Working knowledge by Terry MacKenzie Hoy
I once worked at a plant where the system voltage was 525 volts. The calculation was, if the motor is rated 500 V then if the busbar voltage is 525 volts, one loses 25 volts as they drop in the cable.
However, in common usage in the plant, the motors were referred to as ‘525-V motors’ and not ‘500-V motors’. There were also a number of motors on the plant where the busbar voltage was 400 V and operated at 380 V. These were routinely referred to as ‘400-V motors’. I am writing this to try and clarify matters, especially when you see written on the label of a motor ‘400 V / 380 V 3 ph’ and you think is not the same as a different motor with a label which reads ‘400 V 3 ph’. They are in fact the same motor. There is a wide tolerance for the voltage range over which a motor will continue to operate satisfactorily-in general, motors can operate within +/- 5% of their nominal rating (that's the 400 V or busbar voltage rating). A 400-V motor will operate at between 380 volts and 420 volts without any harm. But beware! This applies to motors that are made to a SABS specification or an IEC specification. Almost all the time, the standard three phase motor will be a TEFC motor (stands for Totally Enclosed Fan Cooled) and will have two, four, six or eight poles. For motors which run on 50 Hz, this means the motors no load speed will be 3000 rpm, 1500 rpm, 1000 rpm or 750 rpm. The full load speed is less than this-2850 rpm, 1425 rpm, 950 rpm and 712 rpm. The speed chosen by the designer depends on the type of load-most fan suppliers like to run fans at about 2850 rpm since the fan diameter is small. Most pump suppliers prefer 1425 rpm motors. Motors that are rated at 60 Hz will run 20% faster than 50 Hz motors. On ships, the power supply is rarely 50 Hz and is normally a 60 Hz voltage supply. The question is, can a 50 Hz motor run on a 60 Hz supply? The answer is generally yes, a 50 Hz motor can run on a 60 Hz supply, but not the other way around because the 60 Hz motor on a 50 Hz supply is running 20% slower and getting 20% less fan cooling ... one thinks. However, the fan air flow rate does not fall off linearly, a fan running at 20% less speed than design is in fact giving nearly 30% less motor cooling.
When looking at motors, when reading the fine print on the label, be cautious of any motor that is not rated S1 (this rating is on the label). Motors rated S3 or S4 are only rated to give full power for intermittent duty. One only finds this on imported equipment. Given the operating conditions in this country, one should never accept any motor which is not rated at S1. On the motor label, one also sees the ‘insulation class’ which can be class B, class F or class H. All these are based on a maximum ambient temperature of 40°C and a permissible temperature rise for B of 80 degrees, F of 105 degrees and H of 125 degrees. In by-gone days, class H and F motors were more expensive than class B and thus rarely specified. These days, they are not expensive and class F is most commonly specified. A final tip - if you have to replace the motor, make sure you know the frame size. This is a number which looks something along the lines of, ‘Frame 90’ or F90 or F112 or similar. All manufacturers work to the same standard frame size. But, if you are not certain, well just take a cell phone photo of the label and SMS it to the supplier. WhatsApp works too. Oh so easy these days...
