Sometimes the hardest part is getting started. Choose the best motor starting method for a particular application by keeping a few guidelines in mind. For example, while across-the-line starting is simple and inexpensive and provides maximum starting torque and acceleration, it has two important limitations: starting current is high and some loads cannot withstand the high starting torque. Various types of reduced-voltage starting are used, mainly on motors 25 hp or larger, to overcome these limitations.
During across-the-line starting, motor input current is five to eight times normal full-load current. This can cause an excessive temporary voltage drop on power lines which, in turn, causes lights to flicker or may even interrupt service. To control these temporary voltage drops, power companies have restrictions such as:
• A specified maximum starting current (KVA)
• A specified limit on KVA/hp
• A maximum hp motor size which can be started across-the-line
• A specified maximum line current that can be drawn in steps (increment starting)
The specified restrictions vary considerably between power companies and even within one company’s service area. It is wise to check local power company restrictions before making a large motor installation.
Reduced-voltage starters operate such that input current and, consequently, torque are reduced during starting. Partwinding and wye-delta starting, frequently referred to as “reduced-voltage starting,” are described in more detail in the following sections.
With increment starting, a special case of reduced-voltage starting, the motor does not accelerate to its normal rated speed on the first step. The term refers primarily to power company requirements specifying the maximum line current that can be drawn in steps. When these requirements are followed, power system corrective devices compensate for the effects of increased line current. Basically, any type of motor starting in which the motor does not accelerate the load to the rated speed on the first step may be termed an increment starting system.
Part-winding starting employs a special two or three-step starter to connect the motor to the line. The two-step starter is more common. Its first-step contactor connects a portion of the winding to the line. Then, after a short, preset time interval (usually 4 sec max), the second-step contactor closes to connect the full winding to the line.
A three-step starter requires an additional step. Step 1 connects the first group of windings plus a resistance in series with them. Then, Step 2 removes this resistance. Finally, Step 3 connects the full windings across the line. The expense of a three-step starter limits its use except where starting currents must be exceptionally low.
The starting torque developed by part winding is considerably less than that developed by full winding and is usually less than the rated load torque. Therefore, part winding will not start a motor requiring rated load torque during start.
Because the torque is lower, partwinding starts give slower acceleration than across-the-line starts. This is an important advantage when the driven machine must be protected from the shock of a high starting torque.
Ideally, part-winding starts should reduce inrush current during the entire starting cycle. To do this, the motor must accelerate to almost its normal rated load speed on the first step. For this to happen, the torque demanded by the load must always be less than the torque produced with only the part winding connected. This occurs when:
• The motor is started with no load.Then, after it has accelerated to normal speed and the full winding is energized, the load is applied.
• The load builds up gradually as the motor accelerates.
The most common of these applications are fans, centrifugal blowers and pumps, and reciprocating pumps and compressors equipped with unloaders to relieve the starting torque requirements.
In cases where load torque exceeds the available torque provided by the part winding at any speed during starting, the motor remains at that speed until after the preset time delay has elapsed and the second contactor connects the remaining windings across the line. At that time the motor develops full winding torque and draws full winding current required at that speed. The slower the speed, the greater the current surge when the second contactor closes.
Thus, when a high torque load causes the motor to run below rated speed, the advantage of low starting current is partially compromised by the second step. Such a starting cycle may hold starting current within increment starting limits on many applications, but operation below rated speed may cause overheating of the partial winding. To avoid excessive overheating at below rated speed, a shortened time delay (from the usual 4 sec down to 1 or 2 sec) may be needed to more quickly engage the second contactor and connect the full motor winding across the line.
The most common part-winding system is the “1/2 winding” system. It uses a starter with two three-pole contactors which connect one half the windings to the line in the first step.
Not all motors have stator windings suitable for part-winding starting. Always check the motor manufacturer’s recommendations before trying to connect a motor for part-winding starting.
Wye-delta starting, or star-delta starting, takes advantage of the fact that a motor wound for normal delta connected operation draws less current when it is wye connected. In this system, Step 1 makes a wye connection and Step 2 makes a delta connection. Because it is necessary for contactors in Step 1 to open before those in Step 2 close, the system has an open transition between the two steps. It is possible to obtain a closed transition by using resistors.
Because the starting torque is below that available with part-winding start, it is particularly important that load torque be low at the start and build up quite gradually or, better still, that the load be applied only after the motor has reached rated speed and connected for delta operation.
It is possible that the load torque could be greater than the available motor torque. If this occurs, the time delay must be reduced so that the second step connects the motor across the line (delta connection) so full available torque accelerates the load to rated speed without further delay.
If the load torque does not exceed the motor torque during the starting period, the motor should be able to accelerate to rated load speed on the first step (wye connection). Under these conditions the inrush current is reduced (1/3 full locked-rotor current) during the entire starting period, and, because of this, the starting time could be considerably longer than with a part-winding start.
Wye-delta starters require three threepole contactors: one used only during starting; one used only during running; and one for both starting and running. Contactor control systems, including time delay for transfer and thermal protection, are established by the starter package supplier.
Autotransformer starting uses auto transformers to reduce the line voltage during the starting period. The auto transformer voltage taps provide three starting currents and torques for different load requirements. In this system, the first step is connected to one of the reduced-voltage taps and results in a reduced starting current and torque. After a preset time delay, the second step applies full rated voltage to the motor for normal operation.
Depending on the voltage tap selection, there is a choice of three starting torque curves: 25, 42, or 64% of full locked-rotor torque. Thus, in many cases, it is possible to choose a torque curve that will exceed the load torque curve and let the motor accelerate to full load speed on the first starter step. This is particularly useful with hard-to-start loads.
The load torque may be greater than the available torque of the motor. If so, the time delay must be reduced so the second step connects the motor acrossthe- line. This lets full available motor torque accelerate the load to rated speed without further delay.
If the load torque does not exceed the motor torque during the starting period, the motor should be able to accelerate to rated load speed on the first step (reduced voltage tap). Under these conditions, the inrush is reduced (25, 42, or 64% of full locked-rotor current) during the entire starting period and, because of this, the starting time could be considerably longer than on part-winding start.
Overload relays and trip time
For proper motor protection the National Electrical Code specifies that there should be an overload relay in each line of the three-phase power supply. Overload relays should be sized per the instructions of the starter manufacturer depending on the type of starting – across-the-line or reduced voltage.
In general, the sizing of overload relays is based on a percentage of motor nameplate rated load current depending on the type of starter. Under normal conditions, overload relays will provide protection between 110 and 120% of their current rating. No extra allowance for service factor is necessary.
On across-the-line starting, the trip time for properly sized overload relays should be approximately 15 seconds under locked-rotor conditions. If lockedrotor time is longer than 15 seconds, the overload relays should disconnect the motor from the line to prevent motor stator overload burn out.
Oversized relays will not eliminate excessive tripping. Cutting excessive voltage drop, reducing starting time, and properly sizing the motor are the proper solutions.
Joe Hanna is a product engineer with Lincoln Motors, Cleveland.