# Thread: Calculating watt usage for a motor startup.

1. Senior Member
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## Calculating watt usage for a motor startup.

Is there a way to calculate watts for motor start up?

Say if I wanted to know how much energy is used to run a motor from turning it on to turning it off 5 minutes later.

2. Originally Posted by SmithBuilt
Is there a way to calculate watts for motor start up?
When a motor starts "across the line," it will draw many times the rated current (hundreds of amps or thousands of amps for larger motors), but only for a fraction of a second. You basically have a dead short until the counter-EMF is established in the motor windings which will limit the current. So I'm not sure this would be a very useful calculation.

3. Yes, with more info on motor type, load, etc. As peter said, the start up will be a small portion of the 5 minute run and the usage in those 5 minutes will depend on the motor load. Why not just use a clamp on amp meter?

4. dicklaxt Guest
Inrush will be 5 to 12 times FLA as a rule depending on manufacturer and will stabilize rather quickly,if you have to have a #, measure the amps convert to watts and calculate to watt hours,,,,,,,,whatcha need this number for?

dick

5. gar
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As pointed out above --- why is this information important?

If it is really important, then get an electronic wattmeter with a voltage output. Create a circuit that gives you binary 1 output during the startup time. Create an integrating circuit, op amp with feedback capacitor. You need a manual reset of the integrator (short the capacitor to bleed to 0 volts). A gating circuit at the integrator input controlled by the binary 1 signal to gate the wattmeter voltage output to the integrator input. A voltmeter on the integrator output.

When the input gate is off (binary gating signal is 0) and the integrator is reset, then the output voltmeter reads 0. At turn on of the motor the gate closes, and the integrator adds up the incoming watt signal. The voltmeter will gradually increase during the startup time. At the end of the startup time the gate is turned off and the integrator output remains constant at its last value. This is a measure of the watt-seconds of energy used during startup.

Obviously you will need to calibrate the circuit. If you are talking about motors in the 1 HP range, then you might calibrate with a 100 W to 1000 W resistor.

.

6. Originally Posted by gar
080726-1006 EST

As pointed out above --- why is this information important?

If it is really important, then get an electronic wattmeter with a voltage output. Create a circuit that gives you binary 1 output during the startup time. Create an integrating circuit, op amp with feedback capacitor. You need a manual reset of the integrator (short the capacitor to bleed to 0 volts). A gating circuit at the integrator input controlled by the binary 1 signal to gate the wattmeter voltage output to the integrator input. A voltmeter on the integrator output.

When the input gate is off (binary gating signal is 0) and the integrator is reset, then the output voltmeter reads 0. At turn on of the motor the gate closes, and the integrator adds up the incoming watt signal. The voltmeter will gradually increase during the startup time. At the end of the startup time the gate is turned off and the integrator output remains constant at its last value. This is a measure of the watt-seconds of energy used during startup.

Obviously you will need to calibrate the circuit. If you are talking about motors in the 1 HP range, then you might calibrate with a 100 W to 1000 W resistor.

.
Or use a Killawatt type kwh recorder and compare the two situations. The results should be about as accurate as you can get.

Trying to figure it out mathematically will still be an estimate. Frequent starting and stopping also heats the motor up and can change it's efficiency.

7. Senior Member
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I was just wondering if it was possible. Not really important, just learning.

I understand it would still be an estimate. I did not think about it changing after the motor heated up.

I take it from the responses it would not be a very good estimate.

8. gar
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SmithBuilt:

With good instrumentation you can get an accurate measurement. That measurement may vary from one cycle to another, and as the motor heats the mean of the average of your measurements will change.

On my DeWalt radial arm saw at the end of 100 ft or so of #12 wire it draws about 75 A for maybe 4 seconds. As a very rough estimate this 80 * 75 * 4 watt-seconds = 24,000 watt-seconds or 24/3600 KWH = 0.007 KWH or 0.08 cents cost. This was with a 10" blade. The cost seen at the main panel is (120/80)*0.08 = 0.12 cents. The 80 above is the voltage at the motor. If this were supplied from 240 with the same wires there would be less cost and faster startup.

If it takes 1 HP and 4 seconds to cut a board, then the approx cost for the cut is 0.12*1*4/3600 = 0.013 cents. That is 1 KW input for 4 seconds at 12 cents per KWH.

Thus, it costs far more to start the motor than to make one cut. These are wild estimates, but if you do a lot of start stop operations it justifies a regenerative drive.

Yes, accurate instrumentation to measure startup energy use could be valuable in a production environment.

If I use my DeWalt 4 times a month, then I do not care about startup cost. I do not even care about rewiring it for 240, but faster startup would be nice.

At one time my son was cutting aluminum extrusion with a cold saw. The extrusion was U shaped, about 1.25 x 1.25 x 0.25 wall. This was cut in 1 second and peak HP during the cycle was about 7.5 HP from a 5 HP or less motor. Of course this motor was never turned off. Part to part cycle time was about 6 seconds. Simultaneously a 1/4-20 hole was drilled and tapped. Each part was about 1.25 long.

The above figures are from memory and may not be real accurate, but the 7.5 peak HP is.

The average power consumed in this cutting operation was less than to light the area.

.
Last edited by gar; 07-26-08 at 04:55 PM.

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