Meeting article 430.32 thermal overload protection

[sselman]

I am generally low power intrinsic safety focused and am not confident in applying NEC to motors.
We currently have a system for Class 1 Div. 1 hazardous area applications. The unit has Qty. 2 - 1/2HP Class 1 Div. 1 approved single phase motors, continuous duty automatically starting. They are NOT marked thermally protected.

Each motor has a thermal switch mounted internally to the winding to be used in conjunction with a motor controller or contractor control circuit.
UL508 HP rated solid state motor controllers are used to control each motor.

Can I wire the motors thermal switch in series with the 24VDC control circuit on the motor controller input and satisfy 430.32 (B) (2)?

 

[jap]

I would think a motors thermal winding switch such as a P1/P2 N/C auxiliary contact would have a different opening characteristic than that of an overload relay sensing the heat generated by the current through the input conductors, but, I may be wrong.

JAP>

 

[sselman]

Thank you for the reply. I would agree with the statement about opening characteristics which have to be determined by FLA etc.
To be more specific, I looking for opinions or confirmation whether the requirement below is met by using the thermal switch integral with and approved for use with the motors in this application connected as described.
430.32
(2) Thermal protector. A thermal protector integral with the motor, approved for use with the motor it protects……Where the motor current interrupting device is separate from the motor and its control circuit is operated by a protective device integral with the motor it shall be arrange so that the opening of the control circuit results in interruption of the current to the motor.

 

[petersonra]

I would think a motors thermal winding switch such as a P1/P2 N/C auxiliary contact would have a different opening characteristic than that of an overload relay sensing the heat generated by the current through the input conductors, but, I may be wrong.

JAP>

A standard thermal overload does not "sense" anything. It essentially does an I^2T calculation to estimate what the temperature is in the motor windings.

The switch that is in the windings actually does sense the temperature there so it is a much better guide to the temperature there than any thermal overload relay.

 

[jap]

I believe the above in bold is simply one of the options you could use should you decide to wire the control circuit through the integral thermal switch in the moor to an external current interrupting device such as a contactor, starter or drive.

There are some applications where a motor may not have an external current interrupting device such as a contactor, starter or drive, and, it uses the integral thermally controlled switch itself to remove the line voltage and shut the motor down in case of an overload.

JAP>

 

[jap]

A standard thermal overload does not "sense" anything. It essentially does an I^2T calculation to estimate what the temperature is in the motor windings.

The switch that is in the windings actually does sense the temperature there so it is a much better guide to the temperature there than any thermal overload relay.

I was referring to a thermal overload on a motor starter that is in series with the motor leads.

JAP>

 

[petersonra]

I was referring to a thermal overload on a motor starter that is in series with the motor leads.

JAP>

The typical thermal overload used with a motor starter senses nothing. It just makes an estimate of the motor winding temperature from an I^2T calculation.

 

[jap]

The typical thermal overload used with a motor starter senses nothing. It just makes an estimate of the motor winding temperature from an I^2T calculation.

If it at some point that thermal overload doesn't trigger the opening of a contact to drop the starter out and shut the motor down then all of those calculations are useless.

JAP>

 

[jap]

We could go back to the old style starter with what were called "heaters" back in my day where the heat from the current of an overloaded motor passing through them actually melted the solder which allowed the knurled pot to turn in the now liquified solder that opened a mechanical normally closed switch that the control power was series through to the coil dropping the power to the coil on the starter and shutting the motor down.

Is the old style setup not considered a thermal overload scenario also?

JAP>

 

[petersonra]

We could go back to the old style starter with what were called "heaters" back in my day where the heat from the current of an overloaded motor passing through them actually melted the solder which allowed the knurled pot to turn in the now liquified solder that opened a mechanical normally closed switch that the control power was series through to the coil dropping the power to the coil on the starter and shutting the motor down.

Is the old style setup not considered a thermal overload scenario also?

JAP>

Essentially ALL overload devices commonly used on motor starters just estimate the motor winding temperature via an I^2T calculation. Old style ones do it with an actual heater like an actual thermal magnetic CB does. Newer ones do the calculation electronically like an electronic trip CB does. But it is just an estimate of the motor winding temperature in either case.

 

[jap]

Essentially ALL overload devices commonly used on motor starters just estimate the motor winding temperature via an I^2T calculation. Old style ones do it with an actual heater like an actual thermal magnetic CB does. Newer ones do the calculation electronically like an electronic trip CB does. But it is just an estimate of the motor winding temperature in either case.

What's the point we're trying to get at here?

JAP>

 

[sselman]

Thanks guys,
I am trying to find an alternate solution to an external overload protection device with the set of parts currently in our system.

Our customers are responsible for meeting the overload protection requirement. The majority complain they do not want to supply and/or size the overload device and bear the costs and wiring from the safe area.
Similar thermally protected Ex motors not requiring external thermal overload are considerably more expensive.

The motors currently used have the winding thermostat present for this purpose but are not being used.

I would like to use the winding thermostats in the motors to open the DC control circuit into the motor controllers when an overload occurs therefore shutting off the motor. Would doing so satisfy option (2) in bold above.

 

[jap]

I'm confused.

Do the UL508 HP rated solid state motor controllers you are using incorporate any overload protection devices ?

JAP>