D.c. Motor Question

I am no d.c. drive expert and hope someone out there has some experience with them. Our customer shut off the blower motor to a 250 HP motor. We do not know how long it was turned off. Now the motor is in the shop to get the field wires repaired. Has anyone experienced something like this before? What damage is common when the cooling sytem is removed? Thanks!

Road_Guy said:

I am no d.c. drive expert and hope someone out there has some experience with them. Our customer shut off the blower motor to a 250 HP motor. We do not know how long it was turned off. Now the motor is in the shop to get the field wires repaired. Has anyone experienced something like this before? What damage is common when the cooling sytem is removed? Thanks!

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Just what you experienced, the motor can not cool so burns up. I prefer when the motor and fan motor are interlocked in some manner, at least an alarm if the motor is running and the fan is not. It surprises me the 250HP motor did not have some form of Thermal overload, that must have been bypassed.

The same applies when an AC motor may be used with an inverter and running at slower speeds i.e. fan should be powered separately to run full speed at all times, if off the motor will overheat and fail.

How long, in many cases it may not take long at all, maybe minutes.

Thanks for the information. We are currently trying to figure out why the thermal protection did not work. From what I am hearing the wires are present however it may be jumpered out in the drive somewhere. The blame game has begun....

As Ron mentioned, they burn up. Usually the insulation melts in the field windings before any damage is done to the armature (in my experience).

Any motor with a blower should be interlocked with either the drive or associated control circuit. A motor of that size will almost certainly have either temperature switches or thermistors embedded in the winding of the motor. These should be used to shut the drive off in case of overtemperature.

That's going to be an expensive one.

Road_Guy said:

Thanks for the information. We are currently trying to figure out why the thermal protection did not work. From what I am hearing the wires are present however it may be jumpered out in the drive somewhere. The blame game has begun....

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Those thermal devices sometimes do fail. Usually they fail in the open position ie the motor isn't hot but it won't run.

If I were a betting man I would guess that at some point the thermal failed and the Maintenance guy decided the motor wasn't hot and jumpered it out "temporarily" and then never reported it to anyone (or did report it and no one listened). Now someone comes along and shuts the blower off and......

allscott said:

Those thermal devices sometimes do fail. Usually they fail in the open position ie the motor isn't hot but it won't run.

If I were a betting man I would guess that at some point the thermal failed and the Maintenance guy decided the motor wasn't hot and jumpered it out "temporarily" and then never reported it to anyone (or did report it and no one listened). Now someone comes along and shuts the blower off and......

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I am a betting man and would not take that bet, even if you find that the thermal is good it could be a situation where it would not start and got bypassed and never put back after the real problem was found.

A little backround on the machine. It has been running
hard approximately 9 months now. So I figure if the
drive was defective or programmed wrong it should have
shown up before this.
Their maintenance man told the motor repairer that
the blower motor had "fallen apart" some time ago
and was never fixed. Thus the circuit breaker being
turned off.
There was no interlock provided going to the PLC
to indicate the circuit breaker was off. That leads
us to the thermal protection which was discussed.
You guys have some good theories.
Now the detective work begins but we will probably
never find out what really happened. The customer is
implicating we screwed up and we are blaming them.

If the blower had "fallen apart" and never got replaced then whoever was responsible for not repairing/replacing it is at fault, regarldess of when it happened. It depends on circumstances, a motor could burn up quickly or last for quite a while, what is inevitable is that without the fan it will eventually burn up.

I can not believe anyone would purposely run a 250HP motor without its fan. I have run some up to about 50HP but we would place a couple of those 36" or larger fans on it and the room was air conditioned. Naturally this was done just long enough to replace the blower. 250HP doubt I would even think about it.

I have no idea how important a customer, sometimes you have to bite the bullet and take some loss. I have made concessions and split costs but then made sure everything was properly connected, interlocked, and documented.

If your customers maintenance man didn't know enough to shut down the motor when the blower failed, he just might have been smart enough to bypass any additional interlocks you might have installed anyway.

Most machines that I see with blower motors use a breaker with an auxillary contact and a motor starter and overload also with an auxillary contact. These contacts are wired in series to a PLC input and shut the motor down, possibly time delayed to flash an alarm and set up for an organized shutdown.

If the blowers overloads had tripped because the fan was plugged or whatever and there was no indication or nothing to shut the motor down then I would have considered that a deficiency on the machine designers part.

I would consider this situation to be negligence on your customers part. However sometimes the customer is always right (even when they are dead wrong).

Had this happened at one of my plants I would be to embarrased to call the OEM and tell them what happened let alone expect them to pay for it.

If your afraid the customer might defeat the interlocks, wouldn't it be a good idea to use some sort of interlock that transitioned during operation, and have the PLC look for the transition. If the plc didn't see the transition then you would shut down. I have seen burner controls that used this feature. For this application a current sensing relay on the blower motor sounds like a good plan. If the contacts were "jumped" then no transition = no permission to run the DC motor.

Pointless. Any interlock can be defeated. And what about where the blower never shuts off? or where air is sourced from another place? We have some DC machines that have their force-ventilation come from our HVAC system.

Best bet, would be to put either additional winding thermals in (motor shop can do that), or put in a thermocouple/RTD sensor.
The simple fact is, if the thermal was defective, it's probably the motor-manufacturers problem.. If it was bypassed, due to a knowingly defective blower, it's the end-user's problem. For that matter, if the end-user ran the motor at all, knowing there was a defective blower, it's their problem. That, or the loss of production would cost more than a replacement motor...Very poor thinking, unless they have a spare on the shelf.

milldrone said:

If your afraid the customer might defeat the interlocks, wouldn't it be a good idea to use some sort of interlock that transitioned during operation, and have the PLC look for the transition. If the plc didn't see the transition then you would shut down. I have seen burner controls that used this feature. For this application a current sensing relay on the blower motor sounds like a good plan. If the contacts were "jumped" then no transition = no permission to run the DC motor.

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rdrast,


I'm not an expert in either thermocouples or DC motors. But wouldn't the high currents in a 250 hp DC motor cause problems with a thermocouple? I have seen and used several AC motors with thermistors. The thermistors have given reliable performance, when used with the proper controler, for me in the past.

For me, the most likely thing here is that the drive/motor was installed without the P1 and P2 leads being connected to the drive. Good chance the motor has them, they are in good working order, but not being used.

The craftly startup tech decided its too much work to program the drive for an external fault loop so, just set up the overload calculation in the drive software and go with that.

Remarkable what a difference there is between measured and estimated heat, isn't there!!

With the P leads connected, and with no other interlocks in the blower circuit, and with no common sense on the part of the person who left the blower for dead, that motor would be in running condition today.

Bottom line----if you've got P leads in the motor, use them ALL THE TIME, EVERY TIME!!! In addition to getting better protection for the motor, you will get between 5 and 10% more output from the motor before faulting.

For me, the most likely thing here is that the drive/motor was installed without the P1 and P2 leads being connected to the drive. Good chance the motor has them, they are in good working order, but not being used.

The craftly startup tech decided its too much work to program the drive for an external fault loop so, just set up the overload calculation in the drive software and go with that.

Remarkable what a difference there is between measured and estimated heat, isn't there!!

With the P leads connected, and with no other interlocks in the blower circuit, and with no common sense on the part of the person who left the blower for dead, that motor would be in running condition today.

Bottom line----if you've got P leads in the motor, use them ALL THE TIME, EVERY TIME!!! In addition to getting better protection for the motor, you will get between 5 and 10% more output from the motor before faulting.