Thursday, August 14, 2014

Why do motors burn up?

my burnt coils

For a given torque and speed you'll need the same phase current through the motor regardless of whether or not you have a low voltage or a high voltage pack. The high voltage battery would have the potential to climb a hill at higher speeds if it is not phase current limited by the controller. And if you use lower gears to get the same slow speeds and watts there will be more kinetic thrust at less amps.

If you want to make a given Permanent Magnet motor spin faster, you can either run it at a higher voltage, or keep the voltage the same and use a faster motor winding [with fewer turns]. The former approach will have your input electrical power at a higher voltage and lower current, while the latter would be at a lower voltage and higher current, but the total power (volts x amps) would be the same, as would the motor efficiency, heating, etc. However, the motor controller and external wiring can get hotter with the fast wind at lower voltage and higher current, unless you appropriately increase the wire gauge and mosfet resistance.

You really need a controller or CycleAnalyst that shows phase amps and voltage.

[The Phase wires are the three wires that the controller sends the power to the motor's multiple sets of magnets to make it turn.]

The amp current delivered to your motor will be greater than the battery current at gravity forced low speeds, at higher speeds of flat land cruising the motor current will equal the battery current. The controller acts as a power converter and produces more output current at low speeds. This output motor current can be several times the battery current at very low hill climbing speeds where the motor's high drain voltage is low. Installing a digital thermometer is about all you can do to keep the motor from burning out, because no one knows how to install a tachometer in an electric motor. Unless it is some kind of high tech device?

Sometimes a controller will feed power to the motor even though it is not spinning. The CA only shows the battery current. [Ask for one that shows phase amps with a tachometer]. The motor voltage is directly proportional to the RPM it is spinning at. So if your battery voltage is 60V, and the motor is 5RPM/V, it will spin at 300RPM. If that same 5RPM/V motor is spinning at 150 RPM, the motor voltage is 30V.

The only way to read phase amps would be with a magnetometer with remote readout. Use a Field Oriented "FOC" controller.

Phaserunner and Baserunner controllers have heat protection emergency turnoff. Also using Stator-aid in a direct drive HUB motor will help it stay cool [but not with a geared motor because of the higher RPMs of the stator [this applies to mid-drive motors in the proper gear also]. If you use a higher voltage just to run the bike faster without watching the amps, it can result in coil burn. I intend to use a 72v battery pack with lower gears just to keep the motor cooler. 

I think I am going to have to drill vent holes in my mid-drive motor and use a fan blowing down a 1” tube to keep out the road crud in wet weather.

Instead of one large motor two smaller ones would be the best for a heavy duty cycle truck. Although it will require two controllers.

Every thing you need to know about hub motors:

Field Weakening, what is it?

Best new controllers:

Installing A Temperature  Sensor

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