Chuck Yung 
EASA Technical Support Specialist 

When a customer calls and orders a motor, he usually specifies only the Hp/kW rating, rpm, frame, enclosure and voltage rating. That leaves at least one critical area where the elec­trician can go wrong: The starting method and number of leads. 

As manufacturers produce more motors with 12 leads to increase versatility and reduce inventory requirements, there is an increased chance of error by the end user. When considering a 20 Hp (15kW) motor with 12 leads, it is usually safe to assume a dual-voltage rating such as 230/460v with an external connection such as a 1 & 2-circuit wye-delta. 

The motor is suitable for wye­delta starting at either voltage rating. It can be connected for the desired voltage and used for direct on line starting or with a VFD. The motor can be connected for part-winding starting (PWS) on the low voltage only. Some manufacturers might also label the motor for other unique volt­ages, to maintain the same magnetic flux densities. See Table 1. 

Consider voltage-connection factor 
The problem occurs when a customer receives a motor with 12 leads rated for a single voltage. For example, a customer purchases a 400 Hp (300kW) 2-pole motor, and it arrives with 12 leads. Intuitively, we know it cannot be a 230/460v dual voltage motor – turns/coil are in­versely proportional to the Hp rating and proportional to the voltage rating. (In other words, the higher the Hp/kW rating, the fewer turns/coil. So motors with higher power ratings and lower voltage ratings tend to have as many circuits as poles.) 

Inspection of the nameplate will confirm that the motor is rated only for a single voltage, most often 460 volts. The manufacturer undoubt­edly installed a prominent nameplate describing the correct connection methods. Nonetheless, some electri­cians assume from the number of leads that this motor is indeed sutiable for for 230/460v use. Connected using the high-voltage connection, the no-load current is unusually low– but that does not always deter the electrician from connecting the motor to the load.

Watch for overloads 
The result is predictable: the electrician has erroneously connected a 460v motor for 920v (by halving the number of circuits, the motor would require twice the applied voltage to produce the correct magnetic flux den­sities and therefore torque). See Table 2. Coupled to a robust load, the motor cannot produce the required torque, so the speed decreases. The additional 
slip boosts the output torque, as the electric motor attempts to provide the required torque. With variable torque loads such as fans or pumps, the de­creased speed (higher slip) means less load, so the motor produces quite a bit of the torque asked of it. The current is less than the nameplate rated amper­age, so the customer does not realize the motor is over-loaded. 

Here is where it gets interesting. Although the motor is drawing less than the full load amps (FLA) listed on the nameplate, the winding is connected for only half the correct number of circuits.  So the winding temperature increases drastically; the higher the motor current is above half of FLA, the shorter the life of the motor. 

Other important questions
This is an area that really empha­sizes the importance of asking the following questions: 

• How long was the motor in service? 
• What is the starting method? (never assume from the lead markings) 
• What is the applied voltage?

If we know that the original motor was used on 460v, with a PWS, then a “standard” 12-lead dual voltage motor is not the correct replacement. If the new motor failed after mere hours or days, we really need to focus on the customer’s starting method. Ask the customer to send a copy of the ma­chine schematic; that is the best way to determine the rest of the story. 

Finally, the correct time to ask these questions is before – not after – ordering the motor. It is far better to take the time up front to make sure you are ordering the correct motor, rather than trying to correct a mistake after the fact. (“Err in haste, repent at leisure.”) 

What if your customer bought the motor elsewhere and wants you to save them from their own mistake? In some cases, a PWS is used because it offers the low-cost option of the vari­ous “reduced torque" starting methods.

Another option    
If the customer just spent a considerable sum of money for a 12-lead motor and you discovered that it is not suitable for PWS, the economical solu­tion might be to install an auxiliary contactor and convert the starting method to wye-delta. That option may cost less – and be faster – than reconnecting or rewinding the motor.