Motor control maintenance and troubleshooting best practices, advice

Motor and drive insights

• Electric motors drive actuators and machines in manufacturing and other industries and keeping them constantly operating is critical.
• Variable frequency drives (VFDs) are among the most common drives and are used in many different settings because of their versatility.

Electric motors are used to drive actuators and machines in manufacturing and other industries. Most motors used in industrial applications are ac, which means they operate off of the frequency of the power supplied. In the US this is 60Hz, while in Europe it is 50Hz.

Starters and overloads

Motor starters are used to start and run ac motors. Smaller motors may only use a single phase of electrical voltage, but three-phase motors are most common in industrial applications, usually at 240 or 480 Vac in the US. Starting a motor “across the line” will run the motor at a speed related to the applied frequency times the number of poles in the rotor, minus a “slip.” In the US most motors run at about 1750 RPM.

A motor starter is composed of a contactor with a coil and an overload device for circuit protection. The direction of the motor’s rotation is determined by the order of the applied voltage phases for a three-phase motor; reversing any two of the phases will change the direction.

Figure 1: Example of a reversing motor starter. Courtesy: Automation LLC

Figure 1 shows the wiring of a reversing motor starter with an overload. The coils are both wired through the overload’s NC contact, which will de- energize the contactors if the overload is tripped. Both coils are also wired through the opposite coil’s NC contact, preventing both coils from being energized simultaneously. The contactors are mechanically interlocked to prevent them from energizing at the same time. Pushbuttons are shown as a means of actuating the starter, but outputs from a programmable logic controller (PLC) or other controller can also be used. In this case, logic would be used to prevent the two outputs from energizing at the same time.

The disconnect is also often fused, or a circuit breaker is used for branch circuit protection. Overloads are often bimetallic strips that uses a heating element to mimic the temperature of the motor based on current. They are made with different adjustable ranges that are chosen based on the full-load current of the motor and the required time delay.

Figure 2 shows a three-phase motor starter. The red button at the bottom is used to reset the overload.

Figure 2: Example of an IEC motor starter. Courtesy: Automation LLC

Three-wire control circuits are used in motor starter pushbutton enclosures to energize the coil of the starter. The neutral side of the coil is often wired through the NC overload contacts as shown. If both pushbuttons are momentary, the start button must be pressed again after a stop to re-energize the coil. The NO contact labeled “Motor” is called the hold-in contact.

Figure 3: Example of a three-wire “hold-in’ circuit. Courtesy: Automation LLC

Soft starters

A soft starter is a device that can be used to control the starting and stopping profile of an ac motor. The traditional method of applying full voltage “across the line” creates a high inrush current, often 6 to 7 times the motor’s rated current. A soft starter temporarily reduces the electric current and torque during start-up, reducing mechanical stress and the effects of high current on the wiring. Soft starters are solid-state devices, but clutches, fluid couplings and autotransformers also are sometimes used.

Figure 4: Example of a Weg-brand soft starter. Courtesy: Automation LLC

The electrical circuit for a soft starter is shown in Figure 5. Silicon controlled rectifiers (SCRs), also known as thyristors, are solid state current-controlling devices. The SCRs are engaged during ramp up, and the bypass or run contactor is pulled in after maximum speed is achieved. This helps significantly reduce motor heating.

Soft starters are simpler devices than variable frequency drives, requiring fewer adjustments. In pump applications, soft starters can be used to reduce pressure surges, also called “water hammering”. They also take up less space and are less expensive than variable frequency drives.

Figure 5: Example of a soft starter circuit. Courtesy: Automation LLC

Variable frequency drive control

A variable frequency drive (VFD), controls the speed of a motor by varying the applied frequency (and voltage). In industrial automation, these are sometimes known as “drives”.

VFDs are used for ac and dc motors and ac drives can be used with single or three phase induction motors. An ac drive operates by first converting the applied ac voltage into dc, then using solid state electronic circuits to rebuild a sinusoidal waveform at the desired frequency to apply to the motor. This is called pulse width modulation (PWM), where the average power delivered to a motor is controlled by chopping up the waveform into discrete parts or pulses of varying durations as shown in Figure 6.

Figure 6: Example of a pulse width modulation for an ac VFD. Courtesy: Automation LLC

Variable frequency drives are made up of three distinct sections; a rectifier bridge converter, a dc link that creates the bus voltage, and an inverter that converts the dc back into ac. Most inverter switching devices are insulated gate bipolar transistors (IGBTs).

VFDs are often operated at frequencies at or below the rated nameplate frequency of the motor. Operation above the rated speed is possible, but the V/Hz factor of the nameplate is derated, reducing the available power.

By varying the voltage and angle from reference as well as the frequency, torque can also be controlled. Some drive manufacturers refer to these as vector or direct torque control (DTC) drives.

An embedded microprocessor controls the overall operation of the VFD. Parameters such as acceleration, deceleration and preset speeds can be programmed, and feedback like bus voltage, current, and speed can be obtained. These parameters can be set and displayed by means of an operator interface built into the drive or exchanged with a controller by communications like Modbus or Ethernet.

In addition to setting and displaying parameters from the operator interface, physical discrete and analog signals can be used to start and stop the drive, select preset speeds, and control the motor speed with a reference (usually 0 to 10 Vdc). Most discrete signals are 24 Vdc.

Starting and stopping a drive can be done using two different wiring schemes. Two-wire control directly energizes a run command input; if the signal is removed, the drive will stop. An enable signal can be used in this configuration to disable the run signal, but if the enable signal returns, the drive could start if the run signal is present. For this reason, two-wire control is not considered as safe as the alternative described next.

Three-wire control as shown in Figure 3 uses a stop signal to enable the drive. The stop signal is usually required to be on for the drive to run; that way, if the wire is cut, the drive will stop. A separate enable or safe-torque off (STO) signal is still often used for safety purposes.

Two and three-wire control is often one of the selectable parameters for VFDs.

Digital inputs can be assigned functions such as selecting a preset speed, clearing alarms or faults, and placing the drive under automatic or manual control. Digital output signals may be assigned to indicate that the drive is at speed, running or faulted. Outputs may be normally open (form A) or form C with both NO and NC contacts. Form C allows dual connections for running/not running or faulted/not faulted. Using relay type outputs allow connections to external pilot lights for drive status.

A safety disable (STO) input may also be used when required to meet industrial safety standards. This is in addition to the enable signal or stop signal for 3-wire control.

The main circuit wiring for a VFD accepts line voltage in and provides load power to the motor. VFDs may be single or three-phase, so it is important to select the correct drive for your motor. Three phase drive power input terminals are generally designated as R, S, and T or L1, L2 and L3, while output terminals to the motor are often labeled U, V and W or T1, T2 and T3.

DC bus terminals aren’t used as often. Components such as dc reactors, power regenerative units, and dynamic braking modules can be interfaced with these terminals, which are often labeled + and -. Even more uncommon uses for these terminals include the addition of capacitance or common bussing.

Besides wiring, parameter changes or dip switch settings are often required for setup of a VFD. Manufacturers use different features and labels, so it is very important to carefully read the manual.

Figure 7 shows typical control wiring connections for a VFD. There are a lot of options shown in this diagram; sink vs. source wiring for discrete signals, 0 to 10 V vs. 4 to 20 mA for the analog I/O, and two wire vs. three wire control as indicated by the jumper from terminal 1 to 11. Digital inputs 1 to 4 are also configurable for preset speed selects or jogging and stopping profiles.

Figure 7: Example of an Allen-Bradley Powerflex 40 control wiring. Courtesy: Automation LLC

The output relay is also programmable as a fault or running/at frequency output, or feedback for direction or logical functions.

Figure 8 shows power wiring for the same PowerFlex 40 drive. EMI is an acronym for electro-magnetic interference. Because VFDs create powerful radio frequency signals, filters are required to minimize the effects on other components.

Figure 8: Example of VFD power wiring. Courtesy: Automation LLC

Proper grounding of the drive, motor and enclosure are also important considerations in VFD installations. There are many internal diagnostic functions to detect problems with VFDs. Fault codes can be displayed on the operator interface on the front of the drive or sent via communications.

Other troubleshooting techniques involve measuring voltage on the control signal wiring and at both the infeed and outfeed of the drive. Because there are many auxiliary components in a VFD system, it is necessary to use electrical schematics and the drive user manual to guide the technician through the system.

Figure 9 shows the parameters that are exchanged with an Allen-Bradley ControlLogix PLC when the PowerFlex 40 VFD shown previously is added to the Ethernet I/O network. The I parameters show feedback from the device, and the O parameters show the commands to the drive. These addresses and parameters can be used just like physical I/O in the PLC to operate the drive.

Figure 9: Example of VFD (PowerFlex 40) parameters in a PLC. Courtesy: Automation LLC

Figure 10 shows the front of the PowerFlex 40. All of the parameters listed in Figure 10 are available for programming and display from the controls shown via menu selection. The drive can be operated via discrete wiring as shown in Figure 7, from PLC commands over Ethernet, or by pressing buttons and adjusting speed from the drive controls shown here.

Figure 10: PowerFlex 40 VFD with front accessible controls. Courtesy: Automation LLC

The bottom cover of the VFD is removed to connect the wiring. Voltage and current in VFD power wiring can be dangerous, follow proper safety precautions.

– This has been edited from the “Maintenance and Troubleshooting in Industrial Automation” book by Frank Lamb, the founder and owner of Automation Consulting LLC and a member of the Control Engineering editorial advisory board.

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