Measuring motor windings

[Ralphy007]

I have used the resistance range on a digital Fluke multimeter to measure the resistance of a motor winding.

The result is 40. Bit what is this related to ? Is it just the dc resistance of the winding ?

[Zero999]

Yes, it's the winding resistance, which sounds quite high. How big is the motor and what's its nominal voltage?

[Benta]

Well, you set the multimeter to measure resistance, so it's probably measuring resistance.

[Doctorandus_P]

If you want to calculate the DC resistance of a motor properly, then do something like:

1. Block the rotor.
2. Push about half the nominal current through the motor.
3. Measure the voltage over the motor.
4. Repeat the measurement a bunch of times with the rotor in different positions.
5. Averge the results.

[Ralphy007]

I took the motor out a tumble dryer or a washing machine cant remember which.

Looks like a 105w motor - 230v.

[WattsThat]

The only way anyone could make sense of the winding resistance reading is know what the motor construction is. Some the details would be from attributes such as:

AC or DC powered
Three phase or single phase
Brushed or brushless
Operating voltage
Operating current
Direct line connect or inverter driven

And the list goes on. It is doubtful you’ll find a true dc machine as they cost more to build but it is easier to build a drive for them. Overall, the age of the machine is a huge factor with regard to the construction used. 30+ year old machines used one or two speed direct line connection ac induction machines. Current technology is brushless inverter driven variable speed with many variations over the years. As igbt based drives evolve, the motors get more complex but cheaper to manufacture. Google “synchronous reluctance ac motor”. Many appliance motor applications have been converted to that technology.

[Benta]

I took the motor out a tumble dryer or a washing machine cant remember which.

Looks like a 105w motor - 230v.

Very unlikely, unless it's the pump motor. The drum motor would be 1...1.5 kW.

[Zero999]

40 Ohm is a reasonable winding resistance for a 230V 100W motor.

[Ralphy007]

Many thanks for getting back.

I am hoping to build a v/f drive for the motor - dont have any experience so probably wont work but I'll have a go.

I have build the attachment with 100k resistors across the caps. which are 100uF.

Strange thing is when I plug the earth connection into circuit, the RCD in my consumer trips ?

[themadhippy]

Strange thing is when I plug the earth connection into circuit, the RCD in my consumer trips ?

maybe has a little something to do with the diode from live to earth

[penfold]

Firstly, if you're working on a mains powered circuit, I would strongly advise using a safety isolating transformer, if not for safety then at least allowing you to connect an oscilloscope to it easily. If it is your first project of this nature, perhaps test it at a safer voltage from a transformer to check the power devices are switching correctly first - mains can deliver enough energy to cause some serious damage long before protection devices have a chance to kick in

[Ralphy007]

Apologies

I didnt explain properly. When I attach the earth from my circuit to the earth on the 13A mains plug, the RCD in the consumer unit trips - strange thing is I have not switched ON the supply voltage ? The 13A plug is in the socket but I havent put the switch down.

I will have to update the schematic to include the 100k resistors.

Yes, I will be using an isolating transformer (down to 15v) to do some testing before full testing with the mains.

If I cant resolve the earthing issue, I may remove the earth connection in the circuit.

[themadhippy]

strange thing is I have not switched ON the supply voltage ? The 13A plug is in the socket but I havent put the switch down

then the problem lies between neutral and earth

[Ralphy007]

schematics

[themadhippy]

were youve got the earth connected is wrong, connect it to the metal frame of the motor only.

[drussell]

strange thing is I have not switched ON the supply voltage ? The 13A plug is in the socket but I havent put the switch down

then the problem lies between neutral and earth

I think there is also some significant PEBCAK going on here!   

I'm not sure that Ralphy should be delving into mains just yet without a bit more overall understanding!   

[wizard69]

strange thing is I have not switched ON the supply voltage ? The 13A plug is in the socket but I havent put the switch down

then the problem lies between neutral and earth

I think there is also some significant PEBCAK going on here!   

I'm not sure that Ralphy should be delving into mains just yet without a bit more overall understanding!   

Maybe a lot more!   This is certainly an area where one mistake puts you on the morgues list.