The Stovebolt Site Forums The Shop Area The Electrical Bay Voltage change / ohms law / motors - most confusin

I have seen a fair amount of confusion with different voltages and the effects on equipment. Let us do ohm law because that is the easiest and is directly applicable to resistive loads.

RESISTIVE LOADS only.
I = E/R
Keeping the resisance constant at 6 ohm for example

On 6 volt I = 6 /6
= 1 amp

12 volt I = 12 / 6
= 2.0 amp

POWER
P = I x E or I squared R

on 6 volt
P = 6 x 1
= 6 watt

on 12 volt
P = 12 x 2
= 24 watt

So power goes up by the square of the voltage change. For this reason you have to select dropping resistors carefully. As one has seen too small a wattage and they burn out.

MOTORS
Here is where things get complicated they obey ohms law but then there are other factors.
HP = 746 watts
HP = (Torque x RPM) divided by 5252

First let us do AC motors since someone mentioned them. A common motor is one for 115 or 230 single phase. Yes it will run PROPERLY CONNECTED on higher or lower voltage. What they do is on low connection the windings are in parallel
SO for a 1 HP motor drawing 746 watts for the 115 volt connection each winding is drawing 373 watt, connected in parallel across 115 and drawing approx 3.24 amp. Since they are in parallel Line current is the total of current and is 6.28 amp.

HIGH VOLTAGE connection each winding is still drawing 373 watt, 3.24 amp and at 115 volt BUT they are in series so the voltage across the two is 230. Line current is now 3.24 amp.

The same holds for 3 phase dual voltage motors. You save almost no power with either connection except for line current drop. Where you do save with the higher voltage is wire size and starter size which are definitely cost factors.

DC motors. One person asked why can DC motors operate on PWM power supplies at a lower voltage (and speed).

ANY DC motor can be operated at a lower voltage. This is the oldest trick in the book of motor control. Halve the voltage and roughly halve the speed and half the current as explained below.

Yes you can run 6 volt motors on 12 volt
PROVIDED
1. The insulation can take it
2. Torque. Let us assume a constant torque load - this is almost anything on wheels cars trains conveyers etc etc.

This does NOT apply to pumps or fans. They obey another confusion factor the affinity laws. Basically on a centrifugal pump or squirrel cage fan double the speed and power goes up by 8.

3. You do not overspeed it and throw windings out of the armature.


For constant torque loads torque does not change with speed. HOWEVER HP will be proportional to speed
HP = T x RPM /5252. Double RPM then double HP. HP = 746 watts so running a motor at double voltage will double wattage and about double amperage. If the motor can keep itself cool you are OK.

The reason you can get away with this is Counter Electro Motive Force aka generator action
ALL MOTORS ARE GENERATORS AND
ALL GENERATORS ARE MOTORS
The generator action in a motor produces CEMF - this opposes the line voltage and reduces the "actual motor voltage". You cannot directly measure this but can get an idea of it. CEMF is proportional to motor speed so if you run at double the design volage and double the speed the increased CEMF will save you by lowering the actual motor run voltage thus current.

All bets are off with a hard startig engine though - I predict a new starter motor if you do this every day.

Take a DC motor and measure resistance line to ground or line to line. Let us say it is a starter motor - I would estimate resistance at about 0.1 ohm or so. At 12 volt this will draw about 120 amp - which if you have a high range curren meter you can see. The motor is not up to speed thus CEMF is low and is zero at start Once the motor speeds up the current drops to let us say 30 amp (not a very big starter motor).

So at speed "run voltage = I x R
= 30 x 0.1
= 3
Therefore CEMF = 9.

You can also see this directly
Take a motor and if you have a variable DC power supply slowly crank the voltage up and watch the current rise. Once the motor starts turning it is generating CEMF and you will see current drop like a rock.

Dan Bentler

This basic electrical theory might make more sence if you explain the terms...

E=Voltage
I=Current
R=Resistance

Well I had to forget something

Dan Bentler

Is it safe to assume we're listening to an engineer, or a government consultant, or both? This thread is WAAY over the heads of at least 95% of the people who will be reading it.

After spending the majority of the past 40 years either working with or teaching automotive DC electrical systems, I had a bit of trouble following some of the terminology, and reading between the lines to make some sense of the comments. Good try, no seegar!
Jerry

"Is it safe to assume we're listening to an engineer, or a government consultant, or both?"

No it is not safe to assume. You dont need to worry about the seegar either.

Just a qualified and competent motor control electrician. I worked with DC also - 250 VDC and up to 450 HP. Same basic stuff as automotive.

Dan Bentler

Well, I got most of it until you got to the CEMF part, which I'm going to have to read again so I can absorb it. I think I'm getting slower with age.

Now this reminds me, a guy in town was saying it's cheaper to run something on 230v than it is on 115v.
Why's that I asked, The current draw is half on 230v so you cut down on your electric bill, he says. REALLY, that's good to know and I left his store not wanting to oppose his view.

What do you think....Back woods analogy or fact ?

I did gain insight from what you explained here though. If I switch my truck to 24v the thing is going to run faster, RIGHT ?

Dan
i'm kiddin about the 24v thing.

Dan - your electric bill is based on power [watts] not current [amps] - a motor has the same power rating on either voltage [like 2hp] so no cost savings on yer electric bill, but the advantage is the motor will start easier on 240 than 120 and have an easier life so last longer trouble free ... so it's both fact and fiction

aside ... odd that so many use 110, 115, and 220, 230, even electricians, when it's actually 120 and 240 as supplied by the power companies

Bill

I have been getting into electric bikes and these exhibit what they are calling bemf= back electromotive force. I think it is where the iron in the motor/stator? saturates. Guys are pushing brushless hub motors to 100+ volts and 100+ amps- on bicycles. I just run 48 v and 22 a on mine. 20 mph. is good enough for me on a bike
They make faster winds, mine is a low speed winding on a mt. bike.

They are actually running them with a pwm controller and hall sensors so the motor is actually 3 phase ac I think. It gets over my head pretty quick too..

WooHoo ! From this discussion, can electric powered Stovebolts be far behind ? Step up. Who'll be the first ?

Me ! As soon as they go 500 mi. on an overnight charge

Most factory electric vehicles use a 3 phase motor. For vehicles these come in two "flavors"
1. Standard induction motor using squirrel cage rotor. Power from battery and speed is controlled with Pulse Width Modulation (commonly called Variable Frequency (Speed also sometimes) Drive
2. Again basically 3 phase except squirrel cage rotor is replaced with high strength magnets (neodynim - dont trust my spelling). Commonly called Brushless DC (BLDC) More properly called Permanent Magnet Synchronous Motor. Same basic setup as for induction except VFD has a different control to control for magnets.
Has better torque than a squirrel cage.

500 mile range ?? Well if you want to drive 500 miles in one day OK with me - my butt gets tired. National average is around 30 to 40 miles per day. That is the average and of course there are those who are higher and lower. Electric vehicles will probably not meet the needs of those who drive longer distance
say > 100 miles per day. For them hybrid would work quite well. Kenworth and International have 26,000 GVW with electric drive - operates on startup to about 30 mph then diesel clutched in.
For long trips some people tow an additional battery or a genset. SO long range can be done if you really need it.

RE 120 vs 240 on motors. Wont make a differance on power draw. Not sure about being able to start faster on 240 but doubtful. Each "half" of the winding does the same thing whether connected series or parallel. 240 has some savings on wire cost and definitely on contactor cost / availability.

Dan Bentler

The one thing that got missed in this discussion is the power loss through the resistance in the wires themselves. The thinner the wire the higher the resistance per unit of length, and the higher the current the more loss as well. When you double the voltage (6 to 12) the current is cut in half, so the power loss in the wire is 1/4 as much. In a 12v system you can use thinner (cheaper) wires and still get better power delivery/efficiency to the loads.

Good points. I guess I don't want to sit in any vehicle for 500 mi. these days. I would like to see 50-100 mi. range with electric but the batteries to do that at 60 mph. are probably 10k $$ even with a small light aero chassis. I'm just tired of doing oil changes on my work/commute rigs I guess. I still want my I.C.E. for traveling though.

Merl

You have it backwards.

As I said with CONSTANT resistance double the voltage = double current and 4 X watts. True on AC or DC

DC motors in theory you are sorta correct BUT speed is proportional to voltage double voltage = double power. ROUGHLY
HP = T x RPM / 5252 double voltage thus RPM you double HP and watts again RUFFLY

AC motors you are reconnecting from paralleled windings ie 120 to seriesed 240. Half current and same wattage.

More gory details in first post.

Dan Bentler