simulate this circuit – Schematic created using CircuitLab


simulate this circuit

Schematic redrawn by Transistor. OP to edit or delete as required.

I have a wireless electricity transmission circuit that is intended to be used to power a toy car. However, the receiver coil does not cause the motor to rotate.

I am using a 9-volt battery in the transmitting coil, which is functioning properly. The receiving coil is receiving electricity and can power an LED. I used a bridge rectifier to convert the alternating current to a direct current to power the motor.

To test it, I connected the motor and LED in series as the output of the bridge rectifier with a capacitor going across the outputs. The LED lights up properly but the motor fails to function.

What can be done to power motors through wireless power transfer and how can I make my circuit work?


closed as unclear what you're asking by Harry Svensson, ThreePhaseEel, pipe, R Drast, laptop2d Nov 20 '17 at 6:05

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Have you attempted to work out how much power is required to run the motor?? \$\endgroup\$ – Jack Creasey Nov 19 '17 at 6:14
  • \$\begingroup\$ Voltages and current would help us have a clue. But I'd be surprised if you are getting the amp or so you may need to get that motor started. AA batteries source a lot more than you think. Also, the LED in series with the motor will steal a lot of volts from the motor. \$\endgroup\$ – Trevor_G Nov 19 '17 at 6:29
  • \$\begingroup\$ @Trevor The transmitter coil has one 9 volt battery. My multimeter reads 12 volts when the output of the bridge rectifier is measured, with a current of 0.3 milliamps. I tried the motor on its own and that did not work either. I was using the LED as a guide to show if a current is passing or not. \$\endgroup\$ – apaul Nov 19 '17 at 6:40
  • \$\begingroup\$ AA battery can source perhaps 1-2 Amps peak. Your motor could be anything. You'd need to hook it up to a power supply to test it (with CC mode being helpful). Just guessing I'd say you won't get anywhere with less than 4-5 W of power transfer, and that's hard to do. Your LED only needs 20 mA or so and your motor may be 1000 times that to start it under load. \$\endgroup\$ – Jack Creasey Nov 19 '17 at 6:42
  • \$\begingroup\$ @Jack Creasey How can I add the additional power to my circuit. I tried connecting 3 9-volt batteries in parallel but this also proved unsuccessful. For background information, this is for a science fair project which is measuring the correlation of the distance between transmitter coils on the road to the speed of the toy car, which is a piece of light wood with 4 wheels and the necessary electrical components. Thanks for your help! \$\endgroup\$ – apaul Nov 19 '17 at 6:47

I figure this will be closed, but I'm going to answer anyway. This is more in the way of things you need to do rather than a solution to your problem.

  1. You need to find out how much power your motor needs to move the car.

I assume it can move the car using a AA battery.

Measure the voltage across the motor while the car is moving. It will be lower than 1.5V.

Now, measure the current through the motor while the car is moving.

Multiply the volts by the amperes (P=IE, power=current*volts.) This gives you the power in watts.

Your power transmitter/receiver system must be capable of transmitting at LEAST that amount of power.

  1. Measure the power going into your transmitter.

Place the transmitter/receiver coils together so that the receiver coil can receive maximum power. Power up the transmitter, and measure the voltage across the battery, then the current flow from the battery. Now, calculate the power going into the transmitter. Your receiver will NEVER get more than this amount of power.

To increase the transmitter power, you need to give it more voltage or more current - or both.

  1. Measure the received power.

Hook up the motor to the receiver. Leave out the LED. It will only confuse you right now. Measure the voltage across the motor and the current through the motor then calculate the power available to the motor.

The difference between the transmitted power and the received power is the loss of the system. You can calculate the efficiency of the system as received power divided by transmitted power (Eff=Pr/Pt). This number will be lower than 1. A REALLY good system might reach 0.9 when the coils are close together. I expect your number to be much smaller.

I expect will measure a received power that is much lower than the power the motor needs to move the car.

There are MANY things which can influence the efficiency of the system. One you already know - distance between transmitter and receiver coil. But, the circuits have losses, and the coils have losses. The coils must be matched to each other and to the circuits, else you will have losses between the coils and losses between the circuits and the coils.

You picked an interesting project, with a lot of things to learn and a lot of room for discovery. It is also a difficult project - note that there are no existing systems that do what you are trying to do.

If something hasn't been invented yet, there's often a good reason. Either there's no basis to operate from (no body can figure out how it could be done) or the basis is well enough understood that engineers can calculate how well it would work before building one, and find that it wouldn't work well enough to be worth while.

If you have trouble carrying out any part of the above, ask your parents or teacher for help.

You can also ask here, but then you must post a clear question that explains what you are trying to do and what you have done yourself to find the answer. You will also need to post a drawing of your circuits illustrating your system and how you tried to make your measurements.

Do try searching the site here for basic questions. Measuring voltage and current as well as calculating power are basic things that have been covered before.

The folks here like to help, but they are like your teachers - they want you to make an effort first.

  • 1
    \$\begingroup\$ +1 - Plus, if, once you have done all of the above, you find it may not be possible, you could use a capacitor and charge it with your wireless power transmission and the motor only be switched on when the voltage across the capacitor reaches the required value. That way the car will be stationary and then shoot forward some distance until the capacitor discharges. You may have more success that way as the starting current for your motor is likely to be higher than what you can transmit :-) \$\endgroup\$ – Rendeverance Nov 19 '17 at 13:34

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