Electric motors are rated for a specific base speed, and if you need your operation to run slower than that, there are a few things you need to consider:
- Electric motor turndown ratio
- If the load is constant torque or variable torque
- What type of control you’ll need your VFD to be capable of
- Does your motor need an encoder
- What happens when running a motor below base speed
How to Calculate Turndown Ratio
The turndown ratio is the ratio of how slow a motor can go below the base speed without overheating and damaging the motor. You can find the turndown ratio on your motor’s nameplate or in the manufacturer’s motor datasheet.
Common turndown ratios you’ll come across are: 4:1, 10:1, 20:1, and 1000:1. The higher the first number in the ratio, the slower the motor can safely go.
A turndown ratio of 4:1 means the motor can safely be slowed to a quarter of its base speed. A turndown ratio of 10 to 1 means the motor can safely operate at one-tenth of its base speed, and so on. This is demonstrated in the calculation below:
Most general-duty electric motors can have a 10 to 1 turndown. Meaning, if your motor is 1800 RPM, it can be slowed down to 180 RPM without overheating the motor.
Basic “inverter duty” or “inverter ready” motors have a turndown ratio of 20:1 when combined with scalar VFD control. And inverter duty motors have a turndown ratio of 1000:1 or higher.
Variable Torque vs Constant Torque
The turndown ratio can be listed as constant torque (CT), variable torque (VT), or both. In cases where CT or VT isn’t indicated, assume the rating is listed as VT.
Constant Torque loads require the same torque output regardless of the RPM. This means that the shaft must rotate with the same force regardless of the speed. In electric motors, the torque produced is proportional to the current drawn and consequently the heat produced. Thus, constant torque loads produce a disproportionately large amount of heat at low RPM and require more robust cooling.
Variable torque loads are loads that vary depending on the speed. Generally, as the speed decreases, the load on the motor decreases as well. VT loads require a torque output that is the square of the speed. For example, a 10HP load at a rated speed of 1800 RPM will only require 2.5HP at 900 RPM. VT loads generally only need minimal cooling ability.
Each motor will have separate VT and CT turndown ratios as the current needed in CT applications produces more heat at low speeds than an equivalent VT application. For TEFC and ODP motors, the cooling performance of the motor degrades significantly at low speeds due to the speed of the fans being tied to the speed of the rotor which makes the low RPM heat load of constant torque applications more problematic.
You’ll find some brands, like WEG, have general duty motors with a VT turndown ratio of 1000:1. Please note that this is not the same as a 1000:1 turndown motor which is rated for CT loads.
Scalar Control vs Vector Control
Scalar control is volts/hertz control. Supply frequency is varied to control the voltage supplied to the motor, and consequently, the motor’s speed. Theoretically, this form of control keeps the torque constant as the speed of the motor increases and decreases. Motors will see reduced efficiency as the speed gets quite low, particularly as the motor reaches a standstill. In this case, the motor will require a voltage boost at lower speeds to maintain its efficiency.
While scalar control isn’t the most accurate type of VFD control, it is effective for turndown ratios of 5:1 or less.
Vector control, on the other hand, is much more accurate and offers a higher degree of control by managing the torque current and the flux current separately. Vector control is a closed-loop system that requires a shaft encoder or hall effect sensor to be added to the motor setup and fed to the analogue input of the VFD. The encoder or sensor detects the rotation of the motor’s shaft and provides feedback to the VFD. Allowing for precise control at low speeds or even at a standstill.
Vector control is highly effective for higher turndown ratios, like 1000:1 and 2000:1, particularly where high levels of low-speed torque are required by the load.
You’ll find that most modern VFDs are capable of both scalar and vector control. Though, a different setup will be required to include the sensors and appropriate settings.
What Happens When Running a Motor Below Base Speed
You must know the speed required by the intended load before selecting your electric motor. Running a motor below the calculated turndown RPM will reduce the efficiency of the cooling system, likely damaging the motor. Specifically in motors where the cooling system is attached to the shaft, like Totally Enclosed Fan-Cooled or Open Drip Proof motors. As the shaft spins slower, so does the shaft-mounted fan, and air will no longer circulate through the machine.
This is amplified even more in situations where the motor is already under full load. When a motor runs under full torque at low speeds, heat builds quickly.
Thermal damage can be either electric or mechanical. The insulation around the motor’s stator and rotor windings can break down, resulting in damaged wiring and short circuits. Excessive levels of heat can break down bearing lubrication, and lead to bearing damage as well.
When Do You Need to Turn Down a Motor?
Since the speed of an AC industrial motor is based on the frequency of the power supply, your average motor will only run at a single speed unless paired with a speed control device.
There are many situations where you’ll want to run your motor at a lower RPM. You may need to slow down a conveyor system, move the load on a crane very slowly, or run a water pump at a slow speed to decrease the psi down the line.
Knowing the speed and torque required by your intended load will help you determine what turndown ratio you’ll need your motor to be capable of. And matching your turndown ratio to the appropriate type of control will help you avoid causing thermal damage to your motor and keep your operation as efficient as possible.
Still unsure what turndown ratio your application needs? Get in contact with our electric motor and VFD specialists. They’d be happy to help you find the requirements for your intended set-up.
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