Electronic – 24V 250W motors

Note that your question states that you want to reduce the starting torque, but you actually want to reduce the starting current.

There are a few ways to reduce the starting current of a three-phase induction motor. In rough order of cost:

1. Reduced Voltage starters

   These methods apply a reduced voltage (say 50%) to the motor on first starting. This reduces the current draw to 50%. After the motor attains enough speed, the voltage is increased from 50% to 100%.

   Note that the torque is reduced as the square of the voltage, so with 50% voltage the motor will only develop 25% torque. You need to ensure that this torque is enough to turn the motor's load from a standing start.

   • Series resistance starter - put resistors in series with the motor's line terminals to limit the amount of current drawn on starting. The resistors are shorted out after the motor has reached approximately half speed.
   • Star-delta starter - the motor is started with its three-phase terminals connected in wye. If you have a 415VAC three phase supply, that means that only 230VAC is applied to each winding and the current is similarly reduced. Once the motor is up to speed, switch from star connection to delta connection, which applies the full 415VAC.
   • Autotransformer starter - uses an autotransformer with two voltage taps to apply a low voltage on motor starting (with corresponding low current), switching to full voltage after the motor has gained some speed.

2. Soft starter - an electronic device that starts the motor at a low voltage, with corresponding low current, and continuously increases the voltage until full speed is reached. The difference between this and rotor resistance, star-delta, and autotransformer starting is that the voltage is increased steadily, as opposed to in one sudden step. This is more gentle to the motor and load.

3. Variable speed drive (also known as VSD, VFD, VVVF) - an electronic device that starts the motor using full rated voltage, but at a lower frequency. The most expensive of all methods, but also the best, as it can apply very high torque from a standing start.

   As the name suggests, VSD's also allow variable speed control, which no other motor starting method does.

It doesn't sound like you have a wound-rotor induction motor, so I will neglect to mention rotor resistance starting.

More on this topic can be found by searching for "three-phase induction motor starting". You should be able to find application notes and guides written by equipment manufacturers, that will go into more detail about how the above methods work and what equipment you can use to implement them.

A 3.5 Volt, 1000 mAh battery can supply 3.5 watt hours. That means the battery can supply 500 watts for 0.007 hours. So, with perfect efficiency of all circuitry, including stepping up the battery's 3.5 volts to the 12-24V the motor needs, you could theoretically supply full power to the motor for 25 seconds.

Of course, it's very unlikely the battery could tolerate being drained in 25 seconds. The fastest draining batteries I've seen can tolerate being fully drained in 72 seconds with only moderate loss of lifespan.

You're going to have to design a circuit to step up the 3.5 volts from the battery to at least the 12V that seems to be the motor's minimum specification.

An 11.1V 30-60C LiPO battery (such as those used for drones) seems like a reasonable match for your application, and could probably run the motor at 1/2 voltage (just under 12V) and 1/2 duty (pulsed). The one I linked to stores about 24 watt hours and could probably run the motor at 1/3 speed or so for about 5 minutes. It's designed to tolerate complete discharge from a fully charged state in as little as one minute.

Update: Since this is a quadcopter application, you should be looking at using a brushless motor with an electronic speed controller. Otherwise, it will be very difficult to adjust the rotation rate of the motor quickly enough to provide a stable platform. You should look at the design of a quadcopter that's similar in size and weight to your requirements and look at what batteries, motors, speed controllers, and props it uses.