Measuring current draw by AC motor.

[Ray Pooley]

Hello guys.

This isn't a job, just so that you know. Just exploring.

I have a couple of AC motors and I am playing with them. One of the things I want to do is measure the current draw.

So I attach them to a power source and switch it on and off they go doing their thing. No problem there.

So I then switch them off and insert a multi in the line, set it to AC amps and switch it on again and motor doesn't operate. I do get a reading on the meter but it's not what I expect it to be.

Here's an example:

35W 50Hz 230V, although actual supply is 240VAC. This is from a dishwasher. It's the drain pump. Of course when I power it up on the bench it's not pumping water so it's not under load.

Anyway, here's the calculation:

Current i amperes = 34W / 240VAC = 0.142 amperes. Or 142 milliamps.

I would expect the measured current to be less than that given that i's not actually pumping water. Right?

So I insert the meter into the circuit and:

Set it to AC Amps I get a reading of 9.9

Set it to AC milliamps I get a reading of 59.8 milliamps
Now that's a possibility, I guess, since it's not under load.

I should point out that I only have two digits before the decimal point on this meter when in milliamp mode.

I didn't bother with microamps mode.

And, btw, the motor doesn't run while the meter in in the circuit. Runs fine when I remove it. Why would that be? Is the resistance of the meter that much that it reduces the current to the extent that the motor no longer functions? That would seem odd to me.

Can anyone offer any suggestions as to what's going on here?

How could these values be so wide of what I am expecting?

Appreciate your input.

Thanks

 

[telectrix]

use a clamp meter instead. you need one with a resolution low enough.e.g. 1mA.

 

[Ray Pooley]

use a clamp meter instead. you need one with a resolution low enough.e.g. 1mA.

Now that's a good idea. Thanks.

 

[Pete999]

Hello guys.

This isn't a job, just so that you know. Just exploring.

I have a couple of AC motors and I am playing with them. One of the things I want to do is measure the current draw.

So I attach them to a power source and switch it on and off they go doing their thing. No problem there.

So I then switch them off and insert a multi in the line, set it to AC amps and switch it on again and motor doesn't operate. I do get a reading on the meter but it's not what I expect it to be.

Here's an example:

35W 50Hz 230V, although actual supply is 240VAC. This is from a dishwasher. It's the drain pump. Of course when I power it up on the bench it's not pumping water so it's not under load.

Anyway, here's the calculation:

Current i amperes = 34W / 240VAC = 0.142 amperes. Or 142 milliamps.

I would expect the measured current to be less than that given that i's not actually pumping water. Right?

So I insert the meter into the circuit and:

Set it to AC Amps I get a reading of 9.9

Set it to AC milliamps I get a reading of 59.8 milliamps
Now that's a possibility, I guess, since it's not under load.

I should point out that I only have two digits before the decimal point on this meter when in milliamp mode.

I didn't bother with microamps mode.

And, btw, the motor doesn't run while the meter in in the circuit. Runs fine when I remove it. Why would that be? Is the resistance of the meter that much that it reduces the current to the extent that the motor no longer functions? That would seem odd to me.

Can anyone offer any suggestions as to what's going on here?

How could these values be so wide of what I am expecting?

Appreciate your input.

Thanks

Use a clamp on meter as Tel said with a low scale

 

[Ray Pooley]

use a clamp meter instead. you need one with a resolution low enough.e.g. 1mA.

Can you recommend a decent budget one? Don't want to go nuts.

 

[Pete999]

Use a clamp on meter

Think screwfix do a cheap one

 

[MJPD29]

Anything fluke for clamp meters. The cheaper ones don't have great accuracy especially if your measuring milliamps

 

[DPG]

As Matthew says, don't expect a cheap clamp meter to be anywhere near accurate enough to read low mA figures.

 

[Lucien Nunes]

You don't need a clamp meter, multimeter is fine. Have you got the meter leads in the right sockets i.e. A and COM? If you have, then there's no circuit through the meter so either the shunt or the fuse is dead. But with a dead fuse I wouldn't expect a reading, and I've never managed to blow a shunt. If you have accidentally left the red lead in the V socket the pump won't run and you'll get a nonsense reading.

BTW the current won't be P/V because of the power factor, and running the pump dry will ruin the seal.