What's the best way to improve torque in a stepper motor?

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What's the best way to improve torque in a stepper motor?
There are 2 ways to drive a stepper motor regardless its coil configuration or size.

1.

Constant Voltage mode:

Its basic way to get motion from the motor if you don’t want to invest in driver (which by the way are becoming cheaper and more functional than ever). Let’s assume that you want to drive a motor rated at 1.2 amp and has coil resistance of 3 ohm. Not though you will drive it in constant voltage, ignore the voltage rating on motor. This is because 90 % time it’s correct. But if you end up with motor from rest of 10 %, expect magic smoke. To avoid this, always calculate the motor’s voltage as

Voltage = current x coil resistance

Now for our motor, 1.2 Amp x 3 ohm = 3.6 Volts.

Get yourself a supply with 3.6 V and 3-4 amp rating. Higher rating since 2 coils will pull current simultaneously.

Now you can use any switching circuit you want. But remember, you cannot use readymade H bridges since they need some margin of voltage over logic level voltage. And our supply voltage is itself 3.6 V. One thing can be done is you can put additional resistance in series and use higher source voltage. But this will result in power wastage.

Using this method, your motor will rotate. But the dynamic torque will be at lower end. But I think it should provide its rated holding torque since while holding, current is constant and the inductance doesn’t come in picture.

Now why don’t people use this often?

Because it’s like racing an F1 car with passenger car driver coz you can’t manage a formula driver.

So people use:

2.

Constant current mode:

In stepper motor, torque depends on the coil current. So higher the current, higher will be the motor torque. In order to get maximum torque, the maximizing current is the way.

Now in this mode, a specific driver is dedicated for the purpose of motor control. Ever wondered how a motor rated at 3.6 volts can run at 36 volts? This is because the motor is actually rated for current and indirectly heat (I^2*R) it can dissipate. So though we apply 36 volts to the coil, the current never goes beyond the rated value. This is done by the driver circuit which often uses chopper techniques with current feedback resistor for current control. Also the inductance plays major role since at fast switching circuits, it offers greater impedance and results in lower motor current. Thus lower torque.

The drivers almost every time use H Bridge. It’s also the reason why drivers offer a minimum supply voltage with respect to logic level input voltage. But the current control is also more sophisticated and the current through the coil is not always kept maximum. But is varied so as to give smooth operation.

The overall current and hence torque of a stepper goes on decreasing as we move from full stepping towards micro stepping. But also the smoothness in motor operation goes on increasing.

Such control is not available in constant voltage mode. And that’s why the F1 driver for F1 car.

3.

The one with danger.

Now if this doesn’t satisfy then the answer for you is if the duty cycle of the motor is studied, then it can be used for even higher current than it’s rated one. Just ensure that whatever cumulative heat generated is lower than motor’s rejection capacity. But doing so increases the danger of coil damage as for a particular task, duty cycle of a particular coil can be lower. But for another task, it may get to maximum which will damage the coils or at least lower their lifespan for sure. Great optimization can be done by this method for repetitive kind of work.
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