Question on Using PTC for Over Current Protection

[Electronic Swear]

I am now using a PTC to behave as a over current protector for
Brushless Dc Motor. However, there are some problems happened. The PTC
is connected on the
front of the main circuit ( after the main V+ ).

1) For normal operation, motor will slowly rise their internal
temperature.
As PTC is sensitive on temperature, after a long operation period (let
say 10min) the PTC will tripped off the connection. However, I do not
expect the PTC operated like this.

2) The PTC is operated as over current protector when lock rotor
happened. If i set a higher current tripped off PTC, the time for
tripped of in lock rotor protection will also increase. It is not a
good appearance for me.

So, what should I do to prevent PTC tripped off because of motor
internal temperature and also function in a short time tripped off for
lock rotor (over current) happened?

Please give me some comments on it and thanks very much.

 

[MikeM]

I am now using a PTC to behave as a over current protector for
Brushless Dc Motor. However, there are some problems happened. The PTC
is connected on the
front of the main circuit ( after the main V+ ).

1) For normal operation, motor will slowly rise their internal
temperature.
As PTC is sensitive on temperature, after a long operation period (let
say 10min) the PTC will tripped off the connection. However, I do not
expect the PTC operated like this.

2) The PTC is operated as over current protector when lock rotor
happened. If i set a higher current tripped off PTC, the time for
tripped of in lock rotor protection will also increase. It is not a
good appearance for me.

So, what should I do to prevent PTC tripped off because of motor
internal temperature and also function in a short time tripped off for
lock rotor (over current) happened?

Please give me some comments on it and thanks very much.

As you have already found out, a PTC is thermal sensor.

It doesn't care who heated it; it just knows that it is hot!

Using it as a "self-resetting fuse" requires that its temperature
be determined only by its own self-dissipation; not by external
heat sources.

Mounting it a long distance from the motor would be good start...

MikeM

 

[John Popelish]

I am now using a PTC to behave as a over current protector for
Brushless Dc Motor. However, there are some problems happened. The PTC
is connected on the
front of the main circuit ( after the main V+ ).

1) For normal operation, motor will slowly rise their internal
temperature.
As PTC is sensitive on temperature, after a long operation period (let
say 10min) the PTC will tripped off the connection. However, I do not
expect the PTC operated like this.

2) The PTC is operated as over current protector when lock rotor
happened. If i set a higher current tripped off PTC, the time for
tripped of in lock rotor protection will also increase. It is not a
good appearance for me.

So, what should I do to prevent PTC tripped off because of motor
internal temperature and also function in a short time tripped off for
lock rotor (over current) happened?

Please give me some comments on it and thanks very much.

The effect you are seeing is not entirely bad. The motor tolerance
for surge current is also related to its temperature before the
surge. A cold motor is less likely to be damaged by a given period of
locked rotor current than a hot motor is. So the problem is not so
much one of keeping the PTC cool during operation so its response to
locked rotor current is repeatable, but to have its operating
temperature model the motor's temperature. They typically would have
very different thermal rises and time constants when exposed to
operating current and ambient changes.

I have seen PTC devices mounted in thick heat shrink tubing to alter
their rise and time constant.

 

[Electronic Swear]

I am now using a PTC to behave as a over current protector for
Brushless Dc Motor. However, there are some problems happened. The PTC
is connected on the
front of the main circuit ( after the main V+ ).

1) For normal operation, motor will slowly rise their internal
temperature.
As PTC is sensitive on temperature, after a long operation period (let
say 10min) the PTC will tripped off the connection. However, I do not
expect the PTC operated like this.

2) The PTC is operated as over current protector when lock rotor
happened. If i set a higher current tripped off PTC, the time for
tripped of in lock rotor protection will also increase. It is not a
good appearance for me.

So, what should I do to prevent PTC tripped off because of motor
internal temperature and also function in a short time tripped off for
lock rotor (over current) happened?

Please give me some comments on it and thanks very much.

Thank you for your reply.

Actually, what I am facing is that the internal thermal rising inside
the motor
have affected my PTC operation.

As I mentioned before, the PTC i add is to monitor the motor whether
it is over current (lock rotor prevention) or not. But it do not want
to be affected by the outside temperature.

Unfortunately my PTC will be tripped off after a long time motor
running because of the motor rising temperature. So, any method for me
to prevent this happened?

I am now considering to use thermal tube cover out of the PTC to
behave as thermal insulator. Is it a good way to do so? THANKS~

 

[MikeM]

Actually, what I am facing is that the internal thermal rising inside
the motor
have affected my PTC operation.

As I mentioned before, the PTC i add is to monitor the motor whether
it is over current (lock rotor prevention) or not. But it do not want
to be affected by the outside temperature.

Unfortunately my PTC will be tripped off after a long time motor
running because of the motor rising temperature. So, any method for me
to prevent this happened?

I am now considering to use thermal tube cover out of the PTC to
behave as thermal insulator. Is it a good way to do so? THANKS~

No. Imagine taking two cans of Coke at room temperature. Place
one into a small cardboard box, and then place the other can of
Coke and the boxed Coke into a refrigerator.

Leave them in the fridge for 24 hours, and then compare the temperature
of the Cokes. They will be at the same final temperure. The only
difference the box makes is the rate at which the boxed Coke cools.

If your PTC is inside, or in intimate thermal contact, with the motor
housing, then it will be at the same temperature as the motor.

MikeM

 

[N. Thornton]

I am now using a PTC to behave as a over current protector for
Brushless Dc Motor. However, there are some problems happened. The PTC
is connected on the
front of the main circuit ( after the main V+ ).

1) For normal operation, motor will slowly rise their internal
temperature.
As PTC is sensitive on temperature, after a long operation period (let
say 10min) the PTC will tripped off the connection. However, I do not
expect the PTC operated like this.

2) The PTC is operated as over current protector when lock rotor
happened. If i set a higher current tripped off PTC, the time for
tripped of in lock rotor protection will also increase. It is not a
good appearance for me.

So, what should I do to prevent PTC tripped off because of motor
internal temperature and also function in a short time tripped off for
lock rotor (over current) happened?

Please give me some comments on it and thanks very much.

If it fits in with your equipment, how about putting the PTC next to a
small fan blade on the motor shaft, with the PTC outside of the motor.
Now if the motor stalls the cooling airflow is lost and you get good
discrimination.

In terms of fan blade the very simplest would do: nothing more than a
flap of tape on the shaft would be sufficient if the tape was of good
enough adhesive characteristics.


Regards, NT