Stepper Motor control not working

[asagohan]

Hello. I am trying to get a 23A108A linear actuator to work using the circuit provided in the attachement. it is rated at 1.4VDC and 3.8 Amps and each transistor is switched on for 500ms then switches off for 500ms, then the next one switches on, etc. in a loop. The pulses to switch the transistors on come from a programmable PICAXE chip. I can feel the stepper motor jerking if I hold it in my hand, but it is not spinning. After taking to a friend with a CRO we noted that the collector of each transistor is not dropping all the way to 0v when it is switched on, it only drops a little bit. However, if I use an LED in place of the motor coil, it does. I am a beginner with electronics so I am not sure what is causing this. Could it be the inductance from the stepper motor? Also, is it correct to drive the motor with 1.4VDC? Because it seems like a weird voltage. I have tried running the motor by placing the wires in order on a battery by hand an it does turn in that situation.

 

[colin55]

To start with, the voltage of 1.4v is too low as the darlington transistors will have a voltage drop across them that is greater than 1.4v.

 

[asagohan]

Schematic mistake

Sorry, I used a circuit I found on the net and forgot to change part of it. I am using 2n3055 transistors not darlington arrays. Does this make a difference?
Here is the updated schematic.

 

[colin55]

What is the resistance of the coils in the motor?

 

[Hero999]

Use logic level MOSFETs such as the IRL540, they have a lower voltage loss than the 2N3055 which is, for all intents and purposes obsolete.

 

[evarobotics]

First, if your motor is jittery and vibrating but not turning then it could just be that one of your phases is connected in reverse. Also confirm the FETs are switching in the right order.

Second, the voltage is too low. Steppers are always driven at higher than rated voltages (24/48 volts are typical) - but they are current controlled, because it is the current that burns the windings. So get the winding resistance from the data sheet (or measure it) and design your circuit so the voltage generates a decent current - about 1 Amp would be a good start (if the FET's are rated to that).

As the switching frequency increases you will find you need to increase the voltage to counter the back EMF in the motor. How fast did you want to get the motor turning?

 

[asagohan]

Thanks for your replies. The phases are correct, I have tried it manually. The transistors are switching in the right order too.

So the data sheet says that the resistance is 0.33 ohms and unipolar inductance is 0.4mH.
I don't quite understand how I can put 24v into it.

V = IR
24 = I * 0.33
I = 72.7A

How do I limit the current to 1A while keeping the voltage at 24V? And why is the motor rated at 1.4V when I should be putting 24V into it? How do I know what voltage is too much?

If I can get it working I want the motor to turn as fast as possible. But I just want to get it to work first.

 

[colin55]

Actually measure the resistance of the windings.

 

[Pommie]

To limit the current you use a series resistor. If you run your motors at 24V then you will need a resistor of approximately 20Ω. As this will be carrying 1A it will need to be a 20W resistor and will get hot. It might be better to start with 12V and 10Ω 10W resistors. You use 1 resistor per phase.

To understand why this works see this page.

Mike.

 

[colin55]

Limiting the current to 1amp will make the motor very weak.

 

[Pommie]

That is what he asked but I see earlier he states 3.8A. So 12V and a 3Ω resistor should do the job. It's going to get mighty hot though dissipating around 30W. He might be better with one of the other methods on the page I linked to.

Mike.

 

[asagohan]

Yes the motor is 3.8 Amps. I was replying to evarobotics who said to try it with 1Amp.

I haven't measured the resistance in the coils yet because the resistance measurement doesn't work on my multimeter. However I have tried running the circuit with a 12V power supply which can be used to jump start a car battery and I still got the same effect. The motor was pulsing very lightly but did not turn at all. The first time I tried it was using a regulator set at 1.4V and 3.8Amps and I got the same result.

 

[evarobotics]

OK, let me clarify what I was saying...

Stepper motors are current controlled. So a stepper motor controller tries to maintain a certain current flow through the windings. On a 3.8A motor, 1 amp will give plenty of torque without risking any damage to the motor.

The current control is normally implemented with PWM (Pulse Width Modulation) - This has the effect of producing a variable voltage. I'll explain why soon. The voltage source used for the PWM is much higher than the rating of the motor (can be up to 96 volts, 24V is typical). The reason for the high voltage is that it produces a current in the motor windings very quickly. That allows a fast PWM (approx 25KHz) which provides smoother control.

For a winding resistance of 0.33 Ohm, you only need 0.33 volts to produce 1 amp. That is when the motor is at standstill. As the motor starts to spin, the generated voltage (Back EMF) increases and opposes the power supply. So, for example, when spinning at 100RPM, the back EMF may be 20 volts, so you need 20.33 volts to produce 1 amp. It is actually mush more because the inductance has greater effect - but let's keep it simple.

So if you are just going to spin the motor very slowly, a 1.5 volt source seems reasonable, as long as it can deliver 1 amp without dropping in voltage (i.e. well regulated supply).
If you intend to spin faster, then you may find you need a higher voltage to accomodate the back EMF - trial and error will help you here.

How fast are you trying to step the motor?