Calculation behind 90V DC from bridge rectified 120vac

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Flicker Index

Flicker Index

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90V DC motors are common and are intended to be thyristor controlled off bridge rectified 120vac mains. For simplicity sake, ignoring the foward voltiage drop, what is the math behind 90v DC output? Is that 90v DC average or RMS?

I would think that output (ignoring the 2 x VF drop of 2-3v) would still be 120V RMS, or in other words, 120v 600W heating element would still absorb 600W of real power when fed through a rectifier, but if fed from a 90v battery, it would only operate at 56% power. (90/120)^2.

Does a 90v DC rated motor fed on a 90v battery pump the same cfm of air against a constant torque load (same psi diff) as when its powered from 120vac through a bridge? @Jraef
 
You can’t compare a heating element to a motor. The work done by a heater for the most part depends on the power supply, while a motor depends on the load. The air flow in your case will be the same as long as the motor can maintain the same speed when fed by the rectifier. And if it’s designed to operate from a rectified source, it will maintain that speed.
 
Flicker Index said:
90V DC motors are common and are intended to be thyristor controlled off bridge rectified 120vac mains. For simplicity sake, ignoring the foward voltiage drop, ...

Does a 90v DC rated motor fed on a 90v battery pump the same cfm of air against a constant torque load (same psi diff) as when its powered from 120vac through a bridge? @Jraef
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Wouldn’t this depend entirely on the ability of the source to maintain voltage and current requirements?
 
RMS sort of can be used but it has no meaning in DC. Volts times Amps is Watts. It’s not AC so there is neither reactive power nor power factor. Power supply doesn’t matter as long as you don’t have a blown diode or bad cap in which case it’s not DC any more.

In DC motors the field is almost immaterial. It has to be there and has to be in the range of 50% to 100% of name plate. Too low is worse than too high (dangerous over speeding). The armature power minus counter EMF which is usually under 10% so it’s a small “loss” is the power output. So since the motor is almost just a resistor armature voltage is approximately equal to speed in strong field (field weakening destroys this relationship) and armature current is approximately proportional to torque...enough that DC techs use this pretty frequently for troubleshooting. Now the engineers will accuse me of grossly oversimplifying but I’m a field engineer not office engineer.
 
retirede said:
You can’t compare a heating element to a motor. The work done by a heater for the most part depends on the power supply, while a motor depends on the load. The air flow in your case will be the same as long as the motor can maintain the same speed when fed by the rectifier. And if it’s designed to operate from a rectified source, it will maintain that speed.
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The RMS and the average voltage are just that - voltage. The speed is another matter. It can zero speed or full speed. Or anywhere between.
 
Flicker Index said:
90V DC motors are common and are intended to be thyristor controlled off bridge rectified 120vac mains. For simplicity sake, ignoring the foward voltiage drop, what is the math behind 90v DC output? Is that 90v DC average or RMS?
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Incidentally, the SCR is usually called that the USA. In other regions, including the UK, it is called the Thyristor.
 
paulengr said:
RMS sort of can be used but it has no meaning in DC. Volts times Amps is Watts. It’s not AC so there is neither reactive power nor power factor. Power supply doesn’t matter as long as you don’t have a blown diode or bad cap in which case it’s not DC any more.
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If the power source is a battery at 200v and it is cycled at 5mS on/5mS off. the voltage is 141vRMS. That is, a 100 ohm resistance would produce 200W. It would be operating at 400W half the time and 0W the other half. I think a DC volt meter would indicate 100v. But what does a DC motor see it as?

You're incorrect there isn't a power factor. At the output of a bridge, the voltage is a continuing string of arches. If the load current doesn't mirror these arches, then the power factor is other than unity. It's not an alternating current if it doesn't change polarity, but you can have a load current that doesn't mimic the voltage waveform.
 
Besoeker3 said:
The RMS and the average voltage are just that - voltage. The speed is another matter. It can zero speed or full speed. Or anywhere between.
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Correct. I only mentioned speed because the OP was questioning how much air his driven machine would move - which is proportional to speed.
 
Flicker Index said:
I would think that output (ignoring the 2 x VF drop of 2-3v) would still be 120V RMS
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Yes. If you input any voltage waveform to a bridge rectifier, which effectively just takes the absolute value of the voltage curve, then V RMS won't change, as x2 = |x|2.

As to the average voltage, if the input wave form is a sine wave with P-P voltage 1V, its RMS voltage with be sqrt(2)/2. The output voltage waveform will have average value the average value of sin(x) over [0,pi], or 2/pi.

So in general for an AC input, the average output voltage will be 2*sqrt(2)/pi of the input RMS voltage, or 90%. Not sure if this notion of average voltage is useful for anything.

Cheers, Wayne
 
Flicker Index said:
If the power source is a battery at 200v and it is cycled at 5mS on/5mS off. the voltage is 141vRMS. That is, a 100 ohm resistance would produce 200W. It would be operating at 400W half the time and 0W the other half. I think a DC volt meter would indicate 100v. But what does a DC motor see it as?
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Hypothetically speaking for simplicity... A 200V battery source would supply 200VDC... period. If you turn on the 200VDC source for 5mS, your 100ohm resistance would draw 2 amps, producing 400 watts for 5mS. 0 Watts when turned off. The meter would 200VDC for 5mS while on and 0V while off. That is basic electronics and ohms law. A DC motor would see the same. Remember... it is DC. Once that DC motor reaches steady state it will see 200VDC (ignoring other factors for simple math).

Now if you start messing with variable speed drives, or other controlling factors, well that changes things.
 
wwhitney said:
As to the average voltage, if the input wave form is a sine wave with P-P voltage 1V, its RMS voltage with be sqrt(2)/2. The output voltage waveform will have average value the average value of sin(x) over [0,pi], or 2/pi.
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Agree, except with the peak voltage instead of P-P.

wwhitney said:
So in general for an AC input, the average output voltage will be 2*sqrt(2)/pi of the input RMS voltage, or 90%. Not sure if this notion of average voltage is useful for anything.
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The average of the rectified voltage and its 90% relationship to the RMS input voltage is relevant with choke input filters that have a sufficiently high inductance to keep the current flowing through the rectifier throughout the AC cycle. The choke input filter type has better inherent voltage regulation and less peak current than a capacitor input filter (and therefore less stress on the rectifier). Choke input filters were somewhat common with higher power vacuum tube equipment. But the weight and cost of such a choke usually makes it unattractive, especially compared to modern switching supplies.
 
2102115-1617 EST

Flicker Index:

Your question brings up a number of considerations.

Assume a sine wave voltage source of 120 V RMS, 60 Hz. This means two wires, and neither needs to be earthed. It also means the full wave DC average voltage from an ideal bridge rectifier is 120 V * 0.636/0.707 or 120 * 0.8996 = 107.9 V .

Connect a resistive load and the average DC voltage across the resistor will be 108 V. Make it a half wave rectifier and the voltage will be 54 V.

Connect a highly inductive load, such as a large solenoid clutch, then the average DC voltage across that coil will be 108 V read by a Simpson 260 meter.

Make the load a permanent magnet DC motor. What is the average voltage across the motor? I don't know, but it is less 169.7 V.

On a Bodine small motor with a worm gear load, and no other load, fed from a 1/2 wave diode rectifier, I read an average DC voltage of about 128 V DC across the motor. Much of the time this was from back EMF when no current was flowing to the motor. Inertia was back feeding the motor making it a generator during the major portion of the cycle when no line current was flowing to the motor.

.
 
Jraef said:
We call them thyristors too, but using that term usually gets guys thinking about "thighs" and you lose them...
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At least in the US a thyristor is the generic term for the whole family of switching devices but generally speaking to current controlled devices. So for example there is the whole family of gate commutated thyristors (GCTs) which can switch at up to several kV but where the IGBT is voltage controlled (low gate leakage) GCTs require significant gate current. Others use the term thyristor generically to refer to all switching devices with a third input so for instance it would include UJTs and MOSFETs but not diodes.
 
Then there's the bi-directional triac.
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Besoeker3 said:
Incidentally, the SCR is usually called that the USA. In other regions, including the UK, it is called the Thyristor.
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We use the term thyristor here Stateside also. Thyristor is just a broader category. It can also mean triac, diac, or several other types of components.
 
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