Reliance DC motor failure - thermostats to blame ?

[woodglass]

We have experienced a failure of a 30HP Reliance dc motor (15 years in operation).

The motor is fitted with an electromagnetic brake (fail safe – energise to hold brake off)

Both the field and armature have been destroyed beyond repair.

One of the cores supplying the brake coil had gone open circuit apparently causing the brake to be applied permanently while the motor was running.

The motor is fitted with thermostat switches (wires P1 & P2), connected to the dc drive, however these didn’t go open circuit & stop the drive.

A check confirmed that the switches hadn’t been manually bypassed (by shorting out anywhere in the control circuit).

Question 1 – Roughly, at what temperature do thermostat switches operate, & shouldn’t they have gone open circuit long before any thermal damage was done to the motor ?.

Question 2 – If the switches do go open circuit due to motor over temperature, when the motor cools down, do the switches close again ?.

Question 3 – Is it possible that these switches could become ‘stuck’ closed through years of inoperation, & therefore never open when required ?.

 

[rovineye]

1. The contacts are intended to open and shut down the motor when the motor is overloaded or overheating from loss of ventilation for instance. Usually a gradual heat rise. They may not help if a bearing wipes or as in your case a brake engages. That overcurrent may burn out the motor before the conatacts have a chance to open.

2. In normal abnormal conditions (overload) the contacts will close again when the motor has cooled off.

3. I guess it is possible but I have never seen it. Similar to bi-metali overlaod in a controller. No real current passing thru to weld the contatcs.

 

[oftenlost]

One major US manufacturer uses a temperature of 115C for the switching temperature in an effort to limit armature heating to below the permanent damage point. I see no real use for it in cases where the motor is locked down by the driven load or the brake.

 

[woodglass]

Thanks for your swift reply guys !.

oftenlost,

Am I correct in thinking that you mean using 'a temperature of 115C for the switching temperature in an effort to limit armature heating to below the permanent damage point' is of no use for motor protection in the situation 'where the motor is locked down by the driven load or the brake. ' ?

Regards,
woodglass

 

[LaffeyDD724]

Your 30 HP motor might be better served by the application of a zero speed switch on the motor shaft wired into the control circuit, this would cause the M coil to De-energize and open the power circuit in the event of a stalled armature. Temperature does not build rapid enough to serve as a safety feature for a stalled armature, the armature and fields can fry before the temperature would reach the trip point under that condition.

The resistance of a DC Motor Armature is usually less than one (1) ohm, you can do the math and see what the armature current would be with a stalled armature. A DC motor needs to begin it's rotation immediately to begin generating Counter Electromotive Force (CEMF) to counter the applied EMF, thereby reducing motor current as the motor speed increases.

From your posting I gathered that there was not a DC current magnetic overload in the power circuit. Perhaps that device should be added also and you may need a time delay configured to get the motor up to speed before it is activated. In any event the Zero Speed Switch and the Magnetic Overload or a combination of both may have saved the 30 HP motor.

 

[itsmoked]

Welcome to Eng-Tips LaffeyDD724.

I concur with either a current sense or rotation monitor as a 'better way'.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LaffeyDD724]

Woodglass,please find the pasted URL for a zero speed switch.

http://www.speedswitch.com/

Thanks, for the welcome Kieth. Tom