Commissioning of DC motors and Recording of Data
In order to ensure correct set up procedures are carried out the following record with accompanying information can help avoid some common mistakes!
Some common faults include:
# Fitting pulleys of too small a diameter on a given shaft size resulting in an excessive bending moment and ultimate shaft breakage. The smaller the pulley the greater the bending moment for a given torque! Check with the supplier when determining pulley sizes or calculate as per the manufacturers instructions!
# Not realizing that force ventilation units will blow air through the motor regardless of the fan motor direction but that only one direction is correct! If the direction of the fan motor is incorrect it will create very little air pressure and only force a very small volume of air through the motor and result in a motor burn out! Check that the direction is as indicated by the arrow on the fan unit and if there is no indication try running the fan motor in both directions as the correct direction will be obvious by the great difference in air volume.
# Having the wrong type of bearing on the drive end of the motor. A direct-coupled motor should ideally have a deep groove ball bearing fitted [especially at speeds greater than 2000rpm] whereas a belt driven machine should have a cylindrical roller bearing. If a ball bearing is suitably sized it will suffice for both applications but a roller operating without sufficient radial load, as can be the case with any direct-coupled application, will generate greater heat and noise and can lead to a premature life.
# Operating motors at current limit for prolonged periods, especially at reduced speeds, and not realizing the RMS or effective dc current will be much greater than that indicated by any instrument not designed for reading such large form factors [most panel meters and many hand held instruments] Again this can result in a motor burn out.
# Setting the current limit too low. The current limit of a DC motor drive can be set at up to 200% of rated motor current for the purpose of overcoming starting inertia, or torque, and yet it is often set at only 100% due to misconceptions about perceived protection. If the starting current is set too low for a particular application then the risk exists for a stall or prolonged start up. If a prolonged stall occurs due to start up conditions and is not detected due to the lower limit setting, then damage may be done to the commutator surface. Damage is often detected at a latter date after the surface condition deteriorates to the point of causing excessive sparking and brush wear. The physical manifestation of this damage is usually that of diametrically opposed flat spots (4 pole machines) or even a series of such. In conclusion, it is safer to set the drive current limit higher rather than too low, especially on extruder applications. The drive can then be programmed to limit the duration of the "over current" level to that specified by the motor manufacturer.
# Operating a motor with a supply form factor greater than that it was designed for. An example of this would be to operate a motor designed for a full controlled bridge rectifier on something less, such as a three phase half bridge, two phase full or half bridge, and not de-rating the motor for the increased copper losses that will occur!
# Failure to interlock the ventilation circuit with the main drive control circuit!
# Failure to connect or interlock the temperature detection devices!
# Operating with a blocked or incorrect air filter; The air flow can be drastically reduced by either of these events and only proper maintenance or an accurate air flow detection system will prevent this happening! Another problem sometimes encountered is where a filter is fitted where it was never designed to be and again resulting is severely reduced airflow. In this case a larger force ventilation unit would be required which could accommodate a filter without reducing the required airflow.
# Use of liquid sealants as gasket material on inspection covers; It is not only silicon sealants that have been known to cause problems for commutation on dc machines but in fact any vapor emitting product could cause problems. Use only proper gasket material if it is required or ensure any liquid sealants are fully cured before operating a machine.
The following commissioning record is recommended as a prompt and guide for those carrying out commissioning work and a completed copy of which could be sent to the supplier for perusal and possible comment.
Complete nameplate details:
Method of drive or coupling:
Diameter of pulley:
Diameter of shaft:
Minimum diameter of pulley required for given shaft diameter:
Type of bearing fitted to drive end of motor:
Mounting position of motor if other than horizontal:
Has vibration analysis been performed and recorded?
Has the direction of fan been proved correct?
Is the air filter in place, clean and of the correct material?
Have any liquid sealants been used on the motor or inspection covers?
Has the motor been connected as per the manufacturers specifications?
Is the tachometer polarity correct?
[If not the motor will ramp to full speed]
Has the field current or voltage been set to specifications?
Method of adjustment:
Has the output wave-form from the drive been checked and proved?
Was it performed under load?
What is the actual or normal load current of the motor?
What is the thermal overload set to?
What is the current limit set to?
What is the time duration of CL set to?
Is the motor de-brushed, or does it need to be, for reduced load?
What is the actual or normal speed or armature voltage?
What is the commutation level according to the Westinghouse scale?
[To be observed at the stated load current]
Are the temperature detectors wired into the control circuit?
Is the fan control circuit interlocked with the main drive?
Are there any other protection facilities provided?
If so, have they been incorporated in the protection circuit?