Overdriving A Stepper Motor limits

Hello all.

I am using a 5 volt stepper motor from adafruit, the 28BYJ-48, found here.

At 5 volts (not coming from the arduino BTW, external supply) It cannot spin the disk that is required. At 6 volts, it is inconsistant. At 7.5 volts, it works like a charm. My question however is this; Will I damage the motor by running it at this speed for extended periods of time? Also, I am using a 1A H-Bridge, but the power supply is 2A. Will this damage the H bridge, or does it regulate the current?

Thanks, Tyler

The motor can be damaged by overheating, if operated above the rated voltage for a long time.

Will this damage the H bridge, or does it regulate the current?

Depends on the H-bridge.

jremington: The motor can be damaged by overheating, if operated above the rated voltage for a long time. Depends on the H-bridge.

Ok, what would be considered a long time? Like, if its running for three hours at a time? Or if it runs 15 minutes at a time two to four times a day?

The H-Bridge is this one from sparkfun. I have been unable to find max input current.

Those small motors will have a thermal time constant of a few minutes, not hours. You can measure it if you want...

have been unable to find max input current.

Look in the data sheet for the continuous output current instead. The data sheet also discusses what happens if that current is exceeded.

A circuit [u]draws[/u] current from the supply, according to the power supply voltage and the circuit resistance.

The supply must be capable of providing [u]at least[/u] the drawn current, and usually, more is desirable.

jremington: Look in the data sheet for the continuous output current instead. The data sheet also discusses what happens if that current is exceeded.

A circuit [u]draws[/u] current from the supply, according to the power supply voltage and the circuit resistance.

The supply must be capable of providing [u]at least[/u] the drawn current, and usually, more is desirable.

So the 2A power supply is better than the 800mA one I was using before? And won't damage it? Also, how could I measure how long the motor can handle 7.5 volts at? Like I said, the project is almost due and can only provide the power necessary to turn the disk at 7.5 volts.

Thanks

tyler_newcomb: Hello all.

I am using a 5 volt stepper motor from adafruit, the 28BYJ-48,

That motor has a winding resistance of at least 40-50ohm, that means it will draw a current of max 188mA at 7.5volts

At 5 volts (not coming from the arduino BTW, external supply) It cannot spin the disk that is required. At 6 volts, it is inconsistant. At 7.5 volts, it works like a charm. My question however is this; Will I damage the motor by running it at this speed for extended periods of time?

well, you can let it spin at desired load and speed and check it if overheats and how fast. I guess you can use the sensors in your fingers for a balpark initial estimate. The motors are damaged by overheating because high currents and/or limited cooling, long before a higher-than-rated voltage will do other kind of damage.

Also, I am using a 1A H-Bridge, but the power supply is 2A. Will this damage the H bridge, or does it regulate the current?

No way, the power supply rated current is the max it can give. It's normal (and required) to be higher than the load will draw. A car battery can provide hundreds of amps, that doesnt mean it will burn an arduino board that draws 100mA. If it is powered through the right pins of course. The current is limited by the motor's resistance, and as I mentioned above, it is high enough to keep both the H-bridge and power supply within safe limits.

All you need is to let it run and check if it overheats.

PS as an assuring exercise - measure the winding resistances of the motor coils at its terminals. Doesnt matter wich wires, measure them all and see what values you get - measure the mA drawn by the H-bridge when the motor is spinning, then hold it with your hand, see if current changes. - measure the voltage that motor windings see from the H-bridge, see if it is still 7.5volts or less. Some bridges have a voltage drop themselves.

Stepper motors are current driven devices. In order to overcome DC winding resistance but more important the impedance of the windings due to their inductance, you will have to put some suitable voltage over it. The amount of tension needed depends on basically two factors: 1) DC resistance 2) frequency and more importantly, duration of the stepper pulse

according to L.di/dt = V.

Thats why at 5V your system did not work: not enough current was allowed due to the high L or too short dt. Suddenly, from 7.5V on there was enough current allowed to be able to spin. But if you want to be in the optimal region, you need to find a good relation between voltage (that will dictate current for a given on-pulse) and the load. The more load on the shaft of the stepper, the more current you will need, so the higher voltage you need to apply.

There are 2 major constraints to this game: 1) Maximum allowed voltage - typically 500!! volts for most steppers 2) Maximum temperature (datasheet)

The temperature will rise in a normal way, almost linearly with increasing current. Until you reach the point where the iron stator gets magnetically saturated. This is the real maximum that you might be able to use.

How do you detect that saturation point? If you have a scope and measure the current through one of the motor windings in full step mode, you should see a gradually increasing current per pulse. When you reach saturation, current will rise much much faster towards the end of the pulse and instead a sawtooth you will see a sawtooth with steep spikes at the end.

This will give you an indication how far you can push it.

Do not worry about driver and power supply. Both are protected from overload. Power supply should be higher in amps than driver or/and motor.