project going? I decided to use one of my old motor control prototype PCB's as a three phase 400Hz low power inverter using some junk box parts.
The 110AC transformers (3 delta to 4 wye) are 40VA per phase, that should be plenty to play around with some old aircraft instruments.

Low power software testing.

PIC32MK using a 360 32-bit word table for 400Hz 3ph sine wave generation using a 10 KHz PWM carrier/interrupt loop on 4 hardware PWM modules. One channel is for neutral wire zero-sequence current generation but I'm not using that here.

I'm using transformers instead for isolation and phase configuration transformation. Small load resistors for power testing.

PWM and 400Hz power after only a 1mH L filter. I have a few DC link caps to add for a proper LC filter later.

Two phases at a few watts output by adjusting the PWM modulation index in software.

Last edited:

#### AK 7466

Joined Aug 25, 2021
13
How's the 400Hz project going? I decided to use one of my old motor control prototype PCB's as a three phase 400Hz low power inverter using some junk box parts.

Hi "nsaspook" Thanks for all the effort you have put in. The results look really promising.

After I received the Hantek LCR Meter, I confirmed that the Y (Star) connected gyro motor windings have the following characteristics (as the central star point is not accessible, these measurements are of necessity across two winding in series)
DC resistance 34 ohms, inductance (parallel resistance model) 119mH, inductance (series resistance model) 76mH.

Within the aircraft instrument (a Litton LN2-3A) the 400Hz 3 phase signals are required to drive several devices such as gyros, gimbal motors and synchro-resolvers at different voltages / phases.

Would you be willing to share your design details so that I can make a replica?

Thanks for your contribution. Much appreciated.

Alan K.

#### AK 7466

Joined Aug 25, 2021
13
Here is an image of the Litton LN2-3A inertial platform, the sealed casing of one of the gyros can be seen in the upper left (with the circular pink heating tape on one end) at the right hand end are two of the gimbal motors.

#### nsaspook

Joined Aug 27, 2009
10,895
That is a nice hunk of gear.
I'm still kicking around ideas with my 3-phase inverter.
Added the C for the LC filter. power signal is much cleaner.

Transformers: Hammond 227P 40VA 400Hz 26V C.T. @ 1.54A 13V @ 3.08A 115V
Inductors: Signal HCTI-1000-2.4 1000uH 0.300ohm DCR
Film Capacitors: 100uF 10% 63volt MKT

24VDC bridge supply signals and outputs. 10KHz pwm carrier

Low duty cycle PWM modulation. Half-bridge output: green, AC power signal after LC filter: yellow

Close to max PWM modulation for full 115AC output voltage@400Hz.

Last edited: