Electrical Engineering - Protection for a permanent magnet dc motor

Author

Protection for a permanent magnet dc motor

Palindrome

2007-01-20, 5:25 pm

Hi,

I'm after suggestions for protecting a DC permanent magnet motor.

The motor is used to drive a hydraulic swash plate pump for typically 30
min -1 hour at a time. Mostly the motor is on light load when running,
but it is intermittently loaded to higher than its continuous rating,
for 5 mins or so.

The failure mode is overheat of the rotor winding - but it isn't going
to be easy to sense the temperature of the winding directly.

I've been thinking of:

1) putting a small value resistor in series with it and bonding a
temperature sensor to that - tripping the supply if it gets too hot.

2) putting a temperature sensor in the airflow from the motor's cooling fan.

3) sensing the motor current and feeding that into an integrator and
setting a trip level on the integrator output.

Any thoughts? The motor only has a fuse* at the moment, plus a nose to
sense that it is "smelling a bit hot"..

* A fuse with the correct i2t curve would do nicely, perhaps. The
present one, fitted by the system manufacturer, is a quickblow one that
doesn't even blow if the motor stalls..
--
Sue

John G

2007-01-20, 5:25 pm

"Palindrome" <me9@privacy.net> wrote in message
news:eotst9$oea$1@aioe.org...
> Hi,
>
> I'm after suggestions for protecting a DC permanent magnet motor.
>
> The motor is used to drive a hydraulic swash plate pump for typically
> 30 min -1 hour at a time. Mostly the motor is on light load when
> running, but it is intermittently loaded to higher than its continuous
> rating, for 5 mins or so.
>
> The failure mode is overheat of the rotor winding - but it isn't going
> to be easy to sense the temperature of the winding directly.
>
> I've been thinking of:
>
> 1) putting a small value resistor in series with it and bonding a
> temperature sensor to that - tripping the supply if it gets too hot.
>
> 2) putting a temperature sensor in the airflow from the motor's
> cooling fan.
>
> 3) sensing the motor current and feeding that into an integrator and
> setting a trip level on the integrator output.
>
> Any thoughts? The motor only has a fuse* at the moment, plus a nose to
> sense that it is "smelling a bit hot"..
>
> * A fuse with the correct i2t curve would do nicely, perhaps. The
> present one, fitted by the system manufacturer, is a quickblow one
> that doesn't even blow if the motor stalls..
> --
> Sue

You did not tell us how big this motor is.

A properly rated magnetic/thermal breaker should do what you want I
thought.

A temperature sensor in the air flow will most likely fail to react if
the motor stalls.

Surely a quickblow fuse must be too big if it does not blow on startup
and a closer rated slowblo fuse would seem more appropriate.
--
John G

Andrew Gabriel

2007-01-20, 5:25 pm

In article <45b29396$0$20000$5a62ac22@per-qv1-newsreader-01.iinet.net.au>,
"John G" <Greentest@ozemail.com.au> writes:
> You did not tell us how big this motor is.
>
> A properly rated magnetic/thermal breaker should do what you want I
> thought.
>
> A temperature sensor in the air flow will most likely fail to react if
> the motor stalls.

A self-heating thermister can solve that -- no airflow and it will
heat up anyway.

> Surely a quickblow fuse must be too big if it does not blow on startup
> and a closer rated slowblo fuse would seem more appropriate.

--
Andrew Gabriel

Paul Hovnanian P.E.

2007-01-20, 8:25 pm

Palindrome wrote:
>
> Hi,
>
> I'm after suggestions for protecting a DC permanent magnet motor.
>
> The motor is used to drive a hydraulic swash plate pump for typically 30
> min -1 hour at a time. Mostly the motor is on light load when running,
> but it is intermittently loaded to higher than its continuous rating,
> for 5 mins or so.
>
> The failure mode is overheat of the rotor winding - but it isn't going
> to be easy to sense the temperature of the winding directly.
>
> I've been thinking of:
>
> 1) putting a small value resistor in series with it and bonding a
> temperature sensor to that - tripping the supply if it gets too hot.

This is the basic principle behind motor overload protection. The
resistor needs to be sized (both in terms of resistance, thermal mass
and dissipation) to mimic the motor's thermal time constant. The temp
sensor is usually as simple as a bimetal switch that opens at a
calculated temp. and de-energizes the control circuit.

> 2) putting a temperature sensor in the airflow from the motor's cooling fan.
>
> 3) sensing the motor current and feeding that into an integrator and
> setting a trip level on the integrator output.
>
> Any thoughts? The motor only has a fuse* at the moment, plus a nose to
> sense that it is "smelling a bit hot"..
>
> * A fuse with the correct i2t curve would do nicely, perhaps. The
> present one, fitted by the system manufacturer, is a quickblow one that
> doesn't even blow if the motor stalls..

Fuses and circuit breakers are usually sized to protect against faults
in the branch circuit or motor winding. There may be a slow-blow fuse
suitable to what you need, but that will require some knowledge of the
motor's maximum time-current characteristics.

--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Misery loves company, especially this one.

Bud--

2007-01-21, 3:25 am

Palindrome wrote:
> Hi,
>
> I'm after suggestions for protecting a DC permanent magnet motor.
>
> The motor is used to drive a hydraulic swash plate pump for typically 30
> min -1 hour at a time. Mostly the motor is on light load when running,
> but it is intermittently loaded to higher than its continuous rating,
> for 5 mins or so.
>
> The failure mode is overheat of the rotor winding - but it isn't going
> to be easy to sense the temperature of the winding directly.
>

In the US, AC motors typically use an overload unit. The mechanical
units have thermal heaters wired in series with the motor and sized to
the motor current. They cause a switch to open when they get hot enough.
I presume the UK has equivalent units which you have probably used. One
advantage is overload units are built to match the thermal
characteristics of a motor. Fuses, circuit breakers and series resistors
probably would not be a good match. Overload units also have many heater
sizes to match the motor current - more variety than fuses.

--
bud--

LinkBot