Hi guys, Im working on a school robotics project, where im using two 4pole 450Watt, 24Volt Wheelchair motors. The competition states that I must be under 216Watts per circuit. Both motors will have individual circuts. Now, here's my problem: I keep blowing fuses. Im running off 12 Volts, which means to get under 216Watts i must use 15-18A Fuses, which i keep blowing. Is there any way I can lower my current or something? Maybe a resistor or something to limit my voltage or something? Its not the initial kick of the motor, the fuses go out after aobut 2-3 seconds of running. I doubt slow blow fuses will help me here.
Electric motors are not resistive loads. Your calculation doesn't apply. Did you measure the actual current? If you are $$$ limited try to find some automotive light bulbs as "resistors". With daylight running lights so common the service shops often have to exchange bulbs where the filament for the high beam is still intact. When the filaments are cold they have low resistance so your motor gets a good startup kick. As they heat up resistance goes up and limits the current. My estimate is that you need an array of at least 300W worth of bulbs (parallel) in series with each motor. The bulbs won't get so hot any more, so don't worry about finger prints on the glass, but they won't live as long as in a car, because the halogen cycle doesn't work well at lower temperatures.
However, as a safety issue, its manditory to use Fuses. Money isn't limited, but space is. We're looking for a space saving solution to this. Any idea whats causing us to blow our fuses?
your problem is probably caused by the high current a motor draws when it starts. To fix this, either use slow-blow fuses(fuses that can exceed the maximum rating for a few seconds before blowing) or use proper motor starting capacitors or both. You need to find a way to allow the high current needed to start the motor, without blowing the fuse. I don't know how to calculate the size of the motor starting capacitors but I'd guess that you could find out with a little googleing. edit: I'm not sure how you are driving the motors, but if you are using pwm, then maybe you could slowly ramp the power to the motors, rather then turning it on and off directly. edit:I might be completely wrong about how to fix this but the problem almost defiantly is the surge of current when the motor starts.
They're not, but DC ones follow fairly fixed rules outlined here. Reducing the voltage by 50% from 24V to 12V should have reduced the wattage by 75% from 450W to 112.5W, not halved it. Speed & torque will have halved. But stall current is always very much higher than current at moderate loads, so technically you're well within the competition rules but need bigger fuses to cope with start current. Instead of fuses, a much slower-acting bi-metallic strip heated by the motor current is common, it bends as it warms up to break the circuit on long overloads.
