Small black wheele on the motorized floor brush broke how can i fix it

Check that no strings or other debris is clogging the brush wheels. If it isnt that check that the belt isnt broken connecting the brush to the motor. If that is ok, most likely the motor is blown and not turning the belt.

Is it an upright? On mine it's very simple.
Unplug vacuum and lay it on floor so that you are looking at the rotating brush. Remove screws that attach the cowling. Remove the cowling. You will then see the rotating brush ends. One end will have the belt, or what's left of on the brush wheel. You will also see the motor shaft. The belt goes between the shaft and the brush wheel. Carefully remove the brush by holding both ends and pulling straight out. Place the new belt on the brush wheel and replace the brush. With the belt on the brush wheel, stretch it to fit on the motor shaft. Rotate the brush to make sure the belt is riding correctly at both ends. Replace cowling and screws.

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HighnTools posted this solution a while ago. This might be related to your problem:
I have finally fixed my sister's dyson dc 15 beater bar problem. It was just 2 tiny broken wires connect to circuit board lead to the beater bar. The wires broke where they go through the black rubber gommet. You need a torx screwdriver tx 15 to remove 2 plastic covers to access the wires. When looking at the front of the vacuum, the gray covers located on the right side of the vacuum. 1 big cover with a black rubber gasket and other small gray cover is on the rotating ball area.

The above was from HighnTools.

This appears to be a loose connection. The below post was answered by highntools a while ago. It probably is related to you problem. That is why moving it back and forth makes it go on.
I have finally fixed my sister's dyson dc 15 beater bar problem. It was just 2 tiny broken wires connect to circuit board lead to the beater bar. The wires broke where they go through the black rubber gommet. You need a torx screwdriver tx 15 to remove 2 plastic covers to access the wires. When looking at the front of the vacuum, the gray covers located on the right side of the vacuum. 1 big cover with a black rubber gasket and other small gray cover is on the rotating ball area.

There have been several people who reported loose or burnt wires to be the culprit for the brush bar not working. The DC15 does not use a belt, but a motor that is control by the circuit board. And I really believe that in your instance, the problem is a loose wire between the circuit board and the brush motor.

If hard to push, only a few things to check:

(1) Rear wheels will get hair or thread wrapped around axles, dust, etc. and be hard to turn. Make sure each wheel turns easily.

(2) Small wheels on newer models (since about 1997-98) toward front are worn, missing or have same problem as item 1.

(3) Brush roll not turning. The brush roll actually helps pull this unit along the floor, as it rotates in the "forward" direction. If the belt broke, and you do not realize it, it may suddenly seem harder to push.

This appears to be a loose connection. The below post was answered by highntools a while ago. It probably is related to you problem.
I have finally fixed my sister's dyson dc 15 beater bar problem. It was just 2 tiny broken wires connect to circuit board lead to the beater bar. The wires broke where they go through the black rubber gommet. You need a torx screwdriver tx 15 to remove 2 plastic covers to access the wires. When looking at the front of the vacuum, the gray covers located on the right side of the vacuum. 1 big cover with a black rubber gasket and other small gray cover is on the rotating ball area.

There have been several people who reported loose or burnt wires to be the culprit for the brush bar not working. The DC15 does not use a belt, but a motor that is control by the circuit board. And I really believe that in your instance, the problem is a loose wire between the circuit board and the brush motor.

This appears to be a loose connection. The below post was answered by highntools a while ago. It probably is related to you problem.
I have finally fixed my sister's dyson dc 15 beater bar problem. It was just 2 tiny broken wires connect to circuit board lead to the beater bar. The wires broke where they go through the black rubber gommet. You need a torx screwdriver tx 15 to remove 2 plastic covers to access the wires. When looking at the front of the vacuum, the gray covers located on the right side of the vacuum. 1 big cover with a black rubber gasket and other small gray cover is on the rotating ball area.

There have been several people who reported loose or burnt wires to be the culprit for the brush bar not working. The DC15 does not use a belt, but a motor that is control by the circuit board. And I really believe that in your instance, the problem is a loose wire between the circuit board and the brush motor.

is your brush roller turning,that would cause it to be hard to push .the brush roller helps it walk across the floor.if your brush roller isn't turning then your belt is broke or the brush itself siezed up.unplug the vacuum and try to turn the brush roller .if it turns real free then your belt is broke.before you replace the belt always clean the motor shaft ,the old rubber from the old belt melts onto the motor shaft .if you don't clean it your new belt won't last very long.please leave feedback

sept 30,08 bought dc14 all flloor. After 3 to 4 uses it started making lots of noise when on carpet setting. I examined the roller brush and it was cracked at one end. I returned it Oct 13 ,08 and got another dc 14 all floor. Now this one after 2 uses has the same problem only a larger crack in the brush and a small piece of the brush broke off. Very dissapointed.