# 3-phase motor driver (DRV8313) only works for high impedance loads? Simple circuit!

#### Elazar

Joined Oct 29, 2019
49
Hi All,

I am experimenting with a simple motor driver circuit, based on the DRV8313 chip, Datasheet

The chip has a built-in overcurrent protection circuit,

PROBLEM DESCRIPTION: The motor driver circuit works as expected ONLY for loads above (about) 8 ohms, if I connect any load less than that the chip stops working (stops supplying voltage to the H- bridges)

NOTE: the current consumption is not the problem, because even if I use PWM and give 0.09 volts to a 4.5-ohm load the driver chip still doesn't work, no meter the voltage supplied to the load.

If the load is less than 8 ohms, the chip doesn't even start to supply any voltage, it acts like it has detected a short circuit.

however, the load I need to work with is a 4.5 ohms motor,

See part of the circuit design:

Is anything wrong with this design, that might couse this problem?

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#### AlbertHall

Joined Jun 4, 2014
12,293
You are trying to supply 24V to an 8Ω load. That requires a current of 3A. The chip is rated to supply a maximum of 2.5A and that is your problem.
Note that it doesn't matter if you use a low duty cycle as during the ON time you will still be exceeding the chip's curretn limit.
You need to choose a higher rated driver.

#### Elazar

Joined Oct 29, 2019
49
It makes sense what your saying,

however, I believe that it takes a certain amount of time for the chip to detect an overcurrent situation,
so the question is will a faster PWM clock speed solve the problem, and if yes, what speed will it need to be?

The datasheet states: tOCP Overcurrent protection deglitch time 5 µs
Does that means that it takes 5us for the chip to detect the overcurrent?

#### AlbertHall

Joined Jun 4, 2014
12,293
Does that means that it takes 5us for the chip to detect the overcurrent?
Basically yes.
Is the 4.5Ω you quote the ststic DC resistance of the motor?

#### Elazar

Joined Oct 29, 2019
49
The motor is a Y type, and each winding is 1.8 ohms,
The supply voltage must be 24 volts, but I will use PWM to reduce the voltage to about 2.5 volts (average)

So the PWM speed needs to be more than 200K?
The Arduino chip can't run so fast so I will need a driver that can handle > 13.33A??
This is very expensive and big?

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#### AlbertHall

Joined Jun 4, 2014
12,293
Can you give us a link to some data on the motor?

#### Elazar

Joined Oct 29, 2019
49
The circuit will need to control a custom-made 3-phase BLDC motor that is designed for an air blower (a simple fan!)
I don't have a datasheet, but it should be simple to drive this motor as it's nothing fancy...
The resistance is 1.8 ohms in all combinations...
I broke one open to see the winding... basically, it's a Y construction...

#### AlbertHall

Joined Jun 4, 2014
12,293
Are you sure about the 24V?
That winding doesn't look like it is expecting 11A.

#### Elazar

Joined Oct 29, 2019
49
No, the motor needs much less voltage than 24V,

Just my circuit gets 24V input and I don't prefer to reduce the voltage with a regulator...
because of the cost and size of the PCB... (high current...)

However, I am not sure if I am correct?
what do you think I should do?

#### AlbertHall

Joined Jun 4, 2014
12,293
The problem with the driver circuit is that with a 24V supply, it will apply 24V to the motor coils, even if only for a brief period of time.
I think you should drop the 24V to whatever the motor actually needs. That could be a switch mode device to keep the heat down.

#### Elazar

Joined Oct 29, 2019
49
The L6234 chip from ST works up to 5 Amps and only haves terminal protection, and does not have over current protection,
See datasheet: Datasheet
so the question is will the L6234 do the job without the need to reduce the input voltage?

Also, maybe we can solve the problem by just placing a current limit resistor on all the PGND (sense pins)?

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