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Friday, 29 June 2012

Treadmill Motors Part 2 - How do they fail?



In Part 1, I explained how treadmill motors are constructed and the basic principles of how they turn.  
When it comes to how they fail, you may be suprised to learn that the root cause of failure is rarely the motor itself. In my experience, the cause of failure tends to be related to either over use or lack of maintenance. Both of these factors can be influenecd directly by the user. Here’s why…
 
Over use
As a treadmill is used, heat builds up inside the motor. If you imagine two treadmills, both identical except that one has a small motor and the other has a large motor. If you use them both in the same way, both machines will need to generate the same amount of torque but to do this the smaller motor will have to work harder than the larger motor and this means that it will get hotter, quicker. As the motor gets hot, the insulation of the windings or the windings themselves can fail which leads to failures  inside the motor. This is often referred to as the motor “burning out”. The windings can fail in an open circuit manner where the motor will just not work at all (since electrical current cannot flow throught the windings if the winding is open circuit!). Alternatively, the winding can short out to earth which often cause your house electrics to trip. 
So if you are using a treadmill with a smaller motor (say 1 to 1.75HP motor) then it becomes important to look up the duty cycle as this will help guide you as to how long it is designed to run. The Duty Cycle is how the motor manufacturers express the recommended amount of “on” time and “off” time for their motor. This is one of the factors which a treadmill manufacturer will consider when selecting a motor for their treadmill. For example, a 30 minute 50% duty cycle motor would be rated to run for a 30 minutes followed by a 30 minute cooling off period to allow the heat in the motor to dissipate. Small dometstic treadmill motors will typically be rated for 20-30 minutes 50% duty. Larger domestic treadmill motors would be rated for 1 hour 50% duty. Full commercial treadmill motors would be rated for continuous use and they often include additional internal cooling to ensure that the motor does not overheat. So the amount of continuous use a treadmill gets is really important in determining its overall life!  
As an example, the picture above shows how heat can build in a motor – the dark red area indicates the hottest part which is around the central shaft and windings. The outer surface of the motor can be easily cooled by increasing the surface area of the motor by designing in fins to allow the heat to dissipate easiy, or often cooling fans are mounted on or near to the motor to help provide additional cooling. However, getting the heat away from the centre of the motor is not so easy to achieve.
Similar problems can occur if the user’s weight exceeds the design weight of the treadmill so when selecting a treadmill it is worth reading through the technical specification to look up the motor continuous HP rating (the higher the better), the duty cycle and the maaximum user weight.
Lack of maintenance
Lack of maintenance can lead to the same kind of failure. If you do not regularly lubricate the running belt, the friction between the running belt and the running platform (deck) will increase. The treadmill motor has to work harder to overcome the increase in friction and as it works harder it gets hotter. Sometimes, you are able to see if this has been a problem by examining the motor brushes for signs of burning or heat build up. These are tell tale signs that the motor has been working hard. This problem can be easily avoided by regular maintenance of the running belt and by changing the running belt when it reaches the end of its life.


Other Failure Modes
Of course, there are also other failure modes for motors which you cannot influence quite so easily. Motor controllers can fail and in certain circumstances they can send a sudden, brief, surge of electrcity to the motor. Unfortunately, the motor cannot accelerate instananeously in response and so the electrical surge can permanently distrupt the magnetic field of the motor magnets. You may recall from your school days that the most efficient way to magnetise a metal object is to pass it through a strong electrical field in order to align the micro structure of the metal object, and in the same way it is possible to de-magnetise objects by changing the electrical field so as to distrupt the micro structure. This is the same effect that occurs in a DC permanent magnet motor when it is presented with an sudden electrical surge.
It is also worth bearing in mind that since the motor magnets are permanent magnets, they will naturally lose some of their magnetism over time in much the same way as a horse-shoe magnet will eventually lose magnetism if you leave off the keeper!
The problem is that if the motor magnetism is reduced too much then the treadmill system has to compensate for this by increasing the amount of electrical current flowing  thorugh the windings in order to get the same amount of torque. This causes more heat to build up in the motor which can lead to the failures described above. Therefore, if your treadmill motor has been subjected to a sudden electrcal surge then it may have suffered permanent damage to its magnets but it may not have burnt out and will probably still be able to turn (although reluctantly). In these circumstances, testing the motor with a battery is not sufficient. All this will tell you is that the windings have not burnt out. It will not tell you anything about the amount of torque the motor is able to generate for given amount of current (remember that magnetism and current directly influence the torque!). The only reliable way to test the torque is with specialist equipment such as a torque meter which can be used in a workshop, or DC motor tester which is used out in the field.

So What?
Why is this important? Lets assume you have a fauty treadmill and you suspect that the problem is with the motor or the motor control board (MCB). If you test your motor on a battery and find that the motor turns ok, you may be mislead into believing that the motor is fine just because it turns. Then, you may conclude that the fault must be with the motor controller and you would naturally invest in a new motor controller. However, if the motor has been demagnetised, then the motor will require excessive current to turn and so it won’t be long before either the controller fails again and/or the motor burns out completely. Result – you’ve wasted both your time and money!
In summary, whenever you suspect a motor or motor controller fault, make sure you have the motor thoroughly tested. If you use the services of a professional Treadmill Engineer, he will have the necessary specialist equipment on the van and he will be able to assess other factors such running belt wear and advise you accordingly. With these factors correctly diagnosed, you are then abe to make a the right decision with regard to the repair.
The Treadmill Engineer
Northwick Associates Limited
Experts in fitness machine technology
T: 01386 555630


4 comments:

denis lambert said...

Can a treadmill motor run without brushes

The Treadmill Engineer said...

Hi Denis. Thanks for your question. DC motors cannot run without brushes. AC motors are brushless and so they do not require brushes. Hope this helps.

Peter said...

Hi,

I have a NordicTrack EXP 1000S and lately I've experienced a 'burning or burnt rubber smell' after a 20-min run. I've looked online at youtube videos showing typical symptoms, but was going to leave the cover un-screwed so to run a short time and quickly take it off and see where the smell is coming from. I did notice the belt on the motor (has ribs in the belt that fit in the grooves in the circular fitting that the belt goes over), that some of the ribs in the belt are ragged, a bit worn in places, and I can see some white fabric (the underlying liner inside the belt?)between the grooves. I also see some black 'stuff' under the motor/belt on the underlying housing, like black dust, possibly coming off the belt ribbing? Is that a possible likely scenario? I bought a silicone lubricating kit for treadmills and applied it twice but it still makes that smell.

The Treadmill Engineer said...

Hi Peter - thanks for your comment. It sounds like the running belt could be worn out causing excessive friction. This could be why the drive belt has worn and could explain the burning smell (could be the drive motor over heating or the drive belt slipping). Apply 15 to 20ml of silicone oil under the running belt and walk it in then see if theat makes a difference.