The drive belt has come off
This is by far the most common cause and should result in the dryer sounding like it's running – although the sound should be a little different to normal. When you open the door and try to turn the drum by hand it’s likely to be freer than usual although not as free as you might expect if a belt has come off (normally a tumble dryer drum is pretty stiff to turn by hand). This method isn't perfect though because you need to know how stiff the drum normally is, or you can't necessarily be sure it isn't right and the belt could be tangled up causing some resistance.
The best method is to simply disconnect the dryer from the mains and take off the lid so you can see if the belt has disappeared leaving a mark on the drum where it normally runs. If the belt is still present, try to lift it clear of the drum because sometimes they can snap but remain in place. If it’s still OK you will not be able to lift the belt off the drum.
NOTE: A seized up drum, or faulty motor or capacitor could also cause similar symptoms but instead of the dryer sounding relatively normal it's likely to make a gentle humming noise. This humming could be the motor attempting to run but being jammed or a motor or capacitor fault. (All dealt with below)
What causes the belt to break?
There’s no point replacing a broken belt if another fault has caused it to break because the same may happen to the new one. Fortunately it’s almost always simply wear on the belt. You need to make sure the drum hasn’t seized up though, a dryer’s drum is normally quite stiff, but it shouldn’t be seized.
Replacing the drive belt on a tumble dryer
Warning: Fitting a new belt on a tumble dryer is a lot harder than you might think, and considerably more involved than replacing a belt on a washing machine. The problem is that the drum on a tumble dryer is encased between the back and front panels. Removing the back panel on a tumble dryer usually just reveals the heating element (if fitted at the rear) or a metal casing with access to very little, so to replace the belt you normally need to strip down the front panel and sometimes the side panels too.
All dryers are different, and some are easier to strip (and more importantly get back together) than others. I would recommend getting an domestic appliance engineer to fit a belt unless you are very confident. [ Related: Find a local appliance repairer ]
I can’t give specific instructions, needless to say if you have the ability you should be able to work it out. It’s not technically difficult, it’s just awkward and involved. Some modern dryers require a special tool to even get the belt on. It’s often possible to do it without, but even as an experienced engineer I’ve had cases where I’ve really struggled.
The drum isn’t turning because the motor isn’t running
If the dryer lights are on, and it clearly has power, but the drum doesn’t revolve plus it doesn’t sound like anything is running then the main motor is probably not rotating.
Motor, or start capacitor failed?
Tumble dryer motors are simple induction motors and often start up using a starting capacitor, therefore a fault on the motor or capacitor can also cause the drum to not turn round.
WARNING: Capacitors can hold a nasty electrical charge after the tumble dryer has been unplugged. Don't mess with them unless you know what you are doing.
The symptoms of a faulty motor and a faulty capacitor can be very similar. Sometimes, you can spin the drum by hand through the open door and quickly close the door, if the motor then runs OK until it stops to reverse and then won't start again it may well be a faulty starting capacitor.