Protection for a permanent magnet dc motor

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

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

--
Andrew Gabriel

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Palindrome wrote:

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.

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: snipped-for-privacy@Hovnanian.com
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Palindrome wrote:

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--
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