# What permanent magnet motor is the best for the bike generator? How to decide about wattage?

I am new here.. I would like to build my bicycle generator with permanent magnet motor similar to this one Bicycle Power Basics

Basically, I want to run my pedal cinema with a bike generator. I am not sure about the load I will need but I will try to reduce it as much as I can because I am aware that I will not be able to produce more than 100W anyway..

I made some research about permanent magnet motors and I found some here http://www.fireworksandscootersdirect.co.uk/category.php?CID=52 They mainly differ in watts.

My question:
- is the wattage of the PM motor important in making the decision about single bike generator? If I buy a motor with 24V 350W RPM2750 will I be able to run appliances which consume more power rather then I would purchase a motor 24V 250W RPM2750? Does it mean that the motor with higher wattage will be more effective in running appliances which consume more power?

As I understand, the adult person is able to run the bicycle with more or less RPM 60, so it means that in an hour I will be able to do RPM 360, which is 7 times smaller then RPM2750. So if I use a motor with 350W, will it give me 50W and if I use a motor with 250W it gives me only 35W? Is this correct or I am completely wrong?

How can I choose a permanent magnet motor?

• Wattage per se doesn't determine the power you'll get, but it's loosely related to the motor's resistance, which will lose some of that precious power. You're better off looking for motors designed to work at your achievable speed, or gearing up to the motor's ideal speed. To get 360rpm you have to gear up 6:1 anyway (an hour has nothing to do with it!) Some info on the relation between motor speed, Kv, and power : electronics.stackexchange.com/questions/177513/… – Brian Drummond Mar 27 '16 at 23:49

I am aware that I will not be able to produce more than 100W anyway.

That's about right. Another useful number to remember is that a servant can give about 1 kWh per eight-hour shift!

If I buy a motor with 24V 350W RPM2750 will I be able to run appliances which consume more power rather then I would purchase a motor 24V 250W RPM2750?

There will be losses in any motor / generator and these will increase with size although they may be countered by improving efficiency. The 250 W unit will have lower inertia and will probably be cheaper.

Does it mean that the motor with higher wattage will be more effective in running appliances which consume more power?

Yes, but as you've stated, your power source (you) is limited in output.

As I understand, the adult person is able to run the bicycle with more or less RPM 60, so it means that in an hour I will be able to do RPM 360, ...

Umm, 60 RPM x 60 minutes = 3,600 revs/hour - but why are you converting to hours?

which is 7 times smaller then RPM2750.

Much worse than you imagine: $\frac {60~rpm}{2750~rpm} = 2\%$ of rated speed giving you 2% of 24 V = 0.5 V out. This won't light an LED! The DC generator's output voltage will be proportional to rotational speed.

The article is too long for me to read but here are a few pointers:

• You need to figure out what voltage your system requires.
• You need to generate in excess of that voltage and regulate it. (It is also possible to generate a lower voltage and boost it with a boost regulator.)
• If you go for 24 V out then you need to gear-up the motor drive to give 2750 RPM at your preferred cadence.
• You need to figure out what you want to happen when you stop pedaling for a second or a minute. Do you want your cinema to reboot? If not you need some short-term storage.

I think a more practical project would be to power your cinema from a mains-powered battery and the bicycle generator. Monitor the generator output and if you fall below a certain threshold for a time then blank the video. If you maintain output for more than ten minutes then dispense a chocolate reward.

• Thank you for your answer. You are right, I made a mistake, it is only 2 %... so does it mean I have to find the ratio between the size of my back wheel and the size of the motor so it gives me enough of turns? I want to use the inverter or power pack so the voltage will be regulated for mains appliances and the energy will be stored. – michaela Mar 28 '16 at 22:38
• Or what do you mean with gearing-up the motor drive to give me 2750 RPM? If I go for 12 V out, I will need to achieve at least 1375 RPM? – michaela Mar 28 '16 at 22:51
• Yes, you would have to gear the motor up or, if you're running the motor from the bike tyre you could calculate as follows: 18 kph = 5 m/s = 300 m/minute. 300 m / 2750 = 110 mm circumference drive roller on motor = 35 mm diameter. Inverting to mains voltage will cause further losses. I think you haven't thought this through. Where are you going to store the power and what happens when you stop pedaling? – Transistor Mar 28 '16 at 22:53
• I am sorry, I do not understand your calculation about how you get the diameter of the drive roller? I think I would like to store the energy in the power pack so when I stop pedaling it will not cause the breakdown. I was also thinking about the option that I would charge my power pack when riding my bike and then stop, put the bike on the stand and attach the generator and pedal, so there should always be the energy to use and the power pack should stay always charged. do you think this is a more achievable option? – michaela Mar 29 '16 at 13:49
• (1) Figure out the road speed you can maintain at 100 W effort. This will be the surface speed of the generator tyre-contact roller. Convert that to m/minute and divide by the required RPM to calculate the roller circumference. Then divide by $\pi$ to get the diameter. (2) How will you charge your power-pack when cycling? i.e., what generator is there on your bike? Don't forget that charging the power-pack will increase the load on you - it will be like cycling with the brakes on to some degree. – Transistor Mar 29 '16 at 13:56

Quick answer is that you will be very much an admired athlete if you can sustain a 60W output from pedal power. Maybe you read this already.

Begin experimenting with something nice an cheap to get an idea of the human problem of power output and generator + circuit inefficiency.

Buy lots of Gatorade and setup a fan to keep yourself cool on your stationary bike. This sounds like a really fun experiment and I used to suggest it to boys in my scout troop all of the time.