# Thread: Motor overamping and Ohm's Law

1. Professional Member
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Originally Posted by GT Jets

The easiest way to keep it straight in your head is this;

Get a vacuum of the Shop-Vac type. Now start the vacuum and listen to the motor speed, then plug the suction hose with your hand... What happens? The motor speeds up.. Why? The motor load drops, which causes a decrease in amperage, or more accurately, wattage.

Now go get a box fan and do the same experiment. You will obviously need a piece of cardboard or something, start the fan and listen to the speed, then block it off... What happened? The motor slows down, why? the restriction of the air on the blades causes the motor to use more wattage.

IMHO you need to pick up an Ugly's Electrical Handbook and keep it by the toilet to read during "quiet time"

http://www.uglys.net/

There is some really good information in there and will come in handy if ever doing some electrical work sizing conduits, conductors, starters etc...

GT
AWESOME!!!!!

Great way to see it.

2. Since you mentioned ohm's law, remember these things:

1) Ohm's law is a resistive circuit law.

2) Motors create counter EMF, which acts like a resistance to limit current, but it is NOT a resistance.

3) Motor calculations use Impedance (Z) in place of Resistance (R) and phase angle Theta (the value used is the Cosine of Theta) to account for the effects of inductive and capacitive reactance used in motor designs.

In this case, fans are the devices used to place a load on the motor in order to do useful work, so it is important to understand the circumstances of when the fan places a load on the motor, as JP motioned.

Any questions?

3. As it pertains to motors (or solenoids), it needs something (a load) to induce a voltage into to counteract the current flow in the windings. Without this, you have a direct short across the windings. A plunger in the case of a solenoid, or the iron bars on the rotor of a motor. As the motor spins faster, the induced magnetism in the iron bars cut the magnetic field from the stator more frequently, thereby counteracting the current flow. Hence, high current at start up (the bars are not turning).
Solenoids and contractors will burn up if anything prevents the plunger from pulling in. On a related note, always put something in a solenoid if you have to remove the plunger, JUST IN CASE it gets energized while it is removed.

4. Something metal, like a nail or screwdriver

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ohm's law is referring to electrical resistance, so it does not apply to the example you are using. Your example is dirty blower wheel (or restricted blower wheel, like bearing doesn't run smooth etc...), having such case, you have to use more WORK to move the wheel, which means the motor is working HARDER than usual, so the amperage goes up. You use the power equation in this case

Power (work done per unit of time) = Voltage * Current

Since voltage is constant, then power is directly proportional to current, so as power goes up (as more work is done), the current goes up.

6. A PSC motor on a radial fan will over-amp if it has no resistance (static pressure) to work against. Why is that?

7. Originally Posted by air1
A PSC motor on a radial fan will over-amp if it has no resistance (static pressure) to work against. Why is that?
Here is a little more on the subject. http://hvac-talk.com/vbb/showthread....1#post15675481

8. First: resistance to air flow Is Not electrical resistance.

Second: resistance to air flow only increases motor loading in the case of a positive displacement situation - which a blower motor typically is not. So a clogged blower wheel would Reduce motor loading - not increase it. You can prove this to yourself by blocking the supply or air while monitor the amp draw of the blower motor.

PHM
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Originally Posted by HVAC_Austin
Hello everyone,

I realize that when a motor is restricted in some way the amps will go up (for example, an excessively dirty blower wheel). According to Ohm's Law, as electrical resistance goes up, amperage will go down. When the blower wheel is dirty (or whatever other restriction is present), this creates resistance to what the motor is trying to do. Why then does amperage increase?

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