I have a single phase motor which is 3 speed, it blows a 3 amp fuse as soon as you run it, the problem maybe the bearings are tight but would like to test with a megger before doing the bearings what do i need to do and look for?
Testing a motor
- Thread starter markymark2406
- Start date
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A megger will only tell you if you have a low IR. For resistance you will need an Avo/Fluke multi tester, ideally you need a NDT Baker test which will show you the wave form of the windings. Why are you only using a 3A fuse?
Do you have a nameplate showing the motor details? i.e. wattage/voltage/speeds/full load current etc. Does it use a capacitor/centrifugal switch etc. Does it have brushes and an armature or is it a squrrel cage rotor? A motor takes around 1.7 times the full load current to start so if it is rated at 3AFLC you will be starting it at over 4.5A with no load, if it is loaded/connected to something it will take more. Is the shaft free wheeling or is there some resistance in it? Why do you think the bearings need changing.
More info needed.
Do you have a nameplate showing the motor details? i.e. wattage/voltage/speeds/full load current etc. Does it use a capacitor/centrifugal switch etc. Does it have brushes and an armature or is it a squrrel cage rotor? A motor takes around 1.7 times the full load current to start so if it is rated at 3AFLC you will be starting it at over 4.5A with no load, if it is loaded/connected to something it will take more. Is the shaft free wheeling or is there some resistance in it? Why do you think the bearings need changing.
More info needed.
In my experience, a motor rated at more than about 1/3 horsepower will give a 3 Amp fuse a very hard time on start up, despite the running current apparently being only about one Amp.
As a motor is an inductive load, the straightforward relationship between Voltage, resistance and current cannot be applied in the same way as for a resistive load.
In the past it was common practise to start a motor off load to reduce the starting current required.
I would not be surprised if DC tests on an AC motor gave some very misleading results.
As a motor is an inductive load, the straightforward relationship between Voltage, resistance and current cannot be applied in the same way as for a resistive load.
In the past it was common practise to start a motor off load to reduce the starting current required.
I would not be surprised if DC tests on an AC motor gave some very misleading results.
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Tim, you are correct in that you can measure resistance with a megger but usually they only read up to about 99.99ohms. Also, unless you are fairly 'au fait' with motors you wouldn't know what kind of resistance readings to expect and they can vary immensely from different manufacturers due to various winding techniques and size of copper wire used/turns per coil etc. This is not knocking you in any way by the way.
Gary, thats not a megger its a portable flaming test bed!!
Seriously though from a winders point of view the more info the better and as I have said different makers use different winding details. Until we have more info as to the type of motor etc we can only offer a rough guidance. One thing I will suggest to Marky, if you have a local rewind firm near you pop in and just ask them if they will do you a favour and give your motor a quick once over. Most firms will give it a quick test and tell you if anything is wrong and how to remedy it if it is. They probably won't even charge you for it though it would be nice to offer them the price of a pint if they do do it for you.
Good luck and let us know how you get on.
PS Don't be tempted to put in a bigger fuse to make it work! Get it tested first!!
Gary, thats not a megger its a portable flaming test bed!!
Seriously though from a winders point of view the more info the better and as I have said different makers use different winding details. Until we have more info as to the type of motor etc we can only offer a rough guidance. One thing I will suggest to Marky, if you have a local rewind firm near you pop in and just ask them if they will do you a favour and give your motor a quick once over. Most firms will give it a quick test and tell you if anything is wrong and how to remedy it if it is. They probably won't even charge you for it though it would be nice to offer them the price of a pint if they do do it for you.
Good luck and let us know how you get on.
PS Don't be tempted to put in a bigger fuse to make it work! Get it tested first!!
Gary, thats not a megger its a portable flaming test bed!!
It's a Megger MFT1552 that also performs an insulation resistance test.
Fluke, Martindale, Metrel, DiLog and many others meters also perform the same insulation resistance test.
/sarcasm
Tight bearings could result in an overload of the motor. At the very least an increase in length of time for which the circuit protection has to handle the starting current.
You can't test for tight bearings with any kind of electrical meter. You can't test for an increase in starting time with a meter either.
If you suspect that the bearings may be faulty then thats where you should start.
Is this 3A fuse part of the equipment or is it in your mains plug top?
You can't test for tight bearings with any kind of electrical meter. You can't test for an increase in starting time with a meter either.
If you suspect that the bearings may be faulty then thats where you should start.
Is this 3A fuse part of the equipment or is it in your mains plug top?
Quite right Nopoke but bearings don't necessarily need to be tight to increase the start load, dry bearings can have the same effect though to a lesser degree. The ultimate danger being that running dry they will very soon sieze up and then watch the current go through the roof if you have fitted a higher rated fuse.
Any more info yet Marky or have you given up?
Any more info yet Marky or have you given up?
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