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Eicor Class-A Inverter

introduction
A rather 'old school' system of generating three phase 115 VAC 400 Hz signals is with a dynamotor inverter. A dynamotor is a device that has a electric motor connected to a generator. The word dynamotor is a composition of dynamo (generator) and motor. A (DC) motor is mechanically connected to a generator that converts the mechanical energy in electrical energy. So no semiconductors or other electronics is needed. Dynamotors were used in old army transceivers to obtain high voltages and (low) filament voltages for the vacuum tubes in the radio. For avionics these dynamotors were used to obtain for example three phase voltages out of a single phase direct current source. The positive side is that these dynamotors are usually very stable and have a long lifespan. The downside is that dynamotors are very noisy (mechanically) and the neutral is missing likely. (Neutral is not always mandatory for equipment...)

I bought this 'Class A Inverter' from the manufacturer 'Eicor Inc.' The device is marked as built by drawing number '1-100A115D-7'. The input voltage is 27,5 VDC according to the type plate and the current draw is 9,2 Amps thus consuming 253 Watts of energy. The output power is three phase 115 VAC 400 Hz power. According to the type plate the duty cycle is continuos, the power factor is 0,95 and the available power is 100 VA.

115 VAC is the usual voltage for avionics. 115 VAC is measured between line and neutral. To obtain the line-line voltage, the line-neutral voltage (115 VAC) has to be multiplied by the square root of three. The square root of three is also the same as three to the power a half (3^0,5). Therefore 115 VAC * 3^0,5 = 199 VAC. Thus the line-line voltage is approximately 200 VAC between the lines.

To calculate the maximum current per phase, the following formula is needed: I = P / (√;3 pf V)
Where I is the current, P is the power in VA, pf is the power factor and V is the (line-neutral) voltage. Filling the formula, I got this result: I = 100 VA / (√;3 0,95 115 VAC). This means I = 528 mA per phase.

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inside view
The inside of the device is rather straight forward. At the left side is the motor part of the inverter and at the right side is the generator part.

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schematic
The schematic if the device is shown below.

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The odd thing is that the connector has only three pins. The device is fed with 27,5VDC (28VDC). The positive wire has to be connected to pin [B]. The negative side if the power input has to be connected to the chassis. By applying power, the motor starts to spin.
The first 115VAC 400Hz phase is available at pin [A]. The second 115VAC 400Hz phase is available at pin [C]. And the third 115VAC 400Hz phase is available at the chassis! This is rather odd, but it works. There are two windings connected to the pins and the third winding is connected to the chassis. So one output phase is connected to the 28VDC input negative wire. Based on the schematic it should be possible to detach the third winging from the chassis to obtain galvanic separation between the generator and the motor. Note that the third phase has no filtering applied since it's connected to the chassis.

The good thing it that every pin to the 'outside' is filtered with a three pole low pass filter to suppress high frequency components.

efficiency
The inverter uses approximately 4,6A at 27,5VDC in unloaded condition. The power consumption in unloaded condition is 4,6A 27,5VDC = 126,5 Watts! The efficiency is therefore rather low. For testing are dynamic inverters fine, but for long term use is the high energy consumption (next to the noisy part) a large drawback.

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