# Torque in relationship to Voltage

Regarding a DC motor designed to operate at a voltage up to 22 volts: If this motor is powered by a 18.5 volt lithium battery, will it not produce more torque than if the same motor is powered by a 14.8 volt lithium battery? I know more volts create more speed, but the increased current available in the 18.5 volt pack will provide the additional torque, right? If this is true, will the same motor stall at different amperage depending on how much voltage is being supplied?

• Torque is in relation to current, not voltage. The higher voltage pack isn't necessarily the one that can supply higher current. – user_1818839 May 18 '16 at 23:09

More voltage means more current which means more torque output from the motor. Different amperage implies you have a current limiter, and can adjust (limit) the drive current separate from the drive voltage. If that is the case the motor speed will reduce under the same load but with less current.

Drop the current low enough and the motor will stall for a given load. Do not exceed the voltage or amp rating for the motor and it should be ok. If it stalls under full current (the maximum allowed for the motor) then the load is too much or the motor is too small.

Make sure the load is well lubricated and free of rust.

When you increase the voltage, the stall torque is higher due to higher current. However the rated torque(current) remains equal and is related to motor construction. In the picture you can see what happens if the voltage is reduced, it can be also viewed opposite what should happen if the voltage is increased. The max speed will be increased, also the torque characteristics is offset, meaning that the same load will find the ballance at higher speed, higher torque -> higher torque means higher current. Therfore your motor might became overloaded, this is not true if measure the current and you determine that motor is running with <= rated current

More voltage means more torque at the same speed.

A useful first-order model of a motor is a resistor in series with a voltage source. That voltage source is a function of the motor speed, and apposes the applied voltage. Torque is proportional to the current.

This simple model tells you a lot. When the speed is 0, the current is just the applied voltage divided by the resistance. That's the stall current, and is when you get maximum torque.

If you let the motor go (no load on it), then it will accelerate due to the torque. That makes it go faster, so now the internal voltage source starts offsetting some of the applied voltage. Eventually the motor will get to the steady state speed for that voltage. That is also called the unloaded speed of the motor at that applied voltage. This is where the internal voltage source has offset most of the applied voltage. The little that's left across the resistance allows just enough current to flow to produce just enough torque to overcome the friction and other losses to keep the motor spinning.

Now you should hopefully be able to understand why the first sentence is true.

One gotcha of this is higher power dissipation. The motor's dissipation comes from the resistance, and is therefore proportional to the square of the current.