I pulled a 12V motor out of a car vacuum which I plan to use for a Van de Graaff generator. I ordered a 12V 3A wall adapter, thinking it would be enough, because I was measuring ~1.5A running the motor off 10V worth of AA's in series, but it seems the starting current is too high and keeps flipping the supply off.

Recommendations for what I should use instead? 12V 5A?

I saw something about an inrush current limiter online. Would that be a good idea, so that I could still use the 3A supply?

Other cheap ways to use the power supply I have with this motor? Maybe some sort of MOSFET current source?

In the future, I also plan to be able to adjust the speed with PWM, in case that makes a difference for what I'd need.

 

The starting current as well as the running current both depend on the mechanical load on the motor. Is it physically connected to a belt or gear drive?
If you use PWM to control the speed, you can limit the inrush current requirements by using a PWM soft-start. That is where you start with very narrow drive pulses and gradually increase the width over a short period of time.

 

Simply measure the resistance of the motor while slowly turning its shaft, pick the lowest measurement, subtract the resistance of your meter leads then use Ohms Law to calculate the peak starting current.

 

The starting current as well as the running current both depend on the mechanical load on the motor. Is it physically connected to a belt or gear drive?
If you use PWM to control the speed, you can limit the inrush current requirements by using a PWM soft-start. That is where you start with very narrow drive pulses and gradually increase the width over a short period of time.

The shaft currently isn't connected to anything. Thanks for the PWM tip, I'll give it a try.

 

Simply measure the resistance of the motor while slowly turning its shaft, pick the lowest measurement, subtract the resistance of your meter leads then use Ohms Law to calculate the peak starting current.

You mean just turning the shaft by hand? That's an interesting idea.

Edit: The resistance is very variable. Its at it's lowest at a stop, at around 1-3 ohms, and turning it at all brings it up to over 40 ohms.

 

If You have an Aluminum-Box, ( mostly for a Heat-Sink ),
this Circuit is more simple than a proper PWM-Motor-Controller,
but produces more Heat, ( around 5-Watts, maybe slightly more ).
To make a smooth speed "ramp-up",
the Circuit shown needs a ~100uf Capacitor connected between
the Speed-Control Pot-Wiper-Terminal, and Ground.
This will create around a ~1-second Ramp.
A bigger Cap will produce a longer Ramp, ( 470uf = ~4-seconds ).
The Current-Capacity is determined by the FET selection.

If You'd rather go straight to PWM,
( which is recommended but more expensive and complex ),
just say so.
.
.
.

 

If You have an Aluminum-Box, ( mostly for a Heat-Sink ),
this Circuit is more simple than a proper PWM-Motor-Controller,
but produces more Heat, ( around 5-Watts, maybe slightly more ).
To make a smooth speed "ramp-up",
the Circuit shown needs a ~100uf Capacitor connected between
the Speed-Control Pot-Wiper-Terminal, and Ground.
This will create around a ~1-second Ramp.
A bigger Cap will produce a longer Ramp, ( 470uf = ~4-seconds ).
The Current-Capacity is determined by the FET selection.

If You'd rather go straight to PWM,
( which is recommended but more expensive and complex ),
just say so.
.
.
.
View attachment 274890

Well, I already have the components necessary for PWM motor control, while I don't have an op-amp, so I'll try PWM.

 

I pulled a 12V motor out of a car vacuum which I plan to use for a Van de Graaff generator. I ordered a 12V 3A wall adapter, thinking it would be enough, because I was measuring ~1.5A running the motor off 10V worth of AA's in series, but it seems the starting current is too high and keeps flipping the supply off.

Recommendations for what I should use instead? 12V 5A?

I saw something about an inrush current limiter online. Would that be a good idea, so that I could still use the 3A supply?

Other cheap ways to use the power supply I have with this motor? Maybe some sort of MOSFET current source?

In the future, I also plan to be able to adjust the speed with PWM, in case that makes a difference for what I'd need.

Your 1.5 A estimate is highly suspect for a couple of reasons. First, 1.5 A is a pretty heavy load for an AA cell. Your terminal voltage is likely dropping quite a bit and you are not getting a useful measurement. Why make your measurement using a bunch of AA cells? Make your measurement using something that you know can drive it adequately -- like a car battery. You don't have to take the battery out of the car, just connect it up to the battery the same way you connected it up to your AA batteries and measure the current.

Even if that weren't the case, the current draw of a motor like this is driven primarily by the load. An unloaded motor is only drawing enough current to overcome its own friction. To get a useful measurement, you need to measure it under something close to the load you anticipate it driving. One way to get a feel for the currents involved is to use a brake on the motor and measure the current as the brake is applied. If the motor has a smooth shaft (or smooth enough), a simple way to do this is to is to wrap a few turns of a cloth (like a wash rag) around the shaft and then use a pair of pliers, such as channel locks, to grip the shaft through the cloth. The cloth is there to protect the shaft and also make for a smoother, more controlled application of braking torque to the shaft. Monitor the current as you progressively increase braking torque over a period of several seconds. If you want to (and can), you could take this all the way up until the motor stalls and you get a measurement for the stall current. But avoid running the motor under sufficiently heavy loads as to stall, or even greatly reduce, the motor speed for very long -- a few seconds at most.

 

Your 1.5 A estimate is highly suspect for a couple of reasons. First, 1.5 A is a pretty heavy load for an AA cell. Your terminal voltage is likely dropping quite a bit and you are not getting a useful measurement. Why make your measurement using a bunch of AA cells? Make your measurement using something that you know can drive it adequately -- like a car battery. You don't have to take the battery out of the car, just connect it up to the battery the same way you connected it up to your AA batteries and measure the current.

Even if that weren't the case, the current draw of a motor like this is driven primarily by the load. An unloaded motor is only drawing enough current to overcome its own friction. To get a useful measurement, you need to measure it under something close to the load you anticipate it driving. One way to get a feel for the currents involved is to use a brake on the motor and measure the current as the brake is applied. If the motor has a smooth shaft (or smooth enough), a simple way to do this is to is to wrap a few turns of a cloth (like a wash rag) around the shaft and then use a pair of pliers, such as channel locks, to grip the shaft through the cloth. The cloth is there to protect the shaft and also make for a smoother, more controlled application of braking torque to the shaft. Monitor the current as you progressively increase braking torque over a period of several seconds. If you want to (and can), you could take this all the way up until the motor stalls and you get a measurement for the stall current. But avoid running the motor under sufficiently heavy loads as to stall, or even greatly reduce, the motor speed for very long -- a few seconds at most.

Thanks for the suggestions. AA's were just what I had on hand at the time. Maybe I'm misremembering the current under no load, but I'm pretty sure that with the vacuum fan still attached to the shaft, I was getting at least 1.5A continuous.

I might try the car battery within the next couple days. But I might also just start putting the PWM components together and see if the PWM soft-start works.

 

A DC motor is stalled when it starts running which is why the starting current is so high. 12V/1 ohm is 12A if the battery and wires have very low resistance. Without a mechanical load, the motor might start running with 6V then the peak current is 6A.

If you can slowly charge a large capacitor then the capacitor can provide enough current to start the unloaded motor running. The capacitor must be slowly charged so it does not overload your low current power supply.
PWM also needs a slowly charged but smaller capacitor.

 

If you can slowly charge a large capacitor then the capacitor can provide enough current to start the unloaded motor running. The capacitor must be slowly charged so it does not overload your low current power supply.
PWM also needs a slowly charged but smaller capacitor.

Oh, the PWM soft-start method from #2 won't work without a capacitor? Would it just be a capacitor and resistor between 12V and ground, in parallel with the motor?

 

Oh, the PWM soft-start method from #2 won't work without a capacitor? Would it just be a capacitor and resistor between 12V and ground, in parallel with the motor?

With PWM, do not connect a large capacitor across the motor. It will not help with the soft start and may cause problems with the switching waveform. A large capacitor may help if it is connected across the output of the power supply but if it is too large, the supply may go into over-current shut-down on power up as the capacitor charges.. Capacitors across the motor terminals are to cut down radiated interference from the switching step and brush arcing. I usually connect a 0.1uF capacitor from each brush terminal to the motor body.

 

With PWM, do not connect a large capacitor across the motor. It will not help with the soft start and may cause problems with the switching waveform. A large capacitor may help if it is connected across the output of the power supply but if it is too large, the supply may go into over-current shut-down on power up as the capacitor charges.. Capacitors across the motor terminals are to cut down radiated interference from the switching step and brush arcing. I usually connect a 0.1uF capacitor from each brush terminal to the motor body.

Ah ok. So with soft start, I'd just set the Arduino to analogWrite with a duty cycle of 1, and slowly increase the duty cycle? If a motor draws the most current starting from a stop, won't it still pull a lot of current for the duration of the tiny pulse?

 

Ah ok. So with soft start, I'd just set the Arduino to analogWrite with a duty cycle of 1, and slowly increase the duty cycle? If a motor draws the most current starting from a stop, won't it still pull a lot of current for the duration of the tiny pulse?

No. The inductive reactance of the armature wil delay the build-up of current at the beginning of the soft start and the smoothing capacitor on the output of the supply will provide a little more current as the motor starts conducting.

 

No. The inductive reactance of the armature wil delay the build-up of current at the beginning of the soft start and the smoothing capacitor on the output of the supply will provide a little more current as the motor starts conducting.

Oh ok, I get the first part, but could you explain why the smoothing capacitor is needed, and how it should be connected? And does it need a resistor to protect the supply when it first starts?

 

Oh ok, I get the first part, but could you explain why the smoothing capacitor is needed, and how it should be connected? And does it need a resistor to protect the supply when it first starts?

Most power supplies have a capacitor connected across the output to filter out ripple and transient load changes. You may be able to add more capacitance across the power supply to help supply the startup current for the motor, but as I mentioned above, The charging current on turn-on may cause the supply to shut down.
Do not connect a resistor in series with the motor. That will limit the current available and cause more problems.

 

Do not connect a resistor in series with the motor. That will limit the current available and cause more problems.

Oh, I meant resistor in series with the capacitor, because doesn't the cap act like a short circuit at first? Or does it not matter since a small cap will charge up very fast so the high current is extremely transient?

 

No resistor in series with the capacitor. That would limit it's usefulness ar an extra source of motor-start current

 

No resistor in series with the capacitor. That would limit it's usefulness ar an extra source of motor-start current

Got it, thank you. Can you recommend a size for the cap?

 

What do you have available? It must be rated 25VDC or more. Start with 470uF if you have one. Try it first across your power supply without the motor connected. If the supply shuts down on power up, try a smaller value. If it works, you could try a larger one.