Tom Bishop, P.E.
EASA Senior Technical Support Specialist
When an AC motor is initially energized at rated voltage (and frequency), the current drawn is many times rated current until the motor attains full operating speed. Consequences of starting current include short time overheating of stator windings and rotors, undesirable operation of overload protective devices and a sag in the supply voltage.
According to both NEMA MG 1 and IEC 60034-1, the locked rotor current is the steady state current for a locked rotor condition. However, per MG 1 clause 12.36, at the moment of energization, there is a one-half cycle instantaneous peak value which may range from 1.8 to 2.8 times the steady state current as a function of the motor design and switching angle. A significant factor in the amplitude of this instantaneous peak value is the residual magnetism in the stator and rotor cores and the instantaneous angles of the applied phase voltages. Although the measurement rate was not fast enough to capture the first half-cycle peak, Figure 1 illustrates the very brief time interval when the starting current is transient before it reaches steady state. This can be clearly seen at the 95ms point, where the amplitudes of the currents in each phase are different because they are out of phase with each other.
A term that should either be avoided or defined is “ inrush current.” It is not defined in the NEMA or IEC motor standards. Thus, in some cases, it may mean the steady state locked rotor current; in other cases, it could be the instantaneous peak value of starting current or something else. Regarding the steady state locked rotor current, NEMA MG 1 clause 10.37.2 assigns letter designations for locked-rotor kVA per horsepower as measured at full voltage and rated frequency as shown in Table 1.
Differences in Starting Current
To better appreciate the differences in the two types of starting current, we will evaluate a sample NEMA frame motor rated 25 hp at 460 volts with a locked rotor kVA code G. Incidentally, the most common code letters for motors rated 10 hp (7.5 kW) and greater are F and G. The formula for locked rotor current for an AC motor shown below. Note that the motor-rated current is not used in the formula.
Thus, guidelines such as “locked rotor current is 5 to 8 times full load current” are estimates at best; based on the formula above, they cannot be relied on for accuracy. A better approach to estimate locked rotor current is to use the formula above whenever possible and use guideline range(s) when the kVA code letter is not present on the motor nameplate. Suppose the motor is in the service center. In that case, the locked rotor current can be determined by following the test procedure described in the “Need Locked-Rotor Current Only?” section of the February 2016 article in Currents titled “Working with Motor Locked-Rotor Test Data.”
The calculated locked rotor current for the 25 hp motor is between 176 and 198 amps. To determine the potential instantaneous peak value of starting current, we multiply the locked rotor amps by 1.8 to 2.8. The result is a range from 317 amps (1.8 x 176) to 554 amps (2.8 x 198).
Types of Protective Devices
Motor protective devices are either fuses or circuit breakers, with fuses being non-time delay or time-delay type and circuit breakers being instantaneous trip or inverse time type. How each of these devices functions can help explain why a protective device interrupts the motor circuit at startup.
Non-time delay fuses have a very high speed of response to overcurrent conditions, thus providing very effective short circuit component protection. However, harmless short-time overloads or transient surge currents may cause nuisance interruptions unless these fuses are oversized. They are best suited for circuits not subject to heavy transient surge currents and the temporary overload of circuits with inductive loads such as motors or transformers. For AC motor loads, a non-time delay fuse may need to be sized at 300 percent of AC motor rated current to withstand the starting current. A better choice for a motor application is a time-delay fuse.
Time-delay fuses can be sized closer to equipment-rated current to provide very effective short-circuit protection and reliable overload protection in circuits subject to temporary overloads and transient surge currents. Depending on fuse type, for AC motor loads, a time-delay fuse can be sized at 125 to 175 percent of an AC motor-rated current to withstand the starting current. Compared to non-time delay fuses, the lower current ratings can provide better short-circuit protection with less let-through energy and a potential reduction in arc-flash hazard.
Types of Circuit Breakers
Instantaneous trip circuit breakers trip immediately when circuit current attains the level for which the breaker trip mechanism is set. These breakers are magnetic trip only and are also referred to as motor circuit protector breakers (MCPs). The magnetic circuit of the breaker consists of an iron core with a coil wound around it, creating an electromagnet. Load current passes through the electromagnet coil so it can trip in the event of a short-circuit. For AC motor loads, an instantaneous trip circuit breaker may need to be sized at 800 percent of AC motor-rated current (up to 1700 percent for Design B energy efficient motors) to withstand the starting current.
Inverse time circuit breakers have a time delay mechanism that allows the breaker to delay the tripping function. The amount of time delay decreases as the current increases. Inverse time breakers are thermal-magnetic. Thermal-magnetic circuit breakers have two separate switching mechanisms; a bimetallic switch and an electromagnet. Current exceeding the breaker overload rating heats the bimetal enough to bend it toward a trip bar. The time needed for the bimetal to bend and trip the circuit varies inversely with the current. The magnetic portion of the breaker functions in the same manner as for an instantaneous trip circuit breaker. For AC motor loads, an inverse time circuit breaker may need to be sized at 300 percent of AC motor rated current to withstand the starting current.
Another type of protective device is a motor protection circuit breaker with an electronic trip unit. These breakers can be configured with Long delay, Short delay, Instantaneous trip and Ground trip (L,S,I,G). These features give the end user the ability to customize trip settings for the application. Additional benefits include reducing incident energy ratings for potential arc flash events and better overall protection for the motor if properly set.
AVAILABLE IN SPANISH
Print